Zooplankton‐mediated transitions between N‐ and P‐limited algal growth1

Abstract

Limitationof algal growth by nitrogen and phosphorus was assessed in three north-temperate lakes with physiological bioassays and nutrient enrichment experiments. In addition, mesocosm experiments were performed in the three lakes to examine the effects of nutrient enrichment and zooplankton biomass and size on algal nutrient status. In situ indicators of N and P availability were inversely related in magnitude and transitions between N and P limitation were abrupt. Physiological bioassay results did not indicate simultaneous limitation by N and P. However, limited responses to single-nutrient enrichment and pronounced responses to simultaneous N and P addition in enrichment experiments suggested that potential limitation by the secondary nutrient was usually in close proximity to limitation by the primary nutrient. Transitions between N and P limitation closely accompanied major shifts in the zooplankton community. The importance of the zooplankton community in regulating the relative degree of N or P limitation was confirmed by the mesocosm experiments, which demonstrated that transitions between algal N or P limitation could be induced by manipulations of zooplankton biomass or size. This result supports a hierarchical view of the function of planktonic systems, in which biotic interactions structure the response of the algal community to a given nutrient load.

Evidence of a mutualistic relationship between an algal epibiont and its host, Daphnia pulicaria

“…Furthermore, the nutrient supply from host excreta could be one reason for the epibiont preference for Daphnia, because cladocerans have higher excretion rates than do copepods (Lehman 1980). In addition, and because algae are probably N-limited in Río Seco lake (Morales et al 1999), K. gracilipes could take advantage of the higher N/P excretion rate of Daphnia with respect to copepods (Elser et al 1988;Urabe 1993). However, no clear effect of DIN : TP on epibiont abundance can be concluded from our results.…”

Zooplankton egg banks as biotic reservoirs in changing environments

“…For ex-ample, within the zooplankton, taxa differ in being either generalists or specialists (DeMott 1988;Pace et al 1990), in consumption efficiency (Hairston and Hairston 1993), in rates and routes of nutrient recycling (Lehman 1984;Sterner et al 1992), and in vulnerability to predation (Zaret 1980;O'Brien 1987). The net result is that the type of grazer present in a lake can have a large impact on trophic transfer efficiencies from algae to zooplankton and from zooplankton to fish (Hairston and Hairston 1993) and a large effect on nutrient dynamics (especially in a system near the N : P ratio at which it shifts between nitrogen and phosphorus limitation; Elser et al 1988).…”

Les réseaux trophiques lacustres: structure, fonctionnement, interactions et variations spatio-temporelles

“…Dans ce cas, le petit zooplancton ne pourrait pas accumuler ni regénérer de fortes quantités de phosphore. Des études récentes ont en effet démontré que les grands herbivores comme les daphnies ont des contenus en phosphore plus élevés que les petits cladoceres et les copepodes (ELSER et al, 1988), ce qui permet un plus grand piegeage du phosphore dans le zooplancton, accroît le phosphore dissous plutôt que particulaire, et entraîne une production plus faible de chlorophylle par rapport au phosphore (MAZUMDER, 1994b L'oxygénation des eaux et le déficit en oxygène dans l'hypolimnion sont des caractéristiques chimiques fondamentales des lacs, indicatrices de leur état trophique et de leur productivité biologique. Les nutriments et la structure des réseaux trophiques peuvent influencer l'oxygénation des lacs et le déficit hypolimnétique en oxygène de différentes façons : (1) en diminuant la transmission de la lumière et en empêchant la production d'oxygène par photosynthèse dans les eaux profondes, (2) en accroissant le déficit d'oxygène dans l'hypolimnion suite à la décomposition des algues, et (3) en réduisant les échanges de gaz au niveau du métalimnion (MAZUMDER et al, 1990b).…”