Modeling the response of the Gulf of Finland ecosystem to changing climate

Direct and indirect effects of changing meteorological conditions include alterations in nutrient cycling, timing and extent of algal blooms, species composition and oxygen dynamics of the Baltic coastal ecosystems. In this study the one-dimensional water column model GOTM coupled with a modified version of the biogeochemical model ERGOM was implemented to study the effects of changes in meteorological forcing associated with climate change on dynamics of nutrients, phytoplankton and dissolved oxygen in the central part of the Gulf of Finland. The modeling period from 1997 to 2008 (11 years) was chosen. Initial distributions of salinity, temperature and biogeochemical parameters from available measurement data were prescribed. As a reference run representing the present conditions of the ecosystem, model results with real atmospheric forcing were used. The model results were validated using observational data from the HELCOM monitoring program. The first three years of the simulation were used as a spin-up period. For the rest of the period modeled ecosystem sensitivity to variations in wind speed, air temperature, cloud cover and precipitation were analyzed separately and in a combination. The ranges of variations were chosen in agreement with recent publications on the assessment of climate change in the Baltic Sea region. Results have shown that increase in precipitation does not have any remarkable effect on the ecosystem. Increase in wind speed intensifies water mixing thus providing more nutrients for phytoplankton, but also slightly decreases water temperature. Change in cloud cover negatively affects phytoplankton growth due to decrease in light availability during biologically active period. Increased air temperature influences phytoplankton growth rates, leading to enhanced sedimentation of organic matter and near-bottom oxygen consumption. The scenario which combined all the previous, showed similar results as change in air temperature only, but slightly closer to the reference run due to action of wind speed and cloud cover. The study has shown that regardless of known limitations of one-dimensional models, they are a valuable tool in the investigation of marine ecosystem properties and their sensitivity to changes in the forcing parameters.