Phytoplankton response to intermittent stirring and nutrient addition in marine microcosms

Abstract

Eight 30 l cylindric culture vessels were filled with sea water from the Masnou Nautical Harbour, Barcelona (Spain), and subjected to different conditions of stirring and nutrient additions. The contents of 4 of the vessels were stirred vigorously for 15 min every 2 to 3 d. Two of the stirred and 2 of the non-stirred vessels received nitrate, phosphate and silicate nutrient additions at the same time intervals. After an initial bloom dominated by centric diatoms, the sequence of phytoplankton assemblages dominating in different phases of the experiments varied according to treatments. In enriched tubes, total numbers of cells and diatoms were much higher than in non-enriched tubes, and the initial bloom involved higher concentrations of large diatoms. Mixing altered the abundance of several species, but its most apparent effect was proliferation and maintenance, in the vessels that also received nutrient additions, of the large-celled diatom Bellerochea yucatanensis