Relative impacts of bottom-up (producer controlled) and top-down (consumer controlled) forces on the biomass and size structure of five major components of freshwater pelagic systems (piscivores, planktivores, zooplankton, phytoplankton, and total phosphorus availability) were estimated. Predictions that emerge are (1) maximum biomass at each trophic level is controlled from below (bottom-up) by nutrient availability, (2) this bottom-up regulation is strongest at the bottom of the food web (i.e. phosphorus → phytoplankton) and weakens by a factor of 2 with each succeeding step up the food web, (3) as energy moves up a food web, the predictability of bottom-up interactions decreases, (4) near the top of the food web, top-down (predator mediated) interactions are strong and have low coefficients of variation, but weaken with every step down the food web, (5) variability around the bottom-up regressions can always be explained by top-down forces, and (6) interplay between top-down and bottom-up effects changes with the trophic status of lakes. In eutrophic lakes, top-down effects are strong for piscivore → zooplankton, weaker for planktivore → zooplankton, and have little impact for zooplankton → phytoplankton. For oligotrophic lakes, the model predicts that top-down effects are not strongly buffered, so that zooplankton → phytoplankton interactions are significant.

For freshwater pelagic ecosystems, the biomanipulation and cascading trophic interaction theories both predict that decreased piscivore populations will result in direct, short-term (a few years) increases in planktivore biomass, reductions in crustacean herbivore biomass, and increases in chlorophyll a concentration and phytoplankton biomass. An alternate view is offered by the bottom-up: top-down theory, which predicts that in eutrophic lakes changes in piscivore biomasses will have strong impacts on planktivore numbers, weaker but observable impacts on zooplankton biomass, and little or no longterm effects on phytoplankton biomass. A partial winterkill at Lake St. George, Ontario, Canada allowed us to test these predictions. The data set comprised measures of: (1) piscivore and planktivore numbers, (2) zooplankton species composition, size structure, and biomass, (3) chlorophyll a concentration and Secchi depth, and (4) water chemistry from 1980 through 1986. Prior to the winterkill of 1981-1982, the piscivore population was high (1000-2000 piscivores/ha), the planktivore population was intermediate (8000-1 0 000 planktivores/ha), zooplankton biomass was intermediate (2400 ~giL), and chlorophyll a concentration was high (5-12 ~giL). In the year following the winterkill (1982), piscivore and planktivore numbers were low, and zooplankton biomass and chlorophyll a concentration were high. During the next 2 yr (1983)(1984) the planktivore population increased rapidly to densities > 20 000 individuals/ha, zooplankton biomass density decreased to < 1600 ~giL and chlorophyll a concentration decreased. During the final 2 yr of the study, piscivores recruited to near prewinterkilllevels, planktivores were reduced to <8000 individuals/ha, zooplankton biomass increased, and chlorophyll a concentration decreased. Over the 7 yr data set, we found a strong negative correlation between numbers ofpiscivores and planktivores, a weaker correlation between numbers ofplanktivores and zooplankton biomass, and no between-year correlation between zooplankton biomass and chlorophyll a concentration. There was, however, a positive correlation between total epilimnetic phosphorus and chlorophyll a concentration. These data are consistent with predictions made by the bottom-up: top-down model, and the implication is that at Lake St. George, the trophic cascade uncouples at the zooplankton __, phytoplankton link. We speculate that this may be due to the combined effects of lake trophy and Daphnia species composition and size.

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Abstract-Hydrogen peroxide (H,O,) formation rates (nM h-l), photoproductive capacity (nM W-l h-l m-*), and H202 formation efficiency (bH202) were measured on water samples exposed to a standard light source with a spectral composition similar to natural sunlight. Samples were from lakes and wetlands with varying levels of dissolved organic C (DOC), P, Fe, Ca, and pH. The relationship between H,O, formation rates and DOC was a power function @ItO = 49.65 DOC1-71; r* = 0.94), whereas the relationships between formation rates and DOC fluorescence (DOCFLJ w202 = 118.32 '+ 33.06 DOCm, r* = 0.98) and absorption coefficients at 3 10 (J&,) were linear (H202 = 185.0 + 55.50 Kaslo; r* = 0.9 1). +H202 was independent of DOC (r* = 0.12). Apparent quantum yields decreased with increasing wavelengths (300-400 nm). However, when apparent quantum yields were corrected with solar irradiance data, values were greatest in the UV-A (320-400 nm) region. The patterns observed were consistent for samples from temperate to arctic regions.

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A comparative study of small temperate lakes (<20 square kilometers) indicates that the mixing depth or epilimnion is directly related to light penetration measured as Secchi depth. Clearer lakes have deeper mixing depths. This relation is the result of greater penetration of incident solar radiation in lakes and enclosures with high water clarity. Data show that light penetration is largely a function of size distribution and biomass of algae as indicated by a relation between the index of plankton size distribution (slope) and Secchi depth. Larger or steeper slopes (indicative of communities dominated by small plankton) are associated with shallower Secchi depth. In lakes with high abundances of planktivorous fish, water clarity or light penetration is reduced because large zooplankton, which feed on small algae, are reduced by fish predation. The net effect is a shallower mixing depth, lower metalimnetic temperature and lower heat content in the water column. Consequently, the biomass and size distribution of plankton can change the thermal structure and heat content of small lakes by modifying light penetration.

Distribution of larval and juvenile yellow perch (Perca flavescens) in Lake St. George, Ontario, changed during development as follows: (1) early in development, the larvae were distributed exclusively offshore, both day and night; (2) larger larvae and juveniles migrated nearshore to offshore at dusk, returning the the nearshore zone at dawn; (3) the extent of this migration weakened as the fish grew, and they spent a greater porportion of their time nearshore and did not migrate as far at night; and (4) by the time juveniles reached approximately 30 mm total length, they were distributed exclusively in the nearshore habitat both day and night. Analysis of published reports on distribution patterns of larval and juvenile yellow perch and the European perch (Perca fluviatilis) suggests that the ontogenetic distribution patterns observed in Lake St. George are typical for the two Perca species. This regularity amongst lakes in ontogenetic distribution pattern occurred despite variation in the selective forces of prey and predator abundance in nearshore and offshore habitats. We conclude that the changes in distribution throughout early development are not in response to the proximate selective forces of prey abundance or predation risk

Hydrophobic organochlorine contaminants were surveyed in net plankton from 33 lakes in southern Ontario. Organochlorines were detected in all lakes. The relative concentrations of 12 common organochlorines were generally similar, suggesting that they derive from a common source, probably atmospheric transport. Variability in relative abundances of these contaminants increased with contaminant concentration, while contaminant concentration was related to plankton biomass, total phosphorus, and conductivity. Lakes with low plankton biomass had high organochlorine concentrations in that biomass, indicating that a large portion of the variation among lakes in the concentration of contaminants in plankton can be ascribed to variable planktonic biomass. This negative relationship with plankton biomass, or biomass dilution effect, was strongest for the more hydrophobic and refractory compounds.