Identification and quantification of plankton bloom events in the Baltic Sea by continuous pCO2 and chlorophyll a measurements on a cargo ship

Continuous measurements of the surface water CO2 partial pressure (pCO2) and the chlorophyll a fluorescence were performed in the Baltic Sea using a fully automated measurement system deployed on a cargo ship. The ship commuted regularly at two day intervals between the Mecklenburg Bight (Luebeck) and the Gulf of Finland (Helsinki). The pCO2 data collected during June 2003 and September 2004 were used to identify biological production events such as the spring bloom and the midsummer cyanobacteria bloom in five different sub-regions. To quantify the net biomass production, the decrease of the total CO2, NCT (normalized to a uniform alkalinity), during the production periods was calculated using the pCO2, temperature and salinity records and the mean alkalinity. Taking into account the CO2 air/sea exchange and the formation of dissolved organic carbon, a simple mass balance yielded the net production of particulate organic carbon which represents the total biomass. The chlorophyll a concentrations obtained from the fluorescence data showed peaks that in most cases coincided with the production maxima and thus supported the interpretation of the pCO2 data. The production during both the spring bloom (2004) and the midsummer nitrogen fixation period (2003) increased by a factor of about three from the southwest to the northeast. For the spring bloom our estimates were significantly higher than those based on the winter nutrient supply and Redfield C / N and C / P ratios. This indicated the existence of additional nutrient sources such as dissolved organic nitrogen, early nitrogen fixation and preferential P mineralization. Midsummer NCT minima were observed only in 2003 and used to quantify the nitrogen fixation activity and to characterize its interannual variability.