The effects of the presence of seagrasses (Zostera SSP.) on benthic intertidal invertebrate communities

Abstract

1. A quantitative investigation of the benthic macrofauna associated with seagrass swards was carried out over a period of four months, on the intertidal mud flats at Lindisfarne, Northumberland. 2. Two species of seagrasses belonging to the genus Zostera are present at this site: Z.noltii, a perennial species which overwinters as rhizomes, and Z.marina, an annual germinating each year from seed. 3. Core samples were collected from three midshore sites at Lindisfarne and from three different habitat types, namely swards of both Zostera species and from adjacent bare mud surfaces. The samples were removed to the laboratory and carefully washed through a sieve of mesh size 40 squares per inch, to extract the macrofauna. 4. Comparisons were made between the Invertebrate faunas associated with the vegetated and unvegetated sites, as well as between the two species of Zostera. 5. Several of the more obvious environmental variables at the sites were measured - Zostera blomass, detrital biomass, RPD depth and sediment granulometiy - in order to Identify any possible causal agents of these variations in community composition. 6. Differences were found in both the densities and the biomass of invertebrates present between vegetated and unvegetated sites and also between the two seagrass species. The highest densities were recorded in Z.noltii samples, due largely to the abundance of oligochaetous annelids. Zmarma cores, despite having the lowest total densities of benthic macrofauna, showed the highest diversity and biomass of invertebrates. 7. Epifaunal taxa, especially the isopod Idotea baltica and the gastropod molluscs, were generally more abundant in the structurally more complex Z.Tnarina sites. 8. Polychaetes were often poorly represented in the study, but two sedentary species - Scoloplos armiger and Ampharete balthica - showed an apparent preference for sediments supporting the aimual seagrass species. 9. The data suggested that Z.noltii sediments maintained an invertebrate community somewhat intermediate to those of bare mud and Zmarina samples. 10. Statistical analysis of the data gathered on several environmental variables, highlighted a number of differences between the different sample sites and also the different habitat types. 11. The influence of the seagrasses on the composition and nature of the sediments was found to account for approximately 26% of the observed variation in species’ densities. 12. The presence of Zostera species on the surface of intertidal sediments was therefore seen to play a potentially important role in determining the composition of benthic macrofaunal communities.