AMMONIUM TOXICITY IN EELGRASS ZOSTERA-MARINA

Seagrasses are declining all over the world, resulting in a substantia l loss of biodiversity, coastal sediment stabilization and nursery are as 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 l ayer on the seagrass Zostera marina L., which is often the sole higher plant inhabiting coastal zones in the northern hemisphere. Plants gro wn in either mud or sand were subjected to various water ammonium and nitrate concentrations, whereby ammonium and nitrate supply were balan ced (both 25 mu M or 75 mu M), or unbalanced (ammonium 125 mu M and ni trate 25 mu M, and vice versa). We used 2 temperatures, 15 and 20 degr ees C. Analyses were made after 2 and 5 wk of exposure. In an addition al experiment, 9 mu M ammonium and 3 mu M nitrate were supplied. An am monium concentration of 125 mu M in the water layer was toxic for Z. m arina: the plants became necrotic within 2 wk. After 5 wk, plants in a ll treatments except for the 9 mu M treatment were either necrotic or had died. This suggests that toxicity occurs at ammonium concentration s as low as 25 mu M. Nitrate treatment had no effect. Ammonium toxicit y effects were more pronounced in plants grown on sand and at the high er temperature. It is argued that the ammonium toxicity effects on Z. marina are expected to be strongest in autumn when irradiance decrease s, temperature is still high, and ambient ammonium concentrations rise .