Regulation of Lake Primary Productivity by Food Web Structure

Abstract

Recent large-scale seagrass declines have prompted experimental investigations of potential mechanisms. Although many studies have implicated eutrophication or reductions of epi-phyte grazers in these declines, few experiments have simultaneously manipulated both factors to assess their relative effects. This study used meta-analyses of 35 published seagrass studies to compare the relative strength of 'top-down' grazer effects and 'bottom-up' nutrient effects on epiphyte biomass and seagrass above-ground growth rate, above-ground biomass, below-ground biomass, and shoot density. A surprising result was that seagrass growth and biomass were limited in situ by sediment nutrients; light limitation has been emphasized in the literature to date. Water column enrichments, which were correlated with increased epiphyte biomass, had strong negative effects on seagrass biomass. Grazers overall had a positive effect on shoot density, but negligible effects on sea-grass biomass and growth rate. However, analyzing epiphyte grazers separately from other grazers revealed positive effects of grazing on seagrass response variables and corresponding negative impacts on epiphyte biomass. The positive effects of epiphyte grazers were comparable in magnitude to the negative impacts of water column nutrient enrichment, suggesting that the 2 factors should not be considered in isolation of each other. Until the determinants of epiphyte grazer populations are empirically examined, it will be difficult to address the contribution that overfishing and cascading trophic effects have had on seagrass decline. Because increases in water column nutrients are documented in many regions, efforts to reduce coastal eutrophication are an appropriate and necessary focus for the management and conservation of seagrass ecosystems. KEY WORDS: Seagrasses · Meta-analysis · Epiphyte · Nutrients · Grazers · Management · Eutrophication · Top-down/bottom-up Resale or republication not permitted without written consent of the publisher herbivory, and substratum turnover. Limnol Oceanogr 32: 986-992 Brooks JL, Dodson SI (1965) Predation, body size and composition of plankton. Science 150:28-35 Camp DK, Cobb SP, Van Breedfield JV (1973) Overgrazing of seagrasses by a regular urchin, Lytechinus variegatus.

Long-term changes of the crustacean zooplankton community in Lake Mjøsa, the largest lake in Norway

“…If so, the results indicate both top down and bottom up effects between planktivores and zooplankton. However, the low abundance of herbivores in 1993 did not cascade down to the phytoplankton level causing enhanced algal biomass as expected by the trophic cascade hypothesis (Carpenter et al 1987). PB and Chl-a concentrations were very low in 1993, thus suggesting a primarily bottom up regulation of the phytoplankton level in Lake Mjøsa.…”

Crustacean zooplankton size structure in aquaculture lakes: is larger size structure always associated with higher grazing pressure?

“…Bachmann et al, 1996;Jeppesen et al, 2000). Intense predation by planktivorous fish on large-bodied zooplankton not only favors the development of phytoplankton and small-bodied zooplankton (Brooks & Dodson, 1965;Carpenter et al, 1987;Mazumder et al, 1990;Yang & Huang, 1992), but also may accelerate the process of eutrophication through direct excretion and increased nutrient recycling rate (Schindler, 1992;Vanni & Layne, 1997). The large inedible or filamentous alga can also reverse the outcome of competition between species of different body sizes (Gliwicz & Lampert, 1990) as well as the abiotic factors, e.g.…”

Phosphorus release of metazoan zooplankton in two bays with different trophic status in Lake Taihu (China)

“…Excretion by zooplankton also is considered an internal source of P (Lehman, 1980b;Urabe et al, 1995;Kowalczewska-Madura and Gołdyn, 2010). Grazing by large zooplankton, particularly cladocerans, can regulate phytoplankton biomass and species composition (Carpenter et al, 1987;Reynolds, 1994;Sommer and Sommer, 2006). However, nutrient excretion by zooplankton may also affect the supply and stoichiometry of nutrients and thereby influence phytoplankton and lake trophic status (Lehman, 1980a;Elser et al, 1988;Sterner and Hessen, 1994;Kowalczewska-Madura et al, 2007).…”