Terrestrial subsidies and light affect an aquatic micro‐ecosystem in unexpected ways

Abstract

The effects of resource subsidies on a recipient ecosystem depends on a variety of factors. Firstly, increasing the supply of limiting nutrients tends to increase the accumulation of biomass across different compartments of the recipient food web. Secondly, in the case of aquatic microorganisms, this accumulation may also be contingent on inputs of energy through light, mediating interactions between heterotrophic bacteria and autotrophic algae. Thirdly, macroscopic consumers feed upon microorganisms and recycle nutrients through excretion and egestion, profoundly affecting nutrient cycling. In order to examine the context‐dependency of nutrient cycling in aquatic systems, we performed an experiment in mesocosms where we followed the responses of dissolved nutrients, microorganisms, and larvae of the cosmopolitan mosquito Culex quinquefasciatus (Diptera: Culicidae) to two terrestrial subsidies differing in carbon (C):nutrient, in combination with two light intensities. We found that the interactions amongst bacteria and algae themselves remained generally unimportant after accounting for treatment effects, and that light did mediate some of the effects of nutrient subsidies on microorganisms. We also found that the presence of mosquito larvae was associated with altered concentrations of nitrogen (N) in some treatments, ultimately benefitting algae. Finally, we found that low C:nutrient subsidies generally increased the development rate of mosquito larvae, and that increased light exposure led to higher mosquito survival. Overall, these patterns may be explained by strong nitrogen limitation in our system, causing both group of microorganisms to respond similarly to variations in nutrient supply without direct covariation, and to the key role of N recycling by mosquito larvae in sustaining algal densities in exposed treatments. The context‐dependency in our study may reflect elemental or molecular constraints other than phosphorus. Our experiment thus calls for greater synthesis amongst ecological theories.