MEPS 120:123-133 (1995)  -  doi:10.3354/meps120123

Nitrate and ammonium depletion potential of the planktonic community was studied during a spring bloom off the SW coast of Finland in the Baltic Sea. Depletion rates were measured after nutrient enrichments with different concentrations and combinations of NO₃, NH₄ and PO₄. Depletion kinetics, N source preferences, mutual inhibition, utilization efficiency and P stimulation of N depletion were estimated. During the bloom peak, the community showed a depletion potential over 24 h equivalent to the total wintertime accumulation of inorganic nitrogen. A clear succession in N source preferences, mutual inhibition and utilization efficiency illustrated the shift from NO₃ to NH₄ as the basis for nitrogenous nutrition of the community. The succession of these potential qualities was gradual, in sharp contrast to the abrupt depletion of NO₃ from the water column and to profound changes in the species composition of the phytoplankton community. This gradual succession allowed the community to effectively utilize both NO₃ and NH₄ as N sources according to their availability. The relative proportions of new and regenerated production seem therefore to be regulated mainly by relative supply rates of NO₃ through physical transport and NH₄ through mineralization processes within the community, while inherent preferences of the community play a subordinate role.

Phytoplankton . Nitrogen sources . Depletion rate . Preference . Inhibition . Spring bloom . Baltic Sea