Variation in populations of Littorina obtusata and L. mariae (Gastropoda) in the Severn Estuary

Populations of both species of flat periwinkle were examined at five discrete sites along the Severn Estuary, and at a marine site in Dyfed. The vertical distribution of the two species showed a clear partitioning of the shore – Littorina obtusata occupied the high/mid shore and L. mariae the low shore area. L. obtusata also extended further up the Severn Estuary than L. mariae. Maximum densities of both species were found in Milford Haven. Biometric investigations revealed intra and inter-specific differences in colour morphs, shell damage and size between populations. The possible influence of differential selection pressures at the various sites is discussed.     

Decomposition in salt marsh ecosystems of the S.W. Netherlands: the effects of biotic and abiotic factors

Decomposition rates, determined with the litterbag technique in salt marshes of the S.W. Netherlands during the past decade are compared; the biotic and abiotic factors influencing these rates are identified and discussed.Tissue composition is the main variable affecting decay rates of halophytes, particularly variations in lignin content between plant parts and between species.Experiments in which the loss of the tensile strength of cotton strips was used as an index of cellulolytic decay, show that there is a conspicuous variation in decay rates on different sites in a salt marsh. Nonetheless, the locally varying environmental conditions within salt marshes of the S.W. Netherlands have less impact on the variation in decomposition rates of halophyte litter than the chemical make-up of the plant material.Larger fauna elements (> 300 m) may increase decomposition rates, but this effect is only limited and depends on location and litter type. The role of small fauna elements such as nematodes, which occur abundantly in association with halophyte litter, remains largely unknown.     

The distribution of Asellus aquaticus and A. meridianus (Crustacea, Isopoda) in Britain

SUMMARY. Asellus aquaticus and A. meridianus are freshwater isopods common more or less throughout Britain. It has been suggested that the former species is replacing the latter. To investigate this, the distributions of the species were determined in the English Lake District, near Merseyside, and elsewhere. Additionally, some field transference experiments were conducted in Anglesey and near Merseyside. Particular attention was given to changes in distribution with time; many of the observations reported relate to a time span of almost 20 years. Taken together, the results provide qualified support for the suggestion that A. aquaticus is replacing A. meridianus .     

Growth and movements of the twaite shad, Alosa fallax (Lacépède), in the Severn Estuary

Between July 1974 and April 1977, appreciable numbers of 0 group twaite shad, Alosa fallax , have been collected from the cooling water intake screens of the Nuclear Power Stations at Oldbury-upon-Severn and Berkeley on the Severn Estuary and at Hinkley Point on the Bristol Channel, England. Young of the year first appeared at Oldbury in July at a size of approximately 32 mm standard length, and grew to approximately 61 mm by October. The downstream migration in the late summer and autumn appeared to be closely correlated with water temperature since movement reached a peak soon after the temperature in the Estuary had declined below 19° C, and ceased altogether below 9° C. Thereafter, there was virtually no movement of shad until temperatures rose sharply above 7° C in the spring. Maturing adult twaite shad were captured in late April and May as they moved into freshwater to spawn, probably in mid-June. Males were recorded slightly earlier during the migration and were generally younger than females. Furthermore, mean lengths and weights for individuals belonging to the different age classes indicated that males were also smaller. Logarithmic relationships between length and weight of young and adult A. fallax and for the variation with size in the number of rakers on the first gill arch are presented.     

History, distribution, status and habitat requirements of the Harvest mouse (Micromys minutus) in Britain

This paper reviews the past known history of the Harvest mouse and presents the results of a survey of the distribution and status of the animal in Britain. Data are included which demonstrate the wide variety of habitats colonized by the Harvest mouse and the diversity of monocotyledonous plants that can be utilized for nest building. The survey has shown that there is no evidence of a decline in the distribution or abundance of this animal in Britain during the last century, and that the sparsity of earlier records was probably due to the lack of competent recorders.     

Phytoplankton composition and spatial distribution of copper and zinc in the Fal Estuary (Cornwall,UK)

In the estuaries near Falmouth (Cornwall, UK) levels of dissolved copper and zinc are high, due to drainage of copper and tin mines. Phytoplankton species composition in the autumn of 1989 deviated in the metal-contaminated Restronguet Creek from that in other estuarine branches,viz. Fal, Tresillian and Percuil. In the riverine part of Restronguet Creek (Carnon River)Euglena mutabilis, known as an acidophilic (pH 3) metal-resistant flagellate, occurred at micromolar Cu and Zn, whereas in the clean riversChlamydomonas sp. andOocystis sp. occurred at nanomolar Cu and Zn. An ordination analysis revealed the following patterns in Cu, Zn and phytoplankton species composition in the poly- and euhaline waters. Seston-bound Zn and dissolved Zn were in equilibrium,Katodinium rotundatum is Zn-tolerant, andSkeletonema costatum occurred in water with high contents of Cu in seston. However, none of the variables in these patterns correlated at a significant level (p>0.05). The results show that algae-metal interactions are complicated, and that statistical correlations foundin situ need experimental verification.Communication no. 542 of the Delta Institute for Hydrobiological Research.     

Coral reefs in a clastic sedimentary environment: Fossil (Miocene,S.W. Turkey) and modern (Recent,Red Sea) analogues

Summary The Miocene sequences of southwestern Turkey and the Recent Red Sea are characterised by an unusual and distinct sedimentary association. In both cases near-shore patch and fringing reefs occur along the margins of alluvial fans and braidplains developed under semiarid climatic conditions. In such an environment reef location, morphology and internal structure are controlled primarily by the clastic sedimentary system rather than the independent growth of the framework builders as in normal reef environments. The primary control is a morphological one, the reefs adopting the overall geometry of the sediment body forming a fan shaped arcuate belt. Irregularities in reef morphology are clearly related to the presence of fluvial channels on the fans. The coarse grained gravel of the fans provide an ideal substrate for coral colonisation, this is reflected in the lack of a pioneer community in the Miocene reefs. Periodic rapid burial of the reefs terminates the production of carbonate debris by mechanical and biological erosion resulting in preservation of a relatively unaltered primary framework. The inter-reef sediment consists exclusively of terrigenous sediment.     

Downstream drift of the larvae of Chironomidae (Diptera) in the River Chew,S.W. England

The drift of larval Chironomidae (Diptera) was investigated at two sites on the River Chew, using a pump filtration system with 50 µm mesh-aperture aerial nets, situated on the river bank.Chironomid larvae were found to drift in far greater numbers than previously reported, even in slow-flowing water. First and second instar larvae dominated the drift.Variation in drifting behaviour between taxa was observed, where Orthocladiinae drifted in all instars and Chironominae predominantly as first and second instars. Proportionally different rates of drift were observed between these taxa from the benthos at the two sites.It is suggested that the majority of chironomid drift represents an active dispersal and colonisation mechanism by which population redistribution and habitat selection occurs.     

Measurements of import and export in the Krabbenkreek,a tidal area in the Oosterschelde (S.W. Netherlands)

Summary The Krabbenkreek is a tidal area of about 11.5 km², surrounded by dikes for 94%. Mudflats cover nearly 65% of the area and salt marshes cover 15%. The area is connected to the Oosterschelde estuary by a main channel, which allows the tides to reach the inner parts; a small channel draines the mudflats near the mouth directly to the estuary.In a series of 10 measurements the fluxes of material were observed in the two channels by measuring the flow velocities each half hour during two tidal cycles. The water was sampled concurrently. A rough calculation of the data indicated that the resulting masses of soluble and particulate material during a tidal cycle between Krabbenkreek and Oosterschelde are of the same magnitude as the error due to incorrect flow velocity measurements. A more refined calculation is under develoment, which together with checks on the flow measurements will reduce the error.A statistical analysis of the data on the soluble and particulate material gives an indication of the direction of the net flow. In this analysis the chloride is considered a conservative parameter and data of ebb and flood are treated separately. This analysis reveals that ammonia is imported over the year while nitrate is exported. Chlorophyll-a is exported throughout the year as is particulate organic carbon. Silicate is imported throughout the year. There is an import of phosphorus in spring and summer and an export during autumn and winter.     

The occurrence of the rock goby, Gobius paganellus L. 1758 and the two-spot goby, Chaparrudo flavescens (Fabricius,1779) in the Blackwater Estuary, Essex (S.E. England)

The capture of a rock goby, Gobius paganellus , and two-spot gobies, Chaparrudo flavescens from the upper Blackwater Estuary during the autumn of 1974 is reported. Reasons for their presence are discussed, since this appears to be the first record of Gobius paganellus in the North Sea .     

Biotopes of recent benthonic foraminifera in Pitt Water,broken Bay,N.S.W. (Australia)

Q-mode cluster analysis has been applied to a foraminiferal fauna from Pitt Water, N.S.W. (Australia), in order to define biotopes and their geographic extent. By successive elimination of the less commonly occurring species several biotope pattenrs have been obtained and mapped.These different sub-populations indicate that a large population has to be taken into consideration before cluster analysis can be successfully used.The biotopes determined show a meaningful relationship with the bottom topography and with the other environmental parameters.     

Spatial and temporal distribution patterns of the macrozoobenthos assemblage in the salt marshes of Tejo estuary (Portugal)

The present study focuses on the spatial and temporal distribution of the macroinvertebrate community of the salt marsh areas of the Tejo estuary, based on surveys conducted from autumn 1998 to summer 2000. Samples were collected quarterly in five different intertidal areas along an elevation gradient in: mudflats, creek mouths, creeks, pioneer salt marsh areas and middle marsh areas. A total of 36 benthic invertebrate taxa were identified. Insect larvae were the most well represented group, with 10 taxa identified. Oligochaetes and ostracods were the most numerically abundant taxa, whereas bivalves dominated in biomass. Benthic macroinvertebrate assemblages were dominated, both in number and biomass, by deposit feeders. Three distinct macroinvertebrate assemblages were distinguished along the elevation gradient, based on species presence, density and biomass: the unvegetated muddy areas with a macrobenthic assemblage composed mostly by infauna; the salt marsh pioneer areas of Spartina maritima in which several epibenthic taxa were found, as well as endobenthic taxa characteristic of muddy sediment; and the creek margins, with epifauna taxa such as insect larvae and crustaceans and a low abundance of benthic infauna. Total biomass in the unvegetated and Spartina areas was higher during spring and summer mainly due to the increase in biomass of Scrobicularia plana and Hydrobia ulvae. No decreases in the salt marsh macroinvertebrate biomass values were observed during the highest densities of their potential nektonic predators (summer). This fact might indicate that macroinvertebrates are not a limiting resource for the nektonic species, and that the natural biomass increment of these invertebrate species could be masking the predation/disturbance caused by the nektonic species.     

Size distributions and trophic relationship of the pelagic ecosystem in the Oosterschelde (S.W. Netherlands)

The theory of SHELDONet al. (1972), stating that in marine pelagic communities biomass is distributed equally over logarithmic size classes, is tested for the inland part of the Oosterschelde. It is shown that none of the data on total particulate matter (including phytoplankton and non-phytoplankton particles), phytoplankton separately and zooplankton abundance and size, on a yearly basis, are distributed equally over the size range in which they occur. Using experimentally determined predation efficiency values for zooplankton grazing on phytoplankton, it is shown that the ratio of zooplankton-to phytoplankton biomass found is nevertheless rather consistent with the model of SHELDONet al. (1977).Communication no. 539 of the Delta Institute for Hydrobiological Research.     

Dissolved and particulate trace metal geochemistry in the scheldt estuary,S. W. Netherlands (water column and sediments)

The geochemistry of dissolved and particulate trace metals has been studied in the water column and the sediments of the Scheldt estuary between 1987 and 1990. A strong seasonal influence on the behaviour of dissolved Cd, Cu and Zn is observed, related to the redox conditions in the upper estuary and phytoplankton activity in the lower estuary (which are both seasonally dependent variables). The dissolved trace metal concentrations in the fresh water end-member are remarkably low during spring and summer, due to metal sulphide precipitation in the anoxic Scheldt river. However, the dissolved concentrations increase rapidly with increasing salinity, due to oxidation of metal sulphides that are present in the suspended matter, accompanied by (e.g. chloro-)complexation of the released metals. Readsorption of Cd and Zn occurs in the lower estuary during the spring phytoplankton bloom. During winter, when the Scheldt river is not completely anoxic, much higher dissolved trace metal concentrations are observed in the fresh water end-member since metal sulphide precipitation in the water column is precluded. Rapid trace metal removal is observed in the low salinity, high turbidity zone, due to adsorption onto suspended matter and freshly precipitated iron and manganese oxyhydroxides. Upon further mixing, desorption is apparent, due to a similar oxidation-complexation mechanism as observed during spring and summer. Pore water infusion may also contribute to the enrichment of dissolved Cd, Cu and Zn in the mid-estuarine region. The trace metal contents of the suspended matter and the sediments show a continuous decrease with increasing salinity. This behaviour is to a very large extent due to physical mixing of contaminated fluvial particulates and relatively unpolluted marine particulates. Desorption of Cd, Cu and Zn can be identified but is of minor importance compared to the conservative mixing process. The distribution of dissolved Cd, Cu and Zn in the pore waters of the mid-estuarine region reflects the impact of early diagenetic processes. Trace metal peaks are observed near the sediment-water interface, and at greater depth in the manganese and iron reduction zones. These peaks are attributed to oxidation of reduced trace metal compounds (e.g. sulphides) and reduction of the (iron and manganese) oxide carrier phases, respectively. At greater depth, the dissolved trace metal concentrations are much lower due to metal sulphide precipitation in the sulphate reduction zone. Analysis of a large sediment dataset indicates severe trace metal pollution of the Scheldt estuary at the end of the fifties. A major reduction of the pollution by As, Cr, Hg, Pb, and Zn has occurred in the seventies, and of Cd and Cu in the eighties. The Ni pollution has increased over the time period considered. In spite of this improvement, the present-day pollution status of the Scheldt estuary is still reason for concern.