Microcrustacean Community Structure and Biomass in Marsh and Lake Habitats of the Okefenokee Swamp: Seasonal Dynamics and Responses to Resource Manipulations

• Published in: Holarctic ecology 1988-02, Vol.11 (1), p.8-18
• Main Author: Schoenberg, Steven A.
• Format: Article
• Language: English
• Subjects: Bacteria; Biomass; Biomass production; Crustacea; Freshwater; Freshwater ecology; Marshes; Summer; Swamps; Wetland ecology; Winter; Zooplankton
• ISSN: 0105-9327

Microcrustacean community and biomass dynamics were studied for two years in a Nymphaea-Eriocaulon macrophyte marsh and a nearby shallow lake which lacked macrophytes in the Okefenokee Swamp. In this blackwater, acidic wetland, micro-crustacean diversity and biomass were similar to other circumneutral lakes and littoral areas, contributing to a productive fish assemblage. In the lake, the annual biomass pattern ($15-1627\ \mu {\rm g}\ {\rm l}^{-1}$) was unimodal and was dominated by the crustaceans Diaptomus sinuatus and Eubosmina tubicen. Rotifers were occasionally important, constituting up to 55% of total biomass. Over the long term, mean annual biomass in this post-drought study are higher than in pre-drought years. In the marsh, biomass ($11-777\ \mu {\rm g}\ {\rm l}^{-1}$) fluctuated biomodally with late winter depressions corresponding to low temperatures and midsummer declines indicative of increasing fish predation. Summer dominance shifted between years from Macrothricidae in 1982 to Sididae in 1983. Variation in biomass correlated most strongly with algal chlorophyll in the marsh and with bacterial density in the lake. In field enclosure experiments in which primary production was reduced by shading, microcrustacean responses varied between lake and marsh habitats and with season. Lake zooplankton were consistently suppressed by reduced algal resources in winter, spring and summer experiments, with greatest responses to shading in the summer. Marsh microcrustacea were most affected in the winter experiment and became less sensitive to manipulated resource levels in spring and summer. Decoupling of these consumers from autotrophic resources in the marsh, but not in the lake, coincides with times of high macrophyte turnover and warming temperatures which promote the conversion of detritus into heterotrophic resources such as bacteria. The conflict between interactions implied by the experimental approach vs statistical criteria emphasizes a need to interpret resource dependence from seasonal dynamics of field populations with caution.