ABSTRACT: Generally, seagrass Zostera marina L. distribution in the Wadden Sea and south-west Netherlands is limited to waters with low to moderate nutrient concentrations. However, it is known that Z. marina also occurs at high nutrient concentrations when growing in low salinity environments. In this study, we investigated the separate and interactive effects of nutrients and salinity on Z. marina plants in a 5 wk experiment. Two populations were tested; one originating from a relatively marine habitat and the other from an estuarine habitat. Supplied salinities were 23, 26 and 30o/oo S, and supplied water nutrient levels were nitrate:ammonium:phosphate, 1:3:2, 3:9:4.5 and 60:9:9 µM at a refreshment rate of 1 d^-1, corresponding with a load of 20, 95 and 625 kg N ha^-1 yr^-1. Z. marina was negatively influenced by high salinity. The estuarine plants showed a decreased 'vitality' (calculated from 6 plant response parameters), whereas the marine plants showed a lesser number of shoots at high salinity. The negative effect acted on the estuarine plants at 26 and 30o/oo S, and on the marine plants at 30o/oo S. At these high salinities, a high nutrient load had no detectable effect on the marine plants, whereas the estuarine plants were negatively influenced by high nutrient loads. At low salinity levels, i.e. marine plants at 23 and 26o/oo S and estuarine plants at 23o/oo S, plants from both populations were positively influenced by higher nutrient loads. It is argued that these results may explain the distribution and decline of Z. marina in many areas of the northern hemisphere. Examples from both sides of the Atlantic Ocean are presented.

KEY WORDS: Eelgrass · Eutrophication · Interaction · Nitrogen · Phosphorus · Population dynamics · Salinity stress · Seagrass · Wadden Sea

Published in MEPS Vol. 190 (1999) on December 14
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1999