Development, Productivity, and Seasonality of Living Planktonic Foraminiferal Faunas and Limacina helicina in an Area of Intense Methane Seepage in the Barents Sea

Although the plankton communities in the Barents Sea have been intensely studied for decades, little is known about the living planktonic foraminiferal (LPF) and pteropod faunas, especially those found at methane seep sites. Along a repeated transect in the "crater area" (northern Barents Sea, 74.9 degrees N, 27.7 degrees E) in spring and summer 2016 the flux of LPF and of the pteropod species Limacina helicina showed a high degree of variability. The LPF had low concentration (0-6 individuals m(-3)) and small tests (x = 103.3 mu m) in spring and a 53-fold increase (43-436 individuals m(-3)) and larger tests (x = 188.6 mu m) in summer. Similarly, the concentration of L. helicina showed a tenfold increase between spring and summer. The LPF species composition remained stable with the exception of the appearance of subtropical species in summer. No relationship was observed between the spatial distribution of LPF, L. helicina, and methane concentrations in the area. The methane plumes in April coincided with elevated dissolved inorganic carbon, low pH, and calcium carbonate saturation states, and the methane concentration seemed to be controlled by lateral advection. The delta C-13 and delta O-18 of Neogloboquadrina pachyderma and Turborotalita quinqueloba are comparable to previous observations in the Arctic and do not show any influence of methane in the isotopic signals of the shells. Although no evidence of direct impact of high methane concentrations on the LPF (size and concentration) were found, we speculate that methane could indirectly enhance primary productivity, and thus biomass, through several potential pathways. Plain Language Summary Planktonic foraminifera and the thecosome pteropod species Limacina helicina are microscopic organisms who live in the water column and build their shells out of calcium carbonate. Little is known about the planktonic foraminiferal faunas in the Barents Sea, and their seasonality in general. This study is the first on planktonic foraminifera and Limacina helicina in an area of intense methane seepage from the seafloor. Sampling at sea took place during spring and summer 2016. We found a significant increase in shell size and concentration (individuals m(-3)) in both planktonic foraminifera and Limacina helicina between spring and summer. In spring, carbon dioxide was being added from the methane plumes, altering the sea-water chemistry at the site of methane release. The addition of carbon dioxide could stimulate primary production in the overlying water column. However, we did not find that the spatial distribution of planktonic foraminifera coincided with where the methane concentration was the highest, or that methane concentration coincided with the elevation of primary production indicators. This study sheds light on the planktonic foraminifera community and their seasonal development in the Barents Sea for the first time with stratified net samples. Key Points Planktonic foraminifera and Limacina helicina increase in size and concentration (no. individuals m(-3)) from spring to summer Methane plumes coincided with elevated dissolved inorganic carbon, low pH, and calcium carbonate saturation Omega No link was found between methane and planktonic foraminifera and Limacina helicina concentrations