Deep chlorophyll-a maxima (DCMs) in Antarctic waters

Abstract

The US Antarctic Marine Living Resources (AMLR) program has, since 1990, conducted annual surveys from early January through mid-March in a large sampling grid around Elephant Island, Antarctica. Approximately 100 hydrographic stations were occupied twice during each field season, with physical, chemical, optical, and biological data acquired from the surface to 750 m depth or to within 10 m of the bottom at shallower stations. During these 14 years, most of the stations in pelagic waters to the northwest of Elephant Island had very low chlorophyll-a (Chl-a) concentrations (<0.2 mg m−3) in the upper mixed layer (UML) of ~45 m, but a deep chlorophyll-a maximum (DCM) existed between 50 and 100 m, with a peak at approximately 75 m. This was in contrast to adjacent stations which had higher Chl-a concentrations (approximately 1.0 mg m−3) in the UML and no DCM. We provide evidence that the higher Chl-a concentrations that occur at depths of 50–100 m result from increased photosynthetic activity and not from a passive sinking of cells from the UML or by the intrusion of Chl-a rich coastal waters. Data to support this conclusion include (1) elevated dissolved oxygen concentrations between 50 and 100 m, (2) evidence of active photosynthesis at the depth of the DCM as indicated by increased natural upwelling radiation at 683 nm, and (3) water samples obtained from the DCM at 75 m and incubated under simulated conditions of temperature and light had assimilation numbers of approximately 1.0–1.5 mg carbon fixed per milligram Chl-a per hour. DCMs occur in the same depth range as the temperature minimum layer (TML) (the winter remnant of Antarctic surface water, AASW), which is known to have elevated concentrations of inorganic nutrients essential for growth of phytoplankton. Our data indicate that a DCM develops as a result of (1) depletion of iron (Fe) in the UML with the onset of the summer season, and (2) growth of phytoplankton in the TML where both Fe concentrations and solar irradiance levels are high enough to permit an increase of phytoplankton biomass.