Influence of food web structure on the biochemical composition of seston, zooplankton and recently deposited sediment in experimental freshwater mesocosms

Abstract

The effects of food web structure on the quantity and biochemical composition of seston, zooplankton and recently deposited sediment in experimental freshwater mesocosms were examined. Food web structure was manipulated by addition of zooplanktivorous fish. Biochemical characterisations were carried out using lipid biomarkers (sterols, fatty acids, chlorophyll-derived compounds and long-chain alkanediols). Fish addition decreased zooplankton biomass and increased seston biomass and deposited sediment through a trophic cascade. Fish presence strongly influenced the biochemical characteristics of seston and sediment. In contrast, food web structure had a minor impact on the lipid biomarker composition of zooplankton. Although the relative abundance of sterols in the different compartments did not differ strongly between treatments, sterol profiles in seston and sediment depended on food web structure. The predominance of D 7 -sterols in seston and sediment in the fish treatment indicated a major contribution of Chlorophyceae. In contrast, the distribution of sterols in seston and sediment in the fishless treatment, dominated by cholesterol, indicated a major zooplanktonic input. The distribution of fatty acids and the relative abundance of chlorophyll-derived compounds and long-chain alkanediols agreed with the predominant contribution of phytoplankton or zooplankton to seston and sediment in the two treatments. The relative abundance of bacterial biomarkers suggested that the contribution of bacteria was rather low. The high relative abundance of polyunsaturated fatty acids (PUFAs) and the absence of stanols in sediments suggested low microbial reworking of organic matter in the recently accumulated sediments. The trophic cascade, generated by the addition of fish, increased the relative abundance of PUFAs in deposited organic matter, thus enhancing sediment quality and potential degradability.

Effects of food-web structure on the quantity and the elemental quality of sedimenting material in shallow lakes

“…As such, the data should be considered to be qualitative. The chemical analysis of the free lipids extracted from seston (0.7-50 lm fraction) and zooplankton fractions (Allard et al, 2011) indicated that seston and zooplankton compartments were adequately separated by our screening size.…”

Influence of food‐web structure on the biodegradability of lake sediment

“…Lipids containing sterols were isolated and analysed as previously described (Allard et al, 2011). Lipids containing sterols were isolated and analysed as previously described (Allard et al, 2011).…”

“…These authors first confirmed that sedimentation rates were higher in fish enclosures than in fishless ones. Moreover, Allard et al (2011) showed that the relative abundance of polyunsaturated fatty acids was higher in RDS from fish enclosures than in RDS from fishless ones. In fish-dominated mesocosms, where zooplankton grazing on phytoplankton was low, sediment N ⁄ P ratio was positively correlated to seston N ⁄ P ratio, suggesting a major contribution of seston to sedimentation.…”

“…By manipulating the presence of planktivorous fish, they modified the specific composition of zooplankton communities and the phytoplankton biomass (increase in Daphniidae and decrease in phytoplankton biomass in fishless mesocosms). In the same experiment, Allard et al (2011) compared the lipid composition of seston, zooplankton and RDS in fish and fishless mesocosms. They also showed that food-web structure influenced the elemental and biochemical compositions of recently deposited sediment (RDS).…”

“…They also showed that food-web structure influenced the elemental and biochemical compositions of recently deposited sediment (RDS). The differences in the relative amounts of labile compounds such as polyunsaturated fatty acids (Allard et al, 2011) or proteins and sugars (Danger et al, 2012) observed between fish treatments led these authors to suggest that biomanipulations might affect the biodegradability of sedimented organic matter (SOM). In contrast, in fishless mesocosms, dominated by a high grazing activity, a negative relationship between N ⁄ P ratios of sediment and zooplankton was found, suggesting that sedimentation was mainly derived from zooplankton egesta.…”