Particulate organic carbon in the southern Baltic Sea: numerical simulations and experimental data

Particulate Organic Carbon (POC) is an important component in the carbon cycle of land-locked seas. In this paper, we assess the POC concentration in the Gdansk Deep; southern Baltic Sea. Our study is based on both a 1D POC Model and current POC concentration measurements. The aim is twofold: (i) validation of simulated concentrations with actual measurements, and (ii) a qualitative assessment of the sources contributing to the POC pool. The POC model consists of six coupled equations: five diffusion-type equations for phytoplankton, zooplankton, pelagic detritus and nutrients (phosphate and total inorganic nitrogen) and one ordinary differential equation for detritus at the bottom. The POC concentration is determined as the sum of phytoplankton, zooplankton and pelagic detritus concentrations, all expressed in carbon equivalents. Bacteria are not simulated in this paper. The observed large fluctuations of POC concentrations are attributed to its appreciable seasonal variability. The maximum concentration of POC varied between 870 mgC m(-3) in May and 580 mgC m(-3) in September, coinciding with the period of maximum dead organic matter and phytoplankton biomass concentrations. The results of the numerical simulations are in good agreement with observed values. The difference between the modelled and observed POC concentrations is equal to 3-28% and depends on the month for which the calculations were made, although no time trend of the difference is observed. The conclusion is that the numerical simulations are a sufficiently good reflection of POC dynamics in the Baltic.