Numerical modelling of POC yearly dynamics in the southern Baltic under variable scenarios of nutrients, light and temperature

Abstract. This paper presents various scenarios of the particulate organic carbon (POC) in the southern Baltic Sea. The study is based on a one-dimensional Particulate Organic Carbon model (1-D POC). Mathematically, the pelagic variables of 1-D POC model are described by a second-order partial differential equations of the diffusion type with biogeochemical sources and sinks. The POC concentration is determined as the sum of phytoplankton, zooplankton and dead organic matter (detritus) concentrations. The temporal changes in the phytoplankton biomass are caused by primary production, mortality, grazing by zooplankton and sinking. The zooplankton biomass is affected by ingestion, excretion, faecal production, mortality, and carnivorous grazing. The changes in the pelagic detritus concentration are determined by input of: dead phytoplankton and zooplankton, natural mortality of predators, faecal pellets, and sinks: sedimentation, zooplankton grazing and biochemical decomposition.

The 1-D POC model was used to simulate temporal dynamics of POC in the southern Baltic Sea (Gdansk Deep, Bornholm Deep and Gotland Deep) under scenarios characterized by different temperature, nutrients and light. Daily, monthly, seasonal and annual variabilities of POC in the upper water layer are presented for the different scenarios. The starting-point of the numerical simulations was assumed as average values of the investigated pelagic variables for 1965–1998 period. Two- to three-fold increases of POC concentrations in late spring were revealed as well as the shift towards postponed maximum POC concentration. It is speculated that, due to POC increase, oxygenation of under-halocline water layer will decrease, while supply of food to organisms from higher trophic level should increase.