Recent work suggests that temperature effects on marine heterotrophic bacteria are strongly seasonal, but few attempts have been made to concurrently assess them across trophic levels. Here, we estimated the temperature sensitivities (using activation energies, E) of autotrophic and heterotrophic microbial plankton net growth rates over an annual cycle in NE Atlantic coastal waters. Phytoplankton grew in winter and late autumn (0.41 ± 0.16 SE d-1 ) and decayed in the remaining months (-0.42 ± 0.10 d-1 ). Heterotrophic microbes shared a similar seasonality, with positive net growth for bacteria (0.14-1.48 d-1 ), while nanoflagellates had higher values (>0.4 d-1 ) in winter and spring relative to the rest of the year (-0.46-0.29 d-1 ). Net growth rates activation energies showed similar dynamics in the three groups (-1.07 - 1.51 eV), characterized by maxima in winter, minima in summer and resumed increases in autumn. Microbial plankton E values were significantly correlated with nitrate concentrations as a proxy for nutrient availability. Nutrient-sufficiency (i.e. >1 μmol L-1 nitrate) resulted in significantly higher activation energies of phytoplankton and heterotrophic nanoflagellates relative to nutrient-limited conditions. We suggest that only within spatio-temporal windows of both moderate bottom-up and top-down controls will temperature have a major enhancing effect on microbial growth. This article is protected by copyright. All rights reserved.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: We are greatly indebted to the Spanish Institute of Oceanography (IEO) for sustaining the RADIALES time-series program, to our colleagues from the Oceanographic Center of Gijón/Xixón, especially Laura Alonso-Díaz and Nestor Arandia-Gorostidi for measuring chlorophyll a and help in the flow cytometer, respectively, and to the crew of R/V “José de Rioja” in her last year of service. This work was supported by the Spanish Ministry of Economy and Competitiveness though the COMITE (Coastal Ocean Microbial plankton and Temperature, CTM-2010-15840) research grant, the PhD scholarship to T.M.H.-S. (BES-2011-048573) and the postodoctoral fellowship to A.C.-D. The authors declare no conflict of interest.