Abstract
Abstract. Net community production (NCP) and carbon to nutrient uptake ratios were studied during a large-scale mesocosm experiment on ocean acidification in Kongsfjorden, western Svalbard, during June–July 2010. Nutrient depleted fjord water with natural plankton assemblages, enclosed in nine mesocosms of ~ 50 m3 in volume, was exposed to pCO2 levels ranging initially from 185 to 1420 μatm. NCP estimations are the cumulative change in dissolved inorganic carbon concentrations after accounting for gas exchange and total alkalinity variations. Stoichiometric coupling between inorganic carbon and nutrient net uptake is shown as a ratio of NCP to a cumulative change in inorganic nutrients. Phytoplankton growth was stimulated by nutrient addition half way through the experiment and three distinct peaks in chlorophyll a concentration were observed during the experiment. Accordingly, the experiment was divided in three phases. Cumulative NCP was similar in all mesocosms over the duration of the experiment. However, in phases I and II, NCP was higher and in phase III lower at elevated pCO2. Due to relatively low inorganic nutrient concentration in phase I, C : N and C : P uptake ratios were calculated only for the period after nutrient addition (phase II and phase III). For the total post-nutrient period (phase II + phase III) ratios were close to Redfield, however they were lower in phase II and higher in phase III. Variability of NCP, C : N and C : P uptake ratios in different phases reflects the effect of increasing CO2 on phytoplankton community composition and succession. The phytoplankton community was composed predominantly of haptophytes in phase I, prasinophytes, dinoflagellates, and cryptophytes in phase II, and haptophytes, prasinophytes, dinoflagellates and chlorophytes in phase III (Schulz et al., 2013). Increasing ambient inorganic carbon concentrations have also been shown to promote primary production and carbon assimilation. For this study, it is clear that the pelagic ecosystem response to increasing CO2 is more complex than that represented in previous work, e.g. Bellerby et al. (2008). Carbon and nutrient uptake representation in models should, where possible, be more focused on individual plankton functional types as applying a single stoichiometry to a biogeochemical model with regard to the effect of increasing pCO2 may not always be optimal. The phase variability in NCP and stoichiometry may be better understood if CO2 sensitivities of the plankton's functional type biogeochemical uptake kinetics and trophic interactions are better constrained.
Highlights
The Arctic Ocean is a key player in global carbon cycling (e.g. Bates et al, 2009) and the Arctic shelves are currently amongst the most productive areas of the world’s oceans (Wassmann et al, 2011)
This study presents results from the first largescale pelagic ocean acidification mesocosm experiment conducted in the Arctic
C : P uptake ratios decreased with increasing pCO2 from 136.3 ± 18.3 in the low and 127.3 ± 16.4 in the intermediate to 92.8 ± 14.4 in the Net community production (NCP) increased with increasing pCO2 in phase I, which was consistent with the higher growth of small-sized phytoplankton (0.8–2.0 μm) stimulated by elevated CO2 (Brussaard et al, 2013)
Summary
The Arctic Ocean is a key player in global carbon cycling (e.g. Bates et al, 2009) and the Arctic shelves are currently amongst the most productive areas of the world’s oceans (Wassmann et al, 2011). Christensen et al, 2007 and references therein) including warming (Loeng, 2005, Trenberth et al, 2007), sea-ice decline (Polyakov et al, 2010; Stroeve et al, 2012), freshening (McPhee et al, 2009 and reference therein) and increasing surface carbon dioxide (CO2) concentrations (Cai et al, 2010) with concomitant ocean acidification (Bellerby et al, 2005; Yamamoto-Kawai et al, 2009, 2011). Model studies show that the Arctic Ocean may become entirely undersaturated with respect to aragonite already by 2050 (Anderson et al, 2010). These chemical changes may induce modifications in organism physiology and ecosystem functioning, as have been observed in many laboratory and mesocosm experiments (Nisumaa et al, 2010). Increasing ambient inorganic carbon concentrations have been shown to enhance primary production and carbon assimilation in various photoautotrophs, including seagrasses (Palacios and Zimmerman, 2007; Hall-Spencer et al, 2008) and freshwater and marine phytoplankton (Hein and Sand-Jensen, 1997; Schippers et al, 2004; Levitan et al, 2007; Riebesell et al, 2007; Engel et al, 2008; Tortell et al, 2008)
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