Cross-ecosystem analyses of pelagic food web structure and processes in the Baltic Sea

Abstract

This study compares Ecopath models of mass flow of carbon through pelagic food webs in the three major basins of the Baltic Sea: Bothnian Bay (BB), Bothnian Sea (BS) and the Baltic proper (BP) including the Gulfs of Finland and Riga. The carbon flows in the models were estimated indirectly based on monitoring data of bacterial and primary productivity as well as on literature data on predator's diets and size-fractionated primary production. Analysis of the carbon flows suggested that in order to present a good balance between inputs and losses of carbon to each system, the most sensitive factor in the models, e.g. averaged monitoring data on bacterial productivity had to be lowered for the BB and BS, whereas it was raised for the BP basin. The final model configuration resulted in fairly realistic productivity estimates and carbon demands for individual compartments in each area. The supply of carbon via autochthonous primary production was highest in the Baltic proper (192 g C m −2 year −1), whereas it was estimated 3 and 11 times lower in the BS and the BB, respectively. The input of allochthonous sources, via terrigenous dissolved organic carbon as well as advection between basins was relatively higher towards the north, being 7.4, 11.6, and 22 g C m −2 year −1 in the BP, BS, and BB, respectively. Along with the higher allochthonous supply there was a gradual increase in bacterial production relative to particulate primary production since that ratio was 20%, 60% and 160% in the BP, BS and BB, respectively. The relatively higher bacterial production as compared to primary production towards the north of the Baltic Sea resulted in systematic differences in carbon flow between basins. The flow from particulate primary production to the classic food chain (zooplankton and Mysids) and the microbial food web was fairly similar between areas. However, the demand for particulate primary production by the microbial food web was 79%, 54% and 29% in the BP, BS and BB, respectively. The present study thus give further indirect support to the view that the carbon flow through the microbial food web is enhanced in less productive aquatic systems with relatively high input of allochthonous carbon such as the Bothnian Sea and the Bothnian Bay.