Effect of elevated CO<sub>2</sub> on organic matter pools and fluxes in a summer Baltic Sea plankton community

Abstract

Abstract. Ocean acidification is expected to influence plankton community structure and biogeochemical element cycles. To date, the response of plankton communities to elevated CO 2 has been studied primarily during nutrient-stimulated blooms. In this CO 2 manipulation study, we used largevolume (∼ 55 m 3 ) 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 f CO 2 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 f CO 2 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 CO 2 -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.

Effects of CO&lt;sub&gt;2&lt;/sub&gt; perturbation on phosphorus pool sizes and uptake in a mesocosm experiment during a low productive summer season in the northern Baltic Sea

“…Nine mesocosms (M1-M9) were moored in the open waters of the Storfjärden (5951.5 • N, 2315.5 • E) at a water depth of ∼ 30 m. Only six of them were included throughout the whole study period since leakages in the remaining three rendered them unusable. Equipment and deployment procedures are described in detail by Paul et al (2015b). Briefly, polyurethane enclosure bags of 2 m in diameter and 18.5 m in length were mounted in floating frames and lowered in such a way that ∼ 17 m of each bag was immersed in the water column and ∼ 1.5 m remained above the water surface.…”

“…The intended CO 2 and pH gradients were reached after the last treatment on day 4. Details are described in Paul et al (2015b). For the two untreated (control) mesocosms, only filtered fjord water was added to adjust the water volume to that of the treated mesocosms.…”

“…Sampling over the entire 17 m depth was carried out using an integrating water sampler (IWS HYDROBIOS-KIEL) that was lowered slowly on a cable by hand. The sampling frequency differed depending on the parameter to be observed as shown in the overview by Paul et al (2015b).…”

Ocean Acidification-Induced Restructuring of the Plankton Food Web Can Influence the Degradation of Sinking Particles

“…While these studies helped in developing a principal understanding of how OA affects the physiology of single species, we are still largely lacking the understanding of how OA effects may manifest on the community level (Riebesell and Gattuso, 2015). Existing "whole community" studies show that both direct effects of OA on single species and indirect effects through changes in plankton community structure (Hall-Spencer et al, 2008;Fabricius et al, 2011;Schulz et al, 2013;Paul et al, 2015;Bach et al, 2016;Gazeau et al, 2016) can significantly influence biogeochemical cycles in the ocean.…”

“…Recent mesocosm experiments showed that OA effects can be particularly pronounced during extended periods of low inorganic nutrient concentrations (Paul et al, 2015;Sala et al, 2015;Bach et al, 2016;Thomson et al, 2016). We therefore conducted an in situ mesocosm experiment in the eastern subtropical Atlantic Ocean off the coast of Gran Canaria (Canary Islands)-a region characterized by low concentrations of inorganic nutrients in particular during fall when the study took place (Arístegui et al, 2001).…”