Epipelic and pelagic primary production in Alaskan Arctic lakes of varying depth (original) (raw)

We compared on eight dates during the ice-free period physicochemical properties and rates of phytoplankton and epipelic primary production in six arctic lakes dominated by soft bottom substrate. Lakes were classified as shallow (z ̅ < 2.5 m), intermediate in depth (2.5 m < z ̅ < 4.5 m), and deep (z ̅ > 4.5 m), with each depth category represented by two lakes. Although shallow lakes circulated freely and intermediate and deep lakes stratified thermally for the entire summer, dissolved oxygen concentrations were always >70% of saturation values. Soluble reactive phosphorus and dissolved inorganic nitrogen (DIN = NO3—N + NH4+–N) were consistently below the detection limit (0.05 μmol l-1) in five lakes. However, one lake shallow lake (GTH 99) periodically showed elevated values of DIN (17 μmol l-1), total-P (0.29 μmol l-1), and total-N (33 μmol l-1), suggesting wind-generated sediment resuspension. Due to increased nutrient availability or entrainment of microphytobenthos, GTH 99 showed the highest average volume-based values of phytoplankton chlorophyll a (chl a) and primary production, which for the six lakes ranged from 1.0 to 2.9 μg l-1 and 0.7-3.8 μmol C l-1 day-1. Overall, however, increased z ̅ resulted in increased area-based values of phytoplankton chl a and primary production, with mean values for the three lake classes ranging from 3.6 to 6.1 mg chl a m-2 and 3.2-5.8 mmol C m-2 day-1. Average values of epipelic chl a ranged from 131 to 549 mg m-2 for the three depth classes, but levels were not significantly different due to high spatial variability. However, average epipelic primary production was significantly higher in shallow lakes (12.2 mmol C m-2 day-1) than in intermediate and deep lakes (3.4 and 2.4 mmol C m-2 day-1). Total primary production (6.7-15.4 mmol C m-2 day-1) and percent contribution of the epipelon (31-66%) were inversely related to mean depth, such that values for both variables were significantly higher in shallow lakes than in intermediate or deep lakes.

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