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

Abstract

Sedimentation is a key process in lake functioning, and plays an important role in nutrient and carbon cycles at both regional and global scales. Several biological processes have been shown as quantitatively affecting sedimentation, but very few works have tried to relate the structure of aquatic communities and the quality of sinking organic matter. We tested in a mesocosm study how food-web structure affects quantitatively and qualitatively sedimentation in eutrophic systems. We carried out a long-term experiment (14 months) in large replicated enclosures either dominated by planktivorous fish or fishless. Food-web structure modified the specific composition of zooplankton communities and phytoplankton biomass, as expected by the trophic cascade theory. Planktivorous fish had a strong positive effect on gross sedimentation rate, but the fraction of suspended particulate material that sank only slightly differed between treatments. The density of Handling editor: Karl E. Havens M. Danger (

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

“…These results indicate that the C ⁄ N ratio was not a valuable proxy for RDS biodegradability in our experiment and support the previous suggestions that sediment C ⁄ N ratio alone does not allow assessment of sediment biodegradability and quality (e.g. proteins, sugars and PUFAs) than RDS from the fishless treatment (Allard et al, 2011;Danger et al, 2012). Our results are opposite to the findings of Sander & Kalff (1993), who found a positive linear correlation between the bacterial production in freshwater sediments and their organic matter content.…”

“…In a recent long-term experiment in mesocosms, Danger et al (2012) confirmed this quantitative effect. Moreover, Danger et al (2012), at the end of the experiment, found higher contents of proteins and sugars in RDS from fish enclosures than in RDS from fishless ones. These authors first confirmed that sedimentation rates were higher in fish enclosures than in fishless ones.…”

“…Maintaining over a sufficiently long period a topdown control of phytoplankton biomass through the biomanipulation of fish communities should not only induce a more efficient grazing of phytoplankton (Danger et al, 2009) and periphyton (Danger et al, 2008) by invertebrate herbivores, but also reduce the quantity (Danger et al, 2012) and biodegradability (this study) of SOM and the stock of carbon in sediments (stocks of Tot-N and OP followed a similar trend but were not significant), ultimately decreasing internal nutrient cycling. Maintaining over a sufficiently long period a topdown control of phytoplankton biomass through the biomanipulation of fish communities should not only induce a more efficient grazing of phytoplankton (Danger et al, 2009) and periphyton (Danger et al, 2008) by invertebrate herbivores, but also reduce the quantity (Danger et al, 2012) and biodegradability (this study) of SOM and the stock of carbon in sediments (stocks of Tot-N and OP followed a similar trend but were not significant), ultimately decreasing internal nutrient cycling.…”

“…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.…”

“…In this study, we tested the hypothesis that the biodegradability of RDS from the experiment of Danger et al (2012) was higher in fish mesocosms than in fishless ones using two different approaches. First, we carried out ex situ respirometric measurements of the potential biodegradability of RDS samples originating from the two treatments, and we analysed the relationship between the potential biodegradability of RDS and its elemental Food-web effects on sediment biodegradability 2391 composition.…”