Effect of CO2 enrichment on bacterial metabolism in an Arctic fjord (update)

The anthropogenic increase of carbon dioxide (CO₂) alters the seawater carbonate chemistry, with a decline of pH and an increase in the partial pressure of CO₂ (pCO₂). Although bacteria play a major role in carbon cycling, little is known about the impact of rising pCO₂ on bacterial carbon metabolism, especially for natural bacterial communities. In this study, we investigated the effect of rising pCO₂ on bacterial production (BP), bacterial respiration (BR) and bacterial carbon metabolism during a mesocosm experiment performed in Kongsfjorden (Svalbard) in 2010. Nine mesocosms with pCO₂ levels ranging from ca. 180 to 1400 μatm were deployed in the fjord and monitored for 30 days. Generally BP gradually decreased in all mesocosms in an initial phase, showed a large (3.6-fold average) but temporary increase on day 10, and increased slightly after inorganic nutrient addition. Over the wide range of pCO₂ investigated, the patterns in BP and growth rate of bulk and free-living communities were generally similar over time. However, BP of the bulk community significantly decreased with increasing pCO₂ after nutrient addition (day 14). In addition, increasing pCO₂ enhanced the leucine to thymidine (Leu : TdR) ratio at the end of experiment, suggesting that pCO₂ may alter the growth balance of bacteria. Stepwise multiple regression analysis suggests that multiple factors, including pCO₂, explained the changes of BP, growth rate and Leu : TdR ratio at the end of the experiment. In contrast to BP, no clear trend and effect of changes of pCO₂ was observed for BR, bacterial carbon demand and bacterial growth efficiency. Overall, the results suggest that changes in pCO₂ potentially influence bacterial production, growth rate and growth balance rather than the conversion of dissolved organic matter into CO₂.

Implications of elevated CO2 on pelagic carbon fluxes in an Arctic mesocosm study – an elemental mass balance approach (update)

Recent studies on the impacts of ocean acidification on pelagic communities have identified changes in carbon to nutrient dynamics with related shifts in elemental stoichiometry. In principle, mesocosm experiments provide the opportunity of determining temporal dynamics of all relevant carbon and nutrient pools and, thus, calculating elemental budgets. In practice, attempts to budget mesocosm enclosures are often hampered by uncertainties in some of the measured pools and fluxes, in particular due to uncertainties in constraining air–sea gas exchange, particle sinking, and wall growth. In an Arctic mesocosm study on ocean acidification applying KOSMOS (Kiel Off-Shore Mesocosms for future Ocean Simulation), all relevant element pools and fluxes of carbon, nitrogen and phosphorus were measured, using an improved experimental design intended to narrow down the mentioned uncertainties. Water-column concentrations of particulate and dissolved organic and inorganic matter were determined daily. New approaches for quantitative estimates of material sinking to the bottom of the mesocosms and gas exchange in 48 h temporal resolution as well as estimates of wall growth were developed to close the gaps in element budgets. However, losses elements from the budgets into a sum of insufficiently determined pools were detected, and are principally unavoidable in mesocosm investigation. The comparison of variability patterns of all single measured datasets revealed analytic precision to be the main issue in determination of budgets. Uncertainties in dissolved organic carbon (DOC), nitrogen (DON) and particulate organic phosphorus (POP) were much higher than the summed error in determination of the same elements in all other pools. With estimates provided for all other major elemental pools, mass balance calculations could be used to infer the temporal development of DOC, DON and POP pools.

Future elevated pCO₂ was found to enhance net autotrophic community carbon uptake in two of the three experimental phases but did not significantly affect particle elemental composition. Enhanced carbon consumption appears to result in accumulation of dissolved organic carbon under nutrient-recycling summer conditions. This carbon over-consumption effect becomes evident from mass balance calculations, but was too small to be resolved by direct measurements of dissolved organic matter. Faster nutrient uptake by comparatively small algae at high CO₂ after nutrient addition resulted in reduced production rates under future ocean CO₂ conditions at the end of the experiment. This CO₂ mediated shift towards smaller phytoplankton and enhanced cycling of dissolved matter restricted the development of larger phytoplankton, thus pushing the system towards a retention type food chain with overall negative effects on export potential.

Contrasting responses of DMS and DMSP to ocean acidification in Arctic waters (update)

Increasing atmospheric CO₂ is decreasing ocean pH most rapidly in colder regions such as the Arctic. As a component of the EPOCA (European Project on Ocean Acidification) pelagic mesocosm experiment off Spitzbergen in 2010, we examined the consequences of decreased pH and increased pCO₂ on the concentrations of dimethylsulphide (DMS). DMS is an important reactant and contributor to aerosol formation and growth in the Arctic troposphere. In the nine mesocosms with initial pH_T 8.3 to 7.5, equivalent to pCO₂ of 180 to 1420 μatm, highly significant but inverse responses to acidity (hydrogen ion concentration [H⁺]) occurred following nutrient addition. Compared to ambient [H⁺], average concentrations of DMS during the mid-phase of the 30 d experiment, when the influence of altered acidity was unambiguous, were reduced by approximately 60% at the highest [H⁺] and by 35% at [H⁺] equivalent to 750 μatm pCO₂, as projected for 2100. In contrast, concentrations of dimethylsulphoniopropionate (DMSP), the precursor of DMS, were elevated by approximately 50% at the highest [H⁺] and by 30% at [H⁺] corresponding to 750 μatm pCO₂. Measurements of the specific rate of synthesis of DMSP by phytoplankton indicate increased production at high [H⁺], in parallel to rates of inorganic carbon fixation. The elevated DMSP production at high [H⁺] was largely a consequence of increased dinoflagellate biomass and in particular, the increased abundance of the species Heterocapsa rotundata. We discuss both phytoplankton and bacterial processes that may explain the reduced ratios of DMS:DMSPt (total dimethylsulphoniopropionate) at higher [H⁺]. The experimental design of eight treatment levels provides comparatively robust empirical relationships of DMS and DMSP concentration, DMSP production and dinoflagellate biomass versus [H⁺] in Arctic waters.

Technical Note: the determination of enclosed water volume in large flexible-wall mesocosms “KOSMOS” (update)

The volume of water enclosed inside flexible-wall mesocosm bags is hard to estimate using geometrical calculations and can be strongly variable among bags of the same dimensions. Here we present a method for precise water volume determination in mesocosms using salinity as a tracer. Knowledge of the precise volume of water enclosed allows establishment of exactly planned treatment concentrations and calculation of elemental budgets.

Technical Note: A mobile sea-going mesocosm system – new opportunities for ocean change research (update)

One of the great challenges in ocean change research is to understand and forecast the effects of environmental changes on pelagic communities and the associated impacts on biogeochemical cycling. Mesocosms, experimental enclosures designed to approximate natural conditions, and in which environmental factors can be manipulated and closely monitored, provide a powerful tool to close the gap between small-scale laboratory experiments and observational and correlative approaches applied in field surveys. Existing pelagic mesocosm systems are stationary and/or restricted to well-protected waters. To allow mesocosm experimentation in a range of hydrographic conditions and in areas considered most sensitive to ocean change, we developed a mobile sea-going mesocosm facility, the Kiel Off-Shore Mesocosms for Future Ocean Simulations (KOSMOS). The KOSMOS platform, which can be transported and deployed by mid-sized research vessels, is designed for operation in moored and free-floating mode under low to moderate wave conditions (up to 2.5 m wave heights). It encloses a water column 2 m in diameter and 15 to 25 m deep (∼50–75 m³ in volume) without disrupting the vertical structure or disturbing the enclosed plankton community. Several new developments in mesocosm design and operation were implemented to (i) minimize differences in starting conditions between mesocosms, (ii) allow for extended experimental duration, (iii) precisely determine the mesocosm volume, (iv) determine air–sea gas exchange, and (v) perform mass balance calculations. After multiple test runs in the Baltic Sea, which resulted in continuous improvement of the design and handling, the KOSMOS platform successfully completed its first full-scale experiment in the high Arctic off Svalbard (78°56.2′ N, 11°53.6′ E) in June/July 2010. The study, which was conducted in the framework of the European Project on Ocean Acidification (EPOCA), focused on the effects of ocean acidification on a natural plankton community and its impacts on biogeochemical cycling and air–sea exchange of climate-relevant gases. This manuscript describes the mesocosm hardware, its deployment and handling, CO₂ manipulation, sampling and cleaning, including some further modifications conducted based on the experiences gained during this study.

High tolerance of microzooplankton to ocean acidification in an Arctic coastal plankton community (update)

Impacts of ocean acidification (OA) on marine biota have been observed in a wide range of marine systems. We used a mesocosm approach to study the response of a high Arctic coastal microzooplankton community during the post-bloom period in Kongsfjorden (Svalbard) to direct and indirect effects of high pCO₂/low pH. We found almost no direct effects of OA on microzooplankton composition and diversity. Both the relative shares of ciliates and heterotrophic dinoflagellates as well as the taxonomic composition of microzooplankton remained unaffected by changes in pCO₂/pH. Although the different pCO₂ treatments affected food availability and phytoplankton composition, no indirect effects (e.g. on the total carrying capacity and phenology of microzooplankton) could be observed. Our data point to a high tolerance of this Arctic microzooplankton community to changes in pCO₂/pH. Future studies on the impact of OA on plankton communities should include microzooplankton in order to test whether the observed low sensitivity to OA is typical for coastal communities where changes in seawater pH occur frequently.

Technical note: a simple method for air–sea gas exchange measurements in mesocosms and its application in carbon budgeting (update)

Mesocosms as large experimental units provide the opportunity to perform elemental mass balance calculations, e.g. to derive net biological turnover rates. However, the system is in most cases not closed at the water surface and gases exchange with the atmosphere. Previous attempts to budget carbon pools in mesocosms relied on educated guesses concerning the exchange of CO₂ with the atmosphere. Here, we present a simple method for precise determination of air–sea gas exchange in mesocosms using N₂O as a deliberate tracer. Beside the application for carbon budgeting, transfer velocities can be used to calculate exchange rates of any gas of known concentration, e.g. to calculate aquatic production rates of climate relevant trace gases. Using an arctic KOSMOS (Kiel Off Shore Mesocosms for future Ocean Simulation) experiment as an exemplary dataset, it is shown that the presented method improves accuracy of carbon budget estimates substantially. Methodology of manipulation, measurement, data processing and conversion to CO₂ fluxes are explained. A theoretical discussion of prerequisites for precise gas exchange measurements provides a guideline for the applicability of the method under various experimental conditions.

Mesozooplankton community development at elevated CO2 concentrations: results from a mesocosm experiment in an Arctic fjord (update)

The increasing CO₂ concentration in the atmosphere caused by burning fossil fuels leads to increasing pCO₂ and decreasing pH in the world ocean. These changes may have severe consequences for marine biota, especially in cold-water ecosystems due to higher solubility of CO₂. However, studies on the response of mesozooplankton communities to elevated CO₂ are still lacking. In order to test whether abundance and taxonomic composition change with pCO₂, we have sampled nine mesocosms, which were deployed in Kongsfjorden, an Arctic fjord at Svalbard, and were adjusted to eight CO₂ concentrations, initially ranging from 185 μatm to 1420 μatm. Vertical net hauls were taken weekly over about one month with an Apstein net (55 μm mesh size) in all mesocosms and the surrounding fjord. In addition, sediment trap samples, taken every second day in the mesocosms, were analysed to account for losses due to vertical migration and mortality. The taxonomic analysis revealed that meroplanktonic larvae (Cirripedia, Polychaeta, Bivalvia, Gastropoda, and Decapoda) dominated in the mesocosms while copepods (Calanus spp., Oithona similis, Acartia longiremis and Microsetella norvegica) were found in lower abundances. In the fjord copepods prevailed for most of our study. With time, abundance and taxonomic composition developed similarly in all mesocosms and the pCO₂ had no significant effect on the overall community structure. Also, we did not find significant relationships between the pCO₂ level and the abundance of single taxa. Changes in heterogeneous communities are, however, difficult to detect, and the exposure to elevated pCO₂ was relatively short. We therefore suggest that future mesocosm experiments should be run for longer periods.

A 13C labelling study on carbon fluxes in Arctic plankton communities under elevated CO2 levels (update)

The effect of CO₂ on carbon fluxes (production, consumption, and export) in Arctic plankton communities was investigated during the 2010 EPOCA (European project on Ocean Acidification) mesocosm study off Ny Ålesund, Svalbard. ¹³C labelled bicarbonate was added to nine mesocosms with a range in pCO₂ (185 to 1420 μatm) to follow the transfer of carbon from dissolved inorganic carbon (DIC) into phytoplankton, bacterial and zooplankton consumers, and export. A nutrient–phytoplankton–zooplankton–detritus model amended with ¹³C dynamics was constructed and fitted to the data to quantify uptake rates and carbon fluxes in the plankton community. The plankton community structure was characteristic for a post-bloom situation and retention food web and showed high bacterial production (∼31% of primary production), high abundance of mixotrophic phytoplankton, low mesozooplankton grazing (∼6% of primary production) and low export (∼7% of primary production). Zooplankton grazing and export of detritus were sensitive to CO₂: grazing decreased and export increased with increasing pCO₂. Nutrient addition halfway through the experiment increased the export, but not the production rates. Although mixotrophs showed initially higher production rates with increasing CO₂, the overall production of POC (particulate organic carbon) after nutrient addition decreased with increasing CO₂. Interestingly, and contrary to the low nutrient situation, much more material settled down in the sediment traps at low CO₂. The observed CO₂ related effects potentially alter future organic carbon flows and export, with possible consequences for the efficiency of the biological pump.

Enhanced gas fluxes in small sea ice leads and cracks – effects on CO2 exchange and ocean acidification

Earth system models generally partition grid cells into an ice covered and an open water area and view the ice covered area as a barrier to gas fluxes. However, observations suggest that exchange in cracks and small leads can be much higher in localized areas than expected under similar conditions in open water. While these models project a significant retreat in sea ice cover, affecting air-sea CO₂ exchange and consequently ocean acidification, the simple grid cell partitioning might underestimate the actual CO₂ exchange. A sensitivity study with the Canadian Earth System Model (CanESM2) shows that enhanced CO₂ exchange in sea ice areas in the Arctic Ocean increases the uptake in fall and winter, allowing more continuous equilibration and hence reduced uptake in summer. The reduction in summer also hints at a limited CO₂-uptake capacity of Arctic surface waters. Retreating sea ice in the future leads to a similar shift in the seasonal cycle. The annual mean carbon uptake of the Arctic Ocean north of 68°N changes only slightly with the enhanced flux parameterisation (<3 %). For the central Arctic (north of 80°N) the change is up to 21% and accelerates ocean acidification, e.g. locally, surface waters could reach aragonite undersaturation 1–2 decades earlier. The differences between standard and enhanced exchange test runs become less pronounced in the future due to the reduced ice cover and extended open water season. For the Antarctic sea ice zone where the CO₂ flux is out of the ocean, the enhanced exchange increases outgassing, slightly slowing down acidification.