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This study investigates the effects of toxic and non-toxic dinoflatellates on two sympatric bivalves, the clam Mulinia edulis and the mussel Mytilus chilensis. Groups of bivalves were fed one of three diets: (i) the toxic paralytic shellfish producing (PSP) Alexandrium catenella + Isochrysis galbana; (ii) the non-toxic Alexandrium affine + Isochrysis galbana and (iii) the control diet of Isochrysis galbana. Several physiological traits were measured, such as, clearance rate, pre-ingestive selection efficiency and particle transport velocity in the gill. The clearance rates of both M. chilensis and M. edulis showed a significant reduction when fed a mixed toxic diet of 50% Alexandrium catenella: 50% Isochrysis galbana. Similarly, when both species of bivalves were fed with the non-toxic diet (50% A. affine: 50% I. galbana), clearance rate was significantly lower compared with a diet of 100% I. galbana. Under all the experimental diets, M. chilensis showed higher clearance rate values, slightly more than double that of M. edulis. M. edulis and M. chilensis have the ability to select particles at the pre-ingestive level, thus eliminating a larger proportion of the toxic dinoflagellate A. catenella as well as the non-toxic A. affine in the form of pseudofaeces. Higher values of selection efficiency were registered in M. edulis than in M. chilensis when exposed to the toxic diet. Similar results were observed when these two species were exposed to the diet containing the non-toxic dinoflagellate, explained by the fact that the infaunal Mulinia edulis is adapted to dealing with larger particle sizes and higher particle densities (Navarro et al., 1993). The lower transport particle velocity observed in the present work for both species, is related to the reduced clearance rate, the higher particle concentration, and the presence of larger, toxic dinoflagellates. In addition, the species differ in their feeding responses to diets, with and without A. catenella or A. affine, largely reflecting their adaptations to different environmental conditions. The results suggest that the presence of a dinoflagellate bloom, whether toxic or non-toxic spp in Yaldad Bay, is likely to have a greater impact on the Mytilus chilensis than the infaunal Mulinia edulis, based on the combined effects on clearance rate, selection efficiency and particle transport velocity.

The aim of this study was to examine the impact of bioturbation by the Manila clam, Ruditapes philippinarum, on sediment stability. A laboratory benthic annular flume system (AFS) was deployed to evaluate the relationship between sediment stability of a subtidal mudflat and density of the infaunal clam under the influence of different current velocities. There was a significant correlation between mean erosion rate and current velocities in all treatments with clams (p < 0.001). There was also a significant correlation between mean erosion rate and R. philippinarum density (p < 0.001), reflecting bioturbation-enhanced sediment erosion. The effects of clam density on sediment erodability were more marked at the lower current velocities. In the control, the critical erosion velocity (^sub crit^) was about 32 cm s^sup -1^. With increasing R. philippinarum density, ^sub crit^ decreased down to the minimum value of about 20 cm s^sup -1^at a density of 206 clams m^sup -2^. This study demonstrated that the burrowing activity of R. philippinarum reduces sediment stability, particularly at relatively low current velocities (25 cm s^sup -1^) and at densities below those found in the clam cultivation areas within the Sacca di Goro lagoon.[PUBLICATION ABSTRACT]

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 bivalve Macoma balthica and the epifaunal bivalve Mytilus edulis on seston and sediment flux at the sediment-water interface. The bioturbator Macoma 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., $>1000\\ \\text{individuals}\\ {\\rm m}^{-2}$). There was a significant correlation between sediment resuspension and Macoma density (r = 0.99; p < 0.001), which supported previous in situ field observations indicating bioturbation by Macoma enhanced sediment erodability. Biodeposition rates ( g m-2 h1) of Mytilus edulis and Cerastoderma 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 $>1400\\ \\text{mussels}\\ {\\rm m}^{-2}$) 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.

The main objectives of this flume study were to (1) quantify density dependent effects of the short-leaf seagrassZostera noltion hydrodynamics and sediment resuspension from a sandy bed, and (2) measure the erodability of 2 contrasting sediments (sandy and muddy) and the extent to which this is modified by the presence of 2 seagrass species,Z. noltii(sandy) andZ. marina(muddy). Field measurements of near-bed tidal currents, turbulence and suspended particulate matter at 2 differentZ. noltiilocations (low energy [sheltered] and higher energy [exposed] environments) were interpreted in the context of the flume results. Skimming flow above the high density bed ofZ. noltiiwas accompanied by a 40% reduction in near-bed flow, but this was offset by a 2-fold increase in turbulent kinetic energy (TKE) and bed shear stress (τ₀). Despite this increase in τ₀ there was an increase in sediment stabilisation with increasing seagrass density (10-fold increase in critical bed shear stress for erosion [τₑ] from 0.1 [bare sediment] to 1.0 Pa at the highest shoot density). This was largely explained by the increased microphytobenthos abundance (reflected in the higher chlorophyllaand carbohydrate contents) and a lower density of the grazer and bio-destabiliserHydrobia ulvae. In contrast, the muddy site was more easily eroded (10-fold higher), withZ. marinahaving little effect on sediment erodability (bare: τₑ = 0.05 Pa;Z. marina: τₑ = 0.07 Pa). This higher erodability was due to differences in hydrodynamics and the physical/biological properties of the sediment.

Our main objective was to investigate the effect of Spartina anglica stems on hydrodynamics, sediment dynamics and their influence on seasonal morphological changes on the upper shore and salt marsh of an intertidal mudflat in the Tavy estuary (southwest England). On an intertidal transect, the greatest seasonal changes in sediment levels occurred at the seaward edge and lower part of the salt marsh, and on the lower shore mudflat, while the smallest changes occurred on the mid- and upper-shore mudflat. Flume studies demonstrated flow attenuation with increasing Spartina stem density, but this was accompanied by an increase in turbulent kinetic energy and bed shear stress ([tau] sub(0)). The critical erosion threshold ([tau] sub(e)), however, remained relatively constant at 0.12 Pa. Field measurements showed that [tau] sub(0) was well below [tau] sub(e) under calm conditions. During periods of wind-wave activity, when daily mean wind speeds were >8 m s super(-1), [tau] sub(0) was greater than [tau] sub(e) in the lower salt marsh for brief periods at the beginning and end of tidal inundation. At the seaward edge of the salt marsh, [tau] sub(0) increased to 1.0 Pa and [tau] sub(0) was greater than [tau] sub(e) throughout the inundation period, causing significant sediment erosion. Wind- induced wave activity was the major driver of sediment erosion on soft muddy shores and this was enhanced by the interaction with S. anglica stems. The number of days per month with mean daily wind speeds >8 m s super(-1) was inversely related to changes in sediment levels at the seaward edge of the salt marsh. During the summer, winds were below this threshold, and there was gradual accretion. Results are discussed in the context of climate changes involving increased storminess leading to increased erosion. We show that the S. anglica salt marsh should not be considered a 'bio- stabiliser' of fine muddy sediment.

We combined 3 different approaches to determine the relative importance of microphytobenthos production as food for intertidal macrobenthic animals: (1) the natural abundance of stable-isotope ratios of carbon and nitrogen, (2) anin situdeliberate tracer addition of13C-bicarbonate, which was transferred through the benthic food chain after its incorporation by benthic algae, and (3) a dual labelling experiment in a flume, where pelagic and benthic algae were labelled with15N and13C, respectively. The results of the 3 approaches confirmed the high importance of microphytobenthos as a food source for (surface) deposit feeders. Despite the clearly demonstrated resuspension of benthic algae at high current velocities, suspension feeders appeared to depend almost exclusively on pelagic algae (and possibly detrital carbon) as a food source. Based on the results of the experiments, we determined an approximate degree of dependence on microphytobenthos for different species of intertidal macrobenthos. The macrobenthic biomass at 5 study locations, when weighted by these coefficients, correlated very well with measured productivity of the microphytobenthos.

This paper compares modeled biotic and physical effects on intertidal sediment transport, using parameterizations that are based on laboratory and field experiments. A one-dimensional model of an intertidal transect is constructed. The model is aligned cross shore and includes movement of water and suspended sediment. Within the model, tidal currents cause erosion, and bioturbation by the clam, Macoma balthica, alters the erodability of the bed sediment. The concentration of chlorophyll a in the surface sediment (which is an indicator of microphytobenthos density) alters the critical erosion velocity. External sediment supply is specified as an offshore suspended matter concentration. The model is applied within Spurn Bight (Humber Estuary, UK). The effects of various tide heights, biota densities, and external suspended sediment concentrations are investigated. Offshore sediment supply dominates the net deposition below midtide level, but factors affecting intertidal sediment erosion and deposition become important at higher shore levels. Changes in erosion or deposition caused by natural variation in biota densities are as large as those caused by changes in tidal range and currents over a spring-neap cycle, or by doubling external supply. Seasonal variations in densities of stabilizing microphytobenthos can alter the magnitude of net deposition on the upper shore by a factor of two. Interannual variation in numbers of bioturbating clams can change net deposition by a factor of five. These results show that biotic influences on transport of sediment within the intertidal zone are significant and will play a role in determining sediment budgets over tidal to monthly timescales.

Nereis diversicolor is a widely distributed estuarine polychaete worm that is particularly abundant in the mid to upper parts of estuaries. Laboratory annular flumes were used to study the effects of N. diversicolor density (0 [control], 100, 300, 1000, 3000 individuals m super(-2)) on erodability of cohesive muddy sediments. At the highest densities (1000 and 3000 ind. m super(-2)) there was evidence of active sediment resuspension under low current speeds (U = 0.05 m s super(-1)). In response to a step-wise increase in U there was a density-dependent increase in the sediment mass eroded (g m super(-2)). At 3000 individuals m super(-2) and at a flow of 0.4 m s super(-1), sediment erosion increased 37-fold compared to the control. Erosion thresholds representing the onset of sediment resuspension (Type 1a erosion) were low and similar for all worm densities, including the control (critical erosion velocity U sub(crit) = 0.11 to 0.12 m s super(-1); critical bed shear stress for erosion [[tau] sub(e)] = 0.02 to 0.036 Pa). However, at the lower densities (0 to 300 ind. m super(-2)) there was little bed erosion until U = 0.4 to 0.45 m s super(-1) (bed shear stress = 0.16 to 0.2 Pa). The burrowing and surface feeding activity of the polychaete destabilised the bed and increased sediment erodability as a function of worm density. The influence of N. diversicolor (300 ind. m super(-2)) on sediment erosion and deposition during sinusoidal tidal current cycles (0.05 to 0.35 m s super(-1)) was also studied in the flumes. There was a consistent cycle of erosion and deposition with worms inducing ~2-fold higher erosion and deposition rates following an erosion threshold of U => 0.11 m s super(-1) ([tau] sub(e) = 0.02 Pa) and a deposition threshold of U < 0.26 m s super(-1) ([tau] sub(d) = 0.07 Pa). Maximum erosion and deposition rates occurred at 0.25 m s super(-1) and 0.11 m s super(-1), respectively. The results suggest that N. diversicolor can make a significant contribution to increased erosion of fine intertidal muddy sediments and to the overall turbidity in the upper estuary.