High organic carbon export precludes eutrophication responses in experimental rocky shore communities

We studied the effect of nutrient inputs on the carbon (C) budget of rocky shore communities using a set of eight large experimental mesocosms. The mesocosms received a range of inorganic nitrogen (N) and phosphorus (P) additions, at an N:P ratio of 16. These additions were designed to elevate the background concentration, relative to that in eutrophic Oslofjord (Norway) waters, by 1, 2, 4, 8, 16, 32 mumol dissolved inorganic nitrogen (DIN)l(-1) (and the corresponding P increase). Two unamended mesocosms were used as controls'. The nutrients were added continuously for 27 months before gross primary production (GPP), respiration (R), net community production (NCP), and dissolved organic carbon (DOC) production were assessed for the dominant algal species (Fucus serratus) and for the whole experimental ecosystem. Inputs and outputs of DOC and particulate organic carbon (POC) from the mesocosms were also quantified. The F. serratus communities were generally autotrophic (average P/R ratio = 1.33 +/- 0.12), with the GPP independent of the nutrient inputs to the mesocosms, and maintained a high net DOC production during both day (0.026 +/- 0.008 g C m(-2) h(-1)) and night (0.015 +/- 0.004 g C m(-2) h-1). All the experimental rocky shore ecosystems were autotrophic (P/R ratio = 2.04 +/- 0.28), and neither macroalgal biomass nor production varied significantly with increasing nutrient inputs. Most of the excess production from these autotrophic ecosystems was exported from the systems as DOC, which accounted for 69% and 58% of the NCP of the dominant community and the experimental ecosystem, respectively, the rest being lost as POC. High DOC release and subsequent export from the highly energetic environments occupied by rocky shore communities may prevent the development of eutrophication symptoms and render these communities resistant to eutrophication.