Mary D. Brinsley

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.

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.

Use of in situ flume to quantify particle flux (biodeposition rates and sediment erosion) for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna

Abstract A portable in situ annular flume was deployed on eight occasions at five stations along a shore normal transect from immediately below the saltmarsh to the mid-tide level of the Skeffling mudflat in the Humber estuary. Both large scale (km) and small scale (m) spatial differences (i.e. between ridges, gullies and pools on ridges) were examined. Biodeposition rates were measured at low current velocities (5 cm s−1), while sediment resuspension and erosion rates were determined in response to step-wise increases in current velocities between 10 and 50 cm s−1 (equivalent to a range in shear stress from 0.02 to 1.6 Pa). Maximum biodeposition rates (6.6 g m−2 h−1) were more than an order of magnitude higher than the mean sedimentation rate in the absence of sediment and biota. There was a significant correlation between biodeposition rate and the density of the suspension feeder Cerastoderma edule (r = +0.90; P < 0.05). The intertidal sites could be separated into three distinct groups on the basis of sediment stability/erodibility. Upper shore sites had the highest stability with critical erosion velocities (ucrit) of 30 cm s−1, whereas the pools and gullies were the most easily eroded with ucrit of 15 cm s−1. Site differences in sediment erodibility were not significantly correlated with measured physical properties of the sediment (bulk density, moisture content, % POM, % sand). However, there was a significant correlation between sediment erodibility and the density of Cerastoderma (r = +0.89; P < 0.01) and an index of the bioturbation activity of Macoma balthica (r = +0.96; P < 0.001).

QSARs for the sublethal responses of marine mussels (Mytilus edulis)

The marine environment is contaminated with many organic compounds, some of which induce deleterious responses in biota. Biological impact can be assessed by measuring the physiological responses of mussels, though the task of establishing which of the bioaccumulated compounds cause the observed effects is complex. To facilitate this task, quantitative structure-activity relationships (QSARs) for the physiological responses are being established. In this paper, the responsiveness of ciliary feeding to alkanes and benzene-substituted alkanes is described and compared with a QSAR established previously for aromatic compounds. Most of the test compounds with aqueous solubilities greater than 70 micrograms dm-3 were toxic to feeding activity when bioaccumulated to similar concentrations, whereas compounds of lower solubility were less toxic. The only exceptions were the polyaromatic hydrocarbons pyrene and fluoranthene, which were less toxic than predicted from their solubility. These data are consistent with the hypothesis that the toxicity cut-off is due to solubility-related phenomena, the effect perhaps being enhanced for aromatic hydrocarbons dosed near to their solubility limits, by sequestration of crystals within the mussel tissues. These observations indicate that many organic contaminants detected by chemical analysis of mussels have no direct effect on filter feeding, whereas the less frequently determined volatile compounds are toxic.

Direct observation of the formation and break-up of aggregates in an annular flume using laser reflectance particle sizing

Experiments have been carried out to examine the aggregation of natural estuarine suspended sediment under controlled conditions in an annular flume programmed to simulate oscillating tidal currents in an estuary. The size distribution of the suspended particles was measured in-situ using a Lasentec P-100 laser-reflectance particle sizer with the sensing probe inserted directly through the wall of the flume. Parallel measurements of the solids concentration were made using a calibrated OBS sensor. The flume was filled with river water collected from above the influence of salt water. Various quantities of natural estuary sediment were added to the flume to provide solids concentrations of nominally 100, 800 and 4000 mg 1 −1 . A series of experiments was performed in which the flume was run through consecutive, four hour cycles where the mean current velocity in the flume changed sinusoidally from 5 to 45 cm sec −1 . For each sediment concentration the experiment was repeated with some of the fresh water replaced by particle-free seawater to give salinities of 0, 0.2, 2.0 and 10. Over a typical velocity cycle, suspended sediment concentrations decreased with decreasing current velocity, initially slowly, and then more rapidly. The concentration and size of material in suspension minimised over the low velocity period as particles settled. After a certain lag, sediment erosion occurred with increasing velocity and suspended solids concentration increased to a point where all the sediment was in suspension. The particle size data showed that during declining velocity conditions the median size of the particles initially increased as velocity decreased and then decreased as settling of the larger particles from suspension outweighed the aggregation process. During the erosion phase the median diameters increased initially but then decreased and levelled off as current velocity increased further. This was interpreted as mobilisation of aggregated particles followed by breakage as velocity increased. In general the degree of aggregation, and thus deposition, increased with salinity and with solids concentration. Solids concentration had by far the greatest effect on aggregation and deposition rate.

An assessment of the potential impact of dredging activity on the Tamar Estuary over the last century: II. Ecological changes and potential drivers

The Tamar estuary is a Special Area of Conservation (SAC) under the EU Habitats and Species Directive and a Special Protection Area under the Wild Birds Directive (1979). The lower Tamar is also the site of the Devonport naval dockyard which requires annual maintenance dredging, as well as occasional capital dredging for new installations. The main objective of this study was to investigate whether there is any evidence of significant temporal changes in key species (intertidal macrofauna, fishes, birds) and habitats (intertidal mudflats and saltmarsh) that could be related to dredging activities in the lower Tamar. Other physical variables, such as Tamar river flow and the North Atlantic Oscillation (NAO) index, representing potential drivers of changes in the abundance of biota, were also examined. Spatial and temporal changes in the abundance of intertidal macrofauna (between 1939 and 2000) were analysed but there was insufficient comparable data to enable us to draw any conclusions about long-term changes in the Tamar estuary. Commercially and ecologically important fish species (salmon and sea trout) showed a steady decline in numbers caught in the Tamar since the 1970’s. There was a significant correlation between the number of salmon caught by rod and the Tamar river flow. The sea trout abundance was significantly negatively correlated with the NAO index, suggesting that sea trout may be adversely affected by mild winters, which have been a feature of the late 1980’s and 1990’s. There were also significant correlations between the number of salmon caught in the Tamar and other rivers of the SW of England. Ten species of wildfowl and wader birds were analysed. There were no significant correlations between over-wintering numbers and dredging activity, but there were significant declines in teal and wigeon over 30 years. These species also showed a negative correlation with the NAO index suggesting the declines were related to the milder winters; possibly reducing their need to migrate south as far as SW England. Aerial photographs of the Tamar showed that the Egypt salt marsh, creeks and mudflat maintained a remarkably consistent structure over a period of >50 years. However, there was evidence of gradual erosion (0.23 m y−1) of the saltmarsh’s front cliff edge that may be related to sea level rise (~1.5 mm y−1 in SW England). The study concluded that there was no evidence of ecological changes related to the dredging activity in the Tamar. However, there were significant changes in salmon and sea trout catches, and the number of over-wintering teal and wigeon, over many decades and these changes appear to be related to large scale climatic events rather than anthropogenic factors within the Tamar estuary.