THE ECOLOGICAL IMPLICATIONS OF TOLERANCE TO SALINITY BY RUPPIA-CIRRHOSA (PETAGNA) GRANDE AND ZOSTERA-CAPENSIS SETCHELL

Laboratory studies showed that maximum growth of Zostera capensis was at a salinity of 15 and 35parts per thousand. Growth of Ruppia cirrhosa continued at salinity ranges between 0-75parts per thousand, but with maximum growth in freshwater. Despite its wide range of salinity tolerance, R. cirrhosa was found in brackish estuaries where the salinity was less than 30parts per thousand. Zostera capensis was common in estuaries with open mouths, characterized by marine conditions (35parts per thousand). When plants of the two species were grown in the same tank, Ruppia cirrhosa expansion and growth was greater than Z. capensis at both 15 and 35parts per thousand. This was true for the experimental conditions of reduced water velocity and constant temperature and irradiance. Ruppia cirrhosa is an opportunistic species and recovered when salinity was reduced after exposure to 75parts per thousand for 8 weeks. Proline concentrations increased with an increase in treatment salinity for both plants, but the proline concentrations in R. cirrhosa were higher than in Z. capensis at 35parts per thousand. Although R. cirrhosa will grow vegetatively in hypersaline water (> 40parts per thousand), this study showed that no seeds germinated where the salinity was greater than 35parts per thousand.