The Irish Sea: nutrient status and phytoplankton

Richard Gowen
Department of Agriculture and Rural Development for Northern Ireland
Agriculture and Environmental Science Division
Newforge Lane
BELFAST
BT9 5PX

Richard.Gowen@dardni.gov.uk

The Irish Sea is a semi-enclosed coastal sea. Exchange with open shelf waters is restricted by narrow channels to the north and south and the residence time of water within the sea is approximately one year. There is considerable potential for nutrient recycling within the Irish Sea although the autumn /winter build up of dissolved inorganic nitrogen is due in part to the introduction of new nitrogen. The main external sources of nitrogen for the Irish Sea are marine (the Atlantic) and anthropogenic (land-runoff and atmospheric). Understanding of exchange processes across the shelf is limited and constrain attempts to quantify the Atlantic source term. The Celtic and Irish seas currently receive 130,000 t N, 6,000 t P and 34,000 t of Si and the Irish Sea is undoubtedly enriched with nitrogen, phosphate and silicate. Denitrification may limit the extent of nitrogen enrichment.

Time-series observations of phytoplankton community structure are lacking for the Irish Sea making it difficult to evaluate the effects of enrichment. A recent study of Liverpool Bay and Irish coastal waters documented elevated spring production and biomass in the Bay but only small differences in species composition were apparent between the two sites. In offshore western Irish Sea waters, diatom and flagellate dominance of the spring bloom switches from year to year and may indicate that the western Irish Sea is undergoing a transition.