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

Nitrogen (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., dissimilatoryNO 3 −reduction toNH 4 + ; DNRA) were evaluated at a river discharge (PRM) and in the central basin (MB). Sediments were a more effectiveNO 3 −sink at PRM than MB. Sediments at both sites were a netNH 4 +source to the water column, but DNRA was not a consistentNH 4 +regeneration pathway. Net N 2 production at PRM in summer reversed to net N 2 consumption/fixation in fall. MB sediments produced N 2 at lower rates than PRM; these rates also reversed late in the season. Denitrification was limited byNO 3 − , especially at MB, and anammox may have contributed up to 10% of total N 2 production. Sediment N 2 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 higherNH 4 +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.