Biomonitoring of eutrophication levels in shallow coastal ecosystems  <B>Biomonitoring of eutrophication levels in shallow coastal ecosystems</B><P><B>Jens Erik Lyngby & Sverre M. Mortensen</B><I><P> Water Quality Institute, 11 Agern Allé, DK-2970 Hørsholm, Denmark</I><B><P> Abstract</B><BR> Biomonitoring by the use of natural phytoplankton assemblages and incubation of discs of the macroalga, <I>Ulva lactuca</I> L., was carried out in the shallow Stege Bay, Denmark, to assess the significance of nitrogen and phosphorus as limiting factors. At fortnightly intervals, samples were collected and the response of phytoplankton to nutrient enrichment analysed. The macroalgae were analysed for growth, nitrogen and phosphorus content.<BR> Nitrogen limited the phytoplankton production from June to October, when the growth rates were reduced to 25-80% of the maximal growth rate. On no occasion did phosphorus limit phytoplankton production. Minimum and critical tissue concentrations for nitrogen and phosphorus in <I>Ulva</I> were identified. The concentrations of nitrogen were generally below the critical concentration level in June-October, showing that nitrogen was also the limiting factor for macroalgae production. The tissue concentrations of phosphorus were not below the critical concentration level at any time, indicating that phosphorus was insignificant as limiting factor. For both nutrients, high storage capacities were recorded, the factors being approximately six times higher than the minimum concentrations of nitrogen and phosphorus.<BR> It was concluded that the combination of bioassays by the use of phytoplankton and macroalgae is suitable for monitoring nutrient availability in shallow coastal waters and provides a time-integrated measure of the nutrient availability. The applicability of this technique is discussed, and it is suggested that the method would be a sensitive tool for monitoring changes in shallow water ecosystems.<I><P> Keywords</I>: nutrient limitation, eutrophication, <I>Ulva lactuca</I>, phytoplankton, nitrogen, phosphorus.<P>