WIND-INDUCED LONG-TERM INCREASE AND SHORT-TERM FLUCTUATIONS OF SHALLOW-WATER SUSPENDED MATTER AND NUTRIENT CONCENTRATIONS, RINGKOBING FJORD, DENMARK

The terrestrial supply of nutrients to the shallow lagoon Ringkobing F jord has shown a significant downward tendency for phosphorus during t he past 10 years. In spite of a decreasing supply, there is at the sam e time an increase in nutrient concentrations, especially for P, but a lso for the suspended matter content in the fjord. Analyses af wind da ta from nearby lighthouses show that the frequency of strong winds fro m the SW (equal to the longest fetch) has gone up in the same period o f time and that both the nutrients and suspended matter concentrations in the water column are well correlated with available wind energy an d hence, with the possibilities for wave-induced resuspension of mater ial from the fjord bottom. During a 6 month period, daily fluctuations of concentrations in the water column were measured in samples taken by automatic samplers at four stations with different exposures. Due t o wave induced resuspension, the day to day variations were between 2- 895 mg DW l(-1) of suspended matter, 0.2.-24.5 mg l(-1) of organic mat ter, 0.1-1.1 mg l(-1) of Total-N, and 0.01-0.18 mg l(-1) of Total-P. F ortnightly measurements of erosion rates at station bottoms showed tha t as much as 3 cm of the sediment surface were resuspended during peri ods of hard wind conditions. The resuspended particles had mode sizes of 30-40 mu, i.e. high sinking rates are assumed. The concentrations a f suspended matter observed in the water column, however, were not bel ow 40 mg DW l(-1) in 50% og the time, leading to a marked effect on li ght penetration in water. The Secchi-depth is on average reduced to 0. 6 m in summer and 0.7 m in winter.