Two-way coupling between Ecosim (EwE-F) and a biogeochemical model of the Baltic Sea


Muller-Karulis B., Akoğlu E. , Tomczak M., Gustafsson B.

Ecopath 35 Years: Making Ecosystem-Based Management Operational, Florida, United States Of America, 4 - 11 December 2019, pp.29

  • Publication Type: Conference Paper / Summary Text
  • City: Florida
  • Country: United States Of America
  • Page Numbers: pp.29

Abstract

The Baltic Sea is heavily affected by eutrophication, with ambitious nutrient load reduction schemes in
place to reduce algal growth and the secondary effects of increased productivity, such as increased
turbidity and the spread of anoxic bottoms. Forcing an Ecosim model of the Central Baltic food web with
observed and predicted productivity trends suggest that changes in primary production channel strongly
into the pelagic food web, modifying the predation pressure on phytoplankton grazers.
To include the feedback of higher trophic levels onto primary producers into simulations, we coupled the
Fortran version of Ecopath with Ecosim (EwE-F) to the BALTSEM biogeochemical model of the Baltic
Sea. In their coupled code, both models share phytoplankton and detritus components: BALTSEM
calculates phytoplankton growth based on nutrient concentrations and available light; and as a loss term,
EwE-F provides predation mortality of phytoplankton that is calculated applying the foraging arena theory.
Similarly, EwE-F components produce inputs to BALTSEM sediment and suspended detritus variables,
such as non-predation mortality or unassimilated food, and use the BALTSEM detritus variables as a food
resource.
Since nutrient turnover within the Baltic pelagic food web is about ten times as large as the riverine input
of nitrogen and phosphorus, small imbalances in simulated nutrient fluxes can generate sources or sinks
that easily reach the magnitude of planned nutrient load reductions. To ensure closed nutrient cycles, we
track the nutrient intake for each consumer based on fixed prey stoichiometries. We have introduced
compensatory nitrogen and phosphorus fluxes into EwE-F that describe the excretion of nutrients that are
not incorporated into predator growth, and included nutrient limitation functions for EwE-F consumers.
Coupling the zero-dimensional EwE-F food web model to the one-dimensional, vertically resolvedBALTSEM biogeochemical model creates vertical fluxes between the BALTSEM depth layers. We will
therefore also discuss strategies to describe the transfer of carbon and nutrients between the food web
and the biogeochemical model.