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

Abstract

Summary 1. Sediment plays a key role in internal nutrient cycling and eutrophication in lakes. However, studies focusing on the efficiency of the biomanipulation techniques for improving the control of primary producers have rarely examined the effects of changes in food‐web structure on the sediment biochemical composition and biodegradability. 2. In a 1‐year experiment conducted in large replicated mesocosms, we tested how the absence or presence of a zooplanktivorous fish (roach, Rutilus rutilus) affected the elemental composition and the potential biodegradability of recently deposited sediment in a eutrophic system. The potential biodegradability of these sediments was assessed in laboratory microcosms by measuring the production of CO2 during 44‐day incubations. 3. The potential biodegradability of recently deposited sediment from the fish treatment was 60% higher than that from the fishless treatment. This higher biodegradability was corroborated by a higher annual loss of sediment in fish enclosures (36%) than in fishless ones (16%). Annual losses of carbon, nitrogen and organic phosphorous were higher for sediment from fish enclosures. 4. Carbon and nitrogen contents of sediment were higher for the fish treatment. In contrast, the sediment C/N ratio, one of the proxies used to estimate sediment biodegradability, did not differ between treatments. No relationship was observed between elemental composition of sediment and its potential biodegradability. This latter appeared to be more probably dependent on the biochemical composition of the sediment and especially on the content of labile compounds such as proteins, sugars and polyunsaturated fatty acids. The use of sterols as biomarkers revealed an important degradation by microorganisms of 1‐year‐old sediment from both fish and fishless treatments. 5. Our results revealed that fish biomanipulations might favour clear water states not only through a stronger top–down control on phytoplankton but also through a lower biodegradability of sediment reducing internal nutrient cycling.

Interactions between the green and brown food web determine ecosystem functioning

“…; Harrault et al . ) therefore affecting the productivity in the brown food web. In terrestrial ecosystems, productive plants have strong effects on microflora through their control on litter quality (Wardle et al .…”

“…Experiments have also shown that brown food webs respond strongly to the quality and quantity of dead organic matter, which is controlled by the structure of the green food web in both aquatic (Harrault et al . ; Danger et al . ) and terrestrial ecosystems (Bardgett & Wardle ; Wardle et al .…”

“…The concept of 'microbial loop' highlights that predators of the brown food web can increase nutrient mineralization in aquatic (Azam et al 1983;Fenchel 1988) and terrestrial (Clarholm 1985;Bonkowski 2004) ecosystems, which can indirectly affect primary production (Azam, Smith & Hollibaugh 1991;Stone & Weisburd 1992). Experiments have also shown that brown food webs respond strongly to the quality and quantity of dead organic matter, which is controlled by the structure of the green food web in both aquatic (Harrault et al 2012;Danger et al 2012) and terrestrial ecosystems (Bardgett & Wardle 2003;Wardle et al 2003Wardle et al , 2005. Cascading effects from one web to another vary in intensity (Wardle et al 2004) and sometimes in sign [e.g.…”

Bottom‐up effects of lake sediment on pelagic food‐web compartments: a mesocosm study

“…Whereas the bottom-up effects of inorganic nutrients or light on aquatic ecosystems are well known (see Urabe et al, 2002;Cebrian & Lartigue, 2004;Hessen et al, 2004;Dickman, Vanni & Horgan, 2006;Spivak et al, 2007), those of SOM have more rarely been studied. Moreover, as the sediment biodegradability seems to be linked to its OM composition (Allard et al, 2011;Danger et al, 2012;Harrault et al, 2012), an interesting issue is whether different sediment compositions could lead to different bottom-up forcing on aquatic food webs. As DOM can originate from sediment biodegradation, sediment could be responsible for a bottom-up control for the whole food web, through the availability of resources for basal organisms (phytoplankton and bacterioplankton).…”

“…According to the results of Spivak et al (2007), Allard et al (2011 and Harrault et al (2012), we hypothesised that: Three treatments were compared: enclosures without added sediment (S 0 ), enclosures with carbon-poor sediment (S 1 ) and enclosures with carbonrich sediment (S 2 ).…”