Are rocky shore ecosystems affected by nutrient-enriched seawater? Some preliminary results from a mesocosm experiment

The response of rocky shore ecosystems to increased nutrient availability was examined in eight land-based mesocosms designed for hard-bottom littoral communities built at Marine Research Station Solbergstrand ( Norway). The average seawater volume in each basin was 9 m(3) with an average water residence time of about 2 h. A tidal regime resembling that in the fjord was maintained in the basins, and waves were generated regularly. NH4NO3 and H3PO4, at a constant molar NP ratio of 16:1, was added into 6 basins at concentrations 1, 2, 4, 8, 16, 32 muM DIN above the background DIN concentration during 1 1/2 years. Two mesocosms were kept as control treatment. Marine communities were introduced into the basins two years prior to the start of nutrient dosage. The effects of nutrient enrichment were few and only marginal during the first year of nutrient addition, while some effects became more obvious during the second year. The growth rate of the periphyton and fast-growing macroalgae communities was stimulated by nutrient enrichment, while the response was less evident among the perennial fucoids. The structure of the macroalgal communities, however, did not change during 16 months' measurements. In contrast, growth on artificial rock substrates during the same period of time revealed intensive growth of the fast-growing Ulva lactuca in high-dosed basins compared with low-dosed and control basins, which were dominated by the fucoid Fucus serratus. The fauna communities exhibited only a minor response to nutrient treatment. The common periwinkle Littorina littorea, however, appeared with increased abundance in the high-dosed basins. The total system metabolism tended to increase slightly, but not significantly, with increased nutrient loading.