Effects of zooplankton on nutrient availability and seston C : N : P stoichiometry in inshore waters of Lake Biwa, Japan

Abstract

Forty-eight-hour experimental manipulations of zooplankton biomass were performed to examine the potential effects of zooplankton on nutrient availability and phytoplankton biomass (as measured by seston concentration) and C : N : P stoichiometry in eutrophic nearshore waters of Lake Biwa, Japan. Increasing zooplankton, both mixed-species communities and Daphnia alone, consistently reduced seston concentration, indicating that nearshore phytoplankton were generally edible. The zooplankton clearance rates of inshore phytoplankton were similar to rates measured previously for offshore phytoplankton. Increased zooplankton biomass led to increased concentrations of nutrients (NH 4 -N, soluble reactive phosphorus [SRP]). Net release rates were higher than those found in previous measurements made offshore, reflecting the nutrient-rich nature of inshore seston. Zooplankton nutrient recycling consistently decreased TIN : SRP ratios (TIN ϭ NH 4 ϩ NO 3 ϩ NO 2 ). This effect probably resulted from the low N : P ratios of nearshore seston, which were lower than those commonly found in crustacean zooplankton and thus resulted in low retention efficiency of P (relative to N) by the zooplankton. Thus, zooplankton grazing inshore may ameliorate algal blooms due to direct consumption but tends to create nutrient supply conditions with low N : P, potentially favoring cyanobacteria. In comparison with previous findings for offshore, it appears that potential zooplankton effects on phytoplankton and nutrient dynamics differ qualitatively in inshore and offshore regions of Lake Biwa.

Functional shifts in lake zooplankton communities with hypereutrophication

“…Our work illustrates the utility of functional trait‐based approaches in testing stoichiometric hypotheses. Further, our results expand on prior findings from a limited set of moderately eutrophic lakes (Elser, Sterner, Galford, et al., ; Elser et al., ; Paterson et al., ; Urabe et al., ) by supporting the top‐down functional importance of the zooplankton community even in highly nutrient‐enriched systems. Future work on the feedbacks between zooplankton and nutrient cycling in hypereutrophic lakes is needed to understand how these systems function and how to manage their potential for developing harmful algal blooms.…”

Direct and indirect effects of zooplankton on algal composition in in situ grazing experiments

“…Among the phytoplankton, Fragilaria crotonensis, a colonial diatom species, forms large blooms in spring, but Staurastrum dorsidentiferum, a large desmid alga, predominates from summer to autumn. Several studies have examined the effects of zooplankton on phytoplankton in Lake Biwa, including the effects of direct grazing (Okamoto 1984;Kawabata 1987;Urabe et al 1996), nutrient regeneration Elser et al 2001), and both grazing and nutrient regeneration (Yoshida et al 2001b). However, these studies focused mainly on total phytoplankton abundance, as quantified by chlorophyll-a or seston concentration.…”

Seasonal variation in biomass and photosynthetic activity of epilithic algae on a rock at the upper littoral area in the north basin of Lake Biwa, Japan

“…Although N and P amounts in epilithic algae were not measured, DIP generally seemed to be the limiting parameter, except in summer when nitrogen and phosphorus decreased and both almost suffered depletion. Total inorganic nitrogen (TIN) and soluble reactive phosphorus (SRP) concentrations measured by Elser et al (2001) at a station in the central area of the north basin offshore of Imazu (water depth, 100 m) during May to September 1999 ranged from 6.7 to 16.9 µM and from 0.11 to 0.13 µM, respectively. Comparing our data to theirs, TIN (DIN) and SRP (DIP) concentrations were similar except in August because our littoral station does not receive nutrient loads from river flows or human activity.…”