Differential response of microbial plankton to nutrient inputs in oligotrophic versus mesotrophic waters of the North Atlantic

Abstract

The effects of inorganic and/or organic (glucose+AAs) inputs on phytoplankton and heterotrophic bacteria were assessed, using a microcosm approach, in two contrasting marine environments: an open ocean oligotrophic site (North Atlantic Subtropical Gyre) and a highly productive coastal embayment (Ría de Vigo, NW Spain). Overall, changes in microbial plankton biomass were smaller than those of metabolic rates. The largest increases in primary production, bacterial production and community respiration were measured in response to mixed () nutrient additions in both sites. Primary production responded to additions only in oligotrophic waters. The distinct autotrophic responses to nutrient additions measured in these environments were related to the different initial composition of phytoplankton populations and, presumably, also to differences in grazing pressures in both marine ecosystems. Heterotrophic bacteria were limited by organic substrates in both ecosystems, although mixed additions further enhanced bacterial growth in the subtropical gyre. The differences detected in bacterial responses to nutrient additions may be related to differences in nutrient limitations and to the prevalence of different relationships between components of the microbial food web (e.g. coupling between heterotrophic bacteria and phytoplankton and predation pressure) in both environments. We found a more relevant role of inorganic nutrients in controlling the efficiency of bacterial growth in oligotrophic regions as compared with highly productive systems. Our results suggest that organic matter inputs into both ecosystems might result in a tendency towards heterotrophy and in increases in bacterial growth efficiency.

Keywords:

• Microbial plankton
• nutrients inputs
• bacteria
• phytoplankton
• organic nitrogen
• microcosms

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

We thank all the people involved in the CARPOS and AddEx projects who helped with the preparation and sampling of the experiments. We thank X.A. Álvarez-Salgado and V. Vieitez for their help with nutrient concentration measurements. We are grateful to D. López Sandoval for her help with drawing maps. We acknowledge A.R. Larrinaga, J.M. Matías and C. Olabarria for their advice with the data treatment. We thank Estación de Ciencias Mariñas de Toralla (ECIMAT) for technical support during the experiments. We acknowledge the crew of the B.O. Hespérides and the R/V Mytilus, for their help during the work at sea. This research was supported by the Xunta de Galicia AddEx contract (PGIDIT06PXIB312222PR). S.M-G. was funded by a F. P. U. MEC fellowship. E.T. was funded by a Juan de la Cierva and a Ramón y Cajal-MEC contract.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark