Increasing temperatures change pelagic trophodynamics and the balance between pelagic and benthic secondary production in a water column model of the Kattegat

Temperature constrains the various processes in marine ecosystems differently and future climate warming of up to 6 °C will therefore change ecosystem functioning at various trophic levels. This study investigates how increased temperatures would change the present time overall trophic interactions with emphasis on the balance between pelagic primary and secondary productions and between pelagic and benthic secondary productions. A 1D coupled hydrodynamic-biogeochemical model was calibrated against data from a monitoring station in the Kattegat from 2004 to 2006 and validated in a hind-cast study for the period 1994–1996. Climate warming scenarios (+ 3 and + 6 °C) showed that the magnitude and duration of the spring bloom were reduced due to higher grazing impact by heterotrophic plankton. Moreover, sedimentation rate decreased up to 44% due to lower sedimentation of phytodetritus after the spring bloom and higher heterotrophic respiration in the water column. The lower food supply to benthos and enhanced respiration due to higher temperatures reduced the biomass of deposit feeders by 23–66% whereas benthic filter feeders were less affected. The onset of the spring bloom was not changed since it was triggered by the light regime above a permanent halocline at 10–15 m depth. This study showed that climate warming presumably will change the trophodynamics of primary and secondary production and will alter the balance of the ecosystem towards a higher pelagic and a lower benthic secondary production.

Research Highlights
► We modelled the effect of increasing temperatures on food web dynamics.
► This was shown to increase the degree of pelagic heterotrophy in the Kattegat.
► Zooplankton grazing pressure terminated the spring diatom bloom 2–3 weeks earlier.
► The benthic–pelagic coupling was weaken.
► The benthos community experienced food limitation and structural changes.