Observations of phytoplankton productivity and growth rates in the Malin shelf break environment

The southern sector of the Malin Shelf, a subregion of the NW European Shelf, is noted for episodic and unimpeded incursions of oceanic water onto the shelf in an area where the northward flowing European slope current interacts with steep bathymetry, yet the in-situ biological consequences of these incursions are largely unexplored. In this study phytoplankton productivity, nitrate assimilation and community growth rates are presented to characterise in-situ biological conditions during a prominent chlorophyll bloom that occurred at the shelf break in July 2013. Surface waters were replete with nitrate (2–7 μmol L⁻¹) and phosphate (0.1–0.4 μmol L⁻¹) but deficient in silicate (Si < 1 μmol L⁻¹). Chlorophyll concentrations were significantly negatively correlated with phosphate concentrations but not correlated with nitrate or silicate. High variability between stations in productivity, nitrate assimilation, and depth averaged phytoplankton community growth rates, which ranged from <0.01 to 0.14 d⁻¹, could be attributed to subsurface gradients in production and biomass distributions. Though variable the magnitude of productivity rates in this sector of the Malin Shelf environment do not appear unusual relative to comparable observations suggesting that despite the uncommon physical conditions of the study site phytoplankton productivity was not significantly modified by proximity to oceanic influences.