Food web interactions determine energy transfer efficiency and top consumer responses to inputs of dissolved organic carbon

Hydrobiologia, Jul 2017

Climate change projections indicate increased precipitation in northern Europe, leading to increased inflow of allochthonous organic matter to aquatic systems. The food web responses are poorly known, and may differ depending on the trophic structure. We performed an experimental mesocosm study where effects of labile dissolved organic carbon (DOC) on two different pelagic food webs were investigated, one having zooplankton as highest trophic level and the other with planktivorous fish as top consumer. In both food webs, DOC caused higher bacterial production and lower food web efficiency, i.e., energy transfer efficiency from the base to the top of the food web. However, the top-level response to DOC addition differed in the zooplankton and the fish systems. The zooplankton production increased due to efficient channeling of energy via both the bacterial and the phytoplankton pathway, while the fish production decreased due to channeling of energy mainly via the longer and less efficient bacterial pathway. We conclude that the added DOC either acted as a subsidy by increasing the production of the top trophic level (mesozooplankton), or as a sink causing decreased top consumer production (planktivorous fish).

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Food web interactions determine energy transfer efficiency and top consumer responses to inputs of dissolved organic carbon

Food web interactions determine energy transfer efficiency and top consumer responses to inputs of dissolved organic carbon 0 R. Degerman R. Lefe ́bure U. Ba ̊mstedt S. Larsson A. Andersson Umea ̊ Marine Science Centre , 905 71 Ho ̈rnefors , Sweden 1 R. Degerman R. Lefe ́bure P. Bystro ̈m U. Ba ̊mstedt S. Larsson A. Andersson (&) Department of Ecology and Environmental Science, Umea ̊ University , 901 87 Umea ̊ , Sweden 2 Handling editor: Jonne Kotta Climate change projections indicate increased precipitation in northern Europe, leading to increased inflow of allochthonous organic matter to aquatic systems. The food web responses are poorly known, and may differ depending on the trophic structure. We performed an experimental mesocosm study where effects of labile dissolved organic carbon (DOC) on two different pelagic food webs were investigated, one having zooplankton as highest trophic level and the other with planktivorous fish as top consumer. In both food webs, DOC caused higher bacterial production and lower food web efficiency, i.e., energy transfer efficiency from the base to the top of the food web. However, the top-level response to DOC addition differed in the zooplankton and the fish systems. The zooplankton production increased due to efficient channeling of energy via both the bacterial and the phytoplankton pathway, while the fish production decreased due to channeling of energy mainly via the longer and less efficient bacterial pathway. We conclude that the added DOC either acted as a subsidy by increasing the production of the top trophic level (mesozooplankton), or as a sink causing decreased top consumer production (planktivorous fish). Food web efficiency; Carbon transfer; Allochthonous dissolved organic carbon; Mesocosm; Planktivorous fish Introduction Knowledge about pathways and constraints of energy transfer through food webs is fundamental for the understanding of ecosystem function (e.g., Dickman et al., 2008; Wollrab et al., 2012) . The complexity of food web dynamics and structure inherently means that both changes in basal production and top–down control can create alternative pathways of energy flow up to the highest trophic level, resulting in patterns of energy transfer and productivity commonly deviating from expectations based on classical food chain theory (Vadeboncoeur et al., 2004, Hulot et al., 2014) . For example, the presence or absence of classic trophic cascades in marine food webs has been suggested to be dependent on the size structure of the phytoplankton community (Stibor et al., 2004) . In aquatic systems, phytoplankton (autotrophs) and bacteria (heterotrophs) are basal producers, acting as energy source for higher trophic levels, and thus shape the food webs depending on their production and composition (Azam et al., 1983; Legendre & Rassoulzadegan, 1995, Jansson et al., 2007) . The balance between autotrophs and heterotrophic bacteria is governed by both bottom–up factors such as nutrient availability, and top–down effects, e.g., trophic interactions via top predators (Carpenter et al., 1985; Hairston & Hairston, 1993, Vanni & Layne, 1997) . These food webs are complex, as many consumers feed on organisms from both the phytoplankton and the bacterial pathway. Both phytoplankton and bacteria are osmotrophic organisms competing for inorganic nutrients, nevertheless phytoplankton utilize inorganic forms of carbon while heterotrophic bacteria are in many systems dependent on autochthonous organic carbon produced by phytoplankton (Cole et al., 1988) . However, in aquatic systems influenced by allochthonous dissolved organic carbon (ADOC), bacteria can be decoupled from autochthonous production (Karlsson et al., 2002, Stibor et al., 2004) . Accordingly, in systems with high inputs of ADOC, heterotrophic bacteria tend to contribute substantially to the total basal production (Pace et al., 2004; Berglund et al., 2007, Jansson et al., 2007) . Phytoplankton are often directly consumed by primary consumers like mesozooplankton, whereas bacteria are too small to be readily eaten by such organisms. Instead, bacteria are consumed by protozoans which in turn are consumed by mesozooplankton (Hessen & Andersen, 1990, Brett et al., 2009) . Hence, the heterotrophic-based pathway, i.e., the microbial food web, will have a more complex pattern of energy transfer compared to systems dominated by autotrophic production (Sommer et al., 2002; Berglund et al., 2007) . At the top of the food web, interactions between predator and prey may also yield strong effects on food web function and structure, but the nature and composition of top consumer organisms in the food web will exert different top–down impacts on lower trophic levels (Hairston & Hairston, 1993, Vanni & Layne, 1997, Hulot et al., 2014) . For instance, in food webs where mesozooplankton function as top predator, consumption rates on phytoplankton and ciliates have been shown to be high (Johan (...truncated)


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R. Degerman, R. Lefébure, P. Byström, U. Båmstedt, S. Larsson, A. Andersson. Food web interactions determine energy transfer efficiency and top consumer responses to inputs of dissolved organic carbon, Hydrobiologia, 2017, pp. 1-16, DOI: 10.1007/s10750-017-3298-9