Polychaete Abundance Patterns in a Marine Soft‐Sediment Environment: The Importance of Biological Interactions

Abstract

Samples of infauna and measurements of temperature, oxygen, salinity, and algal cover were taken from January 1969 to December 1970 at —1.2—ft tidal elevation in a mud flat dominated by polychaetes in Mitchell Bay, San Juan Island, Washington. Mortality of adults after spawning and variable larval settlement success probably explained much of the variation in population numbers of the four large and numerically important polychaete species, Lumbrineris inflata, Axiothella rubrocincta, Platynereis bicanaliculata, and Armandia brevis. No correlations were found between the abundances of numerically important species and physical factors. Exclosures constructed of 3—mm mesh plastic screening placed on the flat became covered with diatoms. Settling juveniles of tube—building species, such as P. bicanaliculata, Axiothella rubrocincta, and L. inflata, built tubes in this layer of diatoms and thus did not reach the enclosed sediment, while settling juveniles of a burrowing species, Armandia brevis, burrowed through the diatom layer and reached the sediment. Thus, cleaning the cage surfaces or removing the cage after settlement reduced abundances of tube—building species without disturbing the sediment since adults of all three numerically important tube builders experience mortality after spawning. The manipulation of tube—builder abundances showed that the burrowing species responded to space vacated by tube builders by increased settlement success. Results from experimental variation of A. brevis numbers per unit volume of sediment in the laboratory and abundance data from unmanipulated natural areas also demonstrated the presence of interspecific and intraspecific competition for space. Changes in physical factors due to algal cover had some impact on population levels but the competitive interactions and behavior patterns, revealed only by observations on the behavior of living organisms and manipulation of the infauna, demonstrated the importance of biological interactions to the determination of species abundance patterns in a soft—sediment environment.