NUTRIENT SOURCES FOR IN-SHORE NUISANCE MACROALGAE - THE DUBLIN BAY CASE

From investigations of nuisance intertidal algae in Dublin Bay, it was noted that algae had a discrete distribution which persisted from season to season; the time of the biomass peak varied, but was frequently in early summer; and that nitrogen was most probably the main limiting nutrient. A study of nitrogen sources to the bay found that particulate nitrogen accounted for one third of the total nitrogen input and that the bulk of the particulates originated in sewage effluent; that water column inorganic nitrogen levels in intertidal areas were depleted in summer months and that the release of nitrogen from sediment during the main period of growth was greater at a site with high algal biomass than at a site with no biomass. From this information, a hypothesis is suggested which states that particulates, originating mainly from sewage discharge, are major carriers of nitrogen for intertidal green algal growth in Dublin Bay through sedimentation and subsequent sediment nitrogen release. The hypothesis accounts for the limited spatial distribution of algae by emphasising the dependence of nitrogen release on sedimentation patterns. Temporal patterns are accounted for by the dependence of mineralization on temperature and on the size of the mineralizable nitrogen pool in the sediment. If the 'particulate nutrient carrier' hypothesis is correct, secondary sewage treatment would greatly reduce algal growth in Dublin Bay. Current work appears to be supporting the hypothesis.