Heath J. Carney

Algal Dynamics and Tropohic Interactions in the Recent History of Frains Lake, Michigan

Analysis of a 10—m sediment core taken from the center of Frains Lake, Michigan, reveals distinct changes in diatom and chrysophycean cyst assemblages following European settlement °1830 AD. The presettlement planktonic diatom flora was dominated by Cyclotella species. The postsettlement flora was dominated first by eurytopic taxa, including Melosira granulata, Fragilaria construens, and F. crotonesis, and then by a complex of small Stephanodiscus species, particularly S. hantzschii. Cluster analysis and principal component analysis indicate three major stages in the recent development of Frains Lake, the greatest shift in the diatom assemblage occurring with settlement. Diatom succession bears a striking resemblance to patterns in other North American and European lakes. Stockners A/C (Araphidineae/Centrales) ratios does not adequately indicate recent changes in the trophic status of Frains Lake. The recent succession of diatoms does agree with predictions based on the Si:P ratio and resource—based theory. Kerfoots hypothesis that an increase in planktivorous fish caused a shift in postsettlement zooplankton composition is evaluated on the basis of algal microfossil data. I propose that shifts in the algal assemblage and other resources have influenced zooplankton community structure.

Seasonal Phytoplankton Demographic Processes and Experiments on Interspecific Competition

Population dynamics of Lake Tahoe phytoplankton species were studied during 1985. Four dominant diatoms were examined: Cyclotella glomerata, Cyclotella ocellata, Synedra amphicephala, and Synedra radians. The role of interspecific resource- based competition during spring and summer was tested in three ways: (1) species distri- bution patterns along the vertical gradient (upper 100 m of the water column), (2) mea- surements of in situ growth and loss (sinking, grazing, natural death, diffusion) processes, and (3) transplants of assemblages between depths. All three tests indicated that resource-based competition was important. Species clearly segregated with thermal stratification and nutrient depletion in the epilimnion. C. glomerata peaked within the epilimnion (0-20 m), S. radians became dominant in the deep chlorophyll maximum region (60-90 m), and the other species peaked at intermediate depths. Resource- limited growth was a dominant process during this time, and sinking and natural death were significant losses. Experimental transplants of assemblages between two depths (15 and 75 m) demonstrated competitive interactions in the phosphate-limited region near the surface, but there was no evidence of competition in the deeper light-limited region. This suggested asymmetrical competition along the vertical gradient. Primary productivity nu- trient-enrichment bioassays and growth kinetic assays (phosphate, nitrate, light) also in- dicated that phosphate and light limitation were important bases of species interactions.

The zooplankton-phytoplankton interface in lakes of contrasting trophic status: an experimental comparison

We report here the results of an experimental study designed to compare algal responses to short-term manipulations of zooplankton in three California lakes which encompass a broad range of productivity (ultra-oligotrophic Lake Tahoe, mesotrophic Castle Lake, and strongly eutrophic Clear Lake). To assess the potential strength of grazing in each lake, we evaluated algal responses to a 16-fold range of zooplankton biomass. To better compare algal responses among lakes, we determined algal responses to grazing by a common grazer (Daphnia sp.) over a range ofDaphnia densities from 1 to 16 animals per liter. Effects of both ambient grazers andDaphnia were strong in Castle Lake. However, neither ambient zooplankton norDaphnia had much impact on phytoplankton in Clear Lake. In Lake Tahoe, no grazing impacts could be demonstrated for the ambient zooplankton butDaphnia grazing had dramatic effects. These results indicate weak coupling between phytoplankton and zooplankton in Clear Lake and Lake Tahoe, two lakes which lie near opposite extremes of lake trophic status for most lakes. These observations, along with work reported by other researchers, suggest that linkages between zooplankton and phytoplankton may be weak in lakes with either extremely low or high productivity. Biomanipulation approaches to recover hypereutrophic lakes which aim only to alter zooplankton size structure may be less effective if algal communities are dominated by large, inedible phytoplankton taxa.

Strength of Zooplankton-Phytoplankton Coupling in Relation to Lake Trophic State

We examine the interactions of zooplankton and phytoplankton in lakes representing a broad range of environments and trophic states. Comparisons focus on the grazing impact and the regeneration rate of the most limiting nutrient during summer stratification in several large lakes: Lake Michigan (USA), Lake Titicaca (Peru), and Lake Tahoe (USA). These lakes range over an order of magnitude in plankton biomass and productivity, and in all of them copepods are the dominant crustaceans. In Lake Michigan (2.2 mg m−3 chl a biomass and 2.86 mg C-m−3· h−1 primary productivity), zooplankton have a very significant grazing impact on phytoplankton, and they can supply the majority of phytoplankton phosphate demand through regeneration. Daily phosphate demand is much greater than the available ambient concentration, so turnover of the available phosphate pool is quite rapid. Lake Titicaca has similar phytoplankton biomass (0.95 mg m−3 chl. α) and productivity (3.5 mg C-m−3·h−1), yet zooplankton concentrations are lower. Thus, zooplankton have less impact, though size-selective grazing may be significant, and nutrient regeneration may reach 20 percent of phytoplankton N demand. Plankton biomass (0.23 mg m−3 chl. a) and productivity (0.25 mg C-m−3· h−1) are much lower in Lake Tahoe than the other two lakes, and the zooplankton-phytoplankton coupling is much weaker. Grazing by zooplankton is two orders of magnitude lower than phytoplankton growth, and zooplankton regeneration appears to be negligible in relation to other fluxes and to the available concentrations of limiting nitrogen.

Nonlinear scale dependence and spatiotemporal variability in planktonic food webs

Zooplankton food webs of Lake Okeechobee, Florida, were analyzed in order to examine spatiotemporal variability and test for scale dependence of major food web properties. 46 webs were constructed from data collected during several years (1988-92) at six locations. For all these webs, the following food web properties were calculated: number of species, total links, links per species, connectance, percentage of top and intermediate species, and food chain length. We did not find any statistically significant spatiotemporal variation in these properties. Still, there were consistent differences seasonally (summer vs winter) and spatially (littoral vs pelagic habitats). There was also a very clear nonlinear scale dependence in most food web properties: links per species, food chain length, and proportions of top and intermediate species vs number of species. The scale dependence was strong for small webs, but became weaker for larger webs. The relatively simple food webs and consistently collected data used in our study provide some of the clearest and most statistically significant results to date. They help reconcile the debate about scale invariance vs dependence in major food web properties.

Nitrogen and phosphorus dynamics and retention in ecotones of Lake Titicaca, Bolivia/Peru

We are studying present conditions and consequences of material movement from land to water in the Lake Titicaca basin, and how fluxes are affected by human activities. The principal objective of this research is to describe and explain the variability in the Andean Altiplano of (a) water, nutrient and sediment fluxes from land and (b) composition, nutrient limitation and other important features of nearshore lake communities, and compare the effects of different agricultural practices (especially traditional and modern) on these factors. We are focusing on a comparison of the impacts of two forms of agriculture in this region: ancient raised fields currently under rehabilitation, and flat pastures and fields, which are more common. Results of the first year of study indicate there is substantial variability in nitrogen and phosphorus dynamics in relation to ecotone complexity (simple vs. intermediate vs. complex). Raised field sites have the beneficial effect of reducing high available nutrient concentrations (nitrate and soluble reactive phosphorus) and sediment load (measured as turbidity) as the water passes through them enroute to the lake. Aquatic vegetation (algae and macrophytes) reflect well ambient total nitrogen and phosphorus concentrations. Experimental nutrient limitation bioassays indicate that nitrogen is the most important limiting nutrient, though there is important spatial variability within the landscape, and phosphorus as well as nitrogen can be limiting.