A comparison of estimations of net community production in the Amundsen Sea polynya, Antarctica

Abstract

In Antarctic Polynya, where productivity is known to be high, it is important to estimate NCP accurately to understand the marine carbon cycle and further air-sea CO2 flux by biological pumps. We used the difference of dissolved inorganic carbon (DIC) contents in the water column between the surface water and underneath the mixed layer deep water to determine NCP assuming that the deep water preserves the winter characteristics of dissolved inorganic content. This is true in the Amundsen Sea polynya (ASP) as the winter water is detected below the mixed layer. Various literature shows ambiguous definition of the reference depth between the surface and deep waters. We estimated NCP in the Amundsen Sea polynya applying various reference depths relevant to the biological production and vertical mixing. We employed the results from the expedition conducted in ASP during austral summer in 2011. Five reference depths were selected to represent the winter water for which biological activity paused; (1) potential temperature minimum depth (pTmin), (2) euphotic depth (Ed), (3) mixed layer depth (MLD), and (4) 100 m and (5) 200 m nominal depths. The ratio of DIC to total alkalinity (DIC/TA) was altered by biological activities in the surface water. When the DIC/TA ratio is close to the value of deep water, it closely matches the pTmin of the reference depth. The NCP is estimated using this method, 94 ± 43 mmol C m-2 d-1 (n=11) in the Amundsen Sea polynya.