Zooplankton effects on phytoplankton in lakes of contrasting trophic status

Abstract

Studies were carried out in three lakes of strongly contrasting trophic state to evaluate how the effects of zooplankton on phytoplankton vary as a function of lake productivity. Chlorophyll and total P concentrations differed by 2-3 orders of magnitude among ultra-oligotrophic Lake Tahoe, meso-oligotrophic Castle Lake, and strongly eutrophic Clear Lake. Three experimental designs involving short-term (4 d) manipulations of nutrients, ambient zooplankton, and the common crustacean grazer Duphnia were performed in each lake. Algal responses were assessed at both the "aggregate," community (Chl, primary productivity, nutrient status) and species levels.Experiments in Lake Tahoe revealed a high degree of nutrient sensitivity but negligible grazing impacts by the very low densities of ambient zooplankton. Daphnia grazing had substantial impacts on Tahoe phytoplankton, however, indicating high susceptibility to grazing in this assemblage of relatively small-sized algal species. Castle Lake experiments revealed strong direct and indirect impacts of both ambient zooplankton and Duphnia on the nutrient-limited algal assemblage. The cyanobacteria-dominated algal community of Clear Lake was resistant to grazing impacts, responding relatively weakly only to the highest densities of Duphnin. Good correspondence between estimates of algal biomass made from chlorophyll measurements and microscopic examination were obtained, and, in Castle Lake, concordance between estimated community productivity tumover times made from species-specific growth determinations and community productivity measurements was observed. The contrasting responses to experimental zooplankton manipulations in the three lakes provide support for the view that the coupling between zooplankton and phytoplankton is strongest in lakes of intermediate productivity and imply that food-web alterations at the top of the food web are most likely to propagate to the level of the phytoplankton (and therefore lake water quality) in lakes of moderate trophic status.The relationship between nutrient loading and lake trophic state or productivity (Vollenweider 1976) is one of the most wellfounded empirical properties of lakes. Having established this relationship and used it in lake management with success, limnologists increasingly have focused their attention on the considerable amount of unexplained variance which remains in these empirical relationships.One explanation proposes that the chain of predatory interactions within the pelagic food web can strongly alter phytoplankton abundance by ' Present address:

Variability and control of carbon consumption, export, and accumulation in marine communities

“…Therefore, it seems that the importance of herbivores as a control of organic carbon recycling and storage as autotrophic biomass, which is reflected by the percentage of primary production consumed (Cebrian and Duarte 1994;Sterner et al 1997;Cebrian 1999), decreases in more productive communities when different types or only communities of coastal phytoplankton or microphytobenthos are compared. Previous comparisons of planktonic communities also suggest this hypothesis (Eppley 1981;Elser and Goldman 1991). Moreover, because it seems possible that the efficiency of herbivore production does not vary systematically among different types of marine communities (Schroeder 1981), herbivore production may be expected to increase as a decreasing percentage of primary production across community types or within communities of coastal phytoplankton and microphytobenthos.…”

Changes in species composition during annual cyanobacterial dominance in a tropical reservoir: physical factors, nutrients and grazing effects

“…Despite the possible effect of toxins and even the low nutritional value of the available food (Von Elert et al 2005) it also appears that the cyanobacteria biomass is extremely high so that the potential top-down effect may never be felt. Elser & Goldman (1991) showed that algal control by grazing was reduced in systems with higher trophic levels. Some studies in tropical regions have also found a weak relationship between phytoplankton and zooplankton in more eutrophic conditions (Von Rückert & Giani 2008), even for species other than cyanobacteria.…”

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

“…En revanche, l'effet des poissons (ou de la structure des réseaux trophiques : absence ou présence de grands herbivores) sur la biomasse algale s'avère toujours statistiquement indépendant de celui des nutriments, tel que démontré par MAZUMDER (1994a) et BRETT et GOLDMAN (1997). Ceci amène à considérer les hypothèses d'atténuation des effets des prédateurs (McQUEEN et ai, 1986 ;ELSER et GOLDMAN, 1991) ou les hypothèses reliant l'effet des res-sources à la longueur des chaînes trophiques (PERSSON et ai, 1988) comme peu probables dans les lacs dominés par des poissons chassant à vue. Très peu d'expériences factorielles croisant nutriments x poissons ont été réalisées avec des poissons ayant un comportement plus opportuniste de filteur omnivore tels que gardons (LACROIX et LESCHER-MOUTOUE, 1991 ;LACROIX et ai, 1996) et alose à gésier (DRENNER et al, 1996).…”

“…Ainsi, on a successivement proposé que les effets top-down des poissons étaient plus prononcés dans les lacs oligotrophes (McQUEEN étal., 1986) ou mésotrophes (ELSER et GOLDMAN, 1991), ou encore dans les lacs eutrophes pauvres en macrophytes (SARNELLE, 1996;JEPESSEN étal., 1997). Dernièrement, MAZUMDER (1994c) a proposé un modèle non linéaire intégrant les effets combinés des ressources et de la structure des réseaux trophiques pour prédire les variations dans la biomasse du plancton en fonction de l'état trophique des lacs (modèle d: fig.2).…”