Dredged sediment is increasingly being used in mudflat recharge schemes and in habitat restoration/recreation to counter the effect of erosion and sea-level rise. For this reason it is necessary to determine the response of indigenous mudflat fauna to anthropogenic sediment deposition and so in 2001 a manipulative experimental laboratory study was used to assess the biological response to the alternative beneficial use of uncontaminated maintenance dredged material. The experiment assessed the response of common temperate macro-benthic organisms to the addition of increasing amounts of simulated dredged material on to the surface of mudflat cores. Between 0 cm and 20 cm of defaunated sediment was added as both high and low frequency treatments and the vertical migration of species per 1 cm or 3 cm sediment increment was determined. The experiment showed that the bivalve Macoma balthica (Linnaeus) was able to vertically migrate into the surface layers of low or high depositions of sediment treatments but the Spionid polychaete Pygospio elegans (Claparede) and nematodes were less able to reach the surface layers with increased sediment deposition. The oligochaete Tubificoides benedii (Udekem) ability to vertically migrating into larger low frequency depositions of fine-grained sediment treatment placements was less when compared to coarser sand treatments. Hence the study showed that specific errant macro-zoobenthic species vertically migrated through increasing depths of sediment overburden.This concept was investigated further in the winter of 2001 as manipulative experimental field studies at the Skeffiing mudflats along the Humber Estuary and included an investigation to assess the biological response to increased depositions of simulated dredged material at the high-shore area. The main focus of these studies was to understand the relationship between the amounts of fine-grained simulated dredged material deposition and macro-faunal re-colonization through vertical and lateral migration. Defaunated sediment treatments were added as single low frequency amounts of 27 cm and 50 cm and the ability of macro-invertebrate species to migrate to a natural position within the vertical profile of the manipulated sediment was assessed. The re-colonization of defaunated fine-grained sediment via the below surface horizontal migration of macro-fauna occurred when 27 cm of sediment was deposited and the main colonizers were M balthica, juvenile Tellinacea and T. benedii. The macro-faunal re-colonization of a 50 cm deposition of defaunated fine-grained material occurred within 6 weeks via vertical migration. The main vertical migration colonizers were M balthica, juvenile Tellinacea and T. benedii. The deposition of a single large amount of fine-grained sediment had a detrimental affect on macro-faunal nematode recolonization.Further experimental investigations concerning the impact of burial following the high-frequency depositions of simulated fine-grained dredged material on a temperate intertidal mudflat community during the spring-summer period were carried out during 2002 and 2003. Additionally, the logistics of dredged material deposition at different tidal heights was investigated. This was achieved by examining the responses of key mudflat macro-fauna to burial by manipulated water content of fine-grained sediment treatments deposited at the upper-, high- and mid-shore areas of an estuarine intertidal mudflat and determining the macro-faunal re-colonization potential via settlement from the water column. Tubificoides benedii demonstrated a high ability to colonize an increased sediment water content treatment throughout the experiment when deposited at the high- and mid-shore areas but colonized the upper-shore sediment treatment from the middle to end period of the experiment. The polychaete Hediste diversicolor (O.F. Muller), in particular the juvenile stage demonstrated a good ability to colonize the upper-shore fine-grained sediment treatment. The high-shore early treatment colonizers included T. benedii and nematodes, other species colonized the treatment microcosms from July onwards; T. benedii, H. diversicolor, the Spionid polychaete Streblospio shrubsolii (Buchanan) and the gastropod Hydrobia ulvae (pennant) and M balthica throughout the experiment. At the mid-shore the early colonizers included T. benedii, M balthica, P. elegans and S. shrubsolii. The colonization ability of M balthica, juvenile Tellinacea and H. ulvae were negatively correlated to an increase in sediment water content especially when deposited at the high-shore. Tubificoides benendii was the only species to show a sediment-associated pattern at the high-shore and was positively correlated to the sediment water content of the treatments. When simulated fine-grained dredged material was deposited as small multiple amounts over time, the mudflat height was slowly recharged and allowed to build up, this allowed the gradual macro-faunal re-colonization of the recharge material over time. In general, the deposition of manipulated water content fine-grained sediment treatments did not inhibit macro-faunal recovery. This information may be used during the decision making process upon the feasibility of the alternative beneficial uses of dredged material such as when determining the type of dredged material used during a sediment recharge scheme or during simulated dredged material deposition studies
