Journal of the Marine Biological Association of the United Kingdom

Research Article

The Annual Cycle of Nutrients in the Menai Straits

P. A. Ewinsa1 and C. P. Spencera1

a1 Marine Science Laboratories, University College of North Wales, Menai Bridge, Anglesey


The annual cycle of silicate, nitrate and phosphorus in the waters of the Menai Straits is described. The concentration of all these nutrients in these waters is apparently markedly affected by occasional incursions of high-salinity low-nutrient water. The concentrations of nitrate-nitrogen tend to show a general increase throughout the winter months as a result of cumulative contributions from land drainage and run off and the absence of appreciable biological utilization. In contrast, an equilibrium concentration of reactive silicate is maintained over this period following the initial autumnal increase in the concentration of this component. The annual cycle of phosphorus in these waters is also anomalous. Relatively high equilibrium concentrations of this nutrient are maintained during the summer months and unexpectedly low concentrations occur in winter. In conjunction with the biological removal of relatively large amounts of nitrate-nitrogen, this causes anomalously high apparent N: P biological utilization ratios. It is difficult to explain the observed variations in the concentration of phosphate in these waters solely in terms of the equilibrium between biological assimilation and regeneration and advective processes.