Ammonium toxicity in eelgrass Zostera marina

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 μM or 75 μM), or unbalanced (ammonium 125 μM and nitrate 25 μM, 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 μM ammonium and 3 μM nitrate were supplied. An ammonium concentration of 125 μM in the water layer was toxic for Z. marina: the plants became necrotic within 2 wk. After 5 wk, plants in all treatments except for the 9 μM treatment were either necrotic or had died. This suggests that toxicity occurs at ammonium concentrations as low as 25 μM. Nitrate treatment had no effect. Ammonium toxicity effects were more pronounced in plants grown on sand and at the higher temperature. It is argued that the ammonium toxicity effects on Z. marina are expected to be strongest in autumn when irradiance decreases, temperature is still high, and ambient ammonium concentrations rise.