Benthic nitrogen regeneration, fixation, and denitrification in a temperate, eutrophic lake: Effects on the nitrogen budget and cyanobacteria blooms

Abstract

AbstractNitrogen (N) transformations and fluxes at the sediment‐water interface (SWI) were measured in Missisquoi Bay, Lake Champlain, to clarify the role of N in cyanobacterial blooms in temperate, eutrophic lakes. N sources (e.g., N fixation and tributary inputs), sinks (e.g., denitrification/anammox), and internal “links” (e.g., dissimilatory reduction to ; DNRA) were evaluated at a river discharge (PRM) and in the central basin (MB). Sediments were a more effective sink at PRM than MB. Sediments at both sites were a net source to the water column, but DNRA was not a consistent regeneration pathway. Net N2 production at PRM in summer reversed to net N2 consumption/fixation in fall. MB sediments produced N2 at lower rates than PRM; these rates also reversed late in the season. Denitrification was limited by , especially at MB, and anammox may have contributed up to 10% of total N2 production. Sediment N2 fixation occurred simultaneously with denitrification throughout the ice‐free season and, on average, compensated 25–30% of total microbial N losses. A bottom‐water hypoxia event at MB in early July 2009 altered the N cycle, with lower actual denitrification rates and higher effluxes (not from DNRA), while phosphorus (P) flux was unaffected. The hypoxia‐altered N transformation pathways enhanced N bioavailability (but not P) and may have contributed to a non‐N‐fixing cyanobacterial bloom (Microcystis) observed 5 d later. A preliminary N budget incorporating sediment and water column sources and sinks and external loads suggested an ecosystem‐wide N deficit.