A conceptual model of the relation between benthic invertebrate communities and eutrophication 

Initial positive effects of eutrophication and organic enrichment on food-limited benthic communities are reflected as higher abundances and biomasses. For example, in the Bothnian Bay, where the background concentrations of nutrients are relatively low, positive relationships between increased nutrients in the water column and benthic communities are observed.

Increasing amounts of nutrients result in a surplus of organic material reaching benthic habitats. This is not tolerated by sensitive, large-sized and longlived species, and the increase in organic enrichment and subsequent disturbance will initially be seen as large fluctuations in benthic diversity, abundance and biomass. Species composition will change as conditions deteriorate, and the advantage gained by smaller-sized, tolerant species will result in decreasing total biomass and diversity of the benthic community. At advanced stages of organic enrichment, most bottom-water oxygen is consumed by the decomposition of organic material (mainly due to bacteria), resulting in hypoxia and anoxia and initiating the release of toxic hydrogen sulphide from the sediments. At these advanced stages of hypoxia and anoxia, macrozoobenthos is eliminated and important ecosystem services are lost.

Perhaps the single strongest factor influencing the biodiversity of benthic communities in the Baltic Sea is the increased prevalence of oxygen-depleted deep water. Hypoxia has resulted in habitat loss and the elimination of benthic macrofauna over vast areas and has severely disrupted benthic food webs. 

A conceptual model describing the relationship between increasing deposition of organic matter (OM) and changes in soft-sediment habitats and macrobenthic communities. S = species, A = abundance, B = biomass.

References

HELCOM 2009a. Eutrophication in the Baltic Sea. An integrated thematic assessment of the effects of nutrient enrichment in the Baltic Sea region. Baltic Sea Environment Proceedings No. 115B.

HELCOM 2009b. Biodiversity in the Baltic Sea. An integrated thematic assessment on biodiversity and nature conservationin the Baltic Sea. Baltic Sea Environment Proceedings No. 116B.

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