Light availability regulates the response of algae and heterotrophic bacteria to elevated nutrient levels and warming in a northern boreal peatland

Abstract

Summary Light attenuation associated with elevated levels of dissolved organic matter (i.e. browning) is likely to affect the response of benthic biofilms to nutrient enrichment and warmer temperatures expected for northern aquatic ecosystems with climate change. To examine how these factors will interact to regulate the association between algae and heterotrophic bacteria in northern peatlands, we manipulated light (100%, 60%, 30%, 20% and 5% of ambient), nutrients (unenriched, enriched) and temperature (ambient, warming), in a full factorial design by shading nutrient diffusing substrates inside mesocosms with or without warming (3 °C) in an Alaskan fen. After 12 days of experimental conditions, there was no effect of light or temperature on the abundance of algae or bacteria in the absence of nutrient enrichment. Enrichment with a combination of nitrogen and phosphorus significantly increased the abundance of algae and heterotrophic bacteria at ambient light levels (418 μmol m−2 s−1), and warming significantly enhanced the positive effects of nutrients on algal accrual. Although warming enhanced the effects of nutrient enrichment on bacterial growth, the magnitude of the effect was not statistically significant compared to nutrient enrichment alone. The positive influence of nutrient enrichment and the synergistic effects of nutrients and warming on algae and bacteria were lost at ≤30% ambient light (below 125 μmol m−2 s−1). We conducted a separate laboratory incubation using recirculating water baths maintained at either ambient or elevated water temperature (warming of 5 °C) to evaluate the effects of warming on the use of algal exudates by heterotrophic bacteria. During the 16‐day assay, the abundance of heterotrophic bacteria increased rapidly in the presence of a common solution of algal exudates and the rate of exudate assimilation by bacteria was significantly enhanced by warming compared with ambient temperature. The positive association between light availability and heterotrophic bacteria, coupled with accelerated bacterial growth in the presence of algal exudates, support bacterial dependence on algal subsidies in the presence of recalcitrant organic substrates in this northern peatland. Within a predictive context, our results demonstrate that greater light attenuation associated with browning of surface water may reduce the positive influence of warmer temperatures and elevated nutrient levels on the microbial loop by reducing algal production on submerged surfaces in northern peatlands.