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Benthic nitrogen regeneration, fixation, and denitrification in a temperate, eutrophic lake: Effects on the nitrogen budget and cyanobacteria blooms
Journal
Limnology and Oceanography
Volume
61
Number
4
Pages / Article-Number
1406-1423
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 NO3- reduction to NH4+; DNRA) were evaluated at a river discharge (PRM) and in the central basin (MB). Sediments were a more effective NO3- sink at PRM than MB. Sediments at both sites were a net NH4+ source to the water column, but DNRA was not a consistent NH4+ 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 by NO3-, 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 higher NH4+ 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.
