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Studies investigating the effect of increasing CO2 levels on the phosphorus cycle in natural waters are lacking although phosphorus often controls phytoplankton development in aquatic systems. The aim of our study was to analyze effects of elevated CO2 levels on phosphorus pool sizes and uptake. Therefore, we conducted a CO2-manipulation mesocosm experiment in the Storfjärden (western Gulf of Finland, Baltic Sea) in summer 2012. We compared the phosphorus dynamics in different mesocosm treatments but also studied them outside the mesocosms in the surrounding fjord water. In the mesocosms as well as in surface waters of Storfjärden, dissolved organic phosphorus (DOP) concentrations of 0.26 ± 0.03 and 0.23 ± 0.04 μmol L−1, respectively, formed the main fraction of the total P-pool (TP), whereas phosphate (PO4) constituted the lowest fraction with mean concentration of 0.15 ± 0.02 μmol L−1 and 0.17 ± 0.07 μmol L−1 in the mesocosms and in the fjord, respectively. Uptake of PO4 ranged between 0.6 and 3.9 nmol L−1 h−1 of which ~ 86% (mesocosms) and ~ 72% (fjord) were realized by the size fraction < 3 μm. Adenosine triphosphate (ATP) uptake revealed that additional P was supplied from organic compounds accounting for 25–27% of P provided by PO4 only. CO2 additions did not cause significant changes in phosphorus (P) pool sizes, DOP composition, and uptake of PO4 and ATP when the whole study period was taken into account. About 18% of PO4 was transformed into POP, whereby the major proportion (~ 82%) was converted into DOP suggesting that the conversion of PO4 to DOP is the main pathway of the PO4 turnover. We observed that significant relationships (e.g., between POP and Chl a) in the untreated mesocosms vanished under increased fCO2 conditions. Consequently, it can be hypothesized that the relationship between POP formation and phytoplankton growth changed under elevated CO2 conditions. Significant short-term effects were observed for PO4 and particulate organic phosphorus (POP) pool sizes in CO2 treatments > 1000 µatm during periods when phytoplankton started to grow. [ABSTRACT FROM AUTHOR]

Nitrogen fixation by filamentous cyanobacteria supplies significant amounts of new nitrogen (N) to the Baltic Sea. This balances N loss processes such as denitrification and anammox and forms an important N source supporting primary and secondary production in N-limited post-spring bloom plankton communities. Laboratory studies suggest that filamentous diazotrophic cyanobacteria growth and N2-fixation rates are sensitive to ocean acidification with potential implications for new N supply to the Baltic Sea. In this study, our aim was to assess the effect of ocean acidification on diazotroph growth and activity as well as the contribution of diazotrophically-fixed N to N supply in a natural plankton assemblage. We enclosed a natural plankton community in a summer season in the Baltic Sea near the entrance to the Gulf of Finland in six large-scale mesocosms (volume ~ 55 m3) and manipulated fCO2 over a range relevant for projected ocean acidification by the end of this century (average treatment fCO2: 365-1231 µatm). The direct response of diazotroph growth and activity was followed in the mesocosms over a 47 day study period during N-limited growth in the summer plankton community. Diazotrophic filamentous cyanobacteria abundance throughout the study period and N2-fixation rates (determined only until day 21 due to subsequent use of contaminated commercial 15N-N2 gas stocks) remained low. Thus estimated new N inputs from diazotrophy were too low to relieve N limitation and stimulate a summer phytoplankton bloom. Instead regeneration of organic N sources likely sustained growth in the plankton community. We could not detect significant CO2-related differences in inorganic or organic N pools sizes, or particulate matter N : P stoichiometry. Additionally, no significant effect of elevated CO2 on diazotroph activity was observed. Therefore, ocean acidification had no observable impact on N cycling or biogeochemistry in this N-limited, post-spring bloom plankton assemblage in the Baltic Sea. [ABSTRACT FROM AUTHOR]

Ocean acidification is expected to influence plankton community structure and biogeochemical element cycles. To date, the response of plankton communities to elevated CO² has been studied primarily during nutrient-stimulated blooms. In this CO² manipulation study, we used largevolume (~55m3/ pelagic in situ mesocosms to enclose a natural summer, post-spring-bloom plankton assemblage in the Baltic Sea to investigate the response of organic matter pools to ocean acidification. The carbonate system in the six mesocosms was manipulated to yield average fCO² ranging between 365 and ~1230 µatm with no adjustment of naturally available nutrient concentrations. Plankton community development and key biogeochemical element pools were subsequently followed in this nitrogen-limited ecosystem over a period of 7 weeks. We observed higher sustained chlorophyll a and particulate matter concentrations (~25% higher) and lower inorganic phosphate concentrations in the water column in the highest fCO² treatment (1231 µatm) during the final 2 weeks of the study period (Phase III), when there was low net change in particulate and dissolved matter pools. Size-fractionated phytoplankton pigment analyses indicated that these differences were driven by picophytoplankton (< 2 µm) and were already established early in the experiment during an initial warm and more productive period with overall elevated chlorophyll a and particulate matter concentrations. However, the influence of picophytoplankton on bulk organic matter pools was masked by high biomass of larger plankton until Phase III, when the contribution of the small size fraction (< 2 µm) increased to up to 90% of chlorophyll a. In this phase, a CO²-driven increase in water column particulate carbon did not lead to enhanced sinking material flux but was instead reflected in increased dissolved organic carbon concentrations. Hence ocean acidification may induce changes in organic matter partitioning in the upper water column during the low-nitrogen summer period in the Baltic Sea. [ABSTRACT FROM AUTHOR]

Ocean acidification is expected to influence plankton community structure and biogeo-chemical element cycles. To date, experiments with nutrient stimulated blooms have been primarily used to study the response of plankton communities to elevated CO2. In this CO2 manipulation study, we used large-volume (~ 55 m³) pelagic in situ meso-cosms to enclose a natural, post spring-bloom plankton assemblage in the Baltic Sea to investigate the response of organic matter pools to ocean acidification. In the meso-cosms, fCO2 was manipulated yielding a range of average fCO2 of 365 to ~ 1231 μatm with no adjustment of naturally available nutrient concentrations. Plankton community development and key biogeochemical element pools were subsequently followed in this nitrogen-limited ecosystem over a period of seven weeks. We identified three distinct phases based on temperature fluctuations and plankton biomass: a warm, productive period with elevated chlorophyll a and particulate matter concentrations (Phase I), a decline in autotrophic biomass coinciding with cooler water temperatures associated with lower incoming photosynthetically active radiation (PAR) and higher zooplankton grazing pressure (Phase II), and a steady state phase with low net change in particulate and dissolved matter pools (Phase III). We observed higher sustained chlorophyll a and particulate matter concentrations (~25% higher) and lower inorganic phosphate concentrations in the water column in the highest fCO2 treatment (1231 μatm) in Phase III. Size-fractionated phytoplankton pigment analyses indicated that these differences were driven by picophytoplankton (< 2μm) and were already established early in the experiment during Phase I. However the influence of picophytoplankton on bulk organic matter pools was masked by high biomass of larger plankton until Phase III when the small size fraction (< 2 μm) contributed up to 90% of chlorophyll a. Furthermore, CO2-related differences in water column suspended matter concentrations were not reflected in sinking material flux. Our results from this study indicate that ocean acidification could have significant and sustained impacts on pelagic biogeochemical element pools in nitrogen-limited ecosystems. [ABSTRACT FROM AUTHOR]

Ocean acidification is expected to influence plankton community structure and biogeochemical element cycles. To date, experiments with nutrient stimulated blooms have been primarily used to study the response of plankton communities to elevated CO2. In this CO2 manipulation study, we used large-volume (~ 55m³) pelagic in situ mesocosms to enclose a natural, post spring-bloom plankton assemblage in the Baltic Sea to investigate the response of organic matter pools to ocean acidification. In the mesocosms, fCO2 was manipulated yielding a range of average fCO2 of 365 to ~ 1231 µatm with no adjustment of naturally available nutrient concentrations. Plankton community development and key biogeochemical element pools were subsequently followed in this nitrogen-limited ecosystem over a period of seven weeks. We identified three distinct phases based on temperature fluctuations and plankton biomass: a warm, productive period with elevated chlorophyll a and particulate matter concentrations (Phase I), a decline in autotrophic biomass coinciding with cooler water temperatures associated with lower incoming photosynthetically active radiation (PAR) and higher zooplankton grazing pressure (Phase II), and a steady state phase with low net change in particulate and dissolved matter pools (Phase III). We observed higher sustained chlorophyll a and particulate matter concentrations (~ 25% higher) and lower inorganic phosphate concentrations in the water column in the highest fCO2 treatment (1231 µatm) in Phase III. Size-fractionated phytoplankton pigment analyses indicated that these differences were driven by picophytoplankton (< 2 µm) and were already established early in the experiment during Phase I. However the influence of picophytoplankton on bulk organic matter pools was masked by high biomass of larger plankton until Phase III when the small size fraction (< 2 µm) contributed up to 90% of chlorophyll a. Furthermore, CO2-related differences in water column suspended matter concentrations were not reflected in sinking material flux. Our results from this study indicate that ocean acidification could have significant and sustained impacts on pelagic biogeochemical element pools in nitrogen-limited ecosystems. [ABSTRACT FROM AUTHOR]

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 single species 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 2.5m wave heights). It encloses a water column 2m in diameter and 15 to 25m deep (~50-75m³ 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. [ABSTRACT FROM AUTHOR]