THE RELATIVE INCREASE OF LARGER PHYTOPLANKTON IN A SUBSURFACE CHLOROPHYLL MAXIMUM OF THE NORTHERN ADRIATIC SEA

The relative contribution of autotrophic picoplankton, nanoplankton an d microplankton to the subsurface chlorophyll maximum (SCM) biomass of the northern Adriatic Sea was analyzed over 3 years, and related to w ater column distributions. Sampling was conducted along a north-south trophic gradient during the stratified summer period. Using fluorescen t and inverted microscopy, the plankton are presented as size classed cell density, cell volume, chlorophyll a biomass and (where possible) species composition. Estimates of carbon biomass were developed using factors from the literature. The SCM was usually located between 40 an d 60 m, well below the pycnocline, at or slightly above the nutricline , and very close to the depth of 1% surface photosynthetically active radiation (PAR). The vertical distribution of biomass was relatively i ndependent of the density structure of the water column. The SCM resul ted from about a 2-fold increase in phytoplankton carbon biomass and a bout a 4-fold increase in chlorophyll a relative to the upper water co lumn. Although picoplankton represented about half of the total carbon and chlorophyll a at the SCM, an similar to 2-fold increase in the re lative contribution of nanoplankton and microplankton carbon and micro plankton chlorophyll a was observed compared to the rest of the water column. At the SCM, an 8-fold increase in the volume and carbon biomas s contribution by picoeukaryotes, relative to picoprokaryotes, was obs erved compared to the upper water column. The vertical distribution of nutrients and dissolved oxygen implied that the SCM was a region of s ignificant 'new' primary production supported by 'new' nutrients from subeuphotic depths and/or the bottom layer. Associated microheterotrop hs followed a pattern very similar to the vertical distribution of the microautotrophs, exhibiting a subsurface population maximum below the 40 m depth. The SCM represents a layer of higher prey density which m ay enable predators to graze with higher energetic efficiencies during a season when prey densities are low, and of smaller size, in the res t of the water column.