Model estimates of the contributions of interannual changes of climate and nutrient load to changes in the Lake Ladoga ecosystem in 1980–2020

Quantitative estimates of the Lake Ladoga ecosystem response to changes in climatic atmospheric forcing and external nutrient loads were obtained for the period 1980–2020. The estimates were obtained using the MITgcm three-dimensional hydrothermodynamic model combined with the SPBEM biogeochemical module adapted to the phosphorus-limited conditions of the lake.

Numerical experiments were conducted using the following three scenarios: 1) a reference scenario with realistic changes in the external nutrient load and atmospheric forcing for 1980–2020, 2) with realistic changes in the external nutrient load and the “average” intra-annual course of atmospheric forcing for this period, and 3) with realistic changes in atmospheric forcing and a constant external nutrient load equal to the average value for the period under consideration.

The results of the study demonstrate a pronounced dominance of the external nutrient load as the main factor determining the dynamics of the lake ecosystem characteristics in 1980–2020. The contribution of climate change to the highly deterministic linear trends of the change in winter phosphate concentrations, summer phytoplankton biomass, and annual phytoplankton production in the photic layer amounted to only 24 %, 10 %, and 21 %, respectively. According to the calculation results, there is a noticeable decrease in these characteristics of the lake ecosystem in the second half of the considered period 1980-–2020. At the same time, a noticeable (more than 20 %) compensating effect of climate change is noted for phytoplankton production, leveling out part of the effect of the decrease in nutrient load.

It is shown that non-diatoms, which make the main contribution (63 %) to the total phytoplankton production, strongly respond to climatic changes in water temperature in the considered period. Diatoms show less dependence on climate change, maintaining a close relationship with winter phosphate reserves.

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