Satellite-derived spatial and temporal biological variability in the Cyprus Eddy

Abstract

The cycling of phosphorus in the Mediterranean (CYCLOPS) team investigated phosphate limitation in the Eastern Mediterranean and conducted a phosphate addition experiment in 2002 at the centre of an anticyclonic eddy south of Cyprus. The 2002 and other cruises generated a small database of chlorophyll-a (chl-a) profiles that enabled investigation of the performance of a variety of standard and regional bio-optical algorithms for remote sensing retrievals of chl-a in the region. The standard SeaWiFS OC4V4 and MODIS chlor_a2 algorithms overestimated chl-a as previously reported while a regional algorithm proposed by Bricaud et al. [2002. Algal biomass and sea surface temperature in the Mediterranean basin: intercomparison of data from various satellite sensors, and implications for primary production estimates. Remote Sensing Environment 81, 163–178] and the semi-analytical MODIS chlor_a3 gave improved retrievals. SeaWiFS mean chl-a maps are presented for the Eastern Mediterranean for each month between September 1997 and August 2004 and as multi-annual “climatological” images. The former showed that chl-a in the region decreased over the duration of the time series with reductions in the centre of the eddy, tracked using a quasi-Lagrangian approach, of approximately 33% between 1997 and 1998 and 2002 and 2003. This was not correlated with deep winter mixing represented as heat loss from the sea-surface or dust deposition represented as daily EP-TOMS aerosol index and annual aluminium deposition on the Israeli coast. It is hypothesised that the variations in chl-a are partly a function of the eddy dynamics. Daily SeaWiFS observations show that the 2002 phosphate release was conducted at a period of decreasing chl-a between the winter maximum and summer oligotrophic conditions; however, the rate of seasonal decrease was less than that observed in situ during the week following the phosphate release.