Seasonal dynamics in colored dissolved organic matter in the Mediterranean Sea: Patterns and drivers

Two autonomous profiling “Bio-Argo” floats were deployed in the northwestern and eastern sub-basins of the Mediterranean Sea in 2008. They recorded at high vertical (1 m) and temporal (5 day) resolution, the vertical distribution and seasonal variation of colored dissolved organic matter (CDOM), as well as of chlorophyll-a concentration and hydrological variables. The CDOM standing stock presented a clear seasonal dynamics with the progressive summer formation and winter destruction of subsurface CDOM maxima (YSM, for Yellow Substance Maximum). It was argued that subsurface CDOM is a by-product of phytoplankton, based on two main characteristics, (1) the YSM was located at the same depth than the deep chlorophyll maximum (DCM) and (2) the CDOM increased in summer parallels the decline in chlorophyll-a. These observations suggested an indirect but tight coupling between subsurface CDOM and phytoplankton via microbial activity or planktonic foodweb interactions. Moreover, the surface CDOM variations observed both by floats and MODIS displayed different seasonal dynamics from what recorded at subsurface one. This implies that CDOM standing stock can be hardly detected by satellite. It is worthnoting that surface CDOM was found to be more related to the sea surface temperature (SST) than chlorophyll-a concentration, suggesting its physical origin, in contrast to the biological origin of YSM and subsurface standing stocks.