Terrestrial carbon is a resource, but not a subsidy, for lake zooplankton

Abstract

Inputs of terrestrial organic carbon (t-OC) into lakes are often considered a resource subsidy for aquatic consumer production. Although there is evidence that terrestrial carbon can be incorporated into the tissues of aquatic consumers, its ability to enhance consumer production has been debated. Our research aims to evaluate the net effect of t-OC input on zooplankton. We used a survey of zooplankton production and resource use in ten lakes along a naturally occurring gradient of t-OC concentration to address these questions. Total and group-specific zooplankton production was negatively related to t-OC. Residual variation in zooplankton production that was not explained by t-OC was negatively related to terrestrial resource use (allochthony) by zooplankton. These results challenge the designation of terrestrial carbon as a resource subsidy; rather, the negative effect of reduced light penetration on the amount of suitable habitat and the low resource quality of t-OC appear to diminish zooplankton production. Our findings suggest that ongoing continental-scale increases in t-OC concentrations of lakes will likely have negative impacts on the productivity of aquatic food webs.

Experimental evidence that subsidy quality affects the temporal variability of recipient zooplankton communities

“…Cole et al (2006) found that the majority of the carbon subsidy in zooplankton was incorporated by direct ingestion of terrestrial particulate organic carbon (POC), and not through ingestion of cDOM-supported bacteria. However, more recent studies have suggested that zooplankton strongly selected against organic matter of terrestrial origin (i.e., used terrestrially-derived carbon in lower proportion relative to its availability) (Marcarelli et al 2011), challenging the idea that terrestrial carbon may not be as important a trophic subsidy for zooplankton as originally believed (Brett et al 2009;Kelly et al 2014).…”

“…Although this result did not support expectations (temporal variability was hypothesized to be lowest in cDOM treatments), it is consistent with previous studies that showed Experimental evidence that subsidy quality affects the temporal variability of recipient… no effect of leaf litter subsidies on zooplankton abundance relative to controls (Rubbo et al 2008) or even showed slight destabilization (Geddes 2004). It is hypothesized that these latter responses may be mediated by the low quality carbon in cDOM (see below) via a trophic pathway and, to some extent, by a non-trophic pathway, through which subsidies affect recipient communities via physical changes of the environment (Geddes 2009;Kelly et al 2014). For example, color in cDOM provides protection from damaging UV radiation to plankton communities (e.g., Williamson et al 2001).…”

Catchment properties predict autochthony in stream filter feeders

“…In streams, food-web autochthony is predicted to be lowest in the headwaters and increase downstream (Vannote et al, 1980;Collins et al, 2016), and there should, therefore, be a simultaneous increase in secondary production downstream (Finlay, 2011;Kelly et al, 2014). However, in small, heterotrophic streams, a peak in secondary production-and therefore, consumer allochthony-can occur soon after the seasonal input of riparian plant litter, which therefore, often coincides with an increased abundance of macroinvertebrate detritivores (Richardson, 1991;Wallace et al, 1999).…”

Altered energy flow in the food web of an experimentally darkened lake

“…These studies suggest that if local resources are depleted and labile alternatives exist, carbon resources that previously did not contribute substantially to consumer biomass could begin to increase their support of the food web. These shifts in support are reflected in the contributions of sources to consumer biomass, but not necessarily the amount of consumer biomass or production, which might decrease (Jones et al 2012, Kelly et al 2014.…”