No abstract

Zooplankton, including ciliated protozoans, were collected from 39 Florida lakes of widely ranging trophic state. Annual mean biomass values for different zooplankton groups were regressed against Carlson's Trophic State Index based on annual mean chlorophyll a concentration. Whereas total zooplankton biomass yielded a significant regression with increasing trophic state, microzooplankton (ciliates, rotifers, and nauplii) accounted for more of the relationship than macrozooplankton (cladocera, calanoids, and cyclopoids). Within the microzooplankton, the regression improved with decreasing body size. Macrozooplankton biomass exhibited a weak statistical relationship with lake trophic state, but the different component groups were variable in their response. The dominance within the zooplankton community shifts from macrozooplankton to microzooplankton with increasing trophic state, and the microzooplankton can constitute between 50 and 90% of the total zooplankton biomass in eutrophic lakes. Changes in zooplanktivore community structure with increasing trophic state show that whereas total fish biomass increases, dominance shifts from visually oriented predators, such as bass and bluegill, to pump filter-feeding planktivores, such as gizzard shad (Dorosoma cepedianum). While Florida zooplankton communities are similar in size structure to tropical communities, no statistically significant differences were found between empirical equations of crustacean zooplankton biomass and trophic state determined from temperate and Florida data bases.

The abundance and biomass of ciliates are both strongly related to lake trophic status as measured by chlorophylla concentrations. Taxonomic replacements occur with increasing eutrophication such that large-bodied forms (predominantly oligotrichs) are progressively replaced by smaller-bodied ciliates (mainly scuticociliates). Highly acidic lakes display a more pronounced dominance of large-bodied forms when contrasted with less acidic lakes of comparable trophy. Community structure of ciliate populations is determined largely by lake trophy with acidic oligotrophic systems being characterized by reduced diversity and species richness compared with hypereutrophic systems. The temporal and spatial distribution of small (< 100μm) ciliate populations is ascribed to lake thermal regimes which provide localized concentrations of food resources. Likewise, in extremely productive lakes, very large (> 100μm) meroplanktonic ciliates enter the water column during midsummer after the development of thermal stratification and associated profundal deoxygenation. Laboratory studies indicate that large zooplankton (crustaceans) are capable of utilizing ciliates as a food source, but there is little direct evidence from field studies documenting this trophic link. Ciliates can be voracious grazers of both bacterioplankton and phytoplankton, and each species has a distinct range of preferred particle size which is a function of both mouth size and morphology. Myxotrophic ciliates may be important components in some plankton communities, particularly during periods of nutrient limitation or after their displacement from the benthos of eutrophic lakes. Evidence regarding the importance of planktonic ciliated protozoa in nutrient regeneration and as intermediaries in energy flow is discussed.

The abundance, biomass, composition, and size distribution of planktonic ciliated protozoans were analyzed monthly during 1979 along a trophic gradient represented by 20 Florida lakes. Both the abundance and biomass of ciliates were positively related to trophic state. Eutrophic assemblages were codominated by members of the Scuticociliatida, Oligotrichida, and Haptorida; oligotrophic lakes were dominated principally by the Oligotrichida. In addition to a compositional shift, large-bodied ciliates (40-50 pm) were progressively replaced by small-bodied ciliates (2030 pm) as trophic state increased. The response is attributed to changes in food availability.Small-bodied (~30 pm) bactivorous taxa are reduced in oligotrophic lakes where bacterial concentrations arc limiting and are replaced by those largerbodied taxa (>30 pm) able to ingest nannoplanktonic algae in addition to bacteria.

The relationships between planktonic chlorophyll a and mean light availability in the mixed layer were examined for Lake Okeechobee, a shallow eutrophic lake in south‐central Florida. There were significant spatial differences in the nature of this relationship. In the northern and central regions of the lake, concentrations of nonalgal suspended solids, tripton, were high due to muddy sediments and polymictic conditions. Light attenuation was predominantly correlated to the concentration of tripton and chlorophyll a concentrations were positively correlated to mean light availability in the mixed layer, Im. The western and southern regions of the lake had lower concentrations of tripton and light attenuation was correlated to chlorophyll a concentrations in summer and fall. Variables which estimate mean light availability in the mixed layer, such as Im, may provide insight into the role of light availability in the control of phytoplankton standing crop.