Pelagic community production and carbon-nutrient stoichiometry under variable ocean acidification in an Arctic fjord

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 <i>p</i>CO₂ 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 <i>a</i> 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 <i>p</i>CO₂. 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 <i>p</i>CO₂ 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

Phytoplankton responses and associated carbon cycling during shipboard carbonate chemistry manipulation experiments conducted around Northwest European shelf seas

“…For example, primary production measured by 14 C fixation or the net production of particulate organic carbon (POC) has variously been shown to be enhanced (Riebesell et al, 2007;Egge et al, 2009;Engel et al, 2013;Silyakova et al, 2013;Eggers et al, 2014), decreased Zondervan, 2007;Yoshimura et al, 2013), or not significantly influenced (Tortell et al, 2002;Delille et al, 2005;Yoshimura et al, 2013) following experimental elevation of pCO 2 . Similarly, significant responses to elevated pCO 2 have been described for varying groups, including both large-celled diatoms Eggers et al, 2014) and pico-/nanoeukaryotes .…”

Marine CDOM accumulation during a coastal Arctic mesocosm experiment: No response to elevated pCO2 levels

“…Added CO 2 -enriched water was injected and evenly dispersed throughout the water column between days t4 and t8 using a device known as the "spider" [Riebesell et al, 2013b]. The range of 8 pCO 2 levels averaged for the course of experiment was between 177 ppm and 1084 ppm [Silyakova et al, 2013]. Nutrients (5 μmol L À1 NO 3 À , 0.32 μmol L À1 PO 4 3À , and 2.5 μmol L À1 Si(OH) 4 ) were added to the enclosures on day t13 to stimulate phytoplankton growth.…”

Impact of elevated pH on succession in the Arctic spring bloom

“…However, in brackish coastal waters, where the buffering capacity is lower and phytoplankton blooms are more intense, pH may be highly variable (Hansen 2002, Hinga 2002, Pro voost et al 2010, Brutemark et al 2011) and may affect phytoplankton growth and species succession (Hansen 2002, Hinga 2002, Pedersen & Hansen 2003a,b, Søderberg & Han sen 2007. There are only a few studies on the role of pH in Arctic marine ecosystems, and these studies indicate that pH may also play a central role in plankton dynamics in these high-latitude ecosystems (Charalampopoulou et al 2011, Søgaard et al 2011, Silyakova et al 2013.…”