Terrestrial subsidies modify the growth rate and biomass of micro- and macro-organisms through altered nutrient cycling

Background/Question/Methods

Ecosystems are often represented as closed systems of energy, matter and information flow, but, in reality, they are rarely isolated from each other. Transfer of organic matter is common across ecosystem boundaries, with varying C:nutrient ratios of such subsidies leading to different outcomes in the recipient system. For example, increased P input is believed to increase the growth rate of organisms, according to the Growth Rate Hypothesis. Moreover, these effects of nutrients can themselves vary following changes in light exposure, according to the Light:Nutrient Hypothesis. In aquatic mesocosms, we performed an experiment where we followed the responses of nutrients, microorganisms, and larvae of the cosmopolitan Culex quinquefasciatus (Diptera: Culicidae) to two terrestrial subsidies differing in C:nutrient in combination with two light levels.

Results/Conclusions

The subsidy treatment with low C:nutrient led to a higher concentration of most nutrients, and increased the quantity of chlorophyll and bacteria in the water. We also found that the same subsidies increased the growth rate of larvae, as well as the biomass and health of emerging adults. However, the survival rate of larvae remained unaffected by the different subsidy treatments. Increased light exposure was positively associated with concentrations of NH4 and PO4, and mediated the sign of interaction between microorganisms and nutrients. Even though we did not find an association between the larvae and the amount of microorganisms, larvae were associated with increased recycling of nutrient and had higher biomass in exposed treatments. This experiment underlines the cyclic nature of element flows, which can sometimes diverge from established hypotheses.