engleski

Diatoms in relation to a subsurface chlorophyll maximum layer in the southern Adriatic Sea.

Subsurface chlorophyll maximum (SCM) is common phenomenon in coastal and open waters, and its formation is explained with different mechanisms. In south Adriatic, SCM was not sufficiently investigated. Seven transect cruises were conducted from west (Bari-Italy) to east (Dubrovnik-Croatia) coast of south Adriatic Sea with the objective of determining phytoplankton populations connected to SCM layer. In investigated area, SCM occurred in layer between 50 and 75 m depth, and exclusively during warmer part of the year (May-August) when water column was thermally and halino stratified. In most cases, SCM layer occurred below thermocline and near the end of euphotic zone. Diatoms were the only taxonomic group of phytoplankton present in SCM what points out that they are very well adapted to conditions of low light intensity. Although concentration of chlorophyll a (Chl a) was 3 to 6 times higher in SCM layer than in layer above or below it, densities of diatom populations were only 1.4 times higher. That indicates that increase in cell Chl a content near the end of euphotic zone was one of the factors determining SCM in south Adriatic. Diatom populations in SCM layer during spring were mostly composed of following species: Pseudo-nitzschia delicatissima (Cleve) Heiden, Leptocylindrus danicus Cleve, Chaetoceros anastomosans Grun., Ch. compressus Laud., Ch. vixvisibilis Schiller and Nitzschia sicula (Castr.) Hust.. During summer Leptocylindrus danicus Cleve, Pseudo-nitzschia delicatissima (Cleve) Heiden and Chaetoceros vixvisibilis Schiller were dominant species in diatom populations in SCM layer. In spring and summer time, above SCM layer, dinoflagellates, diatom Thalassionema nitzschioides Grun. and species of genera Rhizosolenia and Guinardia were dominant. In south Adriatic, selective activity of herbivores and position of nutricline are important additional factors that determine position of SCM layer and phytoplankton community composition in water column.

SCM; diatoms; southern Adriatic

nije evidentirano