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Role of nutrient supply and loss in controlling protist species dominance and microbial food-webs during spring blooms

The species composition of spring blooms varies over open marine regions, displaying both spatial and inter-annual differences. We used semi-continuous cultures to promote species- specific blooms and investigate associated microbial food-web dynamics and inorganic nutrient utilization. Beginning with high-nutrient, low-biomass water from 13 m depth, we compared the changes that took place over 9 d in 2 treatments: (1) NEW, a 'new-nutrient' treatment that simulated horizontal or vertical advection; every 2 d, both particles and dissolved organic matter were removed and ca. 25% of the volume of the container was replaced with nutrient-rich seawater from 200 m depth. (2) REC, a recycling treatment simulating grazing and sinking losses without nutrient replace- ment, i.e. conditions mimicking sharply stratified water columns; in this treatment the same volume of water was removed, but was then returned to the container following filtration through a 2.0 µm filter. In the NEW treatment, diatoms consumed the added nutrients and dominated the production and biomass of the protist community. Total protist community production in the REC treatment was significantly lower than in the NEW treatment, with either a late or no diatom bloom and prymnesio- phytes such as Phaeocystis spp. attaining higher proportional biomass. Total production rates for heterotrophic protists, bacteria and viruses did not differ significantly between treatments. Nutrient consumption by the ensuing communities differed between the 2 treatments, with a significantly greater proportion of total inorganic nutrients consumed in the NEW than in the REC treatment. The results demonstrate that the character of nutrient supply and loss influences protist community structure and subsequent bulk nutrient utilization.