seasonal and spatial variation in zooplankton community structure and their relation to possible...

Freshwater Biology (1996) 36, 45–56

Seasonal and spatial variation in zooplanktoncommunity structure and their relation to possiblecontrolling variables in Lake Okeechobee

J O H N R . B E AV E RBSA Environmental Sciences, Inc., 3620 Ingleside Road, Shaker Heights, OH 44122–5004, U.S.A.

K A R L E . H AV E N SSouth Florida Water Management District, 3301 Gun Club Road, West Palm Beach, FL 33406, U.S.A.

S U M M A R Y

1. Lake Okeechobee is a large (1732 km2), shallow (mean depth 2.7 m), eutrophic,subtropical lake located in southern Florida. Approximately 25% of the lake surface areais occupied by an extensive littoral zone. From August 1988 to June 1992, µ 2000zooplankton samples were collected throughout the lake.2. During the study period, a severe drought lowered lake levels more than 1 m. At lowand normal lake stage, the average lake-wide abundance of rotifers (c. 2000 l–1) wasgreater than during high water periods (c. 1600 l–1). The average abundance of adultcrustaceans (cladocerans and copepods) (c. 30 l–1) varied little regardless of lake stage.3. Although only minor differences were apparent when lake-wide means in rotifersand adult crustaceans for each lake stage were compared, pronounced differences wereevident in the distribution of zooplankton communities within Lake Okeechobee.During high and normal lake stage, both rotifer and adult crustacean populations weremore uniformly distributed throughout the lake. At low lake stage, the densestzooplankton populations were concentrated in the transition area between the centrallake and the littoral fringe. The abundance of all zooplankton groups was inverselycorrelated with lake stage, but the relationship was much stronger for rotifers thancrustaceans. Both rotifer and crustacean zooplankton population densities werepositively related to increased phytoplankton biomass (as measured by chlorophyll a)but the relationship was much stronger for rotifers than crustaceans.

Introduction

The pivotal role of zooplankton in aquatic food webs tions (e.g. Schoenberg & Carlson, 1984; Shapiro &Wright, 1984). Intense grazing by large-bodied Daphniahas long been recognized (Nauwerck, 1963; Brooks &

Dodson, 1965). Through their grazing activities on also can cause a shift in phytoplankton communitystructure to less edible forms (Shapiro & Wright, 1984;phytoplankton populations, zooplankton function as

intermediaries between fish and lower trophic levels. Carpenter et al., 1987; Benndorf et al., 1988)Factors controlling the seasonal and spatialThe importance of zooplankton as food for juvenile

and adult fish is well known (Jones & Hoyer, 1982; dynamics of zooplankton in large tropical/subtropicallakes are not well described when compared with theGuest et al., 1990).

Numerous studies in the temperate zone demon- vast literature on the temperate zone. In an earlierstudy on zooplankton populations in Lake Okee-strate the ability of large-bodied zooplankton (especi-

ally Daphnia) to graze effectively and, in some chobee, Crisman, Phlips & Beaver (1995) determinedthat total zooplankton biomass is positively related tosituations, almost to eliminate phytoplankton popula-

© 1996 Blackwell Science Ltd 45

46 J. R. Beaver and K. E. Havens

total nitrogen and inversely related to water depth. zooplankton composition during different hydro-logical regimes; and (iii) to describe potential inter-However, the results of that study were based on

only four sampling stations that did not represent acting factors influencing the seasonality andcommunity structure of zooplankton populations inadequately the diversity of habitats in Lake Okee-

chobee. Lake Okeechobee.As a part of the Lake Okeechobee Ecosystem Study

(LOES), Phlips, Aldridge & Hansen (1995) definedMethodsfour ecological zones in Lake Okeechobee, based on ten

limnological parameters integrating physical, chemical Study siteand biological characteristics of the lake. These zonesare the north lake area, central lake area, extreme Lake Okeechobee, a large, shallow, subtropical lake

located in south-eastern Florida, has a surface area ofsouthern lake area and littoral fringe, and the transitionarea between the central lake and littoral fringe area. 1732 km2. It is the third largest lake in the United

States. Based on chlorophyll a and total phosphorusThese ecological zones possess different sedimentcharacteristics. The north lake area and central lake concentrations, Lake Okeechobee is considered

eutrophic (Canfield & Hoyer, 1988; Aumen, 1995).area both have muddy sediments. The central lakearea is roughly 3 m or more in depth, while the north Approximately 25% of the lake’s surface area is occu-

pied by a littoral wetland that supports a variety oflake area is between 2 and 3 m deep. The extremesouthern lake area and littoral fringe possess a hard macrophyte species, including exotics (Aumen, 1995).

External phosphorus and nitrogen loading to Lakebottom substratum and there are extensive submergedmacrophyte beds. The transition area between the Okeechobee peaked in 1982 and declined until 1992.

Water column total phosphorus concentrations variedcentral lake and littoral fringe areas varies between 2and 3 m in depth over a hard marl substratum. from less than 70 µg l–1 to more than 100 µg l–1 between

1982 and 1992. Total nitrogen concentrations alsoThe water level of Lake Okeechobee is regulatedfor flood protection during the rainy season (May to increased from 1.5 mg l–1 to a peak of µ 2.5 mg l–1 in

1982, but rapidly declined to near 1.25 mg l–1 in 1992.October) and for water storage during the dry season(November through April). During the study period During the period 1982–92 nitrogen/phosphorus

ratios in Lake Okeechobee declined from µ 30 : 1 to(1988–92), the water level ranged from 3.2 to 5.0 mabove mean sea level (m.s.l.). A severe drought about 20 : 1. After 1980, the average chlorophyll a

concentration in Lake Okeechobee increased, possiblyoccurred in 1989 and 1990 and was followed by normaland high water periods. resulting from a shift to less edible cyanobacteria

(Havens et al., 1995; James, Jones & Smith, 1995; SmithThe majority of the zooplankton data for Floridalakes has been obtained from 1-year studies (Shireman et al., 1995).

Fairly distinct ecological zones in Lake Okeechobee& Martin, 1978; Fry & Osborne, 1980; Beaver &Crisman, 1982; Bienert, 1982; Wyngaard, Elmore & have been defined based on criteria such as chlorophyll

a and nutrient concentrations. The distribution andCowell, 1982; Bays & Crisman, 1983; Elmore, 1983;Elmore, Vodopich & Hoover, 1983; Brezonik, Crisman seasonality of phytoplankton biomass (as measured

by chlorophyll a) in Lake Okeechobee during the& Schulze, 1984; Canfield & Watkins, 1984; Schmitz &Osborne, 1984; Foran, 1986a, 1986b). Multiple-year study period was a function of these ecological zones

(Fig. 1). The north and central lake areas had peaks instudies are less common (Nordlie, 1976; Blancher, 1984;Billets & Osborne, 1985). No studies have addressed chlorophyll a in summer and minima in winter. The

transition area between the central lake area and thethe effect of water level fluctuations on zooplanktoncomposition and seasonality in Florida lakes. littoral fringe had maximum chlorophyll a concentra-

tions in the autumn and winter and lowest concentra-The study had three objectives: (i) to expand theanalyses of Crisman et al. (1995) to include data from tions in summer. The phytoplankton community of

Lake Okeechobee was dominated numerically byall habitats, including the littoral zone, to determineif the zooplankton community of Lake Okeechobee cyanobacteria, principally Microcystis incerta Lemmer-

man, Merismopedia tenuissima Lemmermann, and Lyng-follows the general ecological zones delineated byPhlips et al. (1995); (ii) to delineate spatial patterns in bya limnetica Lemmermann. Cyanobacteria also

© 1996 Blackwell Science Ltd, Freshwater Biology, 36, 45–56

Seasonal and spatial dynamics of zooplankton 47

ton sample for analysis was composited from fivesuch tube samplings. Samples were preserved withLugol’s solution and stored on ice for transport to thelaboratory. After filtering through a 41-µm screen inthe laboratory and dilution to µ 10 ml, 1-ml aliquotswere placed in a Sedgwick–Rafter chamber andzooplankton were counted at 100 3 until at least150 individuals were tallied. Identification followedEdmondson (1959), Deevey & Deevey (1971), Ruttner-Kolisko (1974), and Pennak (1989).

Similarities in zooplankton community structurewere identified using Ward’s minimum variance clus-tering technique (SAS, 1994). Milligan (1981) revieweda variety of clustering techniques and determinedthat Ward’s minimum variance clustering methodprovided the best overall performance for most datasets. Ward’s method is designed to produce clustersof observations having the lowest amount of within-cluster variance. In addition, this approach is sensitiveto outliers (SAS, 1994). Significant differences betweenthe abundance of each taxon in each cluster associationwere evaluated using an ANOVA (Green, 1979).

Ward’s minimum variance technique was firstapplied to the station means for each species fromeach lake stage classification to determine overallpatterns in zooplankton community structure. Theminimum criterion for a station to be included in theanalysis was at least six zooplankton collections duringa particular lake stage period (low, normal, high).Separate analyses were performed for crustaceansand rotifers. Although the raw abundance of eachFig. 1 Maps of Lake Okeechobee, Florida, U.S.A., showing (a)

the ecological zones defined by Phlips et al. (1995) and (b) the zooplankton taxon was log-transformed prior to ana-locations of pelagic and littoral (shaded region) sampling lysis, the selected procedure and, in particular, the typestations where zooplankton were collected between 1988 and of data input (absolute rather than relative densities)1992.

caused the results to be largely driven by total abund-ance. The total abundance of species (absolute) ratherthan the relative abundance of species (percentage)dominated the biovolume of the phytoplankton com-

munity at most locations (Cichra et al., 1995). was chosen because information on the overall abund-ance of each species would be lost in the later approach.

Sampling

ResultsNearly 2000 (1980) samples of zooplankton were takenbetween August 1988 and June 1992. The frequency Variation in lake stageof sampling varied from weekly to bimonthly. Depth-determined zooplankton samples were obtained by Lake stage varied between 5.0 and 3.2 m.s.l. during

the study period (Fig. 2). A lake stage classificationlowering a 3.2 cm diameter PVC plastic tube from thesurface to a depth µ 10 cm above the sediment surface. was determined for each climatic season and year

combination. This categorization was based on theThe tube was stoppered, and the retrieved samplewas placed in a plastic bucket. The final 1-l zooplank- one-third percentile rank of the lake stage for each



the desired number of clusters by one from seven toten succeeded in removing only one station at a timefrom the four main clusters for each of the six clusteranalyses. This suggested that the stations within thefour major clusters were strongly related to each other.

Rotifer communities

The dominant rotifers in Lake Okeechobee regardlessof lake stage, were Keratella cochlearis Gosse, Polyarthravulgaris Carlin and Trichocerca multicrinis Jennings.Cluster analysis identified six clusters of stations forthe low lake stage period based on mean abundance

Fig. 2 Daily stage values in Lake Okeechobee during the study of common rotifer species (Table 2, Fig. 3). Thirty-period. The horizontal lines indicate the manner in which the three of the seventy-two stations analysed for the low-data were divided for statistical analyses into high (. 66%),

stage period were characterized by comparatively lownormal (33–66%) and low (, 33%) stage intervals.abundance of rotifers (cluster 1). The majority of thesestations were located in the central lake area, north

season and year combination (Fig. 2). At the beginning lake area and littoral region. Sixteen of the stationsof the study in August, 1988, lake stage was 4.75 m.s.l. were grouped into three clusters which had the great-A severe drought occurred at Lake Okeechobee from est average abundance of total rotifers (clusters 3, 41989 until the end of 1990 and lake stage dropped to and 6). Most of these stations were located in theless than 3.25 m.s.l. by the spring of 1990. From that transition area between the central lake area and thepoint until the end of the study in June 1992 lake littoral region. The remaining tweny-three stationsstage rose to 5.0 m.s.l. The zooplankton data base was were clustered into two groups which were inter-grouped according to the lake stage (low, normal, mediate in total rotifer abundance and were generallyhigh) at the time of sampling. located in the littoral regions (clusters 2 and 5). The

number of total rotifers in the high abundance clustersaveraged more than four times the rotifer numbers ofAnalyses of zooplankton community structurethe low abundance clusters.

An ANOVA indicated that the mean abundance ofGeneral trends in zooplankton community structurewere determined based on the average abundance of total rotifers at low lake stage in the high abundance

groups (clusters 3, 4 and 6), intermediate abundancethe sixty-one zooplankton taxa as well as calanoidcopepodids, cyclopoid copepodids and nauplii (clusters 2 and 5) and the low abundance group

(cluster 1) were all significantly (P , 0.05) different(Table 1) at each station for each of the three stageintervals (low, normal, high) (Fig. 1). from each other. Of the major rotifer taxa identified

in Table 2, the mean abundances of Anureopsis fissaBased on the lake stage classification scheme out-lined above, the data base was subdivided into a low Gosse, K. cochlearis, P. vulgaris, and T. multicrinis at

low lake stage were significantly higher in clusters 3,water level period (766 sampling events from seventy-two stations), normal water level period (663 sampling 4 and 6 (transition region between littoral and open

water habitats) when contrasted with cluster 1 stations.events from seventy-three stations), and high-waterlevel period (551 sampling events from forty-two During normal lake stage the distribution of high,

intermediate, and low abundance stations was similarstations). Each station was sampled at least six timesduring the study period (1988–92). The number of to that described for low lake stage but the surface

area of Lake Okeechobee occupied by high abundancestations assigned to each lake stage classificationvaried because of differences in sampling regimes stations was reduced. Stations with the highest abund-

ance of rotifers were generally located in the transitionduring the course of the study.The statistical package permitted the desired num- area between the littoral and open water habitats

(clusters 4, 5 and 6). Fewer stations in the transitionber of clusters to be specified. Incrementally increasing



Table 1 The sixty-one dominant zooplankton components of Table 1 (Cont.)Lake Okeechobee during the study period

Platyias quadricornis (Ehrenberg)Ploesoma hudsoni (Imhof)Copepods

Nauplii Polyarthra major (Burckhardt)Polyarthra vulgaris CarlinCalanoid copepodid

Diaptomus dorsalis Marsh Scaridium longicaudum (Muller)Squatinella rostrum (Schmarda)Cyclopoid copepodid

Acanthocyclops vernalis (Fischer) Trichocerca longiseta (Schrank)Trichocerca multicrinis (Jennings)Mesocyclops edax Forbes

Tropocyclops prasinus (Fischer) Trichocerca similis (Wierzejski)Trichocerca spp.Ergasilus spp.

CladoceransAlona spp.

region were classified with the high abundance groupBosminopsis deitersi RichardCamptocercus rectirostris Schødler than at low lake stage. As was the case during lowCeriodaphnia spp. lake stage, an ANOVA indicated that the mean totalChydorus sphaericus (O.F. Muller)

abundance of rotifers was significantly (P , 0.05)Daphnia ambigua Scourfieldgreater at the stations located in the transition regionDiaphanasoma brachyurum (Lie’ven)

Eubosmina tubicen (Brehm) when contrasted with littoral and open water habitats.Macrothrix rosea (Jurine) During high lake stage, the greatest concentrations

Rotifersof rotifers were confined to the Fisheating Bay area.Anureopsis fissa (Gosse)The transition region between the littoral and openAscomorpha spp.

Asplanchna spp. water was characterized by lower total rotifer abund-Brachionus angularis (Gosse) ances than at normal and low lake stages. DuringBrachionus calyciflorus (Pallas) normal and low lake stages, low rotifer populationsBrachionus caudatus (Barrois)

were found throughout the lake’s central region. AtBrachionus havanaensis (Rousselet)Brachionus quadridentata (Hermann) high lake stage this region of Lake Okeechobee wasBrachionus urceolaris (Muller) characterized by both low and intermediate rotiferBrachionus zahniseri (Ahlstrøm) abundance.Brachionus spp.

The highest densities of rotifers (3000–4000 l–1) wereCollotheca libera (Zacharias)found in the transition area between the central lakeColurella spp.

Conochiloides dossaurius (Hudson) area and in the littoral fringe. Under high and normalConochilus spp. lake conditions, the highest rotifer abundances wereConochilus unicornis (Rousselet)

limited to the area between Fisheating Bay and theEpiphanes spp.transition area. When lake stage was low, the zone ofEuchlanis spp.

Filinia longiseta (Ehrenberg) high rotifer abundance was larger and included mostGastropus hypotus (Ehrenberg) of the transition area between the central lake areaGastropus stylifer Imhof

and the littoral zone and the southern lake area. TheHexarthra mira (Hudson)most pronounced difference between high abundanceKellicottia bostoniensis (Rousselet)

Kellicottia longispina (Kellicott) rotifer associations at high, normal and low lake stageKeratella cochlearis (Gosse) was the increased relative importance of K. cochlearisKeratella quadrata (Muller)

when lake stage was low.Keratella serrulata (Ehrenberg)Keratella taurocephala (Ahlstrøm)Keratella testudo (Ehrenberg)

Crustacean communitiesLecane spp.Lecane lunaris (Ehrenberg) The differences in the structure of crustaceanLepadella ovalis (Muller)

zooplankton communities in Lake Okeechobee relativeMacrochaetus spp.Monostyla spp. to lake stage were less striking than for rotifer commu-Notholca caudata (Carlin) nities (Table 3, Fig. 4). In general, Eubosmina tubicenNotholca squamula (Muller) (Brehm) was more abundant at high and normal lakePlatyias patulus (Muller)

stage as compared with low lake stage. The highest



Table 2 Average abundance (nos l–1) of the dominant rotifer taxa and percentage numerical composition for each series of clusteranalyses and lake stage (n 5 number of stations in cluster). Cluster groups correspond to stations depicted in Fig. 3

I I (%) II II (%) III III (%) IV IV (%) V V (%) VI VI (%)

High lake stage (n 5 14) (n 5 9) (n 5 12) (n 5 3) (n 5 3) (n 5 1)

Anuraeopis fissa 91.5 9.6 298.0 13.2 171.4 11.5 387.9 10.5 255.6 10.8 104.7 4.1Brachionus havanaensis 36.3 3.8 107.3 4.7 83.4 5.6 65.0 1.8 82.7 3.5 89.1 3.4Keratella cochlearis 295.6 30.9 796.6 35.2 566.1 38.0 1115.6 30.1 771.4 32.7 1476.5 57.1Lecane spp. 47.8 5.0 39.4 1.7 56.4 3.8 11.9 0.3 36.0 1.5 10.8 0.4Polyarthra vulgaris 179.8 18.8 183.2 8.1 175.4 11.8 149.4 4.0 389.0 16.5 244.2 9.5Trichocerca multicrinis 147.5 15.4 575.5 25.5 237.6 15.9 949.0 25.6 429.5 18.2 299.4 11.6Total rotifers 958.0 2261.0 1491.0 3704.0 2357.0 2584.0

Normal lake stage (n 5 32) (n 5 11) (n 5 11) (n 5 9) (n 5 9) (n 5 1)Anuraeopis fissa 77.8 8.0 89.2 5.7 174.3 8.1 169.3 4.5 165.9 6.3 59.5 2.1Brachionus havanaensis 13.5 1.4 60.4 3.8 47.7 2.2 96.1 2.5 105.6 4.0 52.0 1.8Keratella cochlearis 174.6 17.9 570.8 36.3 428.2 20.0 1010.6 26.7 948.9 36.3 143.0 5.1Lecane spp. 106.1 10.9 42.3 2.7 85.8 4.0 76.9 2.0 32.7 1.3 141.8 5.0Polyarthra vulgaris 164.5 16.9 165.6 10.5 206.8 9.7 452.7 12.0 437.0 16.7 66.5 2.4Trichocerca multicrinis 133.9 13.7 272.3 17.3 662.1 31.0 1146.6 30.3 431.3 16.5 158.2 5.6Total rotifers 975.0 1573.0 2139.0 3781.0 2615.0 2822.0

Low lake stage (n 5 33) (n 5 13) (n 5 3) (n 5 6) (n 5 10) (n 5 1)Anuraeopis fissa 72.3 7.4 183.1 7.6 282.7 6.4 294.6 6.6 129.1 5.9 210.9 1.2Brachionus havanaensis 21.2 2.2 51.3 2.1 130.4 3.0 146.0 3.3 81.4 3.7 873.0 4.8Keratella cochlearis 243.7 24.9 350.7 14.5 1184.5 26.9 2117.0 47.7 744.7 33.9 1000.5 5.5Lecane spp. 61.4 6.3 175.4 7.3 51.9 1.2 31.0 0.7 30.0 1.4 0.0 0.0Polyarthra vulgaris 165.4 16.9 244.0 10.1 454.0 10.3 526.6 11.9 264.5 12.0 1992.5 11.0Trichocerca multicrinis 143.6 14.7 740.8 30.7 988.1 22.4 681.2 15.4 393.6 17.9 1771.5 9.8Total rotifers 978.0 2412.0 4403.0 4434.0 2196.0 18061.0

mean cladoceran abundances were found at high lake the littoral and open water. The abundance of naupliiwas generally reduced in the littoral region.stage throughout the north lake area, the periphery of

the central lake area, Fisheating Bay, and in the littoralfringe. Patterns in cladoceran abundance during nor-

Discussionmal and low lake stage were not apparent.

At low lake stage, cyclopoid copepods were more The taxonomic composition of crustacean and rotifercommunities of Lake Okeechobee is similar to otherabundant in the littoral fringe and the transition area

than at higher lake stage. At normal and high lake subtropical lakes in peninsular Florida (Bays &Crisman, 1983). Florida zooplankton communities arestage, cyclopoid copepods were more uniformly dis-

tributed throughout the lake. Calanoid copepods were typified by a small number of limnetic crustaceanspecies and display substantial overlap in speciesmost abundant at low lake stage throughout the north

lake area, the central lake area, the transition area composition across a broad range of trophic conditions.The dominant cyclopoids in Lake Okeechobee (Acan-between the central lake area and the littoral fringe

and in Fisheating Bay. This pattern also was observed thocyclops vernalis (Fischer), Tropocyclops prasinus(Fischer)) are also the dominant cyclopoid species inat high lake stage, although overall abundances of

calanoid copepods were less than of the other lake the plankton of most Florida lakes (Bays & Crisman,1983). The only calanoid copepod encountered in Lakestages and abundances were much reduced in the

Fisheating Bay area. However, an ANOVA indicated Okeechobee during the study was Diaptomus dorsalisMarsh. The studies of Elmore (Elmore, 1983; Elmore,that there were no significant differences between

clusters in the abundance of any crustacean component Vodopich & Hoover, 1983) suggest that distribution ofthe three species of Diaptomus found in Florida lakeswith the exception of copepod nauplii. The mean

abundances of copepod nauplii were significantly is controlled by competition for food and reducedvulnerability to predation by planktivorous fish. Diap-different between clusters for each lake stage. Copepod

nauplii tended to be most abundant at stations located tomus dorsalis is more successful than other calanoidspecies under eutrophic conditions because of itsin the open water and the transition region between



Fig. 3 The classification of zooplanktonassemblages at discrete locations, basedon the results of cluster analyses forrotifers at: (a) high lake stage; (b)normal lake stage; and (c) low lakestage. The numbers correspond to thecluster designations given in Table 2.

higher population growth rate and stronger ability atures. Mallin & Partin (1989) observed that fecundityof Daphnia ambigua peaks at 25 °C and is stronglyto avoid vertebrate predation (Elmore, Vodopich &

Hoover, 1983). depressed at 30 °C. The water temperature of LakeOkeechobee is generally greater than 25 °C betweenCladocerans were relatively rare in Lake Okee-

chobee during the study period. Eubosmina tubicen was April and October. Winter water temperatures in LakeOkeechobee average between 18 and 20 °C (Havens,the most commonly encountered cladoceran species.

Daphnia ambigua Scourfield, the largest and most 1995). Thus, the peaks in cladoceran abundance inwinter in Lake Okeechobee are consistent with thewidely distributed native daphnid in Florida lakes,

was extremely rare. The low abundance of Daphnia in premise that water temperature, in part, exerts astrong influence on cladoceran seasonality in LakeLake Okeechobee is consistent with the phenomenon

of cladoceran rarefaction with decreasing latitude. Okeechobee. Recently, the exotic tropical daphnidDaphnia lumholtzi (Sars) has become established inFernando (1980) suggested that the general decrease

in daphnid species numbers and body size observed Florida lakes (Havel & Hebert, 1993), including LakeOkeechobee (Havens, unpublished data). Daphnia lum-in lowland tropical and subtropical lakes may result

in part from physiological responses to higher temper- holtzi, however, was not recorded in Lake Okeechobee



Table 3 Average abundance (nos. l–1) of the dominant crustacean taxa for each series of analyses and lake stage (n 5 number ofstations in cluster). Cluster groups correspond to stations depicted in Fig. 3

I II III IV V VI

High lake stage (n 5 11) (n 5 14) (n 5 4) (n 5 9) (n 5 3) (n 5 1)Diaptomus 10.9 7.5 5.3 9.7 10.7 4.0Cyclopoids 21.3 17.9 11.6 8.7 15.2 34.8Nauplii 235.0 175.3 126.5 88.2 203.7 356.8Eubosmina 2.5 5.3 3.8 4.2 6.4 5.1Daphnia 0.8 1.4 0.0 0.2 0.3 0.0Cladocerans 5.7 9.7 6.2 8.7 8.9 6.8

Normal lake stage (n 5 20) (n 5 14) (n 5 12) (n 5 19) (n 5 5) (n 5 2)Diaptomus 6.0 14.0 7.6 13.0 4.8 5.8Cyclopoids 11.9 17.9 13.9 18.7 12.3 33.6Nauplii 81.5 232.4 139.3 200.7 329.5 611.8Eubosmina 3.2 2.0 3.3 4.0 3.2 6.2Daphnia 0.1 1.0 0.3 0.3 1.0 0.0Cladocerans 5.9 5.2 5.6 6.3 7.9 30.7

Low lake stage (n 5 17) (n 5 21) (n 5 19) (n 5 2) (n 5 11) (n 5 2)Diaptomus 5.6 12.2 14.9 9.1 7.8 3.8Cyclopoids 6.4 16.1 17.2 27.8 26.8 19.8Nauplii 64.1 219.3 143.2 192.9 316.1 700.7Eubosmina 2.6 1.2 2.8 0.0 1.5 2.4Daphnia 0.3 0.2 0.4 0.0 0.3 0.0Cladocerans 4.6 5.4 7.2 3.3 3.1 18.6

during the period of this study (1988–92). The rotifer erratic dynamics of zooplankton populations. Anotherzooplankton study conducted under drought and highcommunity of Lake Okeechobee during the study

period also was dominated by taxa characteristic of lake stage is needed to provide replication lacking inthe present study.other lakes in Florida (Bays & Crisman, 1983; Beaver,

unpublished data). Unfortunately, comparable data Crisman et al., (1995) demonstrated a strong positiverelationship between total zooplankton biomass, totalfrom other shallow, tropical lakes do not exist.

The present study suggests that zooplankton popu- nitrogen and chlorophyll a in a subset of intenselymonitored stations during the Lake Okeechobee Eco-lations in Lake Okeechobee are positively impacted

by lower lake stage and its effect on phytoplankton system Study. The same study also indicated that totalzooplankton biomass was inversely related to waterbiomass. The highest zooplankton abundances in Lake

Okeechobee were found in the transition area between depth. Aldridge, Phlips & Schelske (1995) demon-strated that nitrogen was the major limiting nutrientopen water and the littoral fringe. This area is also

the region of the lake where phytoplankton biomass for phytoplankton biomass in Lake Okeechobee.Spatial and temporal patterns in zooplankton popu-was most impacted by nutrients and less impacted by

light limitation. As lake levels declined during the lations in Lake Okeechobee are probably partiallycontrolled by the dynamics of phytoplankton popula-drought in 1989–90, the area of Lake Okeechobee

where the phytoplankton populations were strongly tions. Phytoplankton in areas of Lake Okeechobeeoverlying extensive muddy sediments (the north andinfluenced by nutrients expanded (the transition zone

between open water and littoral habitats). At high to the central lake areas) are strongly light-limited bynon-algal turbidity caused by resuspension of sedi-normal lake stage only the Fisheating Bay area and a

portion of the transition area between the central ments from the shallow polymictic lake (Carrick,Worth & Marshall, 1994). The phytoplankton in otherlake area and the littoral zone exhibited elevated

zooplankton densities, especially rotifers. Alternative, areas of Lake Okeechobee overlying more firmsubstratum (the extreme southern lake area, thebut less likely, explanations are that the lowering of

lake levels caused a concentrating effect on zooplank- transition area between the central lake, and the littoralfringe) were significantly less light-limited and moreton densities or that these results are a reflection of



Fig. 4 The classification of zooplanktonassemblages at discrete locations, basedon the results of cluster analyses forcrustaceans (copepods and cladocerans)at: (a) high lake stage; (b) normal lakestage; and (c) low lake stage. Thenumbers correspond to the clusterdesignations given in Table 3.

strongly influenced by nutrients. The autumn and the transition zone was proportionally much greaterthan the abundance of adult crustaceans when com-winter peaks in chlorophyll a concentrations in these

areas of Lake Okeechobee may be influenced by pared with littoral and open water regions, althoughalgal composition was similar (Cichra et al., 1995).nutrients released from senescing macrophytes and/

or wind-induced redistribution of sediments from the Among the correlations presented in Crisman, Phlips& Beaver (1995) between various zooplankton com-central lake area to the transition area associated with

the littoral fringe. ponents and phytoplankton biomass or nutrient para-meters, the correlations were the strongest for rotifers.Food availability in Lake Okeechobee is a strong

controlling factor for zooplankton abundance, and The relationship between crustacean zooplanktoncomponents and trophic state variables was significantparticularly for rotifers. The highest zooplankton

populations in the present study were found in the but weak. These observations suggest that, althoughpopulations of adult crustaceans respond somewhattransition area between littoral and open water hab-

itats, which was also the area of Lake Okeechobee to increased food availability, other controlling factorsare limiting their populations.where phytoplankton biomass was most strongly

impacted by nutrients. The abundance of rotifers in The distribution of fish in Lake Okeechobee is



heterogeneous but is also strongly influenced by sedi- complement of eutrophic Florida lakes. Vertebratepredators, such as gizzard shad [Dorosoma cepedianumment type and water depth. The fish assemblage

of Lake Okeechobee is numerically dominated by (Le Suer)] and threadfin shad (Dorosoma petenense) arebelieved to be the primary determinants of crustaceanthreadfin shad (Dorosoma petenense (Le Suer)) which

on average constitutes nearly 50% of the total fish zooplankton abundance and mean zooplankton bodysize in productive Florida lakes (Bays & Crisman,population. Bull et al. (1995) reported that the abund-

ance of threadfin shad in Lake Okeechobee was closely 1983; Elmore, Cowell & Vodopich, 1984). Previousexperimental studies on Lake Okeechobee alsorelated to phytoplankton densities, with highest

catches noted in the north-west shore and south- strongly suggest that fish predation is a more import-ant factor than food availability influencing adultsouthwest shore areas. Like phytoplankton, abund-

ance of threadfin shad was also inversely related to crustacean populations, especially cladocerans. Forexample, when freed from planktivore predation, cla-turbidity. Fox et al. (1992) have also shown that blue-

gill sunfish (Lepomis macrochirus (Rafinesque)) were doceran populations in mesocosms increased morethan twentyfold (Crisman & Beaver, 1990). The presentnegatively correlated with water depth and were the

dominant sunfish in littoral regions of Lake Okee- study’s observations regarding the potential control-ling factors on the zooplankton populations of Lakechobee.

It is possible that predation pressure on cladocerans Okeechobee are consistent with previous studies. Asdetermined for other Florida lakes, it is likely thatand copepods is a more important factor than food

availability in structuring the distribution of these the abundance of small-bodied zooplankton (rotifers,ciliated protozoa) in Lake Okeechobee is most stronglyzooplankton components in Lake Okeechobee. Bull

et al. (1995) identified four fish associations in Lake controlled by food availability, while the abundanceof large-bodied zooplankters (cladocerans, copepods)Okeechobee. Although seasonal and spatial hetero-

geneity in fish populations was observed in Lake is primarily limited by predation from fish (Beaver &Crisman, 1982; Bays & Crisman, 1983).Okeechobee, the distribution of threadfin shad closely

tracked phytoplankton densities. Both larval and adult In summary, the results of the present study indicatethat the zooplankton population of Lake Okeechobeethreadfin shad are voracious zooplanktivores. As

adults, threadfin shad function as both filter-feeders displayed a similar pattern to the ecological zonesdefined by Phlips et al., (1995). As observed for phyto-and visual feeders on adult crustaceans, particularly

Daphnia spp. (Holanov & Tash, 1978; Guest et al., plankton biomass, zooplankton populations tended tobe reduced over most of the central lake area but were1990). As threadfin shad are effective grazers of adult

crustaceans, it is likely that the adult crustacean elevated in the transition region between littoral andopen water habitats. The results of the present studyzooplankton populations in Lake Okeechobee are

more strongly controlled by the grazing activities of also suggest that low lake stage tends to promotegreater zooplankton densities over a larger area in theplanktivores such as threadfin shad than by food

limitation. This premise is consistent with the distribu- transition region between littoral and open waterhabitats. Finally, the tendency for elevated zooplank-tion patterns of zooplankton in Lake Okeechobee.

During periods of low lake stage, the phytoplankton ton populations in the transition region is probably aresponse to increased phytoplankton biomass.populations in a larger portion of the surface area

of Lake Okeechobee were stimulated by nutrients Although all elements of the zooplankton communityappear to respond to increased food availability, roti-(Philips, Aldridge & Hansen, 1995). Coincident with

this event, zooplankton abundance, particularly rotifers respond proportionally more than crustaceansbecause predation pressure limits large-bodiedfers, also increased over a greater surface area of

Lake Okeechobee in response to expansion of the zooplankton.phytoplankton population. However, the adult crusta-cean response to increased food availability lagged

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seasonal and spatial variation in zooplankton community structure and their relation to possible...

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