Benthic‐pelagic coupling and trophic relationships in northern Baltic Sea food webs

Mikko Kiljunen,Maiju Lehtiniemi,Laura Uusitalo,Antti J Rissanen,Jyrki Torniainen,Joanna Norkko,Tuula Sinisalo,Heikki Peltonen,Mervi Kunnasranta,Juha Karjalainen

doi:10.1002/lno.11413

Abstract

AbstractUnderstanding marine ecosystem structure and functioning is crucial in supporting sustainable management of natural resources and monitoring the health of marine ecosystems. The current study utilized stable isotope (SI) mixing models and trophic position models to examine energy flow, trophic relationships, and benthic‐pelagic coupling between food web components. Roughly 1900 samples from different trophic levels in the food web, collected during 2001–2010 from four northern and central sub‐basins of the Baltic Sea, were analyzed for SI ratios of carbon and nitrogen. Trophic structure of the food webs among the sub‐basins was consistent, but there were differences between the proportions of energy in different trophic levels that had originated from the benthic habitat. Mysids and amphipods served as important links between the benthic and pelagic ecosystems. Much (35–65%) of their energy originated from the benthic zone but was transferred to higher trophic levels in the pelagic food web by consumption by herring (Clupea harengus). One percent to twenty‐four percent of the energy consumption of apex seal predators (Halichoerus grypusandPusa hispida) and predatory fish (Salmo salar) was derived from benthic zone. Diets of mysids and amphipods differed, although some overlap in their dietary niches was observed. The food web in the Gulf of Finland was more influenced by the benthic subsystem than food webs in the other sub‐basins. The baseline levels of δ13C and δ15N differed between sub‐basins of the Baltic Sea, indicating differences in the input of organic matter and nutrients to each sub‐basin.

Highlights

Large pelagic marine systems are mainly driven by pelagic primary production, in some cases, for example, areas close to river deltas, substantial amounts of energy can be of terrestrial origin, transported into the system by rivers and through the atmosphere as terrestrial particulate organic matter (POM) (Rolff and Elmgren 2000; Vähätalo et al 2011; Woodland and Secor 2013)
A similar spatial pattern was observed in δ13C0 values of the benthic baseline organism L. balthica
Our results emphasize that benthic and pelagic food webs cannot be treated separately (e.g., Marcus and Boero 1998), and that benthic energy sources have to be accounted for when reconstructing food web and energy flow models in shallow seas

Summary

Introduction

The role of benthic-pelagic coupling has been proposed to be important in shallow marine ecosystems where a large proportion of energy from pelagic primary production settles to the benthic layer (Marcus and Boero 1998; Garstecki et al 2000; Woodland and Secor 2013; Baustian et al 2014; Kopp et al 2015; Duffill Telsnig et al 2018) One such shallow system is the Baltic Sea, which is a semi-enclosed brackish water ecosystem with rather simple food webs, containing roughly a tenth or less of the species richness found in the NE Atlantic (Johannesson and André 2006). As herring is an important food source for aquatic predators, prey switching may cause spatial differences in the strength of coupling between benthic and pelagic pathways, and eventually influence the energy flow dynamics and trophic relationships of the brackish water food webs

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Conclusion