Seasonal rates of benthic primary production in a Greenland fjord measured by aquatic eddy correlation

Abstract

We present the first year‐round estimates of benthic primary production at four contrasting shallow (3–22 m depth) benthic habitats in a southwest Greenland fjord. In situ measurements were performed using the noninvasive aquatic eddy‐correlation (EC) oxygen (O2) flux method. A series of high‐quality multiple‐day EC data sets document the presence of a year‐round productive benthic phototrophic community. The shallow‐water sites were on average autotrophic during the spring and summer months, up to 43.6 mmol O2 m−2 d−1, and heterotrophic or close to metabolic balance during the autumn and winter. Substantial benthic gross primary production (GPP) was measured year‐round. The highest GPP rates were measured during the spring, up to 5.7 mmol O2 m−2 h−1 (136.8 mmol O2 m−2 d−1), and even at low light levels (< 80 µmol quanta m−2 s−1) during late autumn and winter we measured rates of up to 1.8 mmol O2 m−2 h−1 (43.2 mmol O2 m−2 d−1) during peak irradiance. The benthic phototrophic communities responded seasonally to ambient light levels and exhibited year‐round high photosynthetic efficiency. In situ downwelling irradiances as low as ∼ 2 µmol quanta m−2 s−1 induced an autotrophic response and light saturation indices (Ik) were as low as 11 µmol quanta m−2 s−1 in the winter. On an annual timescale, the average areal rate of benthic GPP was 11.5 mol O2 m−2 yr−1, which is ∼ 1.4 times higher than the integrated gross pelagic primary production of the ∼ 30–50 m deep photic zone of the fjord. These results document the importance of benthic photosynthesis on an ecosystem level and indicate that the benthic phototrophic compartment should be accounted for when assessing carbon and nutrient budgets as well as responses of coastal Arctic ecosystems to climate change.