Bokn TL, Duarte CM, Pedersen MF, Marba N, Moy FE, Barron C, Bjerkeng B, Borum J, Christie H, Engelbert S, Fotel FL, Hoell EE, Karez R, Kersting K, Kraufvelin P, Lindblad C, Olsen M, Sanderud KA, Sommer U, Sorensen K
mesocosm, hard-bottom organisms, intertidal communities, nutrient enrichment, coastal eutrophication, rocky shore communities, algae, benthic vegetation, Norway, Solbergstrand, Oslofjord, 12 m3
The aim of this study was to determine whether theexperimental nutrient enrichment of littoral rockyshore communities would be followed by a predictedaccumulation of fast-growing opportunisticalgae and a subsequent loss of perennial benthicvegetation. Inorganic nitrogen (N) and potassium(P) was added to eight concrete mesocosms inhabitedby established littoral communities dominatedby fucoids. The response to nutrient enrichmentwas followed for almost 2 1/2 years. Fast-growingopportunistic algae (periphyton and ephemeralgreen algae) grew significantly faster in response tonutrient enrichment, but the growth of red filamentousalgae and large perennial brown algae wasunaffected. However, these changes were not followedby comparable changes in the biomass andcomposition of the macroalgae. The biomass of opportunisticalgae was stimulated only marginally bythe nutrient enrichment, and perennial brown algae(fucoids) remained dominant in the mesocosmregardless of nutrient treatment level. Establishedrocky shore communities thus seem able to resistthe effects of heavy nutrient loading. We found thatthe combined effects of the heavy competition forspace and light imposed by canopy-forming algae,preferential grazing on opportunistic algae by herbivores,and physical disturbance, succeeded by amarked export of detached opportunistic algae, preventedthe fast-growing algae from becoming dominant.However, recruitment studies showed thatthe opportunistic algae would become dominantwhen free space was available under conditions ofhigh nutrient loading and low grazing pressure.These results show that established communities ofperennial algae and associated fauna in rocky shoreenvironments can prevent or delay the accumulationof bloom-forming opportunistic algae and thatthe replacement of long-lived macroalgae by opportunisticspecies at high nutrient loading may be aslow process. Nutrient enrichment may not, in itself,be enough to stimulate structural changes inrocky shore communities.