Bottom‐Up and Top‐Down Impacts on Freshwater Pelagic Community Structure

Abstract

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.

Disentangling the spatial patterns in community composition of prokaryotic and eukaryotic lake plankton

“…Because phytoplankton species occupy a low trophic level, they are under relatively strict environmental or bottom-up control (Paszowski and Tonn 2000). Higher trophic levels, such as zooplankton that graze on phytoplankton, may be more strongly regulated by food-web interactions or top-down forces (McQueen et al 1989). In RDA, the finding that nutrient supply is the dominant factor influencing community composition of phytoplankton agrees with Longmuir et al (2007), who documented that trophic status of the lake was the strongest determinant of phytoplankton community structure in lakes in British Columbia, Canada.…”

Strong bottom‐up and top‐down control of early life stages of macroalgae

“…As shown earlier for pelagic food webs (e.g., Sommer 1988;McQueen et al 1989), bottom-up and top-down forces simultaneously influence macrobenthic populations and communities (Menge et al 1997;Worm et al 2000a). Their relative strength, however, may vary under different environmental conditions.…”

“…Although we admit that no valid generalizations can be drawn from a study involving two sites, we think that our results match similar patterns that emerged in different ecosystems. For example, it was proposed that in lake ecosystems, as productivity increases, top-down control at lower trophic levels should weaken and bottom-up control should become more important (McQueen et al 1989). Similarly, in saltmarsh ecosystems Van de Koppel et al (1996) observed that herbivore control of plant biomass weakened at high productivity levels.…”

Can fish introductions alter nutrient cycles in previously fishless high-latitude lakes?

“…However, this predation will fluctuate in time with variations in population structure, as different sized individuals focus on different prey. It is accepted, however, that trophic cascades exert more influence on lower trophic levels in oligotrophic lakes (McQueen et al, 1989), and so the rather high nutrient concentrations in the study lake may counter such effects. Chlorophylla concentrations suggest that algae have slightly increased overall; this is still likely affected by the increase in benthic primary producers.…”