Aspects of the ecology, behaviour and ecosystem effects of the marine polychaete Nereis diversicolor (O.F. Müller, 1776) in the estuaries of south-east England.

Abstract

The marine infaunal polychaete Nereis diversicolor has been shown to restrict the development of pioneer saltmarsh vegetation in south-east England through bioturbation and herbivory while surface deposit feeding. This can have severe implications for saltmarsh conservation as in the UK the greatest loss of internationally important saltmarsh habitat occurs in these estuaries. This thesis addresses several aspects of the ecology of N. diversicolor as an ecosystem engineer. The main aims were to investigate how it disperses and colonises sediments, what food sources support its often dense populations and whether these are enhanced by nutrient pollution, and what other effects on the habitat and other benthic fauna they may have. A 12 month experiment on the Crouch Estuary demonstrated that N. diversicolor disperse throughout the year, from the water column as late larvae and juveniles, and as adults crawling across the sediment surface. Peak dispersal is during late summer, but was not necessarily density dependant and individuals may disperse several times. There may be population differences in the stages of dispersal as funnel trap data from the Thames estuary indicate that N. diversicolor also disperse as much younger larvae. Multiple stable isotope (δ 13C and δ 15N) analyses were used approach was employed to test the hypothesis that N. diversicolor feeding behaviour was affected by sewage inputs, which would encourage surface deposit feeding on organic detritus, microphytobenthos and benthic macroalgae, rather than suspension feeding on phytoplankton and water column organic matter. At polluted sites N. diversicolor were mainly surface deposit feeders feeding on Ulva sp. and sediment organic matter, and at putative clean sites suspension feeding was more important. Their diet varied spatially as in some locations Spartina anglica detritus was important while in others Nereis was a secondary consumer, feeding on the amphipod Corophium volutator. A seasonal study using stable isotopes revealed differences in feeding behaviour between the Wallasea Island managed realignment site, and in an adjacent natural saltmarsh. At the realignment site benthic algae were the important food source, whereas in the mature marsh Nereis consumed predominantly Spartina anglica detritus, C. volutator, and was suspension feeding on water column organic matter. Preventing Nereis from surface deposit feeding within the recharged sediments in the Wallasea Island managed realignment site resulted in a reduction in sediment water content, increased sediment stability and development of diatomaceous mats. However, the exclusion did not provide refuge for young Nereis and bivalves, or encourage the successful establishment of pioneer marsh species (S. anglica and S. europaea agg.).