Implications of elevated COsub2/sub on pelagic carbon fluxes in an Arctic mesocosm study – an elemental mass balance approach

摘要

Recent studies on the impacts of ocean acidification on pelagic communitieshave identified changes in carbon to nutrient dynamics with related shiftsin elemental stoichiometry. In principle, mesocosm experiments provide theopportunity of determining temporal dynamics of all relevant carbon andnutrient pools and, thus, calculating elemental budgets. In practice,attempts to budget mesocosm enclosures are often hampered by uncertaintiesin some of the measured pools and fluxes, in particular due to uncertaintiesin constraining air–sea gas exchange, particle sinking, and wall growth. Inan Arctic mesocosm study on ocean acidification applying KOSMOS(Kiel Off-Shore Mesocosms for future Ocean Simulation), all relevant element pools and fluxesof carbon, nitrogen and phosphorus were measured, using an improvedexperimental design intended to narrow down the mentioned uncertainties.Water-column concentrations of particulate and dissolved organic andinorganic matter were determined daily. New approaches for quantitativeestimates of material sinking to the bottom of the mesocosms and gasexchange in 48 h temporal resolution as well as estimates of wall growthwere developed to close the gaps in element budgets. However, losses elements fromthe budgets into a sum of insufficiently determined pools were detected, and are principallyunavoidable in mesocosm investigation. The comparison of variability patterns ofall single measured datasets revealed analytic precision to be the mainissue in determination of budgets. Uncertainties in dissolved organic carbon(DOC), nitrogen (DON) and particulate organic phosphorus (POP) were muchhigher than the summed error in determination of the same elements in allother pools. With estimates provided for all other major elemental pools,mass balance calculations could be used to infer the temporal development ofDOC, DON and POP pools.Future elevated pCO₂ was found to enhance net autotrophic communitycarbon uptake in two of the three experimental phases but did notsignificantly affect particle elemental composition. Enhanced carbonconsumption appears to result in accumulation of dissolved organic carbonunder nutrient-recycling summer conditions. This carbon over-consumptioneffect becomes evident from mass balance calculations, but was too small tobe resolved by direct measurements of dissolved organic matter. Fasternutrient uptake by comparatively small algae at high CO₂ after nutrientaddition resulted in reduced production rates under future ocean CO₂conditions at the end of the experiment. This CO₂ mediated shifttowards smaller phytoplankton and enhanced cycling of dissolved matterrestricted the development of larger phytoplankton, thus pushing the systemtowards a retention type food chain with overall negative effects on exportpotential.