PHYTOPLANKTON AND PARTICULATE MATTER IN CARLINGFORD LOUGH, IRELAND - AN ASSESSMENT OF FOOD AVAILABILITY AND THE IMPACT OF BIVALVE CULTURE

In an attempt to assess the impact of bivalve culture in Carlingford L ough, Ireland, the seasonal cycles of nutrients, particulate matter, c hlorophyll a, and phytoplankton in the lough was investigated in 1992. Chlorophyll levels showed an increase in April, corresponding to the annual spring bloom, and levels remained relatively high (2-12 mg m(-3 )) throughout the summer before dropping to a winter minimum by Decemb er. Throughout the summer the phytoplankton community was dominated by diatoms, with microflagellates becoming an increasingly larger fracti on of the biomass in autumn and winter. Dinoflagellates were only pres ent on occasion in low numbers during the summer months. Seasonal vari ations in nitrate, phosphate, and silicate concentrations at all stati ons showed characteristic winter maxima and summer minima. Nitrate con centrations had reached a minimum undetectable level by June, at a tim e when the main freshwater input from the Clanrye River had dropped to <0.3 m(3) s(-1). Particulate organic carbon (POC) composed approximat ely 5% of the suspended matter, with highest values in winter due to r esuspension. Levels of biologically available POC, as determined by a modified BOD technique, were greatest in summer, and an inverse relati onship was observed between total POC and its fraction that was biolog ically available. Most of the labile fraction was considered to by phy toplankton, and remineralization during the summer is suggested as a m echanism for maintaining high productivity during the summer months. A lthough the phytoplankton biology was uncoupled with that outside the lough, it is concluded that ther is scope for expansion of the local b ivalve mariculture industry without altering the ecosystem of the loug h. The upper limit on such expansion would be set by practical conside rations such as availability of space and site suitability due to wate r quality.