AME 39:293-302 (2005)  -  doi:10.3354/ame039293

Enhanced heterotrophic activity in the surface microlayer of the Mediterranean Sea

Ingrid Obernosterer1,*, Philippe Catala1, Thomas Reinthaler2, Gerhard J. Herndl2,Philippe Lebaron1

1Observatoire Océanologique de Banyuls, Laboratoire d’Océanographie Biologique, Université Pierre et Marie Curie, CNRS UMR 7621, BP 44, 66651 Banyuls-sur-Mer Cedex, France
2Dept. of Biological Oceanography, Royal Netherlands Institute for Sea Research (NIOZ), 1790 AB Den Burg, The Netherlands

ABSTRACT: The abundance and activity of heterotrophic and autotrophic organisms were determined in the surface microlayer (SML) at marine and estuarine sites in the NW Mediterranean Sea from May to July 2003. At all study sites, community respiration in the SML exceeded that in the subsurface water (SSW) by factors between 1.7 and 28. The abundance of heterotrophic bacteria and the major groups of the autotrophic community displayed consistent, but low enrichment factors (EF) for the SML (mean 1.3). In contrast, EF for the abundance of heterotrophic nanoflagellates and the concentration of particulate organic carbon were, on average, 4 and 7, respectively. Community respiration rates in the SML were related to the concentration of total organic carbon; however, no relationship was detectable between community respiration and the abundance of the major groups of the heterotrophic and autotrophic community. Net community production, determined at 2 oligotrophic marine sites, displayed negative values for the SML, while in the SSW autotrophic processes balanced or exceeded heterotrophic processes. Similar rates of photochemical oxygen consumption were determined for the SML and the SSW. These results let us conclude that in situ primary production in the SML is not sufficient to support biological and photochemical mineralization at the air–water interface. We suggest that vertical transport of organic matter, mainly in colloidal and particulate form, to the air–water interface is the basis of the food web, thereby supporting net heterotrophy of the SML.

KEY WORDS: Surface microlayer · Community respiration · Net community production · Neuston

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