Controls of Benthic Nitrogen Fixation and Primary Production from Nutrient Enrichment of Oligotrophic, Arctic Lakes

Abstract

We examined controls of benthic dinitrogen (N 2 ) fixation and primary production in oligotrophic lakes in Arctic Alaska, Toolik Field Station (Arctic Long-Term Ecological Research Site). Primary production in many oligotrophic lakes is limited by nitrogen (N), and benthic processes are important for whole-lake function. Oligotrophic lakes are increasingly susceptible to low-level, non-point source nutrient inputs, yet the effects on benthic processes are not well understood. This study examines the results from a whole-lake fertilization experiment in which N and P were added at a relatively low level (4 times natural loading) in Redfield ratio to a shallow (3 m) and a deep (20 m) oligotrophic lake. The two lakes showed similar responses to fertilization: benthic primary production and respiration (each 50-150 mg C m -2 day -1 ) remained the same, and benthic N 2 fixation declined by a factor of three-to fourfold by the second year of treatment (from $0.35 to 0.1 mg N m -2 day -1 ). This showed that the response of benthic N 2 fixation was de-coupled from the nutrient limitation status of benthic primary producers and raised questions about the mechanisms, which were examined in separate laboratory experiments. Bioassay experiments in intact cores also showed no response of benthic primary production to added N and P, but contrasted with the whole-lake experiment in that N 2 fixation did not respond to added N, either alone or in conjunction with P. This inconsistency was likely a result of nitrogenase activity of existing N 2 fixers during the relative short duration (9 days) of the bioassay experiment. N 2 fixation showed a positive saturating response when light was increased in the laboratory, but was not statistically related to ambient light level in the field, leading us to conclude that light limitation of the benthos from increasing water-column production was not important. Thus, increased N availability in the sediments through direct uptake likely caused a reduction in N 2 fixation. These results show the capacity of the benthos in oligotrophic systems to buffer the whole-system response to nutrient addition by the apparent ability for significant nutrient uptake and the rapid decline in N 2 fixation in response to added nutrients. Reduced benthic N 2 fixation may be an early indicator of a eutrophication response of lakes

Benthic community metabolism in deep and shallow Arctic lakes during 13 years of whole–lake fertilization

“…Two O 2 flux methods were used. From 1999 to 2003, O 2 flux was measured in situ using plexiglass chambers placed on the lake floor using SCUBA ( see Gettel et al for method details). From 2001 to 2013 we generated photosynthesis–irradiance (PI) curves by measuring O 2 flux in sediment cores incubated in laboratory facilities at the Toolik Field Station.…”

Mosses in High-Arctic lakes: in situ measurements of annual primary production and decomposition

“…Other studies on primary production in Arctic lakes have been on pelagic and benthic microalgae primary production using 14 C (Vandeboncoeur et al 2003;Wahlen et al 2006) or oxygen development in short-term in situ bottle incubation for pelagic production (Priddle 1980) and in situ chambers incubation for benthic algae primary production (Gettel et al 2013) on a timescale from hours to days. In some cases, these short-term values have been scaled up to whole year production assuming a growth season of 100 days (Wahlen et al 2006) based on measurements from Toolik Lake in Canada (Miller et al 1986).…”

Seasonality of dissolved nitrogen from spring melt to fall freezeup in Alaskan Arctic tundra and mountain streams

“…While considerable attention has been paid to how climate change will affect carbon export from high-latitude river catchments [e.g., Frey and Smith, 2005;Striegl et al, 2005;Guo et al, 2007;Raymond et al, 2007;Walvoord and Striegl, 2007;Hilton et al, 2015], fewer studies have addressed impacts on nitrogen export from the same systems. Changes in fluvial export of particulate and dissolved fractions, in both inorganic and organic forms, are of particular interest because they serve as important resources for downstream Arctic freshwater [e.g., Levine and Whalen, 2001;MacIntyre et al, 2006;Gettel et al, 2013] and coastal ecosystems [e.g., Amon and Meon, 2004;Tank et al, 2012;McClelland et al, 2014].…”