The fine vertical distribution of phytoplankton groups within the deep chlorophyll maximum (DCM) was studied in the NE Atlantic during summer stratification. A simple but unconventional sampling strategy allowed examining the vertical structure with ca. 2 m resolution. The distribution of Prochlorococcus, Synechococcus, chlorophytes, pelagophytes, small prymnesiophytes, coccolithophores, diatoms, and dinoflagellates was investigated with a combination of pigment-markers, flow cytometry and optical and FISH microscopy. All groups presented minimum abundances at the surface and a maximum in the DCM layer. The cell distribution was not vertically symmetrical around the DCM peak and cells tended to accumulate in the upper part of the DCM layer. The more symmetrical distribution of chlorophyll than cells around the DCM peak was due to the increase of pigment per cell with depth. We found a vertical alignment of phytoplankton groups within the DCM layer indicating preferences for different ecological niches in a layer with strong gradients of light and nutrients. Prochlorococcus occupied the shallowest and diatoms the deepest layers. Dinoflagellates, Synechococcus and small prymnesiophytes preferred shallow DCM layers, and coccolithophores, chlorophytes and pelagophytes showed a preference for deep layers. Cell size within groups changed with depth in a pattern related to their mean size: the cell volume of the smallest group increased the most with depth while the cell volume of the largest group decreased the most. The vertical alignment of phytoplankton groups confirms that the DCM is not a homogeneous entity and indicates groups’ preferences for different ecological niches within this layer.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: We thank L. Díaz and C. Cabeza for their technical assistance with flow cytometry and HPLC pigment analyses, and E. Fernández-Guallart for her collaboration in sampling tasks. C. González-Pola provided the hydrographical data. We are grateful to F. Sánchez for inviting us to participate in the INDEMARES 0710 cruises and chief scientists, colleagues and crew for their collaboration. This study has been financed by projects FAMOSO (CTM2008-062621-C03/MAR, Ministerio de Economia y Competitividad, Spanish Government) and GIDEP (GRUPIN14-144, Principado de Asturias). A. M. Cabello was supported by a Spanish FPI grant (BES-2009-0277194). Two anonymous reviewers provided valuable comments.