Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 Combatting nutrient spillage in the Archipelago Sea—a model system for coastal management support H. Lauri% H. Ylinen*, J. Koponen*, H. Helminen* & P. Laihonen* ' Environmental Impact Assessment Centre of Finland Ltd, Finland ^ South-West Finland Regional Environment Centre, Finland Abstract The Archipelago Sea is a sea area in the Baltic Sea between Aland and the Finnish mainland strongly influenced by the Baltic Sea forcing. Archipelago Sea waters are relatively clean but threatened from the southern and eastern sides by more nutrient rich waters from the Gulf of Finland and the Baltic Proper, and also from local nutrient loads from Finnish mainland and fish farming. In order to find out the relative impact of different nutrient sources a hydrodynamic model was applied to the area. Modelling the Archipelago Sea required taking into account large scale variations of the Baltic Sea as well as the small scale topography created by the numerous islands, therefore a nested three- dimensional grid with three grid refinement levels was used. Flows in the Archipelago Sea are dominantly wind-induced but affected by salinity and temperature gradients. Computed flows were compared to flow measurements performed in the Archipelago Sea during the open water periods of 1993 and 1994. These simulated flows were then used to model transport in the area to investigate the effect of local nutrient sources and water exchange between the Archipelago Sea and neighbouring sea areas. Transport computations were further verified using salinity measurements. In order to make the model more usable a model management system was constructed. The system simplifies model input data handling, model computation, and result visualisation. As a basis of the user interaction a map- based view of the modelled area is shown, including a set of interactive symbols that allow geographic location-based data access and model parameter manipulation. Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 194 Environmental Coastal Regions HI Introduction Archipelago Sea is one of the most complicated archipelago areas in the world, numerous small and big islands and rocks make the archipelago highly fractal in character. Large open water area and main channels form the central part of the archipelago that is strongly influenced by the Baltic Sea forcing, while intermediate and inner archipelago areas surround the central part on the Aland Island and Finnish mainland sides. Inner archipelago areas are to a large extent isolated from the main Baltic Sea influence, storm events, however, can be important for the exchange of the inner archipelago waters. Figure 1. Archipelago Sea The Archipelago Sea is affected by nutrient transport from the surrounding seas, mainly Baltic Proper, and nutrient loads from land runoff and point sources, and internal bottom load from sediments. Some areas of the Archipelago Sea suffer from excessive eutrofication demonstrating itself as sliming, extensive growth of filamentous algae, oxygen depletion and algae blooms. In order to better understand the origin nutrients causing the eutrofication, the influence of the surrounding seas to the Archipelago sea was investigated by computing the water flows in the area and water exchange with the neighbouring seas. The goal was to achieve information on the relative importance of local nutrient sources compared to long distance transport (Helminen 1998). The flow computations were verified using set of flow measurements performed during open water period in several locations on years 1993 and 1994. In addition to computation results a modelling support system was built to enable generation and investigation of computation results by the end users, that is, the local environmental office. The complete modelling system consist of a hydrodynamic and water quality models, and a model management system Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 Environmental Coastal Regions HI 195 helping users in the complex task of input data management, model running and result visualisation (Lauri 1998). Computational model The hydrodynamic model used is a 3D primitive equation, z-coordinate, free surface model, with constant eddy viscosity coefficients (Koponen 1992, Simons 1980). The model grid covers the whole Baltic Sea with two nestings that locally increase the model resolution. The grid box sizes used are 24, 6 and 1.5 km, where the finest grid covers the Archipelago Sea. Vertically there are ten layers with thickness of one meter at the surface and increasing downwards. Figure 2. Part of the model grid showing the two finest grid nestings. As the whole Baltic Sea was modelled there are no open boundaries. The water exchange through the Danish straits and fresh water river inputs were either neglected or taken into account as constant concentrations, and in the model initial state. As the tide in the Baltic Sea is negligible, the main current driving forcing in is wind. Comparison of computed and measured flows To investigate model reliability model results were compared to measured flows performed with recording current meters at ten-minute intervals. There were three point measurements from summer -93 and four from summer -94. The measurement points were mostly located on straits on the Archipelago Sea. The comparison between computed and measured flows were performed using along the strait flow component, typically the cross-strait flow components were significantly smaller in magnitude. Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 196 Environmental Coastal Regions III Goodness of fit of computed flow to measured flow was estimated using goodness of fit (R?) coefficient. For the two whole measurement periods the R varied from negative values to 0.51 at different points. Below a figure showing comparisons performed using a moving one-week time range for year -94 data, from which periods of good and bad model fit can be identified. As a reference the lighter colored line in the figures identifies the values of R? achieved when using a constant zero as the computed flow. 940606 940613 940620 940627 940704 940711 940718 940603 940605 940607 940600 940611 940613 940615 940617 940619 Bano Kumlinge Figure 3. Time-dependent R2 coefficient between computed and measured flows The model performance goes in some cases below the reference line. However, in this kind of comparison a slight phase error may cause poor values for the R* coefficient. Also this comparison was done with measured point flow data, while the model always computes average flow of 1.5 x 1.5 km grid box. Generally the modelled results compared to measured values rather well, and in some cases the model fit could be called good, taking into account the above limitations. Water exchange Water exchange was computed using an initial state where Baltic, Gulf of Finland, Bothnian Bay and Archipelago Sea were marked individually. The movement of these waters was then followed for eight months in different years. The water exchange varied greatly between different years, mainly due to strong winds occurring most often during spring and autumn. A difference between outer (left) and inner archipelago is demonstrated in the figure below. 1 Gulf of Finlad 0.8 Baltic proper 0.6 Bothnian Bay Archipelago Sea 0.4 0.2 0 940402940502940601940701940731940830940929941029941128 Kihti, outer archipelago Turku, inner archipelago Figure 4. Water exchange timeseries for eight-month simulation. Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9 Environmental Coastal Regions HI 197 Nutrient dispersion To find out the relative importance of local nutrient sources in different areas of Archipelago Sea the local nutrient sources were mapped and divided into three main categories: 1. loads from Finnish shoreline including river discharges as well as municipal and industrial sources, 2. Loads in eastern archipelago including mostly fish farming, and 3. Loads in Ahvenanmaa archipelago. In the figure below the load locations are shown on the map. Distribution of nutrients was then estimated by letting the released tracker spread with the computed flow and plotting timeseries of tracker concentrations on given locations for the eight month simulation period. A clear distinction can be seen between the inner archipelago, where local source are significant due to smaller water exchange and large loads from shore, and the outer archipelago, where local loading disperse quickly. # Ahvenanmaa archipelago m. Finnish shore O Eastern achipelago Point loads on the Finnish shore, Eastern and Ahvenanmaa archipelago • Ahvenanmaa archipelago I I Finnish shore Eastern achipelago 12 140*01 MO701 940731 MM30 140921 M10M Mil Turku, inner archipelago Kihti, outer archipelago Figure 5. Simulated relative active phosphorous fractions from different load areas in two points on Archipelago Sea during eight-month simulation. Environmental Coastal Regions III, C.A. Brebbia, G.R. Rodriguez & E. Perez Martell (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-827-9