Differential responses of aquatic consumers to variations in leaf-litter inputs

Abstract

Terrestrial support of aquatic food webs is becoming well established in the science of ecology. However, while terrestrial subsidies of energy have been shown to exert strong effects on aquatic food webs, it is less clear how variations in these subsidies, via natural or anthropogenic factors, will affect recipient ecosystems. To assess the influence of variations in terrestrial subsidies on an aquatic food web, we manipulated leaf-litter inputs in artificial ponds. Decreasing litter inputs did not affect any of the response variables in artificial ponds. This may be because the minimal amount of terrestrial carbon present combined with autochthonous production was enough to sustain the food web and/or the food web was altered in ways not detected by the experimental design. However, increasing leaf-litter inputs increased the percent survival and developmental rate of larval wood frogs (Rana sylvatica). Conversely, increasing litter input appeared to have no influence on zooplankton or salamander larvae. Increasing litter inputs also increased the dissolved organic carbon content and decreased the percent saturation of dissolved oxygen in artificial ponds. As system respiration in aquatic systems is frequently dominated by microbial respiration, we hypothesize that the effects of increasing litter input on wood frogs were the result of an increase food resources (i.e., microbes) for tadpoles. The lack of a response by salamander larvae and zooplankton may be due to the densities of zooplankton in tanks providing enough food for salamanders in all treatments, variation among specific zooplankton species in their ability to exploit these resources and transfer energy to salamanders, or omnivory among zooplankton offsetting the affects of leaf-litter inputs. Additional work is needed to determine the influence of litter inputs on zooplankton and salamanders in this community. These data demonstrate that variations in leaf-litter inputs can influence food web structure; however, the importance of these variations will likely be dependent upon the trophic position of various consumers.

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

“…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).…”

Leaf litter resource quality induces morphological changes in wood frog (Lithobates sylvaticus) metamorphs

“…At this time, we also added 150 g of recently senesced and dried leaves to all mesocosms except the no-litter control. This biomass of litter is within the range observed in natural wetlands (Rubbo et al 2008). The mesocosms overwintered until the start of the experiment in the spring.…”

Over-winter decomposition and associated macroinvertebrate communities of three deciduous leaf species in forest streams on the Canadian Boreal Shield

“…Melody and Richardson (2004) found that a 75% reduction in litter inputs to experimental mesocosms resulted in differences in invertebrate densities, suggesting food limitation. Rubbo et al (2008) also found that variations in leaf litter inputs can affect the structure of aquatic food webs in artificial ponds and that increasing the quantity of leaf litter can positively influence the productivity of amphibian larvae, also providing evidence that suggests food limitation by reduced leaf litter. Further studies would be useful to determine to what extent a reduction in deciduous leaf litter inputs to streams would invoke invertebrate community responses as a result of food limitation and to determine if a threshold of leaf litter loading to boreal forest streams exists.…”