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

Abstract

Benthic primary production and oxygen consumption are important components of lake biogeochemical cycling. We performed whole-lake nutrient manipulations in arctic Alaska to assess the controls of lake morphometry, nutrients, and light on benthic community metabolism. One deep, stratified lake (Lake E5) and one shallow, well-mixed lake (Lake E6) in the Alaskan Arctic were fertilized with low levels of nitrogen (56 mg N m-3 y-1) and phosphorus (8 mg P m-3 y-1) from 2001-2013. Benthic primary production was not stimulated by fertilization in either lake. In the deep lake, decreased water clarity is consistent with an increase in phytoplankton biomass during fertilization. Benthic GPP decreased by 7 - 47 mg C m-2 d-1 (not statistically significant) and benthic respiration increased from 87 ± 20 to 167 ± 9 (SE) mg C m-2 d-1. The areal hypolimnetic oxygen deficit increased by 15 mg O2 m-2 d-1 each year during the 13 years of monitoring, apparently driven by lower (more negative) benthic NEP. In the shallow lake, phytoplankton concentration did not change with fertilization. As a result, the light environment did not change and benthic GPP did not decrease. Overall the data suggest that (1) benthic algae are not nutrient limited in either the deep or shallow lake, (2) lake morphometry modulated the overall nutrient impact on benthic metabolism by controlling the response of phytoplankton, and by extension, light and organic carbon supply to the benthos, (3) year-to-year variability in light attenuation explains considerable variability in benthic GPP between lakes and years, (4) correlations between both dissolved organic carbon concentrations and light attenuation coefficients (kd) between lakes suggests a regional control on light attenuation, and (5) the dissolved oxygen concentrations in the deep experimental lake are highly sensitive to nutrient enrichment.