The nutrient economy of a marine inlet: Lough Hyne, South West Ireland

Simple mass balance models have been frequendy used in limnology to study the retention of nutrients by lakes. However, there are difficulties in applying mass balance to marine systems, particularly with regard to parameters involved in tidal flushing. A well defined tidal channel at the entrance to Lough Hyne allows the use of adapted mass balance models to study the nutrient economy, and to make inferences about system scale productivity. Water quality gathered over four years demonstrated 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 dissolved inorganic nitrogen from the sea was also detected. Scaling the net inputs with flushing terms in a mass balance leads to estimates of the net system productivity. From a dissolved inorganic phosphorus budget, the uptake of nutrient in the lough implied a net system production of 11–33 g C m ² yr¹. Budgets for nitrogen suggest that a degree of denitrification occurred. Dissolved inorganic silicon budgets underestimated the net system production determined using dissolved inorganic 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 emphasise the importance of catchment type and size, along with tidal flushing scales in determining the local rates of processes such as net system metabolism.