Baltic clams (Macoma balthica) have increased due to the eutrophication of the Baltic Sea in areas unaffected by oxygen deficiency. Photo: Eva Ehrnsten.

Eutrophication of the Baltic Sea has led to increased production of algae, which also provide food for benthic fauna, the animals living on the seafloor. On the other hand, excessive amounts of sinking algal blooms has caused vast areas of oxygen-deficient bottoms where no animals can survive. To date our quantitative understanding of benthic faunal production has been limited, impeding our ability to understand links between eutrophication, climate change and biodiversity of seafloor communities, and to make quantitative estimates of the role of benthic fauna for carbon cycling.

New way of analysing monitoring data 

The PhD thesis of Eva Ehrnsten provides a new modelling framework that combines a mechanistic model on biomass production with analyses of long-term monitoring data. Computer modelling was used to study the development of benthic fauna in the past and future.

Eva Ehrnsten, Ph.D. student at University of Helsinki and part of the Baltic Nest Institute and the research initiative Baltic Bridge. Photo: Niklas Björling

Benthic fauna are important for the ecosystem as they aid in the degradation of algae and other organic matter sinking to the bottom, bringing it back into circulation to be used by the food-web of algae, zooplankton and fish. The animals are also themselves a food source for many fish species, such as cod and flatfish. Ehrnsten and colleagues estimate that today, the benthic fauna degrade about a fifth of the organic matter sinking to the bottom.

Possible weakening of benthic-pelagic coupling

In the future, reduced nutrient loads may decrease the production of algae. At the same time, climate change is expected to increase water temperatures in the Baltic Sea. According to the modelling results, this will lead to less food for benthic fauna. Not only is the amount of benthic fauna projected to decrease, but also their role in carbon cycling.

The results from the thesis adds new quantitative information on the role of functional biodiversity for ecosystem processes including the cycling of carbon in seafloor communities of the Baltic Sea.

Authors: Eva Ehrnsten, Alf Norkko & Joanna Norkko


More information:

EHRNSTEN, E. 2019. Quantifying biomass and carbon processing of benthic fauna in a coastal sea - past, present and future. Doctoral thesis, University of Helsinki, Faculty of Biological and Environmental Sciences.