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Seagrasses are declining all over the world, resulting in a substantial loss of biodiversity, coastal sediment stabilization and nursery areas of economically important fish. The seagrass decline has often been associated with increasing eutrophication of coastal areas. We tested possible toxic effects of high nitrogen concentrations in the water layer on the seagrass Zostera marina L., which is often the sole higher plant inhabiting coastal zones in the northern hemisphere. Plants grown in either mud or sand were subjected to various water ammonium and nitrate concentrations, whereby ammonium and nitrate supply were balanced (both 25 pM or 75 PM), or unbalanced (ammonium 125 pM and nitrate 25 pM, and vice versa). We used 2 temperatures, 15 and 20 C. Analyses were made after 2 and 5 wk of exposure. In an additional experiment, 9 pM ammonium and 3 yM nitrate were supplied. An ammonium …