Orgaruc ennchment, both natural and anthropogenic, is one of the most common forms of disturbance in the benthos The effects of organic enrichment on the benthos in the vicinity of a large manculture facihty were examined as a general model for enrichment, without the confoundmg effects of toxlcants often associated with anthropogenic inputs Stations nearest the fac~lity were sublect to continuous input of fish feed and fecal matter and gross structural changes in the macrofaunal community ( e g reduced species richness, dominance of opportunislc species) were sim~lar to those commonly reported for other ennched sites More complex community and population responses were Indicated by changes in body size, vertical distnbution of infauna and patterns of trophic dominance Enriched areas are generally assumed to be charactenzed by macrofauna \\nth small body sizes and mean individual size did, in fact decrease m t h proximity to the farm At stations farthest froin the farm however, size distribut~ons were skewed by a few large individuals indicating that trends in mean indimdual size are susceptible to the methodological limitations inherent in adequately sampling rare indimduals Whde lnterspeafic measures of a n ~ m a l size decreased with increasing enrlchment lntras p e a f ~ c measures indicated a tendency for larger ind~viduals to occur at the most ennched sites Ennched areas may represent increased food resources, and thus Increased potential for growth and attainment of larger body sizes in those species capable of exploiting such habitats Organ~cally ennched areas are generally considered to be charactenzed by infauna living at or near the sedimentwater interface Increasing organic enr~chment resulted in the loss of large, deep-dwelling species and dramatically altered the vertical biomass profiles, but because these individuals were numerically few had little effect on the verbcal abundance proflles Changes In trophic dominance did occur along the ennchment gradient, but the general lack of autecological informahon limits current efforts to define trophic groups and assign species to them The utility of trophic grouping approaches in ident~fication of enrichment-induced disturbances is thus limited

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Quantitative examination of macrobenthic community changes along an organic enrichment gradient

The present review provides a framework within which the impact of dredging on biological resources that live on the sea bed ("Benthic" communities) can be understood, and places in perspective some of the recent studies that have been carried out in relation to aggregates dredging in European coastal waters. The impact of dredging works on fisheries and fish themselves, and on their spawning grounds is outside the scope of this review. We have, however, shown that empirical models for shelf waters such as the North Sea indicate that as much as 30% of total fisheries yield to man is derived from benthic resources, and that these become an increasingly important component of the food web in near-shore waters where primary production by seaweeds (macrophytes) and seagrasses living on the sea bed largely replaces that by the phytoplankton in the water column. Because dredging works are mainly carried out in near-shore coastal deposits, and these are the ones where benthic production processes are of importance in supporting demersal fish production, our review concentrates on the nature of ben thic communities, their sensitivity to disturbance by dredging and land reclamation works, and on the recovery times that are likely to be required for the re-establishment of community structure following cessation of dredging or spoils disposal. Essentially, the impact of dredging activities mainly relates to the physical removal of substratum and associated organisms from the seabed along the path of the dredge head, and partly on the impact of subsequent deposition of material rejected by screening and overspill from the hopper. Because sediment disturbance by wave action is limited to depths of less than 30m, it follows that pits and furrows from dredging activities are likely to be persistent features of the sea bed except in shallow waters where sands are mobile. Recent studies using Acoustic Doppler Current Profiling (ADCP) techniques suggest that the initial sedimentation of material discharged during outwash from dredgers does not, as had been widely assumed, disperse according to the Gaussian diffusion principles used in most simulation models, but behaves more like a density current where particles are held together during the initial phase of the sedimentation process. As a result, the principal area likely to be affected by sediment deposition is mainly confined to a zone of a few hundred metres from the discharge chute. Our review suggests that marine communities conform to well-established principles of ecological succession, and that these allow some realistic predictions on the likely recovery of benthic communities following cessation of dredging. In general, communities living in fine mobile deposits, such as occur in estuaries, are characterized by large populations of a restricted variety of species that are well adapted to rapid recolonization of deposits that are subject to frequent disturbance. Recolonization of dredged deposits is initially by these "opportunistic" species and the community is subsequently supplemented by an increased species variety of long-lived and slow-growing "equilibrium" species that characterize stable undisturbed deposits such as coarse gravels and reefs. Rates of recovery reported in the literature suggest that a recovery time of 6-8 months is charactristic

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The impact of dredging works in coastal waters : A review of the sensitivity to disturbance and subsequent recovery of biological resources on the sea bed

Feeding behavior and general ecology of several spionid polychaetes from the Chesapeake Bay

Aim This paper evaluates global collection records, evidence of anthropogenic transport methods, and experimental and distributional data relative to temperature requirements to understand the historical and potential dispersal of a well-known genus of estuarine crab. Location The records analysed are from temperate and tropical coastal ocean areas. Methods The study is based primarily on literature analysis and examination of museum specimens. Results The human-mediated successful global dispersal of the European shore crabs Carcinus maenas (Linnaeus, 1758) and C. aestuarii (Nardo, 1847) occurred in three major episodes: around 1800, in the 1850s–70s, and in the 1980s–90s. The nineteenth century introductions occurred through transport by ships (probably in hull fouling or in solid ballast), while the introductions in the 1980s could have occurred through a greater variety of dispersal mechanisms (ships hull fouling and seawater system fouling; fouling on semisubmersible drilling platforms; ballast water; transport with fisheries products intended for food or bait; scientific research; releases from aquaria maintained for educational or scientific purposes; or intentional non-governmental releases for human food production). These introductions have resulted in Carcinus establishment in five temperate regions outside of its native Europe in Atlantic North America, Australia, South Africa, Japan and Pacific North America, while releases into tropical regions have not established populations. C. maenas range in both its native and introduced regions appears to be regulated by similar temperature parameters, enabling an assessment of its potential distribution. Main conclusions The second episode of Carcinus global dispersal, the period from the 1850s to 1870s, may be part of a broader surge of world-wide invasions caused by an increase in shipping.

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Episodic global dispersal in shallow water marine organisms: the case history of the European shore crabs Carcinus maenas and C. aestuarii

The Proterozoic-Cambrian transition records the appearance of essentially all animal body plans (phyla), yet to date no single hypothesis adequately explains both the timing of the event and the evident increase in diversity and disparity. Ecological triggers focused on escalatory predator–prey “arms races” can explain the evolutionary pattern but not its timing, whereas environmental triggers, particularly ocean/atmosphere oxygenation, do the reverse. Using modern oxygen minimum zones as an analog for Proterozoic oceans, we explore the effect of low oxygen levels on the feeding ecology of polychaetes, the dominant macrofaunal animals in deep-sea sediments. Here we show that low oxygen is clearly linked to low proportions of carnivores in a community and low diversity of carnivorous taxa, whereas higher oxygen levels support more complex food webs. The recognition of a physiological control on carnivory therefore links environmental triggers and ecological drivers, providing an integrated explanation for both the pattern and timing of Cambrian animal radiation.

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Oxygen, ecology, and the Cambrian radiation of animals

Vertical migration and mortality of benthos in dredged material—part I: Mollusca

This account describes the comparative response of four species of benthic invertebrates to burial in terms of vertical migration and mortality, and provides a synthesis of studies and recommendations upon which to assess future impacts. The species featured were the bivalve Mercenaries mercenaria, the amphipod crustacean Parahaustorius longimerus, and the polychaetes Scoloplos fragilis and Nereis succinea.



There was evidence of synergistic effects on burrowing activity and mortality with changes in time of burial, sediment depth, sediment type and temperature. In sediment with silt-clay, N. succinea was the most resistant species followed by M. mercenaria, S. fragilis and P. longimerus. In sediment without silt-clay the order of percent mortality was reversed.



Studies of surface water chemistry and sediment geochemistry showed that dissolved oxygen decreased significantly and ammonia and sulfide increased significantly between the surface and below 2.0 cm within a 15-day period. Based on these results and physiological tolerances from the literature it was concluded that M. mercenaria and N. succinea would be relatively resistant to chemical effects of spoil disposal, whereas S. fragilis and P. longimerus would be less resistant to such effects.



Vertical migration of benthic invertebrates through dredge disposal can be a viable mechanism of recolonization under certain conditions. Some effects of burial of benthos can be predicted based on morphology, behavior and physiology. These biological features were discussed with examples dealing with molluscs, crustaceans, and polychaetes. Finally, recommendations were made concerning the type of studies to provide additional data to aid management agencies in decision making about future dredging and disposal practices.

Vertical Migration and Mortality of Marine Benthos in Dredged Material: A Synthesis

During two consecutive summers, the first quantitative bay-wide survey (207 stations) of benthic invertebrates was conducted in Delaware Bay (USA). In 1972, 109 species were collected at 105 stations; and in 1973, 125 species were collected at 102 stations. A total of 169 different species were collected for both summers. The number of species and number of individuals increased with increasing salinity and increasing median grain size. These relationships were compared and were found similar to those in estuaries and bays throughout the world. Average density was 722 individuals m2, which is low compared to other estuaries. The relationship of low secondary production to pollution, macroscopic algae, sediment transport, predation, and hydrography is discussed. Deposit feeders comparised the major feeding type. Local species composition was similar to that in Chesapeake Bay, and dominant species occurred in estuaries throughout the Mid-Atlantic Bight. The benthic invertebrates of Delaware Bay were related to the cosmopolitan mode of estuarine faunas. Faunal assemblages were identified by cluster analysis. The assemblages were associated with sediment type and salinity. It was concluded that Delaware Bay comprises a mosaic of animal assemblages, some of which have relatively sharp boundaries similar to classical level bottom type communities, whereas the boundaries of others are almost impossible to detect, and these represent species distributed along an environmental continum.

Benthic invertebrate assemblages of Delaware Bay

Vertical migration and mortality of benthos in dredged material: Part III—polychaeta☆

Based on 408 quantitative samples distributed over two seasons (winter and spring) and 42 stations, from Georges Bank, New England, USA, the feeding guild classification presented by Fauchald and Jumars was examined. Hypotheses involving the association of particular feeding guilds with environmental variables were posed and tested. In this study, herbivore, motile, jawed (HMJ); filtering and surface deposit feeding, sessile or discretely motile, and tentaculate (F-SD-SDT); burrowing, sessile, non-jawed (BSX); surface deposit feeding, motile, non-jawed (SMX); carnivore, motile, jawed (CMJ), and filter feeding, discretely motile, non-jawed (FDT) were the major feeding guilds of polychaetes. There were more significant associations between feeding guilds and depth, dissolved oxygen, and mean phi and gravel than any other environmental variables. Some significant relationships between feeding guilds and depth, fine-grained sediment, nutrition in sediment (carbon, nitrogen, bacterial biomass, and microbial biomass) emerged. Burrowing, motile, non-jawed (BMX), surface deposit feeding, discretely motile, tentaculate (SDT) and FDT increased with depth. BMX, BSX, and FDT increased with carbon, microbial biomass, and nitrogen, respectively. Some preliminary polychaete feeding surfaces were posed. Nantucket Shoals and Georges Bank were characterized by HMJ and F-SD-SDT; Northern Slope by FDT; Southern Slope by FDT and HMJ; Southwestern Slope by HMJ, SMX, BMX; and the Gulf of Maine by F-SD-SDT, SDT and surface deposit feeding, sessile, tentaculate (SST).

Wayne Leathem

Papers

Vertical migration and mortality of benthos in dredged material—part I: Mollusca

Vertical Migration and Mortality of Marine Benthos in Dredged Material: A Synthesis

Benthic invertebrate assemblages of Delaware Bay

Vertical migration and mortality of benthos in dredged material: Part III—polychaeta☆

Polychaete feeding guilds from Georges Bank, USA