Seasonal and interannual phytoplankton production in a sub-Arctic tidewater outlet glacier fjord, SW Greenland

This study describes seasonal patterns and proposes likely drivers of an unusual phytoplankton primary production pattern in the outer-sill region of a tidewater outlet glacier-influenced fjord (Godthabsfjord) in SW Greenland. It is based on monthly measurements of pelagic primary production and hydrographic conditions during a 7 yr period. Total annual primary production during 2005 to 2012 was between 84.6 and 139.1 g C m(-2) yr(-1). Two phytoplankton blooms of similar magnitude reoccur in the fjord every year. A 'classical' spring bloom of up to 1743 mg C m(-2) d(-1) occurred in late April/early May in a water column almost fully mixed due to tidal forces at the fjord sill. After the spring bloom, primary production decreased in June, after which a summer bloom of up to 1383 mg C m(-2) d(-1) built up. This bloom coincided with the development of a pycnocline caused by substantial runoff from the Greenland Ice Sheet every year during midsummer. This observation supports a hypothesis that fjord circulation modes and subglacial freshwater discharge, leading to upwelling of nutrient rich water, stimulate primary production in the fjord. Future changes in the timing or magnitude of meltwater runoff from the Greenland Ice Sheet are thus likely to affect phytoplankton dynamics in the fjord.