Citation
Diel Variations in the Vertical Distribution of Zooplankton in Lake Monroe, Florida

Material Information

Title:
Diel Variations in the Vertical Distribution of Zooplankton in Lake Monroe, Florida
Creator:
BURNES JR, ROBERT M.
Copyright Date:
2008

Subjects

Subjects / Keywords:
Cloud cover ( jstor )
Lakes ( jstor )
Nauplii ( jstor )
Oxygen ( jstor )
Phytoplankton ( jstor )
Predators ( jstor )
Standard error ( jstor )
Taxa ( jstor )
Vertical distribution ( jstor )
Zooplankton ( jstor )
Lake Monroe ( local )

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright Robert M. Burnes, Jr. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Embargo Date:
5/31/2008
Resource Identifier:
659874712 ( OCLC )

Downloads

This item is only available as the following downloads:


Full Text

PAGE 1

1 DIEL VARIATIONS IN THE VERTICAL DI STRIBUTION OF ZOOPLANKTON IN LAKE MONROE, FLORIDA By ROBERT M. BURNES, JR. A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTERS OF SCIENCE UNIVERSITY OF FLORIDA 2006

PAGE 2

2 Copyright 2006 by Robert M. Burnes, Jr.

PAGE 3

3 ACKNOWLEDGEMENTS I would like to thank my major professor Dr. Ed Phlips as well as my entire committee of Dr. Charles Cichra and Dr. Thomas Crisman for a ll of their help and s upport during the duration of this experiment. I would also like to thank Mrs. Mary Cichra for her guidance, support, and also her help with the phytoplan kton portion of the project. In addition, I would like to thank Mark Rogers and Dr. Mike Allen for their aid w ith fish data, as well as Dr. Roger Bachmann for his help with my questions on wind.

PAGE 4

4 TABLE OF CONTENTS page ACKNOWLEDGEMENTS.............................................................................................................3 LIST OF TABLES................................................................................................................. ..........5 LIST OF FIGURES................................................................................................................ .........6 ABSTRACT....................................................................................................................... ..............7 CHAPTER 1 INTRODUCTION................................................................................................................... .9 2 MATERIALS AND METHODS...........................................................................................13 Study Site..................................................................................................................... ...........13 Methods........................................................................................................................ ..........13 Collection Methods............................................................................................................. ....13 Laboratory Analysis........................................................................................................14 Statistical Methods..........................................................................................................15 3 RESULTS........................................................................................................................ .......19 Physical-Chemical Parameters...............................................................................................19 Zooplankton Taxa............................................................................................................... ....20 Zooplankton Distribution Patterns..........................................................................................21 4 DISCUSSION..................................................................................................................... ....38 LIST OF REFERENCES............................................................................................................. ..43 BIOGRAPHICAL SKETCH.........................................................................................................47

PAGE 5

5 LIST OF TABLES Table page 2-1 Light availability regimes in terms of percent cloud cover in Lake Monroe from 1217 June, 2004. Date (time) indicates the dates and times for when cloud cover was present........................................................................................................................ ........17 2-2 Mixed-layer depth (m) distribution in Lake Monroe from 12-17 June, 2004. Date (time) indicates the dates and times fo r when estimated mixed-layer depth was present........................................................................................................................ ........17 3-1 Mean, standard deviation, and range of dissolved oxygen (mg/L), pH (standard units), temperature(C), and chlorophy ll-a (mg/m3) in Lake Monroe from 12-17 June, 2004..................................................................................................................... .....24 3-2 Mean, standard deviation, and range, by depth, for dissolved oxygen (mg/L), pH, temperature (C), and chlorophyll-a (mg/ m3) in Lake Monroe from 12-17 June, 2004........................................................................................................................... .........24 3-3 Scheffe test by depth, for dissolved oxygen (mg/L), pH, temperature (C), and chlorophyll-a (mg/m3) in Lake Monroe from 12-17 June, 2004.......................................25 3-4 ANOVA results for the effects of light availability, mixed-la yer depth, and their interaction on dissolved oxygen (mg/L), te mperature (C), and chlorophyll-a (mg/m3) by depth in Lake Monroe from 12-17 June, 2004. Only results that were significant (at P 0.05) are shown.......................................................................................................25 3-5 Zooplankton taxa broken down by presen ce (X) or absence at each experimental depth and overall prevalence (e.g., comm on, rare, and common/abundant in Lake Monroe from 12-17 June, 2004.........................................................................................26 3-6 Zooplankton mean abundance (individuals/L ), standard error, and range in Lake Monroe from 12-17 June, 2004.........................................................................................27 3-7 ANOVA results for the effects of light availability and mixed-layer depth on zooplankton proportional abundances, by dept h, in Lake Monroe from 12-17 June, 2004. Only values that were signi ficant (at P< 0.05) are shown.......................................27

PAGE 6

6 LIST OF FIGURES Figure page 2-1 Bathymetric map (in meters) of Lake Monroe , Florida. The site used in this study is located at 50x17.5 on this map..........................................................................................18 3-1 Chlorophyll-a values (mg/m3) by depth with corresponding estimated mixed-layer depth (m) in Lake Monroe from 12-17 June, 2004............................................................28 3-2 Dissolved oxygen values (mg/m3) by depth, with corresponding estimated mixedlayer depth (m) for each time period in Lake Monroe from 12-17 June, 2004.................29 3-3 Zooplankton numerical abunda nce for A) copepod nauplii, B) adult copepods, and c) rotifers by depth with corresponding estimated mixed-layer depth (m) for each time period in Lake Monroe from 12-17 June, 2004.................................................................29 3-4 Mean proportional abundance at each de pth (m) for A) copepod nauplii, B) adult copepods, and C) rotifers by time in Lake Monroe from 12-17 June, 2004......................31 3-5 Mean proportional abundance of A) copepod nauplii, B) adult copepods, and C) rotifers by day and night in Lake Monroe from 12-17 June, 2004. Vertical bar indicates one standard error of the mean...........................................................................34 3-6 Mean proportional abundance for A) c opepod nauplii, B) adult copepods, and C) rotifers by light code at post-dawn (0800) a nd pre-dusk (1900) in Lake Monroe from 12-17 June, 2004. Vertical bar indicates one standard error of the mean..........................35 3-7 Mean proportional abundance for A) c opepod nauplii, B) adult copepods, and C) rotifers by light code during midday (1100 and 1530) in Lake Monroe from 12-17 June, 2004. Vertical bar indicates on e standard error of the mean....................................36 3-8 Mean proportional abundance for A) c opepod nauplii, B) adult copepods, and C) rotifers by mixed-layer depth (in meters ) in Lake Monroe from 12-17 June, 2004. Vertical bar indica tes one standard error of the mean.......................................................37

PAGE 7

7 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Masters of Science DIEL VARIATIONS IN THE VERTICAL DI STRIBUTION OF ZOOPLANKTON IN LAKE MONROE, FLORIDA By Robert M. Burnes, Jr. May 2007 Chair: Edward J. Phlips Co chair: Thomas L. Crisman Major Department: Fisher ies and Aquatic Sciences Diel vertical migration (DVM) is one of the most important behaviors of zooplankton in terms of their structure and function in aquatic ecosystems (O hman 1990). The rationale for diel vertical migration seems to be a long-term trade-off between predator avoidance and prey availability, thus enabling the migr ating organisms to maintain the highest level of fitness (Zaret and Suffern 1976; Stich and Lampert 1981; Iwas a 1982; Gliwicz 1986; Eiane and Parisi 2001). The vertical distribution pattern s of DVM in zooplankton are aff ected by the distribution of both their predators (e.g., fish) and prey (e.g., phytoplankton) (Iwasa 1982; Ohman 1990). These vertical distribution patterns can change on a daily basis in res ponse to stochastic environmental variability, mainly in the form of light intens ity and wind re-suspension, both of which can cause a shift in the amplitude of migration and position in the water column (Dodson 1990). This study explored the effects of changes in light and wind intensity on the diel vertical distribution patterns of zooplankt on in a shallow eutrophic Florid a lake. This was accomplished by collecting zooplankton from June 12-17, 2004 us ing a 2-L Van Dorn sampler at depths of 0.5m, 1m, 2m, and 3m at the following times inte rvals: midnight (0200), one hour before sunrise (0600), one hour after sunris e (0800), mid-morning (1100), mid-afternoon (1530), one hour

PAGE 8

8 before sunset (1900), and one hour after sunset (2100). Vertical distri bution patterns were determined by analyzing the pr oportional numerical abundance at each depth to determine if differences occurred based on time of da y, light intensity, and mixed layer depth. The overall zooplankton community did not sh ow observable trends for diel vertical distribution patterns. However, when broke n down into adult copepods, copepod nauplii and rotifers, the copepods did show so me observable diel vertical dist ribution patterns, while rotifers, the dominant organisms of the zooplankton communit y, did not. Light intens ity had some effect on the positioning by depth of the crustaceous zo oplankton, as would be expected. Changes in wind intensity did not appear to have a str ong effect on the verit cal positioning of the zooplankton community within the lake. Othe r factors such as the abundance of algae and shallow lake depth may have influenced th e depth profile of the zooplankton community.

PAGE 9

9 CHAPTER 1 INTRODUCTION Behavior is an important factor in zo oplankton population dynamics (Steele and Henderson 1981; McCauley and Murdoch 1987). Diel vertical migration (DVM) is one of the most important behaviors of zooplankton in term s of their structure and function in aquatic ecosystems (Ohman 1990). The vertical distri bution patterns of DVM in zooplankton are affected by the distribution of both the predator s (e.g., fish) and prey (e.g., phytoplankton) (Iwasa 1982; Ohman 1990). The rationale for diel vertical migration seems to be a long-term trade-off between predator avoidance and prey availabil ity, thus enabling the migrating organisms to maintain the highest level of fitness (Zaret and Suffern 1976; Stich and Lampert 1981; Iwasa 1982; Gliwicz 1986; Eiane and Parisi 2001). For example, zooplankton in some lakes do not carry out vertical migration when there is no difference in food concentration between the upper and lower strata (Pijanowska and Dawidowicz 1986). In such situ ations, inverteb rates avoid the upper strata where there are higher rates of predation by visual f eeders. Zooplankton can offset the loss of daytime foraging opportunity by moving up into the water column to graze at night, when predation by visual predators is greatly reduced (Zaret and Suffern 1976, Iwasa 1982). Vertical migration changes over the course of the year in response to shifting environmental factors such as day length and te mperature, as well as factors such as fish reproductive cycles and phytoplankton abundance. DVM can also change on a daily basis in response to stochastic environmental variability, mainly in the form of light intensity and wind re-suspension, both of which can cause a shift in the amplitude of migrat ion and position in the water column (Dodson 1990). Even the lunar cycle can cause a change in the amplitude of DVM (Gliwicz 1986).

PAGE 10

10 For lakes where DVM occurs, most organisms move into the upper layers of the water column in the evening, with peak abundance arou nd the middle of the night. Individuals then descend slowly and rise again, with another peak usually occurring just before sunrise, followed by a rapid descent after sunris e (Pennak 1944; Wetzel 1975). In 1944, Pennak delineated three categories of movement: The population as a whole moves to the surface after dark and returns to deeper waters during the early morning hours. The population as a whole shows small vert ical movements of a lesser amplitude. The population does not move as a whole. Ra ther, a portion of the population migrates an appreciable distance, pro ducing a vertical distribu tion in the dark hours. Later, Wetzel (1975) suggested that all diel vertical migra tion falls into two categories: Twilight: where two maxima occur near the surface, one at dusk and one at dawn. Nocturnal: characterized by a single peak that is reached sometime between dusk and dawn. The importance of light on influencing daily vertical migratio n patterns has been thoroughly studied, generally showing that, with increased light intens ity, downward movement occurs, and upward movement occurs during decr eased light intensity (Bollens et al. 1994; Rhode et al. 2001) . There are a few proposed driving factor s for this activity; with the most commonly stated reason being pr edator avoidance (Zaret and Suffern 1976; Clark and Levy 1988; Lampert 1989; Bollens and Frost 1991). In la kes with reduced light penetration, either from atmospheric cloudiness or high water turbidity, it is not necessary for zooplankton to move deep during the day as they woul d if light penetrated to greate r depths (Gliwicz 1986; Ringelberg 1995). With lower light levels, there is less of a chance of predator /prey interactions, and therefore zooplankton are not as easily harvested by visual feed ing predators. Therefore, the

PAGE 11

11 tradeoff between predator avoidance and food consumption is lessened. It is clearly advantageous from a fitness standpoint for zoopl ankton to spend more tim e feeding in the upper water column where phytoplankton are more a bundant (Clark and Levy 1988; Lampert 1989). Another, less studied factor that may explai n why zooplankton migra tion is linked to light penetration is avoidance of damage from ultravio let radiation. This was first examined because zooplankton in some fishless lakes still exhi bited diel vertical mi gration (Williamson 2001). Rhodes et al. (2001) showed that cladocerans exhibit vertical mi gration in response to harmful UV radiation. The extent of migr ation for cladoceran species is inversely proportional to their pigmentation (Rhode et al. 2001). Besides the influence of ultraviolet radiati on and predators on migration downward in the water column, predatory invertebrates, mainly Chaoborus , can have the opposite effect (Ohman et al. 1983; Ohman 1990). As z ooplankton migrate down to avoid ri sks in the upper waters, they face a new problem, that of non-visu al invertebrate predators. In systems with large numbers of predatory invertebrates, zooplankton will remain at intermediate depths to avoid predation instead of seeking deeper dept hs (Boeing et al. 2004). In shallow-water systems, sediment re-suspe nsion due to episodic wind events can play a role in altering the environment from the stand point of light penetration and concentration of suspended matter (Luettich et al. 1990). Re-sus pension can greatly redu ce visibility and also liberate nutrients trapped in the sediment (Luetti ch et al. 1990). When episodic sediment resuspension occurs, zooplankton generall y tend to exhibit less vertical migration. This is in part a response by the zooplankton to le ss vertically stratified prey in a well-mixed water column (George 1983; Dodson 1990; Ringelberg 1995). More suspended solids in the water column also

PAGE 12

12 reduce predation on zooplankton by visual feeders, mainly fish, because of reduced visibility (Werner and Hall 1984). Many Florida lakes are shallow and highly productive; therefor e, light intensity and wind re-suspension can play an important role in DVM by zooplankton. The purpose of this study was to determine the effects of light intens ity and wind re-suspensi on on zooplankton diel vertical migration in Lake Monroe, a shallow eutrophic lake in Florida. The study focused on three hypotheses: Variations in irradiance w ill alter vertical distribution patterns of zooplankton, with zooplankton exhibiting a more stratified ve rtical distribution pa ttern as irradiance increases. With an increase in suspended particles and turbidity in the water column, due to an increase in wind, zooplankton wi ll exhibit a less stratified vertical distribution pattern. The alternate hypothesis is that, due to the relative shallow depth and abundant prey supply, zooplankton vertical distribution patterns will not be affected by variations in light intensity or wind mixing.

PAGE 13

13 CHAPTER 2 MATERIALS AND METHODS Study Site Lake Monroe (Figure 2-1) is a natural lake located in Seminole and Volusia counties at 28 50’ N and 81 15’ W. It covers nearly 40 km2, with an east to west width of 8.5km and north to south width of 6km (Ali et al. 2002). The St. Johns River flows into the lake from the east and leaves via the southwest side of the lake. Water flow through the lake is rather slow and nearly unnoticeable to the naked eye (Ali et al. 2002). Wa ter depth ranges from less than one meter to nearly four meters. Bottom sediment in littoral areas is predominat ely sand, with the rest of the lake bottom consisting of fine silt and mud oo ze (Ali et al. 2002). Data from 24 September 2000 to 15 November 2001 showed a mean Secch i depth of 0.7m, mean total phosphorus concentration of 89 g/L, mean total nitrogen co ncentration of 1797 g/L, and mean total chlorophyll concentration of 27.2 g/L (Florida Lakewatch 2002). In June 2004, the Lake Monroe phytoplan kton community was dominated (>95%) by cyanobacteria, both numerically and biovolumetrically. Numerica lly, the dominant taxa were Microcystis incerta and Merismopedia tenuissima . Biovolumetrically, Cylindrosperopsis sp., Chroococcu s spp., and M. incerta were dominant (Mary Cichra, University of Florida, Unpublished Data). Methods Collection Methods Field sampling was carried out from June 12-17, 2004. These dates were selected to maximize the diel variation of light intensity due to moon phase. 0.5-m, 1-m, 2-m, and 3-m samples were collected at a 3-m bottom depth site located approximate ly 100m southwest of channel marker 8 (Figure 2-1). Four liters of wa ter were collected seven times per day using a 2-

PAGE 14

14 L Van Dorn sampler at the following times in tervals: mid-night (0200), pre-dawn(0600), one hour after sunrise (0800), midmorning (1100), mid-afternoon ( 1530), one hour before sunset (1900), and one hour after sunset (2100). For the purposes of this study, 0800, 1100, 1530, and 1900 were classified as daytime, and 2100, 0200, and 0600 were classified as ni ght. Three liters were passed through a 41-m mesh filter, and the filtered contents washed into a 125-ml glass amber bottle. The concentrated zooplankton sample was then preserved with 1% Lugol’s solution. Two 100-ml aliquots of water from each sampling depth were filtered for later chlorophyll-a analysis, at 0200, 0800, 1530, and 2100. A Quanta Hydrolab was used to measure di ssolved oxygen (mg/L), te mperature (C), and pH at 0.5-m intervals. Light r eadings were taken using a Licor light meter, measuring downward light penetration and upward lig ht penetration at 0.5-m interval s. Wind measurements were recorded at nearby Orlando-Sanf ord International Airport usi ng a NOAA anemometer that logs hourly wind data. On-site measurements were also taken using a hand-held anemometer. Samples were not collected on 13 June, 2004 at 0200 due to boat mechanical failure. Also, water chemistry parameter values using the Quan ta Hydrolab were not collected on 15 June, 2004 at 0200 due to Hydrolab malfunction. Laboratory Analysis From each zooplankton sample, a minimum of th ree sub-samples were settled in a 10-ml settling chamber and enumerat ed using an inverted com pound microscope (100x total magnification). For each sample, a total of no le ss than 250 individuals were counted, with at least 100 being of the same taxonomic classi fication. The raw abundance values of the organisms enumerated in each samples were standardized into the number of individuals per liter.

PAGE 15

15 Chlorophyll-a concentrations (ug/L) were obtained from samples (100 ml) filtered onto Gelman A/E glass filters and froze n. These filters were then placed into test tubes with 8.0 ml of 95% ethanol and heated in a water bath at 25.6C (Sartory and Grobbelaar 1984). After a passive extraction period of 24 hours, the filters were remove d from the test tubes, and the remaining test tube liquid left in the test t ubes were then centrifuged to exclude particulate debris. Chlorophyll-a concentrations were then de termined using a Hit achi U2000 dual beam spectrophotometer. Concentrations were corr ected for phaeophytin us ing the acidification method (APHA 1998). Statistical Methods For the purpose of determining the effects of varying light availability and wind on zooplankton population trends, 2way ANOVAs and stepwise multiple regression analyses were conducted using SAS statistical package v8 (SAS 2003). To determine if differing trends occurred between zooplankton groups (rotifers an d crustaceans), as well as for the entire population, all statistical test ing was conducted independently. To accomplish this, the proportions of the total zooplankton abundance f ound at depths of 0.5, 1, 2, and 3 meters were used. This was done to test the hypothesis of changes in the proportion of the population at each depth due to changes in environmental variables. Stepwise multiple regressions were performed at each depth to determine if a relations hip existed between zooplankton proportional abundances and mixed-layer depth, disso lved oxygen, pH, and chlorophyll-a. Two-way ANOVAs were performed to determin e if a difference in the proportion of the zooplankton population at the speci fied depths occurred between day and night, as well as for changes in light availability and wind re -suspension during the daytime hours sampled. Variations in light availability were classified into two categorie s based on percent cloud cover at the time of sampling. The categories were numbered one and two, with one being between 0-

PAGE 16

16 50% cloud cover and two being between 51-100% cl oud cover (Table 2-1). For analysis, data was further categorized by whethe r they occurred at mid-day (1100 and 1530) or post-dawn and pre-dusk (0800 and 1900) to compensate for tem poral differences in li ght availability. Over the course of the experiment, light avai lability changes based on cloud cover were as follows: 9 occurrences of 0-51% cloud cover an d 11 occurrences of 51-100% cloud cover (Table 2-1). Rainfall occurred during five days of the study but was mainly confined to the afternoon. This is common during summer in this part of Fl orida, as nearly 60% of rain occurs between 1400 and 1900 hours (Henry 2000). The effects of wind were measured by conve rting wind speed into mixed-layer depth, which was determined from the formula given in Carper and Bachmann (1984), at the exact location sampled within the lake. The mixed-la yer depth (m) was then seperated into three categories with ranges of 0-1, 1-2, and 2+ (Table 2-2). Estimated mixed-layer depths over the study we re as follows: 21 times for 0-1m, 6 times for 1-2m, and 7 times for >2m (Table 2-2). Prevai ling winds came out of the southeast during the study period, which, due to the location of the stu dy site, allowed for a greater chance to obtain greater mixed-layer depths.

PAGE 17

17 Table 2-1. Light availability regimes in terms of percent cloud cover in Lake Monroe from 1217 June, 2004. Date (time) indicates the dates and times for when cloud cover was present. Category Percent Cloud Cover Date(Time) 1 0-50 12 June (1530), 13 June (0800), 13 June (1100), 13 June (1900), 16 June (0800), 16 June (1530), 16 June (1900), 17 June (0800), 17 June (1100) 2 51-100 13 June (1530), 13 June (1900), 14 June (0800), 14 June (1100), 14 June (1530), 14 June (1900), 15 June (0800), 15 June (1100), 15 June (1530), 15 June (1900), 16 June (1100) Table 2-2. Mixed-layer depth (m) distribution as defined by Carper and Bachman (1984) in Lake Monroe from 12-17 June, 2004. Date (t ime) indicates the dates and times for when estimated mixed-layer depth was present. Category Estimate MixedLayer Depth (m) Date(Time) 1 0-1 12 June (1530), 12 June (2100), 13 June (0600), 13 June (0800), 13 June (1100), 13 June (1900), 14 June (0200), 14 June (0600), 14 June (2100), 15 June (0200), 15 June (0600), 15 June (0800), 15 June (1100), 15 June (1530), 15 June (1900), 15 June (2100), 16 June (0200), 16 June (1100), 16 June (1530), 17 June (0200), 17 June (1100) 2 1-2 13 June (2100), 14 June (0800), 14 June (1100), 16 June (0600), 16 June (0800), 17 June (0800) 3 2+ 12 June (1900), 13 June (1530), 14 June (1530), 14 June (1900), 16 June (1900), 16 June (2100), 17 June (0600)

PAGE 18

18 Figure 2-1. Bathymetric map (in meters) of Lake Monroe, Florida. Used with permission of Dr. Richard J. Lobinske. The site used in this study is located at 50x17.5 on this map.

PAGE 19

19 CHAPTER 3 RESULTS Physical-Chemical Parameters Mean dissolved oxygen, pH, and temperature d ecreased with depth (Table 3-1). In contrast, chlorophyll-a showed th e opposite trend, with mean valu es increasing with depth and the highest values recorded at the 3m depth (Table 3-1). Th e mean values for water quality parameters for the entire water column were 7.1 mg/L for dissolved oxygen, 8.6 for pH, 29.9 C for temperature, and 34.6 mg/m3 for chlorophyll-a (Table 3-2). Temporal and spatial trends were observ ed for pH. Higher pH was observed during daytime, especially during early morning, and decreased with depth. Temperature decreased during night. Chlorophyll-a values did not exhi bit any defined temporal patterns (Figure 3-1). Dissolved oxygen (DO) showed diurnal and de pth trends (Figure 3-2). DO was highest in midday and afternoon (1530 and 1900) and lo west during the early morning (between 0200 and 0800). DO was generally lowest at 3.0m and highest at 0.5m (F igure 3-2). On 13 June at 1100, at 2.5 and 3.0m, oxygen was low enough to be considered hypoxic; however, there is some uncertainty about the validity of these two observa tions due to possible mach ine error. A Scheffe test was used to determine what significant differences existed among depths for DO, pH, temperature, and ch lorophyll-a. For DO, a signifi cant difference was found between the means at depths of 1.0m and shallower and the mean at 3.0m (Table 3-3). For pH, surface and 0.5-m means were significan tly different from 2.5-m a nd 3.0-m means and 1.0 and 1.5-m means were significantly different from the 3.0-m mean (Table 3-3). Temperature and chlorophyll-a means did not significantly differ among depths (Table 3-3). ANOVA tests were conducted to determine if any water quality parameters were affected by irradiance (i.e., time of day and cloud cover) and/or mixed-layer depth (Table 3-4).

PAGE 20

20 Irradiance had a significant impact on mean su rface DO and mixed-layer de pth had a significant impact on mean DO at 2.0m (Table 3-4). The inte raction of light availa bility and mixed-layer depth was significant for mean DO at 2.0, 2.5, a nd 3.0m (Table 3-4). Changes in light availability and mixed-layer de pth had significant effects on mean chlorophyll-a at 3.0m. No other significant trends we re observed (Table 3-4). In terms of chlorophyll-a, as mixed-layer de pth increased, there was a greater disparity between values (Figure 3-1). Mixed-layer depth appeared to affect dissolved oxygen, in that during long periods of calm weather, DO became stratified (Figure 3-2). Zooplankton Taxa Eighteen Rotifera, 7 Copepoda, and 4 Cladocera taxa were identified (Table 3-5). Of those, 4 taxa, all rotifers, were only found in single samples from individual depths. They were Brachionus calyciflorus, Conochilloides sp ., Conochillus sp ., and Testudinella sp . (Table 3-5). Rotifers that were found at every depth included Anuraeopsis sp ., Brachionus havanensis, Filinia longiseta, Hexarthra sp . , Keratella cochlearis, Notommata sp ., Polyarthra vulgaris , and Synchaeta sp . The latter taxa were found in at least 30% of samples. The 3.0-m depth had the greatest number of taxa for all three of the major taxonomic groups (Table 3-5). Five copepods were commonly found at every depth during the study, including Arctodiaptomus dorsalis , calanoid copepodids , cyclopoid copepodids , Mesocyclops edax, and nauplii spp. (Table 3-5). Two taxa were not found throughout the water column, Acartia sp., and harpactacoid sp., with the latter taxa rarely found in samples, i. e., less than 10% of the time. The cladocerans Bosmina longirostris , Daphnia sp . , and Diaphanosoma sp . were all common and found at every depth (Table 3-5). Ceriodaphnia sp . were also commonly found but at only three of the depths (Table 3-5).

PAGE 21

21 Rotifera was the most numerically abundant gr oup by nearly 3:1 (Table 3-6). The next most abundant group was Copepoda, and the least abundant of the three major taxonomic groups was Cladocera. Based on mean densities over the study period, Ro tifera averaged 506 individuals/L, Copepoda 203 individuals/L, and Cladocera 4.8 individuals/L (Table 3-6). Zooplankton Distribution Patterns Nauplii and adult copepods exhibited differing temporal trends in vertical distribution. Mean nauplii densities for the water column were lowest at night, irrespective of mixed-layer depth (Figure 3-3A). Conversely, mean adu lt copepod densities were low during the day, irrespective of mixed-laye r depth (Figure 3-3B). No observabl e trends were observed for rotifer densities (Figure 3-3C). The major taxonomic groups of copepod naupl ii, adult copepods, and rotifers were further tested by breaking down their mean propor tional abundance by sampling time, day/night, light availability, and wind code . Mean proportional abundance is defined as the percentage of the community at a given depth (e.g., 0.5, 1, 2, and 3m) at a given sampling time. Since cladoceran densities were low, mean of 4.8 indivi duals/L (Table 3-6), it was hard to distinguish any trends between mixed-layer depth and time of day. Therefore, this group was excluded from further statistical and gr aphical analyses. Mean proportional abundance of copepod na uplii were highest at 0.5m at 1100, for 1.0m and 2.0m at 1900, and for 3.0m at 1530 (Figure 34A). For adult copepods, proportional abundance was greatest at 2100 for 0.5m, at 0200 for 1.0m, at 1900 for 2.0m, and at 0800 for 3.0m (Figure 3-4B). Greatest mean proportional abundances for rotifers were at 1100 for 0.5m, at 0800 for 1.0m, at 1530 for 2.0m, and at 0200 for 3.0m (Figure 3-4C). Comparing night versus day, mean propor tional abundance for copepod nauplii was similar (Figure 3-5A). The great est and least proportions of the community were located at 0.5m

PAGE 22

22 (0.27) and 3.0m (0.23) during the day (0800, 1100, 1530, and 1900) and at 1.0m (0.26) and 2.0m (0.24) at night (2100, 0200, and 0600), respectively (Figure 3-5A). For adult copepods, the mean proportional abundances were higher deeper in the water column during the day, with values of 0.32 (at 3.0m) and 0.36 (at 2.0m) and lowest at 0.2m with a value of 0.10. While at night, they were more evenly distributed with in the water column (Figure 3-5B). Mean proportional rotifer abundance was similar be tween day and night (Figure 3-5C). In terms of light availability at post-dawn and pre-dusk, during times of mostly clear skies (0-50% cloud cover), copepod nauplii had a slightly greater proportion of the nauplii located in the shallower depths (Figure 3-6A). However, dur ing times of greater than 50% cloudiness, this trend does not hol d true, in that the nauplii were located more at the middle of the water column, at 1m and 2m (Figure 3-6A). For adult cope pods, there appears to be some slight difference between the vertical distribu tion with different cloud cover (Figure 3-6B). Rotifers were located shallower during periods of less than 50 % cloud cover and deeper when cloud cover was greater than 50% (Figure 3-6C). During midday, copepod nauplii were located deeper in the water column during periods of less than 50% cloud cover than during periods of greater than 50% cloud cover (Figure 3-7A). Adult copepod trends differed between cloud cove r in that during periods of low cloud cover (less than 50%), adult copepods we re deeper in the wa ter column, while duri ng periods of great cloud cover, the adult copepod dist ribution became more even (Figure 3-7B). Rotifer trends differed in that during periods of greater than 50% cloud cover, rotifers were found shallower (Figure 3-7C). For copepod nauplii, vertical distribution was relatively ev en during periods of low to moderate wind intensity, less than or 2m mixed-layer depth (Fi gure 3-8A). However, during

PAGE 23

23 periods of greatest wind intensity, a greater propor tion of the nauplii were located in the middle of the water column (Figure 3-8A ). Adult copepod proportional abundances were similar during periods of extreme calm and extreme wind (Figur e 3-8B). No apparent trend was observed between the proportional abundances for rotifers w ith changes in mixed-layer depth (Figure 38C).

PAGE 24

24 Table 3-1. Mean, standard deviation, and ra nge of dissolved oxygen (mg/L), pH (standard units), temperature(C), and chlorophyll-a (mg/m3) in Lake Monroe from 12-17 June, 2004. Variable Mean Standard Deviation Range DO 7.1 0.10 0.4-11.0 pH 8.6 0.02 6.9-9.1 Temperature 29.9 0.10 29.4-30.4 Chlorophyll-a 34.6 1.09 11.0-59.5 Table 3-2. Mean, standard deviation, and ra nge, by depth, for dissolved oxygen (mg/L), pH, temperature (C), and chlorophyll-a (mg/m3 ) in Lake Monroe from 12-17 June, 2004. Variable Mean Standard Deviation Range Dissolved Oxygen Surface 7.7 0.23 5.5-11.0 0.5m 7.6 0.21 5.5-10.2 1.0m 7.4 0.20 5.3-9.7 1.5m 7.2 0.21 5.2-9.6 2.0m 7.1 0.21 5.0-9.6 2.5m 6.5 0.28 0.6-9.5 3.0m 6.2 0.30 0.4-10.0 pH Surface 8.7 0.04 8.2-9.1 0.5m 8.7 0.04 8.1-9.1 1.0m 8.7 0.05 8.0-9.0 1.5m 8.6 0.05 7.9-9.0 2.0m 8.5 0.05 7.8-9.0 2.5m 8.4 0.07 7.4-8.9 3.0m 8.3 0.08 6.9-8.8 Temperature Surface 30.2 0.13 29.3-32.9 0.5m 30.1 0.10 29.2-31.4 1.0m 29.8 0.18 29.6-31.3 1.5m 30.3 0.66 29.4-30.4 2.0m 29.8 0.08 29.0-30.7 2.5m 29.7 0.08 28.9-30.7 3.0m 29.6 0.08 28.8-30.5 Chlorophyll-a 0.5m 30.9 2.26 11.0-50.1 1.0m 34.8 2.10 13.6-49.5 2.0m 35.0 2.00 17.3-51.6 3.0m 37.4 2.25 17.3-59.5

PAGE 25

25 Table 3-3. Scheffe test by de pth, for dissolved oxygen (mg/L), pH, temperature (C), and chlorophyll-a (mg/m3) in Lake Monroe from 12-17 June, 2004. Variable Depths Surface 0.5m 1.0m 1.5m 2.0m 2.5m 3.0m Dissolved Oxygen A___________________________________ (N=245) B_____________________ Surface 0.5m 1.0m 1.5m 2.0m 2.5m 3.0m pH A_____________________________ (N=245) B ____________________ C________________ Surface 0.5m 1.0m 1.5m 2.0m 2.5m 3.0m Temperature A__________________________________________ (N=238) Surface 1.0m 2.0m 3.0m Chlorophyll-a A_______________________ (N=80) Table 3-4. ANOVA results for the effects of li ght availability, mixedlayer depth, and their interaction on dissolved oxygen (mg/L), te mperature (C), and chlorophyll-a (mg/m3) by depth in Lake Monroe from 12-17 June, 2004. Only results that were significant (at P 0.05) are shown. Depth Environmental Variable N F Value P Value Dissolved Oxygen Surface Light availability 34 3.90 0.01 2.0m Mixed-layer depth 34 5.86 0.01 2.0m Light availability x Mixed-layer depth 34 5.11 0.01 2.5m Light availability x Mixed-layer depth 34 2.80 0.05 3.0m Light availability x Mixed-layer depth 34 2.78 0.05 Temperature Surface Light availability 33 10.92 <0.01 0.5m Light availability 33 3.25 0.03 2.0m Light availability 33 5.38 <0.01 2.5m Light availability 33 5.63 <0.01 3.0m Light availability 33 2.94 0.04 Chlorophyll-a 3.0m Light availability 18 4.24 0.04 3.0m Mixed-layer depth 18 4.92 0.04

PAGE 26

26 Table 3-5. Zooplankton taxa broken down by pr esence (X) or absence at each experimental depth and overall prevalence (e.g., comm on, rare, and common/abundant in Lake Monroe from 12-17 June, 2004). Great Group Genus/ Species Depth (m) 0.5 1.0 2.0 3.0 Commona/Rareb/ Common/Abundantc Cladocera Bosmina longirostris X X X X Rare Ceriodaphnia sp . X X X Rare Daphnia sp. X X X X Rare Diaphanosoma sp. X X X X Common Copepoda Acartia sp . X X Rare Arctodiaptomus dorsalis X X X X Common Calanoid spp. X X X X Common/Abundant Cyclopoid spp . X X X X Common/Abundant Harpactacoid sp . X X X Rare Mesocyclops edax X X X X Common Nauplii spp. X X X X Common/Abundant Rotifera Anuraeopsis sp . X X X X Common Asplanchna sp . X Rare Brachionus angularis X X Rare Brachionus calyciflorus X Rare Brachionus havanensis X X X X Common/Abundant Conochiloides sp . X Rare Conochilus sp. X Rare Filinia longiseta X X X X Common Harrangia sp . X X Rare Hexarthra sp . X X X X Common Keratella cochlearis X X X X Common/Abundant Keratella quadrata X X Rare Monostyla bulla X Common Notommata sp . X X X X Common/Abundant Polyarthra vulgaris X X X X Common/Abundant Synchaeta sp. X X X Rare Testudinella sp . X Rare Trichocerca sp. X X Rare aCommon is greater than 10% of the time bRare is less than 10% of the time cCommon/Abundant is greater than 10% of the time a nd having at least 10 individuals/L more than 50% of the time.

PAGE 27

27 Table 3-6. Zooplankton mean abundance (individua ls/L), standard error, and range in Lake Monroe from 12-17 June, 2004. Taxonomic Group Mean Standard Error Range Rotifera 506 33 0.0-1790 Cladocera 4.8 3.2 0.0-433 Copepoda 203 20.9 0.0-2690 Table 3-7. ANOVA results for the effects of li ght availability and mixed-layer depth on zooplankton proportional abundances, by dept h, in Lake Monroe from 12-17 June, 2004. Only values that were signi ficant (at P< 0.05) are shown. Depth Environmental Variable N F Value P Value All Groups 1.0m Light availability 20 4.35 0.03 3.0m Light availability 20 7.53 <0.01 Adult Copepod (0800 and 1900) 3.0m Light availability 10 8.10 0.02 Adult Copepod (1100 and 1530) 0.5m Light availability 10 5.91 0.04 All Groups 1.0m Mixed-layer depth 20 5.96 0.02 Rotifera 3.0m Mixed-layer depth and Light availability Interaction 20 6.22 0.01

PAGE 28

28 0 1 2 315:30 21:00 2:00 8:00 15:30 21:00 2:00 8:00 15:30 21:00 2:00 8:00 15:30 21:00 6:00 8:00 15:30 21:00 2:00 8:0012Jun 12Jun 13Jun 13Jun 13Jun 13Jun 14Jun 14Jun 14Jun 14Jun 15Jun 15Jun 15Jun 15Jun 16Jun 16Jun 16Jun 16Jun 17Jun 17Jun Time/DateWave Depth (m)0 10 20 30 40 50 60 70Chlorophyll-a (mg/m3) Wave Depth 0.5m 1.0m 2.0m 3.0m Figure 3-1. Chlorophyll-a values (mg/m3) by dept h with corresponding estimated mixed-layer depth (m) in Lake Monroe from 12-17 June, 2004.

PAGE 29

29 0 1 2 315:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:001212121313131313131314141414141414151515151515151616161616161617171717 Time/DayWave Depth (m)0 2 4 6 8 10 12D.O. (mg/m3) Wave Depth 0.5m 1.0m 2.0m 3.0m Figure 3-2. Dissolved oxygen values (mg/m3) by depth, with corresponding estimated mixe d-layer depth (m) for each time period in Lake Monroe from 12-17 June, 2004. A) 0 1 2 315:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:001212121313131313131314141414141414151515151515151616161616161617171717 Time/DayWave Depth (m)0 200 400 600 800 1000Abundance (#/L) Wave Depth 0.5m 1.0m 2.0m 3.0m 2553 #/L Figure 3-3. Zooplankton numerical abundance for A) copepod nauplii, B) adult co pepods, and c) rotifers by depth with correspon ding estimated mixed-layer depth (m) for each time period in Lake Monroe from 12-17 June, 2004

PAGE 30

30B) 0 1 2 315:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:001212121313131313131314141414141414151515151515151616161616161617171717 Time/DayWave Depth (m)0 50 100 150 200 250Abundance (#/L) Wave Depth 0.5m 1.0m 2.0m 3.0m C) 0 1 2 315:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:00 15:30 19:00 21:00 2:00 6:00 8:00 11:001212121313131313131314141414141414151515151515151616161616161617171717 Time/DayWave Depth (m)0 200 400 600 800 1000 1200 1400 1600 1800 2000Abundance (#/L) Wave Depth 0.5m 1.0m 2.0m 3.0m Figure 3-3. (continued)

PAGE 31

31 A) 0200 0600 0800 1100 1530 1900 2100 Figure 3-4. Mean proportional abundance at each depth (m) fo r A) copepod nauplii, B) adult copepods, and C) rotifers by time in Lake Monroe from 12-17 June, 2004. Legend 0.5 1 2 3

PAGE 32

32 B) 0200 0600 0800 1100 1530 1900 2100 Figure 3-4. (continued) Legend 0.5 1 2 3

PAGE 33

33 C) 0200 0600 0800 1100 1530 1900 2100 Figure 3-4. (continued) Legend 0.5 1 2 3

PAGE 34

34 A) 0 0.1 0.2 0.3 0.4 0.5 DayNight Day/NightProportional Abundance 3.0m 2.0m 1.0m 0.5m B) 0 0.1 0.2 0.3 0.4 0.5 DayNight Day/NightProportional Abundance 3.0m 2.0m 1.0m 0.5m C) 0 0.1 0.2 0.3 0.4 0.5 DayNight Day/NightProportional Abundance 3.0m 2.0m 1.0m 0.5m Figure 3-5. Mean proportional abundance of A) copepod nauplii, B) adult copepods, and C) rotifers by day and night in Lake Monroe from 12-17 June, 2004. Vertical bar indicates one standard error of the mean.

PAGE 35

35 A) 0 0.1 0.2 0.3 0.4 0.5 0-50%51-100%Cloud CoverProportional Abundance 3m 2m 1m 0.5m B) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0-50%51-100%Cloud CoverProportional Abundance 3m 2m 1m 0.5m C) 0 0.1 0.2 0.3 0.4 0.5 0-50%51-100%Cloud CoverProportional Abundance 3m 2m 1m 0.5m Figure 3-6. Mean proportional abundance for A) copepod nauplii, B) adult copepods, and C) rotifers by light code at post-dawn (0800) a nd pre-dusk (1900) in Lake Monroe from 12-17 June, 2004. Vertical bar indicates one standard error of the mean.

PAGE 36

36 A) 0 0.1 0.2 0.3 0.4 0.5 0-50%51-100%Cloud CoverProportional Abundance 3m 2m 1m 0.5m B) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0-50%51-100%Cloud CoverProportional Abundance 3m 2m 1m 0.5m C) 0 0.1 0.2 0.3 0.4 0.5 0-50%51-100%Cloud CoverProportional Abundance 3m 2m 1m 0.5m Figure 3-7. Mean proportional abundance for A) copepod nauplii, B) adult copepods, and C) rotifers by light code during midday (1100 and 1530) in Lake Monroe from 12-17 June, 2004. Vertical bar indicates on e standard error of the mean.

PAGE 37

37 A) 0 0.1 0.2 0.3 0.4 0.5 0-11-22+ Mixed-Layer Depth (m)Proportional Abundance 3.0m 2.0m 1.0m 0.5m B) 0 0.1 0.2 0.3 0.4 0.5 0-11-22+ Mixed-Layer Depth (m)Proportional Abundance 3.0m 2.0m 1.0m 0.5m C) 0 0.1 0.2 0.3 0.4 0.5 0-11-22+ Mixed-Layer Depth (m)Proportional Abundance 3.0m 2.0m 1.0m 0.5m Figure 3-8. Mean proportional abundance for A) copepod nauplii, B) adult copepods, and C) rotifers by mixed-layer depth (in meters ) in Lake Monroe from 12-17 June, 2004. Vertical bar indica tes one standard error of the mean.

PAGE 38

38 CHAPTER 4 DISCUSSION The purpose of this study was to determine if differences in light availability and/or mixed layer depth had effects on the ver tical distribution of zooplankton in a shallow, eutrophic Florida lake. The null hypothesis for the study was that the shallow depth and high productivity of the lake would lead to uniformity in the vertical distribution of zooplankton community, despite temporal differences in light availability and mixed layer depth. The results of the study show that the extent of vert ical stratificatio n of zooplankton differ among groups. Overall zooplankton abundance of approximately 500 individuals/L was similar to those found during the same season in Lake Okeechobee by Beaver and Havens (1996) and in Lake Mize by Nordlie (1976). Rotifer, copepod and cla doceran species composition were similar to those found in Lake Monroe by Yount and Bela nger (1988) and in Lake Okeechobee by Beaver and Havens (1996). It was not unexpected, th at rotifers were the numerically dominant zooplankton group. Several studies (Nordlie 197 6; Shireman and Martin 1978; and Bays and Crisman 1983, Crisman et al. 1995) characterize eu trophic Florida freshwater systems as being dominated by rotifers during summer. Direct numerical comparisons of Lake Monroe data between the current study and Y ount and Belanger (1988) are not possible, as collection mesh sizes differed, thus skewing abundances. This lake appears to have similar zooplankton composition and abundance as other shallow, eutrophic systems in Florida Nordlie 1976; Shireman and Martin 1978; and Bays and Cris man 1983; Crisman et al. 1995; Robert Burnes, University of Florida, Unpublished Data). One factor that could possibly aid in rotif er dominance is the composition of the fish community in the lake (Mike Allen and Mark Roge rs, University of Florida, Unpublished Data). Lake Monroe contains many life-long pl anktivores such as various killifish including, bluefin

PAGE 39

39 ( Lucania goodei) , Seminole ( Fundulus seminolis) , rainwater ( Lucania parva) , and least ( Heterandria formosa ) (Keast and Fox 1990), Gambusia (Hurlbert and Mulla 1981), inland silversides ( Menidia beryllina ) (Lemke et al. 2003), blue tilapia ( Oreochromis aurea ) (Zohary et al. 1994), threadfin shad ( Dorosoma petenense ) (Holanov and Tash 1978), and black crappie ( Pomoxis nigromaculatus ) (Carlander 1977). These fish se lectively feed on zooplankton. Neither copepod nauplii nor the rotifers indi cated any significant differences among the various depths in proportion of the community duri ng day or night. This is quite surprising in a lake with, predation by a dense fi sh community, since it is genera lly assumed that diel vertical migration is a major behavioral mechanism that optimizes a trade-off between prey abundance and predation risk (Stitch a nd Lampert 1981; Iwasa 1982). In terms of adult copepod di el vertical distributions, se veral trends emerged. Adult copepod proportional abundance at the surface was gr eatest during the darkest periods of the day (Figure 3-4B), which is expected as this behavior wa s also observed by Zaret and Suffern (1976) and Iwasa (1982). However, this trend is some what obscured when examined in terms of day and night (Figure 3-5B). This is due to th e 0600 time period, which was categorized as night, but with the closeness of this time period to dawn, when increasing irradiance may have triggered a downward movement of the copepods prior to sampling. The daily vertical distribution cycles of c opepods were further obscured, when taking into account their proportional abundances, due to varia tions in their densitie s. By observing the densities of copepods at each depth, a few trends become apparent in terms of the cyclical values in density. The lower densities of nauplii durin g night and adults during day may indicate that there is little inverteb rate predation, which would allow for th ese organisms to live in the area of the bottom sediment (Ohman et al. 1983).

PAGE 40

40 Though it appears that zooplankton diel vertical migration did not exist for all groups in this lake, daytime fluctuations in light availabi lity did have some eff ect on zooplankton vertical distribution. While rotifers exhi bited little variation in vertical distribution based on changes in light availability, the proportions of copepod naup lii at 1.0, 2.0 and 3.0m varied, in that as light availability decreased, a greater proportion of the population wa s found at shallower depths. This same trend is found to some extent for adu lt copepods (Figure 3-6B). From these results, it is not unreasonable to say that co pepods were more driven by cha nges in irradiance than were rotifers. These results were wh at would be expected since it has been shown that zooplankton try to maintain their location in the water column at some optim al light intensity (Forward 1988; Richards et.al. 1996). The results of wind-driven changes in mi xed-layer depth on zooplankton were not unexpected considering that the crustacean zooplankton have relativ ely stronger swimming abilities than rotifers (Richard son 1992). Copepods were still ab le to regulate their position in the water column, as there were no observable e ffects based on changes in mixed-layer depth. One possible factor for the overall lack of va riation in zooplankton vertical distribution may be the spatial trends and abundance of chlo rophyll-a within the lake. Chlorophyll-a values increased with depth, perhaps indicating a subs urface phytoplankton maxima. Because of this, variations in phytoplankton verti cal distribution may be less amplified in this lake (Dini and Carpenter 1982; Pijanowska and Dawidowicz 1987). This would occur since there is an ample food supply (e.g., phytoplankton) at greater dept hs, thus decreasing the necessity for the zooplankton to travel into the upper reaches of the water column to feed (Dini and Carpenter 1982).

PAGE 41

41 The phytoplankton community composition and bi omass may have affected the vertical distribution pattern of the zooplan kton community within the lake. As previously mentioned, the abundance of phytoplankton at all depths may have influenced the lack of observed variation in zooplankton vertical distribution. The domin ant phytoplankton in the lake during this study, Microcystis and Merismopedia, have both been shown to have moderate to low selectivity by copepods for feeding (Demott and Moxter 1981). This has also been reported for other herbivorous zooplankton (Fulton 1988). It is extremely important, when dealing with large shallow lakes like those in Florida, to take into account wind-driven forces. Howeve r, when observing the effects of wind-driven mixed layer depth on the vertical distributi on of chlorophyll-a and dissolved oxygen, it was apparent that no observable trends were present. The apparent lack of influence by mixed layer depth, on chlorophyll-a is quite in teresting. This is contradict ory to what one would think because of their relative light weight; phytoplan kton would be more influenced by wave action and would exhibit different patt erns of vertical abundance unde r different mixed layer depth conditions such as seen in deeper lakes (Battoe 1 985). It is hypothesized that this could be due to the relative shallowness of Lake Monroe and the high frequency, approximately 40% of the time, that there were wind events that affect ed more than half of the water column. The observed trends in zooplankt on relative abundance, due to changes in light availability and mixed-layer depth, seem to support some of the precepts of my hypothesis. Though there was little variation in verti cal stratification in abundance between times of day, copepods fluctuated in their vertical distri butions during the day in relation to light availability. This is due to their greater ability to self-regulate their pos ition in the water column. Several factors that may have influenced this apparent lack of va riation in vertical di stribution patterns were

PAGE 42

42 identified. When taking into account inhere nt traits, mainly size and swimming ability, between crustaceans and rotifers, it is quite apparent why some of the trends may have been masked in this experiment, if they in fact do exist. The relatively shallow and eu trophic nature of Lake Monroe may also have played a ro le. It does not take much to disturb the entire water column and the abundance of food resources for the smalle r rotifers are more even ly distributed within the water column. This makes it not only tougher for the smaller zooplankton to self-regulate their position, but may in fact ma ke diel vertical migration less important to them as prey (e.g., phytoplankton) are more evenly distributed within the water column. Also, a higher density of crustacean zooplankton might show more distinct va riations in vertical distribution pattern, as they are more affected by changes in light availability (Forward 1988). In summary, the overall zooplankton community did not show observable trends for diel vertical distribution patterns. However, when broken down into adult copepods, copepod nauplii and rotifers, it was apparent that copepods did sh ow some observable diel vertical distribution patterns, while rotifers, the dominant organisms of the zooplankton community in the lake, did not. Light availability had some effect on depth position of both copepod nauplii and adults, as would be expected, within this lake. Changes in wind intensity did not ap pear to have a strong effect on the positioning of zooplankton community. Other factors, such as the abundance of edible food and lake depth may have influenced the depth profile of the zooplankton community.

PAGE 43

43 LIST OF REFERENCES Ali, A., Frouz, J., and Lobinske, R.J. 2002. Spatio-Temporal Effects of Selected PhysioChemical Variables of Water, Algae and Sedi ment Chemistry on the Larval Community of Nuisance Chironomidae (Diptera) in a Natural a nd a Man-MadeLake in Central Florida. Hydrobiologia . 470:181-193. APHA. 1998. Standard Methods for the Analysis of Water and Wastewater, 19th edition. American Public Health Asso ciation, Washington, D.C. Battoe, L.E. 1985. Changes in Vertical Phytoplankton Distribution in Response to Disturbances in a Temperat e and a Subtropical Lake. Journal of Freshwater Ecology . 3(2):167-174. Bays, J.S., and Crisman T.L. 1983. Zooplankton and Trophic State Relati onships in Florida Lakes. Canadian Journal of Fisheries and Aquatic Sciences . 40:1813-1819. Beaver, J.R., and Havens, K.E. 1996. Seas onal and Spatial Variat ion in Zooplankton Community Structure and Their Relation to Possible Controlling Variables in LakeOkeechobee. Freshwater Biology . 36:45-56. Bollens, S.M., and Frost, B.W. 1991. Diel Vert ical Migrations in Z ooplankton: RapidIndividual Response to Predators. Journal of Plankton Research . 13:1359-1365. Bollens, S.M., Frost B.W., and Cordell, J.R. 19 94. Chemical, Mechanical and Visual Cues in the Vertical Migration Behavior of the Marine Planktonic Copepod Acartia-hudsonica . Journal of Plankton Research . 16(5):555-564. Carlander, K.D. 1977. Handbook of Freshwater Fishery Biology , Volume 2. Iowa State University Press, Ames, IA. 431pp. Carper, G.L., and Bachmann, R.W. 1984. Wind Re-suspension of Sediments in a Prairie Lake. Canadian Journal of Fisher ies and Aquatic Sciences . 41:1763-1767. Clark, C.W., and Levy, D.A. 1988. Diel Vertical Migrations by Juvenile Sockeye Salmon and the Antipredation Window. American Naturalist . 131:271-290. Comp, G.S. 1979. Master’s Thesis: Diel And Seasonal Patterns in the Vertical Distribution of Zooplankton in Lake Conway, Florida . University of Florida, Gainesville, FL. 93pp. Crisman, T.L, Phlips, E.J., and Beaver, J.R. 1995. Zooplankton seasonality and trophic state relationships in Lake Okeechobee, Florida. Archiv fur Hydrobiologie , Special Issues Advanced Limnology. 45:213-232. Demott, W.R., and Moxter, F. 1991. Fora ging Cyanobacteria by Copepods: Responses to Chemical Defense and Resource Abundance. Ecology . 72 (5):1820-1834.

PAGE 44

44 Dini, M.L., and Carpenter, S.R. 1992. Fish Pr edators, Food Availabili ty, and Diel Vertical Migration in Daphnia. Journal of Plankton Research . 14:359-377. Dodson, S. 1990. Predicting Diel Ve rtical Migration of Zooplankton. Limnology and Oceanography . 35(5):1195-1200. Eiane, K., and Parisi, D. 2001. Toward a Robust Concept for Modeling Zooplankton Migration. Sarsia . 86:465-475. Florida LAKEWATCH. 2002. Florida LAKEWATCH Annual Data Summaries from 1986 through 2001 . Department of Fisheries and Aquatic Sciences, University of Florida, Institute of Food and Agricultural Sciences, Li brary, University of Florida, Gainesville, FL. Forward, R.B., Jr. 1988. Diel Vertical Migrat ion: Zooplankton Photobiology and Behaviour. Oceanography and Marine Biology Annual Review . 26:361-393. Fulton, R.S. 1988. Grazing on Filamentous Algae by Herbivorous Zooplankton. Freshwater Biology . 20(2):263-271. George, D.G. 1983. Interrelations be tween the Vertical Distribution of Daphnia and Chlorophyll-a in Two Large Limnetic Enclosures. Journal of Plankton Research . 5:457475. Gliwicz, M.J. 1986. Predati on and the Evolution of Verti cal Migration of Zooplankton. Nature . 320(24 April):746-748. Henry, J.A. 1998. Weather and Climate in Water Resources of Florida . Fernald, E.A. and Purdumm E.D. eds. Institute of Science and Public Affairs, Florida State University, Tallahassee, FL. 276pp. Hurlbert, S.H., and Mulla, M.R. 1981. Impacts of Mosquitofish ( Gambusia affinis ) Predation on Plankton Communities. Hydrobiologia . 83(1):125-151. Holanov, S.H., and Tash, J.C. 1978. Particulat e and Filter Feeding in Threadfin Shad, Dorosoma petenense at Different Li ght Intensites. Journal of Fish Biology . 13(5):619625. Iwasa, Y. 1982. Vertical Migration of Zoopl ankton: A Game between Predator and Prey. American Naturalist . 120(2):171-180. Keast, A., and Fox, M.G. 1990. Fish Community Structure, Spatial Distribution and Feeding Ecology in a Beaver Pond. Environmental Biology of Fishes . 27(3):201-214. Kerfoot, W.B. 1970. Bioenergetic s of Vertical Migration. American Naturalist . 104(940):529546.

PAGE 45

45 Lampert. W. 1989. The Adaptive Significance of Diel Vertical Migration of Zooplankton. Functional Ecology . 3:21-27. Lemke, A.M., Stoeckel, J.A., and Pegg, M.A. 2003. Utilization of the Exotic Cladoceran Daphnia lumholtzi by Juvenile Fishes in an Illinois River Floodplain Lake. Journal of Fish Biology . 62(4):938-954. Luettich, R.A, Harleman, D.R.F., and Somlyody, L. 1990. Dynamic Behavior of Suspended Sediment Concentrations in a Shallow Lake Disturbed by Episodic Wind Events. Limnology and Oceanography . 35(5):1050-1067. MacCauley, E., and Murdoch, W.W. 1987. Cyclic and Stable Populations: Plankton as Paradigm. American Naturalist . 129:97-121. McLaren, I.A. 1974. Demographic Strategy of Vertical Migration by a Marine Copepod. American Naturalist . 108:91-102. Nordlie, F.G. 1976. Plankton Communities of Three Central Florida Lakes. Hydrobiologia . 48(1):65-78. Ohman, M.D., Frost, B.W., and Cohen, E.B. 1983. Reverse Diel Migrati on: An Escape from Invertebrate Predators. Science . 220(4604):1404-1407. Ohman, M.D. 1990. The Demographic Benefits of Diel Vertical Mi gration by Zooplankton. Ecological Monographs . 60(3):257-281. Pennak, R.W. 1944. Diurnal Movements of Zooplankton Organisms in Some Colorado Mountain Lakes. Ecology . 25(4):387-403. Pijanowska, J., and Dawidowicz, P. 1987. The Lack of Vertical Migration in Daphnia-The Effect of Homogenously Distributed Food. Hydrobiologia . 148(2):175-181. Richards, S.A., Possingham, H.P., and Noye, J. 1996. Diel Vertical Migration: Modelling Light-Mediated Mechanisms. Journal of Plankton Research . 18(12):2199-2222. Richardson, W.B. 1992. Microcrustecea in Flowi ng Water: Experimental Analysis of Washout Times and a Field Test. Freshwater Biology . 28(2):217-230. Ringelberg, J. 1995. Changes in Light Intensity a nd Diel Vertical Migrat ion: A Comparison of Marine and Freshwater Enviroments. Journal of the Marine Biol ogical Association of the United Kingdom . 75:15-25. Rhode, S.C., Pawlowski, M., and Tollrian, R. 2001. The Impact of Ultraviolet Radiation on The Vertical Distribution of Zooplankton of the Genus Daphnia. Nature . 412(July 5):69-72. Sartory, D.P., and Grobbelaar, J.U. 1984. Ex traction of Chlorophyll-a from Freshwater Phytoplankton for Spectrophotometric Analysis. Hydrobiologia . 114:177-187.

PAGE 46

46 SAS. 2003. User’s Guide , Version 8. SAS Institute, Inc. Cary, NC, USA. Shireman, J.V, and Martin, R.G. 1978. Seasona l and Diurnal Zooplankton Investigations of a South-Central Florida Lake. Florida Scientist . 41(4):193-201. Steele, J.H., and Henderson, E.W. 1981. A Simple Plankton Model. American Naturalist . 117:676-691. Stitch, H.B., and Lampert, W. 1981. Predator Ev asion as an Explanation of Diurnal Vertical Migration by Zooplankton. Nature . 293:396-398. Viner, A.B., and Kemp, L. 1983. The Effect of Vertical Mixing on the Phytoplankton of Lake Rotongaio (July 1979-January 1981). New Zealand Journal of Ma rine and Freshwater Research . 17:402-422. Werner, E. E., Hall, D. J. 1988. Ontogenetic Habitat Shifts in Bluegill: The Foraging RatePredation Risk Trade-Off. Ecology . 69:1352-1366. Wetzel, R.G. 1975. Limnology. W.B. Saunders Comp., Philadelphia, PA. 734pp. Williamson, C.E., Olson, O.G., and Lott, S.E. 2001. Ultraviolet Radiation and Zooplankton Community Structure Following Deglaci ation in Glacier Bay, Alaska. Ecology . 82 (6):1748-1760. Yount, J.R., and Belanger, T.V. 1988. Populatio n Trends in the Phytoplankton and Zooplankton of the Upper and Middle St. John’s River, Florida, 1983-1984. Florida Scientist . 51 (2):7685. Zaret, T.M., and Suffern, J.S. 1976. Vertical Mi gration in Zooplankton as a Predator Avoidance Mechanism. Limnology and Oceanography . 21(6):804-813. Zohary, T., Erez, J., Gophen, M., Berman-Frank, I., and Stiller, M. 1994. Seasonality of Stable Carbon Isotopes Within the Pelagic Food Web of Lake Kinneret Limnology and Oceanography. 39(5):1030-1043.

PAGE 47

47 BIOGRAPHICAL SKETCH Robert Michael Burnes, Jr. was born on July 30, 1978 in Patuxent River, Maryland. The oldest of two children, he grew up mostly in Cl earwater, Florida and graduated from Armuchee High School in Rome, Georgia in May of 1997. He earned his Bachelors of Science in Wildlife Conservation and Ecology at the University of Florida in 2003. Upon completion of his Masters of Science degree in December 2006, Robert in tends to pursue work for a state wildlife management agency, focusing on zoopla nkton behavioral and population dynamics.