Ecological investigations of blooms of colonial Phaeocystis pouchetti—I. Abundance, biochemical composition, and metabolic rates

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Abstract
A winter bloom of the colonial stage of the prymnesiophyte Phaeocystis pouchetit was studied in the 13-m3 mesocosms of the Marine Ecosystem Research Laboratory on Narragansett Bay, Rhode Island The tanks were temperature regulated at 4±2°C but differed in their nutrient concentrations and in situ irradiances. One of the tanks was a control without added nutrients, one received a temporary nutrient spike and two others received daily N/P/Si inputs. Photosynthesis and growth rates of colonies exposed to a range of natural light levels were measured at weekly intervals. Particulate carbon production and release of dissolved organic carbon (DOC) by the entire plankton community was determined concurrently. Photosynthesis and growth rates of Phaeocystis in tanks receiving daily nutrient additions were asymptotic functions of irradiance. Light-saturated rates exhibited asymptotic relationships with dissolved inorganic nitrogen (N) levels. N-Limited populations showed more variable responses. Although irradiance and N availability regulated the population dynamics of Phaeocystis, the presence or absence of silicate (S1) influenced its relative importance in each tank. Phaeocystis dominated community metabolism in the absence of Si, but co-occurred with extensive stands of diatoms when Si was available. A significant positive correlation was found between the contribution by Phaeocystis to community production and the proportion of photosynthate released as DOC In all tanks, Phaeocystis populations exhibited cycles of abundance in which division of cells within colonies preceded the multiplication of colonies. The production of new colonies apparently occurred via two mechanisms: the formation of colonies from solitary cells, and the cleavage of larger colonies into smaller daughter colonies. Phaeocystis in tanks with near undetectable nutrient levels contained C:N, C:Chl a, and C:ATP ratios several times higher than colonies in nutnent-replete tanks. Phaeocystis C:Chl a and C:ATP ratios were substantially greater than those of non-gelatinous phytoplankton due to carbohydrate storage in colony gelatin In contrast, C:N ratios in Phaeocystis and non-gelatinous phytoplankton were similar, suggesting a storage depot of organic N outside of the cells. The results support the notion that Phaeocystis colonies function as biological entities rather than as passive aggregations of cells.

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