NITRATE AND AMMONIUM DEPLETION RATES AND PREFERENCES DURING A BALTIC SPRING BLOOM

Nitrate and ammonium depletion potential of the planktonic community w as studied during a spring bloom off the SW coast of Finland in the Ba ltic Sea. Depletion rates were measured after nutrient enrichments wit h different concentrations and combinations of NO3, NH4 and PO4. Deple tion kinetics, N source preferences, mutual inhibition, utilization ef ficiency and P stimulation of N depletion were estimated. During the b loom peak, the community showed a depletion potential over 24 h equiva lent to the total wintertime accumulation of inorganic nitrogen. A cle ar succession in N source preferences, mutual inhibition and utilizati on efficiency illustrated the shift from NO3 to NH4 as the basis for n itrogenous nutrition of the community. The succession of these potenti al qualities was gradual, in sharp contrast to the abrupt depletion of NO3 from the water column and to profound changes in the species comp osition of the phytoplankton community. This gradual succession allowe d the community to effectively utilize both NO3 and NH4 as N sources a ccording to their availability. The relative proportions of new and re generated production seem therefore to be regulated mainly by relative supply rates of NO, through physical transport and NH4 through minera lization processes within the community, while inherent preferences of the community play a subordinate role.