Comparision of in situ and satellite-derived chlorophyll pigment concentrations, and impact of phytoplankton bloom on the suboxic layer structure in the western Black Sea during May–June 2001

CTD and chlorophyll data collected during May–June 2001 R.V. Knorr and June 2001 R.V. Bilim cruises have been complemented by satellite altimeter sea-level anomaly, AVHRR sea-surface temperature and SeaWiFS chlorophyll data sets in order to infer temporal evolution and spatial variability of the circulation, water mass, and chlorophyll pigment concentration in the western Black Sea. Four specific features of the upper layer water-column physical and biogeochemical structures have been addressed in the present study. (i) A broader view of the large- and mesoscale-circulation characteristics, and thus more detailed interpretation of the limited set of measurements was provided by the use of satellite data; (ii) no appreciable Cold Intermediate Layer was detected in response to mild winter conditions prevailing over the basin prior to the cruise. The mild winter in 2001 was in fact prolongation of the similar winter conditions persistently observed during the second half of the 1990s; (iii) a weak phytoplankton bloom development was initiated along the peripheral zone towards the end of May, and then expanded over the entire western basin during the mid-June, soon after the completion of the R.V. Knorr surveys. The R.V. Bilim survey was able to partially capture this intense bloom phase. The SeaWiFS chlorophyll algorithm overestimated surface chlorophyll concentrations by a factor of 4 with respect to in situ measurements within four different regions of the western Black Sea (the southern coastal and Rim Current zones, the interior basin and the northwestern shelf); (iv) An immediate impact of the enhanced plankton activity was increased oxygen consumption within the upper 75 m of the water column due to remineralization of the increased flux of particulate organic material. This process apparently caused an upward rise of the upper boundary of the suboxic zone to shallower depths and lower density levels. It was located at sigma-t levels of σt∼15.2–15.3 kg m−3 within the cyclonic western central basin, at σt∼15.4 kg m−3 within the anticyclonic Sevastopol eddy and at σt∼15.6 kg m−3 within the anticyclonic southern coastal zone. The previous assertion of the stability of the SOL upper boundary position at σt∼15.6 kg m−3 irrespective of the seasons and circulation features (i.e. geographical locations) is therefore not supported by the May–June 2001 data set. It suggests that temporal and spatial variability in the SOL occurs during the year depending on varying local conditions imposed by the physical and biogeochemical processes.