Impact of changing ice cover on pelagic productivity and food web structure in Disko Bay, West Greenland: a dynamic model approach

A rise in global temperatures could potentially lead to less ice in the Arctic, including a reduction in the ice-covered period. The consequence of a changing ice cover on the food web structure and production in Disko Bay, Western Greenland, is analysed through application of a dynamical model for the planktonic food web. The model is successfully calibrated and tested for sensitivity, using a detailed data set for 1996–1997. Model scenarios are (1) extended ice cover and (2) no ice. These scenarios are compared to model runs with measured ice cover in two normal years. In the extended ice scenario, assuming unchanged copepod behaviour, copepods are starving or feeding in the ice/water interface from the time they ascend to the surface layer from over-wintering depths until the ice break-up in June. The total annual primary production reaches the same level as it does in the average year, but copepod ingestion and, as a consequence, vertical carbon export is reduced by app. 40%. In the ice-free situation, an early diatom bloom is initiated by stratification of the water in March, before the copepods ascend. The diatom bloom is grazed upon by protozooplankton, which reach a high biomass before the copepods ascend in April. Annual primary production increases by 52% while copepod ingestion and vertical loss of carbon is reduced by 57%. This study illustrates how a change in the ice cover in Arctic areas can potentially create a mismatch between spring primary production and copepod grazers. The result may be a planktonic food web dominated by protozooplankton, resulting in lower export of organic material out of the photic zone despite increased primary productivity, or alternatively lead to changes in species composition or behaviour.