The influence of local and regional oceanographic processes on phytoplankton distribution in continental shelf waters off north-western Ireland

The distribution of phytoplankton is related to local and regional oceanographic features off the north-western coast of Ireland. A thermohaline front known as the Irish Shelf Front (ISF) that separates coastal and oceanic water was observed in the study area. Phytoplankton biomass levels, as indicated by chlorophyll concentrations, were higher on the coastal side of the frontal boundary. Inshore of the front the phytoplankton species composition was dominated by diatoms such as Leptocylindrus danicus, Guinardia flaccida and Pseudo-nitzschia cf seriata. The flora of the shelf region between the ISF and the outer shelf was characterised by the presence of Halosphaera minor, Oscillatoria sp., Ptychodiscus noctiluca, Ceratium fusus and Amphidoma caudata. Further offshore along the margins of the continental shelf the floral assemblage was characterised by Gonyaulax polygramma, Ceratium furca, Oxytoxum scolopax, Podolampes palmipes, Prorocentrum compressum and Prorocentrum dentatum. The presence of Gonyaulax polygramma, an indicator species for North Atlantic Current (NAC) slope waters, indicates the presence of this current at the shelf edge. The highest Karenia mikimotoi cell concentrations were found in close proximity to bottom density fronts located inshore. The biological observations presented here further highlight the potential role of local processes such as the presence of strong, jet-like, density-driven coastal flows in transporting potentially harmful phytoplankton populations northwards along the western coast of Ireland.