Colony size, cell number, carbon and nitrogen contents of Phaeocystis pouchetii from western Norway

Phaeocystis pouchetii is an ecologically important colony-forming marine phytoplankton species in northern hemisphere cold waters. It plays a central role in the processing of biogeochemically important elements in the upper ocean during spring blooms. Here, we report highly significant quantitative relationships among colony size, cell number and particulate carbon and nitrogen contents of field populations of P. pouchetii, which provide the means to quantitatively convert its biological expression into units of biogeochemical significance. Populations were sampled from mesocosms incubated in situ in western Norway and either fertilized with nitrate and phosphate or left unamended. Phaeocystis colony blooms developed in both scenarios, but were much greater in fertilized treatments. Colonies from the latter treatments were larger, contained higher concentrations of cells and more particulate carbon and nitrogen than those in the unfertilized mesocosm. Considering all data, log cell number increased linearly with log colony volume with a slope of 0.54. Log carbon and nitrogen increased with log colony volume, with respective slopes of 0.92 and 1.22, indicating a significant component of non-cellular carbon and nitrogen within the colonies. Carbon and nitrogen contents of colonies were linearly related, and fertilized colonies contained more nitrogen relative to carbon than unfertilized colonies. These equations are particularly applicable to contemporary studies of P. pouchetii because they represent colonies growing in environments that mimic a continuum from natural to eutrophicated ecosystems.