John Widdows

Impact of biotic and abiotic processes on sediment dynamics and the consequences to the structure and functioning of the intertidal zone

Abstract This paper reviews field and laboratory studies using flumes to quantify the erodability of undisturbed intertidal sediments as a function of changes in (1) the natural benthic community structure and sediment properties, and (2) the abundance of key intertidal species. Sediment erodability, which varies spatially and temporally, is dependent on the interactions between physical processes, sediment properties and biological processes, particularly the balance between two functional groups of biota, the stabilisers and the destabilisers. Bio-stabilisers can influence the hydrodynamics and provide some physical protection to the bed (e.g. mussel beds, macroalgae, salt marsh macrophytes), or can enhance cohesiveness and alter the critical erosion threshold (e.g. microphytobenthos). In contrast, bio-destabilisers (e.g. bioturbators such as Macoma balthica, Hydrobia ulvae ) increase surface roughness, reduce the critical erosion threshold and enhance the erosion rate. Field studies in the Humber (England) and Westerschelde (Netherlands) have shown that interannual changes in sediment erodability were a result of a shift from a stabilised sediment dominated by microphytobenthos to a destabilised sediment dominated by M. balthica . Interannual changes in key biota, their influence on sediment erosion, and the consequences for intertidal ecology and morphology, appear to be driven in part by climatic factors (primarily a shift from mild to cold winters). Quantification and understanding of these benthic processes has been used to parameterise mathematical models of intertidal sediment dynamics, and this has provided insight into the relative importance of biological and physical factors in determining sediment erosion/accretion in the intertidal zone.

Physiological and biochemical responses of bivalve molluscs to exposure to air

Abstract 1. 1. Aerial rate of oxygen consumption by Mytilus edulis and M. galloprovincialis is 4–17% of the aquatic rate. 2. 2. For Cardium edule and Modiolus demissus the aerial rate of oxygen uptake is between 28 and 78% of the aquatic rate. 3. 3. These species differences are related to the degree of shell gape during air exposure. 4. 4. All species show an apparent oxygen debt after exposure to air, the extent of which is not simply related to either the level of aerobic respiration or the degree of anaerobiosis during exposure. 5. 5. Anaerobic end-products accumulate in the tissues of Mytilus during aerial exposure, but not in Cardium. 6. 6. The relative energy yields by aerobic and anaerobic means in M. edulis are discussed.

EFFECTS OF POLLUTION ON THE SCOPE FOR GROWTH OF MUSSELS (MYTILUS GALLOPROVINCIALIS) FROM THE VENICE LAGOON, ITALY

Mussels (Mytilus galloprovincialis) were sampled from six sites in the Venice Lagoon in May 1993 for measurement of their physiological responses (clearance rate, absorption efficiency and respiration rate) under standard laboratory conditions. Scope for growth (SFG) calculated for a ‘standardised ration level’ (0.4 mg particulate organic matter l−1) reflected the underlying pollution induced stress caused by the toxicants accumulated in the body tissues. Mussels from Chioggia had the highest SFG (16 J g−1 h−1) with slightly lower rates at Lio Grande, Crevan and Alberoni (9–10 J g−1 h−1), whereas mussels from the inner part of the lagoon (Salute and Canale Vittorio Emanuele) had significantly reduced SFG values (2 and −4 J g−1 h−1) which indicated severe pollution induced stress. There were significant negative correlations between SFG and the tissue concentrations of petroleum hydrocarbons, PCBs, DDT and HCH, but there were no significant correlations between SFG and the tissue concentrations of metals (Cd, Co, Cr, Cu, Fe, Hg, Mn, Fe, Ni, Pb and Zn). SFG was also calculated on the basis of measured POM levels prevailing at each site and this demonstrated that the inhibitory effects of toxicants on the growth of mussels living in the more polluted inner sites may be partially offset by the positive effects of eutrophication and the higher food/ration levels at these sites. There were also differences in the physiological responses and SFG of mussels from the two main cultivation sites within the Venice Lagoon (i.e. Alberoni < Chioggia) and these reflected the higher contaminant levels and lower food levels at Alberoni.

Use of annular flumes to determine the influence of current velocity and bivalves on material flux at the sediment-water interface

A benthic annular flume for both laboratory and in situ deployment on intertidal mudflats is described. The flume provides a means of quantifying material flux (i.e., biodeposition of suspended particulates, sediment resuspension, nutrients, oxygen, and contaminants) across the sediment-water interface in relation to changes in current velocity and benthic community structure and/or population density of key macrofauna species. Flume experiments have investigated the impact of the infaunal bivalveMacoma balthica and the epifaunal bivalveMytilus edulis on seston and sediment flux at the sediment-water interface. The bioturbatorMacoma was found to increase the sediment resuspension and/or erodability by 4-fold, at densities similar to those recorded at the Skeffling mudflat (Humber estuary) (i.e., >1000 individuals m−2). There was a significant correlation between sediment resuspension andMacoma density (r=0.99; p<0.001), which supported previous in situ field observations indicating bioturbation byMacoma enhanced sediment erodability. Biodeposition rates (g m−2 h1) ofMytilus edulis andCerastoderma edule were quantified and related to changes in population density in a mussel bed (Cleethorpes, Humber estuary). Biodeposition rates were up to 40-times the natural sedimentation rates. At the highest mussel bed densities (i.e., 50–100% cover or >1400 mussels m−2) the physical presence of this epifaunal bivalve on the sediment surface reduced erosion by 10-fold. The shift from net biodeposition to net erosion occurred at current velocities of 20–25 cm s−1. These results demonstrate that infaunal and epifaunal bivalves can have a significant impact on seston flux or sediment deposition and on sediment resuspension or erodability in estuaries where there are extensive mudflats.

Role Of Physiological Energetics In Ecotoxicology

1. The role of physiological energetic measurements combined with chemical analyses of contaminants in body tissues of mussels in fundamental toxicological studies and pollution monitoring programmes is outlined. 2. Important features of this toxicological approach are briefly reviewed, including aspects of bioaccumulation, sensitivity, quantitative concentration-response relationships, QSARs, mechanistic interpretation, ecological relevance, integration of the consequences of multiple mechanisms of toxicity and effects of contaminant mixtures and application to laboratory and field studies. 3. This review focuses particularly on recent advances in understanding and predicting the effects of complex mixtures of contaminants.

Measurement Of Physiological Energetics (Scope For Growth) And Chemical Contaminants In Mussels (Arca-Zebra) Transplanted Along A Contamination Gradient In Bermuda

Mussels (Arca zebra) were transplanted to two sites in Castle Harbour and five sites along a contamination gradient in Hamilton Harbour, Bermuda. After 11–12 days mussels were sampled for measurement of physiological responses (such as feeding rate, food absorption and respiration rate) and analysis of chemical contaminants in their body tissues (metals, alkyltins, hydrocarbons and their polar oxygenated derivatives and polychlorinated biphenyls). Physiological responses were integrated by means of the energy balance equation and performance was assessed in terms of “scope for growth”. Mussels sampled 50 m from the Castle Harbour “dump site” showed a slight, but not significant (P > 0.05), decline in scope for growth and a slight increase in accumulated contaminants (tributyltin, petroleum hydrocarbons and PCBs) in comparison with the Castle Harbour “reference site”. In contrast, mussels sampled from sites along the length of Hamilton Harbour showed a marked decline in scope for growth (P < 0.05), due to a reduction in feeding rate and an increase in metabolic energy expenditure. This was accompanied by a significant accumulation of contaminants (Pb, tri- and di-butyltin, petroleum hydrocarbons and their polar oxygenated derivatives and PCBs). There was a significant negative correlation between the scope for growth of Arca and the concentration of Pb (r = −0.76), TBT (r = −0.91), aromatic hydrocarbons (r = −0.92) and PCBs (r = −0.95) in their tissues. Arca accumulated petroleum hydrocarbons and their polar oxygenated derivatives to tissue concentrations that were sufficient to explain the recorded decline in feeding rate, through the mechanism of “nonspecific narcosis” (based on relationships established for the mussel, Mytilus edulis). Similarly, tributyltin was accumulated to concentrations that could induce the observed increase in energy expenditure, through the mechanism of uncoupling of oxidative phosphorylation. Toxicological interpretation of the coupled physiological and tissue residue chemistry data therefore indicates that petroleum hydrocarbons and tributyltin are the major toxic contaminants causing the reduction in scope for growth of Arca in Hamilton Harbour.

Effects of tributyltin and dibutyltin on the physiological energetics of the mussel, Mytilus edulis

Abstract Sublethal physiological responses (clearance rate, oxygen uptake, absorption efficiency and scope for growth) of mussels (Mytilus edulis) were measured following exposure to tributyltin (TBT) and dibutyltin (DBT). Effects were related to the concentration of TBT and DBT in the mussel tissues. Rate of oxygen uptake increased two-fold with increasing TBT concentration from 0·5 to 10 μg TBT g−1 (dry weight). Clearance (= feeding) rate was significantly reduced above a threshold concentration of 3 to 4 μg TBT g−1. Absorption efficiency was unaffected at concentrations below 10 μg TBT g−1. The overall effect, in terms of scope for growth, indicated a severe inhibition of growth above 4 μg TBT g−1. DBT was shown to be an order of magnitude less toxic than TBT, when expressed in terms of the tissue concentration inducing toxic effects. The threshold concentrations causing a reduction in the rates of feeding and oxygen uptake were 23 and 36 μg DBT g−1, respectively. These sublethal effects are discussed in relation to mechanisms of toxicity, previous alkyltin studies and environmental concentrations.

Quantitative structure-activity relationships for the effect of hydrophobic organic chemicals on rate of feeding by mussels (Mytilus edulis)

Abstract The effect of hydrophobic organic chemicals on the rate of feeding by mussels ( Mytilus edulis ) was investigated. The effect was expressed as the toxicant concentration in water required to reduce feeding rate by 50% (WEC 50 ). A quantitative structure-activity relationship (QSAR) was derived in which WEC 50 was negatively correlated with log 10 octanol-water partition coefficient (log K ow ) and positively correlated with aqueous solubility, indicating that hydrophobicity has a major influence on toxicity. QSARs calculated between bioconcentration factor, and log K ow and aqueous solubility showed, that hydrophobicity influences toxicity largely through its effect on bioconcentration. This observation was confirmed by expressing toxicity as the toxicant concentration in mussel tissue required to reduce feeding rate by 50% (TEC 50 ). For the compounds tested which have log K ow values 50 was relatively constant, irrespective of molecular structure. Compounds with log K ow values > 5 could be accumulated to much greater concentrations before feeding rate was affected, indicating that there is a ‘molecular weight cut-off’ in the toxicological response. These observations are characteristic of a non-specific narcotic mode of toxic action. The application of the reported QSARs to interpreting results derived from combined chemical contamination and biological effects environmental monitoring studies with mussels is discussed.

Temporal and spatial variation in levels of alkyltins in mussel tissues: A toxicological interpretation of field data

This study presents results of mussel tissue tributyltin (TBT) analyses from four study areas; the Lynher River, Cornwall, UK; Sullom Voe oil terminal, Shetlands, UK; Langesundfjord, Norway; Hamilton Harbour, Bermuda. To illustrate temporal variations in TBT concentrations, mussels Mytilus edulis were sampled and analysed for alkyltins at 1–3 month intervals between 1987 and 1989 from the Lynher River. The results show an inter-annual variation in [TBT] of lower winter values and higher late-spring-early-summer values, consistent with probable inputs from seasonal recreational boating superimposed on a background of dockyard-shipping TBT inputs and seasonal water exchange variations. An overall decrease in TBT inputs from 1987 to 1989 was observed. For the remaining three study areas, the spatial variation in tissue TBT concentration was related to scope for growth (SFG), a measure of net energy available for growth and reproduction. For both Sullom Voe oil terminal and Langesundfjord, Norway, the measured TBT concentration in M. edulis from the field were below the TBT concentration shown to have threshold of effect on SFG in laboratory studies. In both cases, observed variations in SFG could be related to other toxicants such as aromatic hydrocarbons. In Hamilton Harbor both petroleum-derived toxicants and TBT were identified as the major toxic contaminants causing a reduction in SFG in Arca zebra. Toxicological interpretation of the contaminants and their effects on the components of SFG indicated that TBT accounted for 35% of the observed decline in SFG from the innermost harbour site and petroleum-derived toxicants, the remaining 65%. The overall results show that it is possible to use toxicological relationships between tissue TBT concentrations and a physiological stress indicator such as SFG to interpret the likely toxic effects of TBT in field populations of mussels.

Disruption of swimming in the hyperbenthic mysid Neomysis integer (Peracarida: Mysidacea) by the organophosphate pesticide chlorpyrifos

Mysids are used routinely by regulatory authorities for conducting 96 h LC50 toxicity tests to evaluate the potential hazards of pollutants to aquatic ecosystems. Data from these acute tests suggest that the European estuarine mysid Neomysis integer (Peracarida: Mysidacea) is comparatively sensitive to trace metals and organophosphate pesticides, and may be a suitable alternative to the frequently-used sub-tropical American mysid Americamysis (≡Mysidopsis) bahia for testing the toxicity of chemical contaminants to European estuarine biota. There is, however, growing demand for the development of toxicity tests which are more representative of the effects of toxic contaminants on natural populations, and which provide results that are more readily extrapolated to natural ecosystems, than acute tests. Behavioural disruption, particularly of swimming ability, is used increasingly in laboratory toxicity studies as a sensitive endpoint for assessing the effects of contaminants on aquatic biota. This paper describes a sensitive laboratory technique, using an annular flume, to determine the effects of an organophosphate pesticide on the swimming behaviour of N. integer. Following 7-day exposure to 0.038 μg chlorpyrifos l−1, mysids became hyperactive and more swam forward into a slow current (3 cm s−1) than control mysids. Despite this hyperactivity, pesticide-exposed mysids were unable to swim faster than 15 cm s−1, whereas control mysids were able to swim faster than 18 cm s−1. Other changes in swimming behaviour following pesticide exposure included fewer mysids maintaining position, and more mysids swimming with the current at high current velocities (18 cm s−1), than control individuals. These responses of chlorpyrifos-exposed N. integer are predicted to cause reduced ability of N. integer to maintain position in the natural estuarine habitat. Furthermore, the swimming behaviour of N. integer was affected at pesticide concentrations below the 7 day LC50 (0.084 μg chlorpyrifos l−1), highlighting the importance of using sub-lethal toxicity studies for predicting environmental consequences of pollutant discharge.