Factors controlling the distribution of Marenzelleria cf. viridis, Pygospio elegans and Streblospio shrubsoli (Polychaeta: Spionidae) in the southern Baltic Sea, with special attention for the response to an event of hypoxia

Abstract

The large-scale distribution patterns of the estuarine spionid polychaetes Marenzelleria cf. viridis, Pygospio elegans, and Streblospio shrubsoli were studied in the Pomeranian Bay (southern Baltic Sea) at 20 stations in April 1993 and their relationship with hydrographic factors and sediment composition investigated. The densities of M. cf. viridis and S. shrubsoli decreased rapidly with increasing distance offshore. A corresponding decrease in phytoplankton concentration offshore is suggested as the main cause for the observed spatial patterns. P. elegans was more evenly distributed. At 10 selected stations further samples were collected between April 1993 and July 1996 to study the response of abundance and biomass of the three polychaetes to changes in environmental conditions and to investigate interspecific interactions within the macrofauna. Salinity, pelagic food supply, and sediment parameters did not change. At three of these stations redox potential of the sediment and macrofauna composition were affected by an unusual hypoxic/anoxic event in July/August 1994. All three spionid species were able to survive moderate seasonal hypoxic conditions, but died after exposure to severe anoxia. Special attention was focused on the re-establishment of the spionids. The rate with which M. cf. viridis and P. elegans re-colonised defaunated stations varied between a few weeks and two years. These differences in rate of re-establishment were attributed to the distance from undisturbed recruitment areas, and to the severity of the oxygen deficiency. Increasing densities of P. elegans and S. shrubsoli after the hypoxic event coincided with a reduced abundance of the bivalve Mya arenaria, suggesting a negative interspecific interaction.