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Discs of the macroalga, Ulva lactuca L., were transplanted around an ocean outfall and at a reference site in KCge Bay, Denmark, to assess the influence of the outfall on the nutrient availability, At 2-wk intervals, samples were collected and analyzed for growth, nitrogen, and phosphorus content.The tissue concentrations of nitrogen and phosphorus decreased with distance to the outfall, showing that the tissue concentrations are suitable for monitoring nutrient availability in coastal areas and provide a time-integrated measure of the nutrient availability. The lowest tissue concentrations of nitrogen were recorded at the reference station, where the internal concentrations generally were below the critical concentration level, showing that nitrogen limited the growth. At the station located close to the outfall, the flux of nitrogen was sufficient to maintain the maximum growth rate. The tissue concentrations of phosphorus were only below the critical concentration level on one occasion, and the result showed a net uptake throughout the study period.It was concluded that in the Kcge Bay, nitrogen was the main limiting factor for macroalgae growth during the summer. The applicability of tissue concentrations for assessment of nutrient availability is discussed and it is considered that the method, when evaluated against established critical concentrations, provides a valuable tool for assessing ecosystem health with regard to eutrophication.

Abstract. The possible effects of fine‐grained deposits on the photosynthesis, growth, and nutrient uptake of Laminaria have been studied in field and laboratory experiments. Some of the plants were treated with fine‐grained material for two hours and then transplanted in rows together with untreated plants. At two‐week intervals the treatment was repeated and samples collected. The macroalgae were analyzed for growth as well as chlorophyll a, nitrogen, and phosphorus content. In the laboratory experiments, plants of a uniform size were incubated in Plexiglas containers after leaf‐marking and treatment in a sediment suspension. At weekly intervals, photosynthesis, growth, chlorophyll a, and nutrients were analyzed. The field study showed that Laminaria growth was significantly affected by the fine‐grained material. The growth in the control experiment was 20% higher than in the treated plants. The chlorophyll a concentrations in the treated plants were significantly lower than in the control experiment, and the results indicated a reduced nutrient uptake in the treated plants. In the laboratory experiments the growth of the treated plants was only 30% of the controls, but no differences in photosynthetic capacities were detected. Nitrogen uptake was reduced in the treated plants, but no effects were observed on phosphorus uptake. Thick layers of fine‐grained material caused direct harmful effects and rotting. The results are discussed in relation to dredging activities; it is concluded that in heavy sedimentation areas, Laminaria growth may be significantly reduced beyond the indirect effect of shading from suspended material in the water.

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