Kraufvelin P, Lindholm A, Pedersen MF, Kirkerud LA, Bonsdorff E
mesocosm, Solbergstrand, Oslofjord, Norway, 12 m3, macroalgae, nutrent enrichment
The littoral zone of temperate rocky shores isnormally dominated by perennial macroalgae (e.g. Fucus,Ascophyllum, Laminaria), but nutrient enrichment and/orpermanently decreased wave action may lead to structuralcommunity changes from dominance of perennials toincreased amounts of annual opportunistic species (mainlygreen algae). Macroalgal biomass, diversity and productionas well as relationships between the two latter were studiedusing Solbergstrand’s rocky shore mesocosms in SENorway in connection with a long-term experimentalmanipulation of nutrient addition and wave action (high andlow levels of both factors applied in a crossed way to eightoutdoor basins). After more than 2 years of experimentaltreatment, the total standing stock of macroalgae was largerin low nutrient than in high nutrient treatments as well as inhigh wave compared to low wave treatments (in autumnonly). For macroalgal functional groups, bushy and filamentousbrown and filamentous red algae were generallyfavoured by low nutrient concentrations, while annualfilamentous and sheet-like green algae were stimulated bythe nutrient enrichment. There was only one significantinteraction between nutrient enrichment and wave action(for brown filamentous algae in autumn) and also only onesignificant main effect of the wave treatment (for bushybrown algae in autumn). Surprisingly, the high nutrienttreatments supported a higher diversity of macroalgae,whereas the low nutrient treatments generally showedhigher production rates. Moreover, significantly negativecorrelations were found between macroalgal diversity andprimary productivity in both summer and autumn. Thisstudy shows that it is the biological components of thecommunities subjected to external forcing (nutrient additionor decreased wave action) that regulate production and thiscontradicts the common misperception that resource productionin natural systems simply can be fast-forwarded byfertilization. The negative relationships between diversityand productivity, although a consequence of unexpectedresults for diversity and production, are also novel and hinttowards species identities having more important functionalconsequences than general species dominance patterns andthe amount of species per se. These results also emphasisethe context dependency of findings within the field ofbiodiversity and ecosystem functioning.