Quantifying feedback of trophic structures on biogeochemical cycles

Food-web structure and processes modulate biogeochemical cycles and material flows, but are only sparingly implemented in state-of-the-art (mainly biogeochemically focussed) ecosystem models. An „end-to-end“ approach requires that we gather data for suspect processes and experimentally determine their impact on the routes and flows of energy and matter in the ecosystem. In this, sensitivity experiments with available models pinpoint weaknesses in current understanding and identify those processes that are responsible for variability at higher trophic levels in an ecosystem, and to their feedback on primary producers and biogeochemical cycles.

Empirical investigations of GENUS focus on a critical trophic level with respect to mass flows, namely pelagic herbivores: Their heterotrophic biomass is sufficiently large to influence lower levels by controlling particle transfer and export, but is in turn regulated by predation of higher trophic levels that strongly modulates their biomass. That is why a shift in bottom-up (biogeochemical) or top-down (predation) control leads to significant effects in overall material cycling at this crucial ecosystem level that may be key to so-called regime shifts. Model output of the NPZD-model ERGOM concerning primary producers will be used as input to a model of trophic energy flows (ECOPATH/ECOSIM) to evaluate possible reactions on higher trophic levels and to improve their representation, which will then be fed back as a boundary condition to ERGOM. Sufficiently long observational data are available to validate these model results, or can be/have been reconstructed from sediments in the shelf mud belt.