Abstract
Abstract. Ocean acidification resulting from the uptake of anthropogenic carbon dioxide (CO2) by the ocean is considered a major threat to marine ecosystems. Here we examined the effects of ocean acidification on microbial community dynamics in the eastern Baltic Sea during the summer of 2012 when inorganic nitrogen and phosphorus were strongly depleted. Large-volume in situ mesocosms were employed to mimic present, future and far future CO2 scenarios. All six groups of phytoplankton enumerated by flow cytometry ( < 20 µm cell diameter) showed distinct trends in net growth and abundance with CO2 enrichment. The picoeukaryotic phytoplankton groups Pico-I and Pico-II displayed enhanced abundances, whilst Pico-III, Synechococcus and the nanoeukaryotic phytoplankton groups were negatively affected by elevated fugacity of CO2 (fCO2). Specifically, the numerically dominant eukaryote, Pico-I, demonstrated increases in gross growth rate with increasing fCO2 sufficient to double its abundance. The dynamics of the prokaryote community closely followed trends in total algal biomass despite differential effects of fCO2 on algal groups. Similarly, viral abundances corresponded to prokaryotic host population dynamics. Viral lysis and grazing were both important in controlling microbial abundances. Overall our results point to a shift, with increasing fCO2, towards a more regenerative system with production dominated by small picoeukaryotic phytoplankton.
Highlights
Marine phytoplankton are responsible for approximately half of global primary production (Field et al, 1998) with shelf sea communities contributing an average of 15–30 % (Kulinski and Pempkowiak, 2011)
Ocean acidification resulting from the uptake of anthropogenic carbon dioxide (CO2) by the ocean is considered a major threat to marine ecosystems
Due to the low buffering capacity of the Baltic Sea and the paucity of data regarding ocean acidification (OA) impact in nutrient-limited waters, the results presented here are pertinent to increasing our understanding of how projected rises in fugacity of CO2 (f CO2) will affect the microbial communities in this region
Summary
Marine phytoplankton are responsible for approximately half of global primary production (Field et al, 1998) with shelf sea communities contributing an average of 15–30 % (Kulinski and Pempkowiak, 2011). Along with warming sea surface temperatures and changing light and nutrient conditions, marine ecosystems face unprecedented decreases in ocean pH (Doney et al, 2009; Gruber, 2011). K. Crawfurd et al.: Alterations in microbial community composition with increasing f CO2 ification is considered one of the greatest current threats to marine ecosystems (Turley and Boot, 2010) and has been shown to alter phytoplankton primary production with the direction and magnitude of the responses dependent on community composition OA has been reported to increase the abundances of small-sized photoautotrophic eukaryotes in mesocosm experiments (Engel et al, 2008; Meakin and Wyman, 2011; Brussaard et al, 2013; Schulz et al, 2017)
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