Simulation of bioavailable phosphorus and nitrogen loading in an agricultural river basin in Finland using VEMALA v.3

- Modelling of bioavailable nitrogen and phosphorus loading to the Archipelago Sea.
- Phosphate represents 15-55% (average: 38%) of the total phosphorus load.
- Inorganic nitrogen represents 35-85% (average: 77%) of the total nitrogen load.

The water quality model VEMALA v.3 reconciles the complexity of a freshwater ecosystem model with the terrestrial and marine ecosystem models already implemented in Finland. This model unifies VEMALA-ICECREAM, VEMALA-N and VEMALA 1.1 as it uses their independent terrestrial loading and implements a simplification of the biogeochemical model RIVE and phytoplankton model AQUAPHY in the river network. VEMALA v.3 simulates the transport and transformations of nitrate, ammonium, organic nitrogen, phosphate, particulate inorganic phosphorus, organic phosphorus, phytoplankton, suspended sediments and total organic carbon during their travel to the sea. The model's results were satisfactory in the Aurajoki river basin (South-Western Finland) in simulating nutrients daily loads, with all Nash and Sutcliffe coefficients (NSE) ranging from 0.51 to 0.89 against observations, and monthly loads (NSE = 0.4-0.97 against estimations). Simulations of total nitrogen and total phosphorus loads were comparable to estimated annual loads with the exception of a few exceptional years. The quantification of the nutrient cycling river processes were consistent with the Aurajoki river ecosystem with maximum summer rates for mineralisation 0.1 mgC L−1 d−1 and denitrification 55 mgN m−2 d−1 resulting in a loss of 4% of the annual nitrogen load entering the river. VEMALA v.3 unites the previous versions of VEMALA to better predict the co-impact of dissolved inorganic nitrogen and phosphate on algal growth and therefore eutrophication. Simulation of bioavailable nutrients rather than total nutrients will allow the distinction in the quality of the various loading sources, farming actions and loading reduction actions. In the Aurajoki simulation, nitrate and ammonium represented on average 74% and 3% respectively of the total nitrogen load to the Baltic Sea, while phosphate constituted 38% of the total phosphorus load to the sea. Thus, the biological impact in the river and the sea will be better assessed to help water managers implement the Water Framework Directive in Finland.