Abstract
Ongoing acidification of the ocean through uptake of anthropogenic CO2 is known to affect marine biota and ecosystems with largely unknown consequences for marine food webs. Changes in food web structure have the potential to alter trophic transfer, partitioning, and biogeochemical cycling of elements in the ocean. Here we investigated the impact of realistic end-of-the-century CO2 concentrations on the development and partitioning of the carbon, nitrogen, phosphorus, and silica pools in a coastal pelagic ecosystem (Gullmar Fjord, Sweden). We covered the entire winter-to-summer plankton succession (100 days) in two sets of five pelagic mesocosms, with one set being CO2 enriched (~760 μatm pCO2) and the other one left at ambient CO2 concentrations. Elemental mass balances were calculated and we highlight important challenges and uncertainties we have faced in the closed mesocosm system. Our key observations under high CO2 were: (1) A significantly amplified transfer of carbon, nitrogen, and phosphorus from primary producers to higher trophic levels, during times of regenerated primary production. (2) A prolonged retention of all three elements in the pelagic food web that significantly reduced nitrogen and phosphorus sedimentation by about 11 and 9%, respectively. (3) A positive trend in carbon fixation (relative to nitrogen) that appeared in the particulate matter pool as well as the downward particle flux. This excess carbon counteracted a potential reduction in carbon sedimentation that could have been expected from patterns of nitrogen and phosphorus fluxes. Our findings highlight the potential for ocean acidification to alter partitioning and cycling of carbon and nutrients in the surface ocean but also show that impacts are temporarily variable and likely depending upon the structure of the plankton food web.
Highlights
The ocean is a major sink for anthropogenic carbon dioxide (CO2) by absorbing more than 2 Pg carbon per year from the atmosphere [1,2]
In this study we investigated the influence of simulated ocean acidification on the development and partitioning of the C, N, P, and Si pools in a coastal pelagic ecosystem
Our mass balance approach over 100 days, covering a natural winter-to-summer plankton succession, has highlighted important challenges and uncertainties in elemental mass balance calculations, and revealed significant changes of element pool partitioning under realistic end-of-the-century CO2 concentrations (~760 μatm pCO2):
Summary
The ocean is a major sink for anthropogenic carbon dioxide (CO2) by absorbing more than 2 Pg carbon per year from the atmosphere [1,2]. This uptake of atmospheric CO2 leads to both carbonation (increasing CO2 concentration) and acidification (decreasing seawater pH) of the surface ocean [3,4] Changes of both environmental factors are expected to impact marine biota from the organism [5] to the ecosystem level [6,7]. For instance changes in stoichiometry and fatty acid composition of primary producers as a consequence of increasing CO2 have already been shown to impact mesozooplankton reproduction and development [23,24] This implies direct consequences for element cycling within the ocean’s food webs
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