Nitrogen subsidies to pelagic food webs through profundal methane‐oxidising bacteria in oligotrophic fresh water

Abstract

Nutrient subsidy of methane‐oxidising bacteria (MOB), an alternative basal resource, to plankton communities through pelagic‐benthic coupling is expected to alter carbon‐to‐nitrogen (C/N) ratios and biomass of zooplankton, and affect biological processes in aquatic food webs. Previous studies indicate that most MOB nutrient subsidies to zooplankton are focused largely on C subsidies; nevertheless, considering that N is important to zooplankton and phytoplankton growth, a better understanding of MOB N subsidies to plankton communities is needed. We examined changes in nutrients, phytoplankton mass‐specific productivity, MOB reliance, C/N ratios and biomass of zooplankton between summer stratification and winter mixing periods based on in situ investigation in a subtropical deep oligotrophic freshwater ecosystem during 2010–2014. The results revealed that with increasing reliance on MOB during the winter mixing period, zooplankton C/N ratios decreased and had a significantly nonlinearly inverse relationship with zooplankton biomass. The negative relationship between zooplankton biomass and phytoplankton mass‐specific productivity was further observed in synchronous time. The profundal MOB enhanced pelagic‐profundal coupling and served as an alternative N support for pelagic plankton food webs during the winter and/or less productive seasons, adding to greater understanding of the role of methanotrophs in aquatic food webs.