Model study on the ecosystem impact of a variable C:N:P ratio for cyanobacteria in the Baltic Proper

Abstract

The goal of this work is to estimate the significance of changes in the elemental composition of cyanobacteria on the ecosystem of the Baltic Sea, especially on oxygen depletion in the deep layers of the Baltic Proper, by means of mathematical modeling tools. In solving this problem a coupled physical–biogeochemical model is used. The physical basis for this model approach is the water column model GOTM (General Ocean Turbulence Model, see www.gotm.net). The biogeochemical basis for this model study is the ERGOM model (The Baltic Sea Research Institute’s Ecosystem Model) with some modifications. The ERGOM model was modified to allow the dynamics of N:P and C:P ratios in cyanobacteria. These ratios are controlled by phosphate concentrations in ambient water. Variable C:N:P ratios are used for cyanobacteria, detritus and sediment detritus. It has been found that the changes in the elemental composition of cyanobacteria from constant C:N:P = 106:16:1 ratio to variable C:N:P = (106–400):(16–60):1 ratio lead to larger consumption of oxygen in the deep layers of the Baltic Proper at the long time scale (50 μM after 15 years, that amount to approximately 10%). This may be a considerable value in multi-decadal prognostic model calculations.