Experimental recolonization by macrozoobenthos in a bay polluted by municipal sewage, oil and heavy metals compared to an unpolluted Baltic bay

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Abstract
Ambient communities and recolonization pattern of soft-bottom macrofauna was studied at two localities on the Swedish east coast (Baltic Sea). One was polluted by a steelworks and a municipal sewage treatment plant (Oxelösund), and the other was a reference site (Tvären). The naturally occurring macrofauna at the polluted site was less diverse and had lower abundance and biomass compared to the reference site. These differences could, by deduction, be attributed to toxic effects of heavy metals, oils and/or PAH since there were no differences between the sites regarding depth, bottom substrate, oxygen, Eh, salinity or temperature. Recolonization by macrofauna in boxes containing an artificial sediment was faster at the polluted site. Abundance and biomass increased more rapidly and to a higher level compared to the reference site. This could be explained by enhanced nutrition conditions, measured as higher sedimentation rate and a higher nutritive value (organic content and nitrogen) of the sedimenting material. However, diversity was higher at the reference site. Recovery time (defined as time required to reach the abundance, biomass and number of species of the surrounding macrofauna) was shorter at the polluted site. This indicates that the populations/communities of the surrounding sediment, which were adapted to a fluctuating (stressed/polluted) environment, were more resilient than those inhabiting the more stable conditions of the reference site.