THE NUTRIENT ECONOMY OF A MARINE INLET LOUGH-HYNE, SOUTH-WEST IRELAND

Simple mass balance models have been frequently used in limnology to s tudy the retention of nutrients by lakes. However, there are difficult ies in applying mass balance to marine systems, particularly with rega rd to parameters involved in tidal flushing. A well defined tidal chan nel at the entrance to Lough Hyne allows the use of adapted mass balan ce models to study the nutrient economy, and to make inferences about system scale productivity. Water quality gathered over four years demo nstrated a net input to the lough for three different nutrients. There was a net tidal input of dissolved inorganic phosphorus. Significant inputs of dissolved inorganic nitrogen and dissolved inorganic silicon came from freshwater sources. In two of the years a net input of diss olved inorganic nitrogen from the sea was also detected. Scaling the n et inputs with flushing terms in a mass balance leads to estimates of the net system productivity. From a dissolved inorganic phosphorus bud get, the uptake of nutrient in the lough implied a net system producti on of 11-33 g C m(-2) yr(-1). Budgets for nitrogen suggest that a degr ee of denitrification occurred. Dissolved inorganic silicon budgets un derestimated the net system production determined using dissolved inor ganic phosphorus by about 50%. An examination of chlorophyll transport suggests that about 28% of the net system production may be exported to the open sea as phytoplankton. Mass balance can be used as a simple starting point for comparing different systems. This is likely to emp hasise the importance of catchment type and size, along with tidal flu shing scales in determining the local rates of processes such as net s ystem metabolism.