Net community production and stoichiometry of nutrient consumption in a pelagic ecosystem of a northern high latitude fjord: mesocosm CO 2 perturbation study

Abstract

Net community production (NCP) and carbon to close to Redfield, however they were lower in phase II and Sciences Open Access nutrient uptake ratios were studied during a large-scale meso- higher in phase III. Variability of NCP, C:N and C: P up- cosm experiment on ocean acidification in Kongsfjorden, take ratios in different phases reflects the effect of increasing western Svalbard, during June–July 2010. Nutrient depleted CO2 on phytoplankton community composition and succes- fjord water with natural plankton assemblages, enclosed in sion. The phytoplankton community was composed predom- Ocean Science nine mesocosms of ∼50m3 in volume, was exposed to inantly of haptophytes in phase I, prasinophytes, dinoflagel- Open Access pCO2 levels ranging initially from 185 to 1420 µatm. NCP lates, and cryptophytes in phase II, and haptophytes, prasino- estimations are the cumulative change in dissolved inorganic phytes, dinoflagellates and chlorophytes in phase III (Schulz carbon concentrations after accounting for gas exchange and et al., 2013). Increasing ambient inorganic carbon concentra- total alkalinity variations. Stoichiometric coupling between tions have also been shown to promote primary production inorganic carbon and nutrient net uptake is shown as a ratio and carbon assimilation. For this study, it is clear that the Solid Earth of NCP to a cumulative change in inorganic nutrients. Phyto- pelagic ecosystem response to increasing CO2 is more com- Open Access plankton growthwas stimulated by nutrient addition halfway plex than that represented in previous work, e.g. Bellerby et through the experiment and three distinct peaks in chloro- al. (2008). Carbon and nutrient uptake representation in mod- phyll a concentration were observed during the experiment. els should, where possible, be more focused on individual Accordingly, the experimentwas divided in three phases. Cu- plankton functional types as applying a single stoichiometry mulative NCP was similar in all mesocosms over the dura- to a biogeochemical model with regard to the effect of in- The Cryosphere tion of the experiment. However, in phases I and II, NCP creasing pCO2 may not always be optimal. The phase vari- Open Access was higher and in phase III lower at elevated pCO2. Due ability in NCP and stoichiometry may be better understood to relatively low inorganic nutrient concentration in phase I, if CO2 sensitivities of the plankton’s functional type biogeo- C:N and C:P uptake ratios were calculated only for the pe- chemical uptake kinetics and trophic interactions are better riod after nutrient addition (phase II and phase III). For the constrained. total post-nutrient period (phase II+phase III) ratios were Published