Effects of salinity and nutrient load and their interaction on Zostera marina

Abstract

Gcnerally, seagrass Zostera manna L. distnbution in the Wadden Sea and south-west Netherlands is iimited to waters with low to moderate nutnent 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. manna plants in a 5 wk experirnent. Two populations were tested, one onginating from a relatively manne habitat and the other from an estuanne habitat. Supplied saiiniti.es were 23, 26 and 30%'~ S, and supplied water nutrient levels were nitrate:ammonium:phosphate, 1:3:2, 3:9:4.5 and 60:9:9 pM at a refreshment rate of 1 d-', corresponding with a load of 20, 95 and 625 kg N ha-' yr-' 2. rnarina 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 estuanne plants at 26 and 30% S, and on the manne plants at 30'::~ C. At these high salinities, a high nutrient load had no detectable effect on the marine plants, ivhereas the estuanne plants were negatively influenced by high nutnent loads. At low salinity levels, 1.e. manne plants at 23 and 26"*, S and estuanne plants at 23% 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. manna in many areas of the northern hemisphere. Examples from both sides of the Atlantic Ocean are presented