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

Abstract

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., dissimilatory (Formula presented.) reduction to (Formula presented.) ; DNRA) were evaluated at a river discharge (PRM) and in the central basin (MB). Sediments were a more effective (Formula presented.) sink at PRM than MB. Sediments at both sites were a net (Formula presented.) source to the water column, but DNRA was not a consistent (Formula presented.) 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 (Formula presented.), 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 (Formula presented.) 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.