Lagoonal invertebrates

A range of invertebrates are found within saline lagoons in the UK including a number that are specialised to the habitat. Of these specialist lagoonal invertebrates, the small amount of information found on potential sensitivity to changes in water quality of concerns the starlet sea anemone Nematostella vectensis.

The starlet sea anemone Nematostella vectensis is a small edwardsiid anemone found in sheltered brackish habitats on the Atlantic and Pacific coasts of North America and on the south and east coasts of England. It is plentiful throughout its range in North America, but is considered vulnerable to extinction through habitat loss in England, where its distribution is restricted to brackish and saline ponds or lagoons. At present there is insufficient data to permit informed management of populations of the species and its habitat. It is found in shallow waters on fine sands and muddy sediments with shingle, where the mud has a high organic content. It occurs in shallow brackish waters, with low near-bottom flow rates, and where freshwater inputs are relatively low and regular seawater inflows occur. It is tolerant of conditions where the sediment surface may become anoxic, but where water column oxygen concentration remains high, as it can climb onto macrophytes (such as Chaetomorpha and Ruppia spp.) into oxygenated water. Salinity at the sites in England where it is found varies from 2-42 ppt, with the largest populations found where salinity varies between 16-36 ppt. Nematostella vectensis is eurythermal (-1EC to 28EC). It feeds on juvenile molluscs (Hydrobia and Littorina) and chironomid larvae, as well as ostracods and copepods. Populations at sites studied on the south coast of England reach their highest densities at the end of the year, falling in late winter back to previous spring/summer levels of abundance. These populations have been found to consist entirely of females reproducing asexually (Sheader et al 1997). No information is available on effects of nutrients on Nematostella vectensis.

No information was found concerning the sensitivity to changes in water quality of other specialist lagoonal species such as the polychaete Armandia cirrhosa or the amphipod Gammarus insensibilis. There is also a paucity of studies from saline lagoons of impacts on other invertebrates. However, a number of invertebrate species found in lagoons either occur in estuaries (or indeed are more characteristically estuarine), e.g. the surface feeding spionid worm Streblospio shrubsolii, or form species pairs with estuarine species, e.g. lagoonal cockle Cardium glaucum (cf the closely related common cockle Cardium edule). Observations from estuaries therefore provide an indication of likely effects within saline lagoons. For example, Scott et al (1999) provide an overview of impacts on benthic communities from nutrient enrichment (and/or organic enrichment as the effects of the two are often difficult to separate). Invertebrates are impacted by elevated nutrients particularly through oxygen depletion in the water column and sediments as a result of excessive benthic and planktonic algal growth and decay. Invertebrates may also be impacted by smothering of the sediment by macroalgal mats, as observed in the Ythan estuary where both Corophium and Cardium were affected (Raffaelli et al 1989). Different taxonomic groups appear to exhibit different sensitivity to hypoxia, ie in order of increasing sensitivity from polycheates to molluscs/bivalves to echinoderms and crustaceans (Scott et al 1999).

Studies on the sensitivity of seagrass Zostera to oil pollution referred to above (see Davison and Hughes 1998) highlighted that the associated fauna, including epiphytic grazers, seem to suffer more damage than the eelgrass itself.

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