Green seaweeds as biomonitors in coastal areas influenced by nutrient rich effluents

Abstract

The organisms along the mediolittoral zone endure a wide range of environmental factors / pressures - fluctuating salinity, nutrient pulses, high exposure / desiccation, human disturbance, changes in emergence and submergence, temperature and wave exposure, and possibly substratum loss due to a possible change in the seasonal hydrodynamics of the area. The green macroalgal, community, mainly Ulvophycean as represented by Entermorpha spp. and Ulva spp. are opportunistic, r-strategist species that are tolerant to varying salinity and thrive in disturbed and polluted waters. They are rapid colonizers with a short generation time of bare substrates, such as slipways. Their response to these environmental factors was monitored to investigate whether their growth could be indicative of nutrient enrichment. Six algal samples from each of the four sites chosen were taken at every collection session, and their biomass recorded. Water testing for the presence of nitrates at each site, was done and any possible correlation between an increase in the macroalgal biomass density and nitrate concentration was examined. The physical enviromnental factors affecting these algal communities during the seven-month investigation period was also taken into consideration. The four sites were specifically chosen to represent a highly touristic area (i.e. Bugibba) and lesser ones with possible less disturbance, sewage effluence and nutrient input due to human influence. Although it must be said that the high population density, the increased summer activity and the smallness of the island, work synergistically to exert great stresses on the coastal areas, almost leaving no accessible coastal area free from pressure and disturbance. The results showed a fluctuation of algal biomass density and nitrate levels during the stipulated observational period. The initial comparatively high biomass density during the month of May was followed by a downward trend in algal biomass density during the following months. It was only at Bugibba and Ghadira that algal biomass density increased again during the months of August till October. The nitrate level seemed to be erratic and not conclusively consistent with the algal productivity. The time span available for this investigation was limited to seven months; consequently results could be less significant than they would otherwise be if carried out over a longer period of time, thus including seasonal productivity changes and verification of the results obtained.