A box model approach for a long-term assessment of estuarine eutrophication, Szczecin Lagoon, southern Baltic

We develop a layered "box, model" to evaluate the major effects of estuarin e eutrophication of the Szczecin lagoon which can be compared with integrat ing measures (chlorophyll a (Chl a), sediment burial, sediment oxygen consu mption (SOC), input and output of total. nutrient loads) and use it to hind cast the period 1950-1996 (the years when major increase in nutrient discha rges by the Oder River took place). The following state variables are used to describe the cycling of the limiting nutrients (nitrogen and phosphorus) : phytoplankton (Phy), labile and refractory detritus (D-N, D-Nref, D-P, D- Pref), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus ( DIP), and oxygen (O-2). The three layers of the model include two water lay ers and one sediment layer. Decrease of the carrying capacity with respect to the increased supply of organic matter of the system with advancing eutr ophication over the period studied is parameterized by an exponential decre ase of the sediment nitrogen fluxes with increasing burial, simulating chan ging properties from moderate to high accumulating sediments. The seasonal variation as well as the order of magnitude of nutrient concentrations and phytoplankton stocks in the water column remains in agreement with recent o bservations. Calculated annual mean values of nutrient burial of 193 mmol N m(-2) a(-1) and 23 mmol P m(-2) a(-1) are supported by observed values fro m geological sediment records. Estimated DIN remineralization in the sedime nts between 100 and 550 mmol N m(-2) a(-1) corresponds to SOC measurements. Simulated DIP release up to 60 mmol P m(-2) a(-1) corresponds to recent me asurements. The conceptual framework presented here can be used for a seque ntial box model approach connecting small estuaries like the Szczecin lagoo n and the open sea, and might also be connected with river box models. (C) 2000 Elsevier Science B.V. All rights reserved.