The turbidity maximum in the Tamar estuary

Many estuaries of medium to high tidal range exhibit an accumulation of fine cohesive material in their upper reaches in the region of the limit of saline intrusion. Much, or all, of this material is suspended each tidal cycle and the entire region undergoes a seasonal variation which appears to depend on fluvial input. Two factors which are throught to influence the formation and maintenance of turbidity maxima are the differing magnitudes of the bed shear stress (τ₀) on flood and ebb tides and the large vertical density gradient which developes on the ebb tide. Crucial to the importance of the first factor is that τ₀ exceeds a critical value, at which erosion occurs, for a greater period on the flood than on the ebb. The effect of the density gradient is that upward propagation of bed generated eddies is inhibited and the sediment is not transported into the upper part of the flow where it will be most effectively transported. It is not clear which, if either, of these mechanisms is dominant. Data consisting of vertical profiles of velocity, salinity and suspended solids were collected at four stations in the Tamar estuary during a high range tidal cycle. One station, at which the depth mean salinity (S _d ) varied from 0.0 to ∼ 12.0‰, was occupied permanently. The other stations were occupied such that data were collected asS _d varied in the range 0.0 to ∼ 4.0‰. In this way each station was occupied for a period of time on the ebb and flood tide. Observations show that during the early ebb, when the flow is relatively deep and slow, stratification persist untilS _d ∼ 0.0‰ and that no significant transport occurs while the flow is saline but that there is a rapid increase in suspended solids concentrations after this time. During the later ebb the shallower faster flow allowed the density gradient to be erode and significant transport was observed atS _d ∼ 5.0‰. On the flood tide the flow in the low salinity region is well-mixed troughout. Computation of the fluxes and total transport per unit breath of estuary show that on the ebb tide the quantity of solid material being transported by the low salinity (0–3‰) region remains nearly constant as this region of the flow is advebted seaward. On the flood tide, however, as the same region is advected landward the quantity of material being transported increases. It is concluded that in the Tamar estuary the early ebb tide stratification contributed to the formation and maintenance of a turbidity maximum which is strongly associated with the low salinity region of the flow. It is also speculated that the differences in the ebb and flood tide transport are caused by differences in the availability of mobile material on the bed at different stages of the tidal cycle.     

Aperiodic variations of the turbidity maxima of two German coastal plain estuaries

Suspended particulate matter dynamics in estuaries can be split into more or less regular and periodic phenomena dominated by the tide and aperiodic events like river spates which have lasting effects on the suspended matter distribution. The catchment areas of the estuaries of both the River Elbe and R. Weser (Germany) are subject to almost the same meteorological conditions. But the mean freshwater runoff of R. Elbe is about twice the mean R. Weser runoff. In the turbidity maxima of both estuaries, suspended matter dynamics are dominated by the tide most of the year. The turbidity maxima are associated with the low salinity regions, and the locations of both the mixing zones and turbidity maxima depend on runoff. In both estuaries, mixing zone and turbidity maximum react almost immediately and simultaneously on strong increases of runoff. During river floods no turbidity maxima can be observed in the inner estuaries. With decreasing runoff after a river flood the re-establishment of the turbidity maximum lags behind the return of the mixing zone. The restoration of the turbidity maximum to its normal magnitude lasts for months. For the 7 river floods presented here the restoration period varied from 1 to 6 months for the Weser and from 5 to 7 months for the Elbe estuary.     

Effects of river flow on larval growth and survival of Japanese seaperch Lateolabrax japonicus (Pisces) in the Chikugo River estuary, upper Ariake Bay

The abundance and growth history of larval and juvenile Japanese seaperch Lateolabrax japonicus were investigated in the Chikugo River estuary, upper Ariake Bay, from 1990 to 2000. Growth during the larval period (up to 15 mm standard length, L _S, the size at recruitment into the estuary) was backcalculated using sagittal otolith microstructures by the biological intercept method. Growth rates in length declined at body sizes >14 mm L _S. High freshwater discharge through the Chikugo River was associated with high temperatures of the upper Ariake Bay where the larvae spend their planktonic life. Mean larval stage duration (days) from hatch to 15 mm ( D ₁₅) varied between 48·8 and 76·2 days and was inversely correlated with the estimated mean temperature history [mean daily temperature (° C) experienced by the larvae during the period from hatch to 15 mm, T ₁₅]. Mean abundance (number m⁻²) of larvae and juveniles was highest in years when T ₁₅, D ₁₅ and freshwater discharge were at intermediate levels. Although the abundance was not correlated with either of these variables, an exponential relationship between abundance and D ₁₅ was found when data collected during the highest river discharge years (1990, 1991 and 1998) were excluded. The increase in freshwater discharge through the Chikugo River probably had the potential to enhance or diminish Japanese seaperch recruitment in two ways: 1) it could increase recruitment probability by increasing temperature and larval growth and 2) high river flow also had the potential to decrease the probability of immigration into the river by increasing larval seaward dispersion, predation due to decreased turbidity and starvation due to decreased zooplankton prey abundance in the estuary.     

Seasonal variations of P compounds and their concentrations in two coastal lagoons (Herault,France)

This article concerns seasonal variations in the phosphate concentrations in two coastal lagoons near Montpellier (Mediterranean coast, France). The o-P concentration in the overlying water is highest during summer. The role of the sediment, particularly that of the different P fractions in the sediment, is discussed. Significant variations, especially in the FeOOH ≈ P fraction, occur. For both Tot-P_sed and the Fe00H≈P fraction a gradient from surface to bottom is observed, as well as a distinct decrease in the FeOOH≈P fraction in the surface sediments during summer and autumn. Variations in the FeOOH≈P fraction appear to be compensated by variations in the CaC0₃≈P fraction. These variations appear to be determined by the ferric hydroxide concentration. This compound represents only a small part (maximally 15%) of the total iron in the sediments and is related to the dissolved oxygen content of the immediately overlying water. Besides the fractions o-P, Fe(OOH)≈P, a large part of the CaC0₃≈P fraction is potentially bioavailable. A large proportion of the Tot-P_sed is therefore bioavailable.[/p]     

Temporal and spatial trends in heavy metal concentrations in the marine mussel Mytilus edulis from the Western Scheldt estuary (The Netherlands)

Since the first North Sea Conference (1984, Bremen), all countries bordering the North Sea made commitments to reduce discharge of hazardous substances into the North Sea. From Belgium and The Netherlands, large reductions (upto 90) in heavy metal emissions from land-based sources have been reported between 1985 and 2000. Recently, some studies in the Western Scheldt estuary (WS) have shown that total metal concentration in the water, sediments and suspended particles have decreased compared to levels in the 70s. However, data on aquatic organisms is still very limited and it is therefore difficult to confirm whether the reductions in pollution input and generally improving water quality in the WS have a corresponding impact on the levels of heavy metals in aquatic organisms. The current study measured metal concentrations in the soft tissues of mussels, Mytilus edulis (known to be good indicators of environmental metal pollution) during the period 1996–2002. Spatial (salinity and pollution gradients), temporal and seasonal variations were also studied. Results showed a down-stream decreasing trend for the metals studied (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) during all sampling campaigns. There was also a significant seasonal effect on tissue metal concentrations, with a peak observed around spring in both WS and the nearby less polluted Eastern Scheldt (ES). On temporal trends, a clear drop of metals in mussels was observed in the early 80s coinciding with the start of the efforts to reduce chemical pollution input into the North Sea. Since those early reductions, metal concentrations in mussels generally remained unchanged upto mid 90s. However, in recent times metal concentration in mussels have increased significantly, for example Cd in 2002 was almost 10 times the values in 1983 and similar to levels seen during the peak in the 70s. Other metals also increased in the 90s also reaching levels seen in the 70s. As there is no indication of recent increase in metal input into the estuary, we suggest that increased metal concentrations observed in mussels in recent years especially in the upper estuary are most likely a result of changes in physical and chemical speciation and metal bioavailability. Such changes may be caused by changes in some water quality parameters in the estuary (i.e. increased dissolved oxygen, concentration of organic matter), resulting in conditions that favour releases of sediment-bound metals into the water column. The relationship between metal content and season showed very similar annual profiles in the polluted WS and less polluted ES. Thus, seasonal variations in metal concentrations appear to be largely controlled by biological processes, while total body burdens are dependent on environmental levels and bioavailability.     

Diversity of the dsrAB (dissimilatory sulfite reductase) gene sequences retrieved from two contrasting mudflats of the Seine estuary, France

The diversity of sulfate-reducing microorganisms was investigated in two contrasting mudflats of the Seine estuary, by PCR amplification, cloning and sequencing of the genes coding for parts of the alpha and beta subunits of dissimilatory sulfite reductase (dsrAB). One site is located in the mixing-zone and shows marine characteristics, with high salinity and sulfate concentration, whereas the other site shows freshwater characteristics, with low salinity and sulfate concentration. Diversity and abundance of dsrAB genes differed between the two sites. In the mixing-zone sediments, most of the dsrAB sequences were affiliated to those of marine Gram-negative bacteria belonging to the order of Desulfobacterales, whereas in the freshwater sediments, a majority of dsrAB sequences was related to those of the Gram-positive bacteria belonging to the genus Desulfotomaculum. It is speculated that this is related to the salinity and the sulfate concentration in the two mudflats.     

Typical features of particulate phosphorus in the Seine estuary (France)

During the 2001–2002 hydrological cycle, 8,000 tons P year⁻¹ (44% as particulate phosphorus) originating from the Seine basin entered the Seine estuary. P content in suspended sediments (SS) is 2.9 g P kg⁻¹ (80% as inorganic form) at Poses (the upstream limit of the Seine estuary). During the transfer from Poses to Caudebec (the limit of saline water intrusion), Particulate Inorganic Phosphorus (PIP) in SS decreases by 40% whereas Particulate Organic Phosphorus (POP) remains stable. This decrease is explained as the result of (i) a dilution by SS poorer in P, originating from storage zones (mudflats) within the estuary, and (ii) a loss of P by sedimentation, especially in the Rouen harbour where 15% to 20% of SS are yearly trapped and extracted. Downstream, in the turbidity maximum of the estuary, P content in SS is twice as low as at Poses (1.5 g P kg⁻¹, 70% as inorganic form). PIP content is fairly homogeneous both vertically and throughout an annual survey. On the other hand, POP varies by season. Higher POP content is observed during vernal period with phytoplankton accumulation. In a salinity gradient from 0 to 30 (PSS78), PIP content further decreases by 30%. POP does not vary much in this gradient, leading to the assumption that PIP is submitted to desorption in response to the dilution by marine waters (poor in orthophosphates). Using ³²P isotopic method, we establish a mathematical formulation of P exchangeable between the suspended solid and water phases of the Seine estuary. Particulate P in the turbidity maximum zone is shown to represent a possible source of dissolved P, available for algal growth in the Seine Bight. Instead of playing a role of nutrient retention, the turbidity maximum zone of the Seine estuary could favour coastal eutrophication.     

The influence of salinity on the cercariae of three species of Schistosoma

Donnelly F. A., Appleton C. C. and Schutte C. H. J. 1984. The influence of salinity on the cercariae of three species of Schistosoma. International Journal for Parasitology14: 13–21. The effect of salinity on the longevity and infectivity of cercariae of Schistosoma mattheei, Schistosoma haematobium and Schistosoma mansoni was determined. No significant differences in cercarial longevity occurred (p > 0.05) in low salinities (0–5.25%), whereas further increases in salinity resulted in progressive decreases in survival. In salinities ? 17.5%, cercariae were incapable of surviving for longer than 11 min. A maximum life-span of up to 122 h was recorded for some S. mattheei cercariae. Cercarial infectivity, as indicated by worm returns, was reduced progressively with increasing salinity up to a lethal limit of 10.5%. Differences in the salinity tolerance of the cercariae of the three species were discussed.     

Spatial distribution and trophic ecology of dominant copepods associated with turbidity maximum along the salinity gradient in a highly embayed estuarine system in Ariake Sea, Japan

The present study aimed to investigate into the feeding ecology of the dominant copepods along a salinity gradient in Chikugo estuary. Copepod composition was studied from samples collected from stations positioned along the salinity gradient of the estuary. Copepod gut pigment concentrations were measured by fluorescence technique and hydrographical parameters such as temperature, salinity, transparency, suspended particulate matter (SPM); pigments such as chlorophyll-a (Chl-a), phaeopigment; and particulate nutrients such as particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured. Two distinct zones in terms of nutrient and pigment concentrations as well as copepod distribution and feeding were identified along the estuary. We identified a zone of turbidity maximum (TM) in the low saline upper estuary which was characterized by having higher SPM, higher POC and PON but lower POC:PON ratios, higher pigment concentrations but lower Chl-a/SPM ratios and higher copepod dry biomass. Sinocalanus sinensis was the single dominant copepod in low saline upper estuary where significantly higher concentrations of nutrients and pigments were recorded and a multispecies copepod assemblage dominated by common coastal copepods such as Acartia omorii, Oithona davisae and Paracalanus parvus was observed in the lower estuary where nutrient and pigment concentrations were lower. Copepods in the estuary are predominantly herbivorous, feeding primarily on pigment bearing plants. However, completely contrasting trophic environments were found in the upper and the lower estuary. It was speculated from the Chl-a and phaeopigment values that copepods in the upper estuary receive energy from a detritus-based food web while in the lower estuary an algal-based food web supports copepod growth. Overall, the upper estuary was identified to provide a better trophic environment for copepod and is associated with higher SPM concentrations and elevated turbidity. The study demonstrates the role of estuarine turbidity maximum (ETM) in habitat trophic richness for copepod feeding. The study points out the role of detritus-based food web as energy source for the endemic copepod S. sinensis in the upper estuary, which supports as nursery for many fish species.     

Dissolved and suspended mercury. species in the Wabigoon River (Ontario,Canada): seasonal and regional variations

Seasonal and regional variations in the speciation, sediment-water partitioning, and dynamics of mercury (Hg) were studied at selected sites along the Hg-polluted Wabigoon River, and at unpolluted headwater and tributary sites, during April–September, 1979. ‘Dissolved’ and ‘particulate’ forms of Hg in the water were separated by continuous-flow centrifugation in the field. The Hg and other pollutants such as wood chips and salt had been discharged from a chlor-alkali plant and paper mill at Dryden, Ontario. Concentrations and loadings of particulate methyl mercury (CH₃Hg⁺) and total particulate Hg (and loadings of total ‘dissolved’ Hg) were greatest during the spring flood (April-May) owing to accelerated resuspension and transport of sediments. Concentrations of ‘dissolved’ CH₃Hg⁺, however, were highest in the summer (July–September), probably reflecting stimulation of microbial methylating activity by elevated temperatures, together with factors such as reduced levels of metal-scavenging particulates and minimal dilution by runoff. Total dissolved Hg concentrations were relatively high in September at polluted sites only, possibly because of desorption from sediments due to elevated concentrations of Cl⁻ ions. Loadings of dissolved CH₃Hg⁺ tended to be high in the summer but were generally depressed (suggesting sorption by suspended particles) during the major spring-flood episode in May. During July–August dissolved CH₃Hg⁺ was a function of total dissolved Hg, suggesting rapid biomethylation of desorbed inorganic Hg; but in general dissolved and suspended CH₃Hg⁺ levels depended on environmental variables and were unrelated to total Hg concentrations. In the summer only, total dissolved Hg was a function of dissolved Cl⁻. Hg species in particulates were associated with sulfides, hydrated Fe and Mn oxides, organic matter (notably high molecular weight humic and humic-Fe components), and selenium (Se); but CH₃Hg⁺ and total Hg differed in their specific preferences for binding agents, implying that binding sites discriminate between CH₃Hg⁺ and Hg²⁺ ions. CH₃Hg⁺ was associated with sulfide and (in the spring only) with Fe oxides, whereas total Hg was associated with organic matter and Se and with DTPA- and NaOH-extractable Fe in the spring but with Mn oxide and NaOH-extractable organics in the summer. Sulfides were most abundant in May, indicating that they were eroded from bottom sediments, but Fe and Mn oxides were most abundant in the summer, probably owing to activities of filamentous iron bacteria and other micro-organisms. Particulate Hg was 98–100% nonextractable by mild solvents such as Ca acetate, CaCl₂, dilute acetic acid, and (at polluted sites only) DTPA solutions, suggesting that the particulate Hg mobilized in the spring may not be readily available to organisms; association with Se and high molecular weight humic matter also supports this hypothesis. Hg probably becomes more bio-available in the summer, as suggested by the upsurge in dissolved CH₃Hg⁺ and total dissolved Hg levels, and by increases in the solubility of particulate Hg in acetic acid, DTPA, H₂O₂, and NaOH solutions, as well as an increase in the relative importance of lower molecular weight fractions of NaOH-extractable Hg (in September). Regional variations in Hg speciation and partitioning reflected a gradient in sediment composition from wood chips near Dryden to silt-clay mud further downstream. Hg in silt-clay mud relatively far (> 35 km) downstream from the source of pollution or in unpolluted areas appeared to be more readily solubilized by Cl⁻ ions or chelators such as DTPA, more readily methylated (as indicated by downstream increases in dissolved CH₃Hg⁺ levels and CH₃Hg⁺/total Hg ratios), and was to a greater degree organically bound (H₂O₂-extractable), and thus was probably more bio-available, than Hg in wood-chip deposits. Possible explanations include weaker binding of Hg by the mud, the more finely divided state of the mud, and improved microbial growth at lower concentrations of toxic pollutants. Owing to enrichment in sulfides and Fe oxides, resuspended wood-chip sediments were especially efficient scavengers of CH₃Hg⁺. The results indicate that in any pollution abatement plan aimed at lowering the Hg levels in the biota of lakes fed by the Wabigoon River, immobilization, removal, or detoxification of dissolved as well as particulate forms of Hg in the river would probably have to be considered. Possibly, Hg species could be ‘scrubbed’ from the river water by increasing the suspended load and by sedimentation and treatment with Hg-binding agents in special receiving basins.     

Seasonal variations in the production of antifungal substances by some dictyotales (brown algae) from the french mediterranean coast

Hexane extracts of some algae belonging to the Dictyotales collected over a 12 month period were tested for their antifungal activity using human pathogenic fungi (yeasts, moulds and dermatophytes) and phytopathogenic fungi responsible for diseases in Mediterranean plants and trees. The three algal species tested (Dictyota dichotoma, Dictyota dichotoma var. implexa, Dilophus spiralis) exhibited a wide spectrum of antifungal activity which varied during the seasons.     

Influence of changes in salinity and light intensity on growth of phytoplankton communities from the Schelde river and estuary (Belgium/The Netherlands)

In the Schelde continuum, a succession in the phytoplankton community is observed along the transition from the river to the freshwater tidal reaches of the estuary and from the freshwater to brackish reaches of the estuary. The goal of this study was to experimentally evaluate the contribution of changes in salinity and light climate to this succession. In summer 2000 and in spring 2001, phytoplankton communities from the river, the freshwater tidal reaches and the brackish reaches of the estuary were incubated under high or low light intensities and exposed to a change in salinity. HPLC analysis was used to evaluate the response of different algal groups to changes in light intensity and salinity. When incubated at a light intensity corresponding to the mean underwater light intensity of the freshwater tidal reaches, growth of phytoplankton from the river as well as from freshwater tidal reaches was significantly lower than when incubated at a light intensity corresponding to the mean underwater light intensity of the river. The phytoplankton community from the freshwater tidal reaches did not appear to be better adapted to low light intensities than the phytoplankton community from the river. Although diatoms were expected to be less sensitive to a reduction in light intensity than green algae, the opposite response was observed. Freshwater and brackish water phytoplankton were negatively affected by respectively an increase or decrease in salinity. However, the effect of salinity was not strong enough to explain the disappearance of freshwater and brackish water phytoplankton between a salinity of 0.5 and 10 psu, suggesting that other factors also play a role. In the freshwater phytoplankton communities from the river and the freshwater tidal reaches, green algae and diatoms responded in a similar way to an increase in salinity. In the brackish water phytoplankton community, fucoxanthin displayed a different response to salinity than lutein and chlorophyll a.     

The role of salt marshes in the Mira estuary (Portugal)

The Mira estuary is a narrow entrenched pristine estuary of the Ria type, about 30 km long. It comprises an area of 285 ha of salt marsh, of which250 ha have been proposed for reclamation for aquaculture. Dredging, village and recreation development menace the yet undisturbed estuarine ecosystem. To assess the biological importance of this wetland, a multidisciplinary study was conducted in apart of the salt marsh, considered as being representative of the whole area. Halophytic vegetation covering 75% of the total salt marsh site is dominated by Spartina maritima (28% of total vegetation area). Total primary production attains63,766 kg/yr (dw). A net export of 1541 kg/yr of COM to the relatively oligotrophic adjacent waters was also found. Insects and birds are described for the first time in the saltmarsh. Macrobenthic communities are dominated by Hediste diversicolor, Nepthys caeca and Scrobicularia plana. The fiddler crabUca tangeri attains here its north distribution limit. The mud flats and creeks associated with the salt marsh act as a nursery for 40.8% of the fish species present. The food web is dominated by detritivorous species like the grey mullets. The results obtained in this study support the need for an effective conservation of this area. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of salinity on water balance in 0-group flounders, Platichthys flesus (L)

The water balance of 0-group flounders was investigated in a range of static [0–100% sea water (SW), 100%≡ 34%‰] and cycling salinities (2–98% SW, 12 h 25 min period). The permeability coefficients of these juvenile fish were found to be higher than those quoted for adults. The permeability of juveniles in fixed salinities decreased with increased salinity whereas animals in the tidally cycling regime showed permeability changes that were directly proportional to the ambient salinity. However, comparison of the two groups showed that animals in a cycling salinity regime were less permeable to water than animals acclimated to fixed salinities. Drinking and urine production rates fluctuated within a tidal salinity regime, and 0-group flounders were found to modify their water permeability, urine production and drinking rates simultaneously, so maintaining their blood osmotic concentration and total water content within narrow limits throughout the range of salinities.
The relevance of measurements made in fixed and tidally cycling salinities to water regulation under natural estuarine conditions is considered.     

The influence of salinity on the ova and miracidia of three species of Schistosoma

Donnelly P. A., Apleton C. C. and Schutte C. H. J. 1984. The influence of salinity on the ova and miracidia of three species of Schistosoma. International Journal for Parasitology14: 113–120. The effect of salinity on the hatchability of the ova and the longevity and infectivity of the miracidia of schistosoma mattheei, Schistosoma haematobium and Schistosoma mansoni was determined. Complete inhibition of hatching of ova occurred in an upper salinity of 14%, whilst salinities < 3.5% did not significantly affect hatchability (p > 0.05). Miracidial survival decreased progressively in salinities? 7 %. However, in salinities of 1.75% and 3.5%, survival was significantly greater than in fresh water (p ? 0.05), although this did not appear to infer an increase in infectivity. Miracidia of S. mattheei and S. mansoni were capable of establishing mature infections in salinities of up to 3.5%. only and S. haematobium in ? 2.5%. Differences in the salinity tolerance of the ova and miracidia of the three species were discussed.     

The influence of turbidity on growth and survival of fish larvae: a numerical analysis

Growth and survival through the early larval phase probably limit the production potential of many commercially important fish stocks. Attempts to increase the production of these stocks by restocking of juveniles have generally failed. Here, we analyse how enhanced concentrations of phytoplankton and zooplankton affect the survival of fish larvae during their early life stages. The analysis is developed for larvae feeding on copepod eggs and nauplii, with fish and invertebrates as major predators. A model of feeding and growth of fish larvae is applied to assess the benefit of enhanced phytoplankton and zooplankton abundance. The analysis shows that the shading effect of higher phytoplankton concentration may reduce predation rates on fish larvae substantially. This `top-down' effect may be more important for the cohort survivorship than the `bottom-up' mechanism in situations when larval food is sufficiently abundant. However, while increased algal biomass will improve recruitment at high zooplankton concentration, it may also reduce recruitment at low zooplankton concentrations and shallow mixing depths. Both the larvae and their vertebrate predators are dependent on light to detect their prey, and the longer reactive distance of the predators make them more susceptible than the larvae to reduced light levels and increased turbidity. We discuss the implications of reduced predation and increased zooplankton abundance on recruitment and production of fish larvae, and point at environmental conditions where changing algal biomass is likely to affect recruitment success.     

Impact of turbulence and turbidity on the grazing rate of the rotifer Brachionus calyciflorus (Pallas)

The impact of turbulence and turbidity on Brachionus calyciflorus grazing rate was determined in short feeding periods (10 min), using labelled Chlorella pyrenoïdosa. The response to water motion of B. calyciflorus depends on it physiological state: the grazing rate of recently fed amictic females stomach green (with one or two eggs) is significantly reduced in agitated environments compared with non-agitated environments. In contrast, the grazing rate of starved amictic females is not reduced by water motion, whatever its velocity (V1=0.18 m s?1 and V2=0.22 m s?1). In the presence of suspended particles (3–6-μm silica beads), a larger reduction in grazing rate is observed in agitated water at any water velocity (V1=0.18 m s?1 or V2=0.22 m s?1), than in stagnant water. A synergy between turbulence and turbidity is unfavourable to feeding of rotifers.     

Effects of changes in turbidity and phosphate influx on the ecosystem of the Ems estuary as obtained by a computer simulation model

In the Ems estuary the gradients in the concentration of nutrients and in turbidity (the factors that mainly determine the amount of carbon assimilated by phytoplankton) are steep. The effects of changing the turbidity in the estuary and the amount of phosphate discharged by the rivers Ems and Westerwoldsche Aa were analysed, using the simulation model developed by BOEDE (Biological Research Ems-Dollard estuary). The results of several sensitivity runs were compared with the standard run.A 50% reduction of turbidity led to a strong increase in phytoplanktonbiomass, especially in the inner parts of the estuary where turbidity is high. On average, the effects are two to three times larger for the inner part than for the outer part of the estuary. When the turbidity doubles the opposite occurs resulting in a significant decrease of phytoplankton biomass in the upper reaches. In the lower reaches of the estuary a 50% reduction in the river discharge of phosphate is largely compensated for by changes in phosphate transport from the North Sea. This results in a nearly unchanged primary production in the lower reaches as compared with the standard run.In the upper reaches a 50% reduction of phosphate loads results in a strongly reduced primary production.In general, the zooplankton groups (copepods and microzooplankton) are influenced less than the phytoplankton. Benthic fauna is hardly influenced, except for filter feeders; which are strongly affected by the total density of the particles, a parameter which also is directly related to turbidity.     

Biodiversity of aquatic gastropods in the Mont St-Michel basin (France) in relation to salinity and drying of habitats

Seventy-seven polder and marsh stations were sampled for aquatic and riparian gastropods in the terrestrial basin of Mont St-Michel from March 1994–March 1995 and during a drought in 1996. In addition, four ponds with riparian coppice (bocage) were investigated. Twenty-one stations, mostly in polders, lacked snails; high values of conductivity, and by implication salinity, appeared to make the habitats unfavourable. Only the euryhaline hydrobid Potamopyrgus antipodarum and the amphibious lymnaeid Lymnaea truncatula survived in areas of high conductivity. In associated lagoons, the maximum level of salinity tolerated by freshwater gastropods was 35. Of the 14 species of gastropods collected, Lymnaea peregra and Anisus leucostoma were the most abundant. Stations had 1–7 species and the average gastropod species richness was 3.32 (±1.67). The canals that were the most susceptible to drying had a community of desiccation-resistant species including L. truncatula, Aplexa hypnorum, A. leucostoma and L. peregra. It appears that gastropod assemblages in the terrestrial basin of Mont St-Michel have achieved their current diversity by surviving strong ecological constraints.     

Effects of the river discharge management on the nursery function of the Guadalquivir river estuary (SW Spain)

Within the Guadalquivir estuary, young recruits of marine species seem to respond to changes in freshwater flow by moving with the mass of estuarine water that is most “suitable” for them. The control of the river flow, from a dam 110 km upstream from the river mouth, has an immediate effect on the estuarine salinity gradient, displacing it either seaward or upstream. Consequently, there is a reduction or enlargement of the estuarine area that is used as nursery grounds. The analysis of the temporal estuarine recruitment and spatial distribution of young stages of marine species, during six annual cycles, provides evidence that the estuarine zone used as nursery grounds is mainly that part situated seaward from an isohaline value of 5. The relationship between the position of that isohaline (D₅) and the freshwater discharges from the dam was also examined during high and low tides. It was found that a high percentage of the isohaline position variation (75% and 73% at high and low tides, respectively) can be explained by the freshwater volume discharged from the dam during the previous week. These preliminary results suggest that an accurate model of the relationship between these two variables may be a useful tool for future management strategies of freshwater discharges to the estuary.