Seasonal changes in Baltic Sea seston stoichiometry: the influence of diazotrophic cyanobacteria

We studied nutrient deficiency of Baltic Sea phytoplankton, as indicated by C:N:P ratios in filamentous N-2-fixing (diazotrophic) cyanobacteria and size-fractions of seston. Samples were collected during an annual cycle in the NW Baltic proper (1998, Landsort Deep) and in 2 summers in the central Baltic proper (1997 and 1998, Gotland Basin). Generally, seston C:N:P ratios in the top 20 m were close to Redfield values. There was a transient increase in the C:N ratio at the end of the phytoplankton spring bloom (to similar to 9 mol:mol), concomitant with the depletion of dissolved inorganic N (DIN), but not dissolved inorganic phosphorus (DIP), indicating that phytoplankton became N-limited at the end of the spring bloom. In early summer (May-June), seston C:N:P ratios indicated weak N limitation of the community of small phytoplankton. From June through summer, seston size-fractions < 10 mu m accumulated mainly C and N, resulting in a September peak in seston C:P and N:P ratios (201 and 23 mol:mol, respectively) in the surface water. This change occurred in parallel with an increase in C:P and N:P ratios of diazotrophic filamentous cyanobacteria to levels indicative of severe P limitation of cyanobacterial growth. This suggests that the cyanobacterial P demand and new N from their N-2 fixation caused a weak P deficiency also in non-diazotrophs. However, the small increase in seston N:P, as well as a seston C:N above the Redfield ratio, indicate that N and P are nearly co-limiting for non-diazotrophs at the culmination of the cyanobacterial bloom in late summer.