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.

Two sequential cohorts of age-0 yellow perch (Perca flavescens), differing approximately threefold in density, were observed for their ontogeny of spatial distribution, growth, and diet in Lake St. George, Ontario. The lower density cohort exhibited typical ontogenetic spatial behaviour patterns consisting of an early pelagic phase followed by a complete migration to the littoral zone by midsummer. The high-density cohort appeared to split into a faster growing littoral component and a slower growing pelagic component which persisted throughout the growing season and into the next spring. The differences in spatial behaviour between these two cohorts could not be explained by spatial patterns in prey availability. The overwinter mortality rate for the high-density cohort was approximately double that observed for the low-density cohort. No direct observations of behaviour were made of the two cohorts, but distribution and growth data, along with arguments from competition theory, support the hypothesis that interference competition is the density-dependent mechanism responsible for the aberrant ontogeny of spatial distribution of the high-density cohort. The occurrence of interference competition in which competitively superior individuals can monopolize optimal habitats, relegating other individuals to suboptimal habitats, infers an asymptotic stock and recruitment relationship.

1990. Vertical migration in Daphnia galeata mendotae (Brooks): demographic responses to changes in planktivore abundance. Can. 1. Fish. Aquat. Sci.47: 395400.We tested the hypothesis that changes in vertical migration patterns in Baphnid gsledta rnendstde were correlated with changes in planktivore numbers. A 4-yr data set from bake St. George showed that during 1982, when planktivores numbered approximately 2500-ha-', adult D. galeata mendstag were tsund in the epilimnion at both midday and midnight. During 1984 and 1985, when planktivores were airnsst an order of magnitude more abundant, adult D. galeata mendstae were detected in the epilimnion only at night, and the amplitude sf migration increased by a factor of two. Changes in die1 population densities were associated with these interannual differences in vertical migration patterns. These results suggest that between-year differences in vertical migration patterns were strongly associated with changes in planktivore density whish may have influenced the selective value of increased hypolirnnetic residence time.Nous avons tent6 de verifier l1hiPoth$se selon laquelle les variations des modes de migration verticale de Daphnia galsata mendstae etaient en cortt?lation avec les variations du nornbre de planctonsphages. Une serie de donnees provenant du lac St-George, et $optant sur quatre ans, a montr6 qu'au cours de I'annee 1982, au moment oh les planctonophages se ckifraient a Bnviron 2500 ha-', les adultes 5. g. mendotae s'observaient dans Ir6pifimnion 2 midi et a minuit. En 1984 et 1985, lorsque Bes planctonophages ktaient pr&s d'un ordre de grandeur plus abondants, les adultes D. g. mendatae n'etaient presents dans I'epilimnion que la nuit; de plus, I'ampteur de la migration a double. Les variations des densitks de populations nystkern6rates ktaient associees a ces dif@rences interannuelles des modes de migration verticale. Ces resultats semblent montrer que les diff6rences interannuelles dans les modes cle migration verticale etaient etroiternent associees a des variations de la densit6 des planctonsphages, ce qui aurait pu avoir une influence sur I'importance du facteur (( peaiode de s6jour accrue )) dans I'hypolimnion.

A winterkill at Lake St. George, Ontario eliminated most of the piscivorous fish and some of the planktivores and also influenced zooplankton community structure. This disturbance and the subsequent recovery of the populations provided the opportunity to evaluate potential impacts of food and predators on the population structure and abundance of the dominant planktivore; the golden shiner (Notemigonus crysoleucas). interpretation of the 8-yr data set suggested that there was a positive relationship between adult golden shiner condition and zooplankton abundance, but that this did not translate into a significant stock–recruitment relationship between female numbers and young-of-the-year (YOY) recruitment. Also, there was no significant relationship between golden shiner survival and zooplankton abundance. However, there were significant negative correlations between YOY and adult golden shiner survival estimates and predator abundances. We conclude that at Lake St. George, golden shiner recruitment and population density is strongly influenced by predator density and is not influenced by food abundance.

We used standardized methods to analyze a 14-yr data set from Oneida Lake and a 10-yr data set from Lake St. George. We estimated mean summer concentrations of several trophic level indicators including piscivores, planktivores, zooplankton, phytoplankton, and total phosphorus, and we then investigated the relationships between these variables. Both data sets yielded similar long-term and short-term trends. The long-term mean annual trends were that (1) the relationships between concentrations of planktivores and zooplankton (including daphnids) were always negative, (2) the relationships between concentrations of zooplankton and various measures of phytoplankton abundance were unpredictable and never statistically significant, and (3) the relationships between total phosphorus and various measures of phytoplankton abundance were always positive. Over short periods, the data from both lakes showed periodic, strong top-down relationships between concentrations of zooplankton (especially large Daphnia) and chlorophyll a, but these events were unpredictable and were seldom related to piscivore abundance.