Alkaline Phosphatase Activity and Relative Importance of Picophytoplankton in Autumn and Early Spring (Mljet Lakes, Eastern Adriatic Sea)

Trophic status, nutrient limitation, alkaline phosphatase activity APA and model which predicts picophytoplankton share in Chl a were tested during autumn and early spring in Mljet Lakes. The ecosystem was confirmed as oligotrophic with mesotrophic conditions developed below 20 m in Small Lake (autumn). The euphotic zone was characterised by frequent P-limitation of phytoplankton growth and absence of P-limitation in PO sub(4) concentration maxima in autumn. Temperature had great impact on chemical and biological variables. Abundance and biomass of picophytoplankton were significantly different between lakes rather than seasons. This was mainly assigned to nutrient limitation and specific trophic interactions which differ two lakes mutually and Mljet Lakes from the coastal sea. Picophytoplankton Chl a was dominantly attributed to autotrophic picoflagellates and did not show deviation from model estimated importance in total Chl a. sAPA maxima in Small Lake were greater than in Great Lake. In autumn it was most probably related to significantly greater abundance of cyanobacteria with low content of Chl a (depth 10 m), while in spring difference was weak and most likely defined by photoadaptation of phytoplankton (low Chl a) and hazardous impacts of UV-B radiation on APA in surface layer. Autotrophic picoflagellates, autotrophic nanoflagellates and nanoplanktonic cyanobacteria showed significant correlations with inorganic nitrogen species. Minimum of average Chl a concentration and APA were found together with maximum of average PO sub(4) concentration (Great Lake, autumn).