Contrasting responses of phytoplankton and benthic algae to recent nutrient enrichment in Arctic tundra ponds

Abstract

Summary Tundra ponds are a dominant feature on the Arctic Coastal Plain, but their response to warming, especially in the long‐term, is largely unknown. Our study assessed changes in algal nutrient limitation over a 40‐year period in historically studied ponds that may have been affected by either human development in the region or increased temperatures leading to permafrost thaw and nutrient release. We also compared nutrient limitation of algae in the pelagic and benthic zones at a landscape level. Nutrient diffusing substrata (NDS) and bottled nutrient incubations, which expose natural algal communities to known quantities of nitrogen (N) and phosphorus (P), were utilised to determine benthic and phytoplankton algal nutrient limitation, respectively, in a series of ponds on the north slope of Alaska, USA. This included ponds where phytoplankton nutrient limitation was examined in 1971–1973 as part of the International Biological Program (IBP), as well as ponds in a remote protected area and others near the village of Barrow. Nutrient limitation status of phytoplankton has changed since the original IBP study 40 years ago from P limitation to NP co‐limitation. One‐third of regional phytoplankton enrichment experiments indicated no nutrient limitation of algal growth, and none exhibited single‐nutrient P limitation. This shift in nutrient limitation was coincident with increased water column nutrients due to degrading permafrost, and to the expansion into the ponds of macrophytes, which may compete with algae for available nitrogen. A comparison of pelagic and benthic experiments across the landscape revealed differences among these zones with a predominance of NP limitation of phytoplankton and absence of nutrient limitation in the benthic zone, reflecting contrasting nutrient limitation status within the same ponds. Permafrost thaw is probably reintroducing previously frozen stores of P or N to the sediment surface, which are quickly taken up by periphyton but become limited in the water column. Grazing by invertebrates, which were not excluded from the benthic assays, may also have influenced the results. While ponds within the village of Barrow had higher nutrient and algae levels, there was no obvious effect of urban development on nutrient limitation status, nor did development appear to have influenced the historic IBP pond sites.

Nutrient Release From Permafrost Thaw Enhances CH4 Emissions From Arctic Tundra Wetlands

“…The control solution included only deionized water with no added nutrients. The low nutrient solution contained 1.5 mg N/L, 0.6 mg P/L, similar to the maximum observed concentrations at enriched sites ponds within the village of Barrow during the growing seasons between 2010 and 2015 (1.7 mg DIN/L; 0.55 mg TP/L, where DIN = dissolved inorganic nitrogen, Ttotal phosphorus; Lougheed et al, and Lougheed, ). The high nutrient solution contained 7.5 mg N/L, 3.0 mg P/L, which was 5 times higher than the “low” solution.…”

Linking permafrost thaw to shifting biogeochemistry and food web resources in an arctic river

“…The relative supply of nutrients from upland environments to surface waters can have important implications for food web properties and consumer productivity (Cross, Benstead, Frost, & Thomas, ; Elser et al, ). The productivity of our study system, like many arctic rivers, is P‐limited, and decreasing P supply may thus be reducing biofilm productivity and nitrate uptake, further driving the increasing ratio of N:P. Changes to nutrient concentrations and nutrient limitation have also been documented in Arctic tundra ponds where Lougheed et al () found increases in dissolved nitrogen and phosphorus and a shift from P to N and P colimitation of benthic biofilms over a 40‐year period. Decreased P concentration in the current study could also be explained by increased uptake by biofilms and thus the removal of P from the water column, rather than a reduced supply of P from upland environments.…”

“…As the limiting nutrient of many aquatic ecosystems in the Arctic, changes in the supply of P to surface waters have the potential to greatly alter the structure and function of aquatic ecosystems (Lougheed, ; Slavik et al, ). Our results, however, do not necessarily support the hypothesis that increased exposure of phosphate‐rich mineral soils will leach P into groundwater, ultimately leading to increases in P supply to surface water ecosystems (Frey & McClelland, ; Hobbie et al, ; Pearce et al, ).…”

Rising plant‐mediated methane emissions from arctic wetlands

“…Nutrient addition has been identified as drivers of increased plant biomass in experimental studies in wet and moist tundra (Chapin et al ., ; Shaver et al ., ). In tundra ponds, warmer water temperatures and permafrost thaw have been linked with elevated nutrient availability (Lougheed et al ., ; Heikoop et al ., ; Reyes & Lougheed, ) and algal biomass (Lougheed et al ., ). We assert that thawing permafrost is also contributing to elevated plant biomass in aquatic habitats of the arctic tundra.…”