Horizontal heterogeneity of seston, organic carbon and picoplankton in the photic zone of Lago Maggiore, Northern Italy

The horizontal heterogeneity of seston, dissolved (DOC) and particulate (POC) organic matter, Chlorophyll-a (Chl) and of autotrophic (APP) and heterotrophic (HPP) picoplankton was evaluated seasonally during a two-year period in Lago Maggiore, a large deep oligotrophic lake in Northern Italy. The dissolved fraction resulted homogeneously distributed in the epilimnion during the whole sampling period. The particulate fraction showed the higher heterogeneity in winter 2000 and 2001. For those years, POC and Chl heterogeneity was, respectively, 3-5 and 5-7 times higher than DOC. A similar trend was evident for Chl and for APP. The spatial heterogeneity of HPP resulted always smaller than the analytical variability, thus preventing any consideration about the spatial distribution of HPP. POC and Chl concentration exhibited a recurrent pattern of their spatial heterogeneity, possibly in relation with the location of the part of the drainage basin conveying higher amounts of nutrients to the lake.