Warming and hypoxia threaten a valuable scallop fishery: A warning for commercial bivalve ventures in climate change hotspots

Marine molluscs constitute the second largest marine fishery and are often caught in coastal and estuarine habitats. Temperature is increasing in these habitats at a rate greater than predicted, especially in warming “hotspots”. This warming is accompanied by hypoxia in a duo of stressors that threatens coastal mollusc fisheries and aquaculture. Collapses of the northern bay scallop (Argopecten irradians irradians) fisheries on the Atlantic coast of the USA are likely to be driven by rapid rates of coastal warming and may provide an ominous glimpse into the prospects of other coastal mollusc fisheries in climate warming hotspots.     

Behavioural approaches to demonstrate the ecological significance of exposure of juvenile Pacific salmon (genus Oncorhynchus) to the antisapstain fungicide TCMTB

Simulated stream conditions were used to expose underyearling Pacific salmon (genus Oncorhynchus) to sublethal doses of TCMTB (2-thiocyanomethylthio) benzothiazole; a fungicide used by the lumber industry to protect against discolouration by sapstain fungi. Observed changes in schooling, swimming, and respiration were studied in more detail utilizing progressively more complex and ecologically relevant experimental designs. Laboratory troughs (600 litre) were used to quantify innate shelter-seeking behaviour and susceptibility to predation during salinity stress. Swimming speed, schooling behaviour and responses to salinity and hypoxia were studied in a 4500-litre Water Column Simulator under vertically stratified conditions (fresh water overlying sea water). Finally a similarly stratified 15,500-litre outdoor tank was designed to compare susceptibility to predation during a volitional salinity challenge. Relative to controls, TCMTB-exposed underyearling salmon exhibited a reduction in cover response, were slower to seek shelter from bright light, were more erratic in their swimming behaviour and more likely to be eaten by marine predators. Videotape analyses confirmed that TCMTB reduced exploratory behaviour and swimming speed under both normoxic and hypoxic conditions in the WCS. The ecological consequences of these overt behavioural changes were experimentally linked to a clear increase in risk from predation by yellowtail rockfish (Sebastes flavidus) inhabiting the marine zone below the halocline of the 15,500-litre tank. These techniques illustrate our most recent efforts to correlate cumulative sublethal physiological stress with ecologically meaningful behavioural dysfunction.     

The soil environment of the intertidal area in the Westerschelde

Hydrodynamic forces and sediment discharges determine the sedimentary environment and surface morphology of the intertidal area in the Westerschelde estuary in the S.W. Netherlands. Sandflats (clay content <8%) are found in the central part, mudflats (>8% clay) and especially salt marshes (>15% clay) occur in sheltered places.The semi-terrestrial soil of the intertidal area is a complex environment where hydrosphere, biosphere, lithosphere, and atmosphere interact. Surface morphology and the semi-diurnal tide may significantly modify this interaction pattern. Organic matter and constituents of the clay-size fraction are the most reactive compounds in the soil.The pathway and rate of organic matter decomposition determines the nature of many biogeochemical processes. Sulfur transformations are dominant processes in the sulfidic grey-black coloured horizons underneath the generally thin yellow-brown surface layer. The sulfur geochemistry also affects the sorption-behaviour of metals. Oxygen intrusion by diffusion, burrowing fauna and plant roots cause a local and sometimes temporary oxidation of reduced inorganic compounds. Thus, in the surface layers, and in and around tubes and roots at greater depth in the soil, a dynamic cycling occurs of redox active compounds, which affects the sorption mechanism ofe.g. metals-ions.     

COMMENTS ON THE WATER QUALITY OF THE MHLATHUZE ESTUARY IN RELATION TO DETERMINING THE ECOLOGICAL INTEGRITY CLASS

Summary

Water quality forms an integral component of the determination of the Ecological Reserve of a water resource. This paper presents the available historical and current information on the water quality of the Mhlathuze estuary, The data were assessed in order to determine the forcing factors that characterise the water quality of Mhlathuze estuary. These factors were used in conjunction with a previously-developed water quality indexing system to assign a water quality integrity class to the Mhlathuze estuary. Three reaches of the estuary were classified as being largely natural with few modifications. Major issues relating to water quality data gaps were identified. By addressing these issues an integrity class could in future be assigned with greater confidence.     

The Effect of Zostera noltii, Spartina maritima and Scirpus maritimus on Sediment Pore-water Profiles in a Temperate Intertidal Estuary

The objective of the present work was to study the effect of plants common in temperate latitudes (Zostera noltii, Spartina maritima and Scirpus maritimus) on sediment nutrient profiles, and to compare it to sand- and mud-flats without vegetation. The study focused on the organic matter contents, the concentration of dissolved inorganic nutrients (PO₄–P, NH₃–N, NO₃–N), an on the estimation of the total amount of these nutrients during day and night conditions and their potential net-fluxes. It was also hypothesised that in an estuarine system, different plants may have specific effects, and consequently different contributions to the system nutrient dynamics as a whole. Sediment profiles of loss on ignition (LOI) showed an increase of the organic matter contents from sand-flat, to Zostera, Spartina, mud-flat and Scirpus. Statistically, there were significant differences between sediment profiles of phosphate, ammonia and nitrate (Mann-Whitney test, p<0.05), during day and night periods. These results suggest that there is an intense mobility of nutrients in the sediment, showing a day-night variation of nutrient concentrations in the pore-water. In the plants’ rhizosphere, the day-night variation of nutrients seemed dependent on plant biomass and penetration of the roots. Additionally, coupling between plant and sediment seems to be a species-specific process. In spring, Scirpus salt marsh reaches the maximum density and biomass, and despite the higher organic matter contents in the plant covered sediment, Scirpus acts as a sink of nutrients. In contrast, the top 10 cm of the sediment in the Spartina salt marsh and in the Zostera beds may contribute to the efflux of nutrients during the night period, especially phosphate.     

Interpreting ecological patterns in an intact estuary, South-west New Zealand World Heritage Area

Vegetation pattern, soil attributes, and salinity regimes along with several other environmental factors were studied in a small (11 ha) but unmodified estuary in south Westland, New Zealand. Part of an unmodified 40 km² catchment within the South-west New Zealand World Heritage Area, the Hapuka Estuary, formed behind a 5-km long barrier beach, provides considerable ecological value as well as baseline, conservation advocacy and educative potentials both in a national and international context. Seventy-four quadrats were sampled randomly along five transects at right angles to the main Hapuka River. Eight plant communities comprising 141 native and thirteen exotic vascular species, were differentiated using multivariate analyses, from bare mud (mean elevation 1.23±0.24 m above Mean Low Water Spring (MLWS)), saltmarsh and shrubland associations through to the adjacent podocarp-broadleaved rain forest (4.53±0.74 m above MLWS). Bulk density of the upper 10 cm of substrate varied from a maximum of 1.15 g cm^?3 in the lowest elevation association to a minimum of 0.15 g cm^?3 in the rain forest. pH showed a similar trend with values of 6.97 and 4.31, respectively. The reverse pattern was evident with organic matter, with the highest content (53% ODWt) in the rain forest substrate. The water which irrigates the saltmarsh at high tide reaches 15–19» salinity on calm days but may be much less saline when moderate to strong southerly winds counter the tidal influence. Northerly winds, or a southern outlet through the barrier beach, intermittently evident in the past, are likely to enhance salinity of the tidal waters across the saltmarsh. Ordination of the vegetation samples indicated a very strong gradient associated with Axis 1 (eigenvalue=0.872) and Axis 2 (eigenvalue=0.461). Vector fitting of nine measured environmental factors indicated a strong positive correlation with Axis 1 of the ordination, of soil pH, sodium and conductivity, and negative correlations with elevation, soil water, organic matter and potassium contents.     

Larval release rhythms and tidal exchange in the estuarine mudprawn, Upogebia africana

The mudprawn, Upogebia africana is common in intertidal regions of many South African estuaries. The life cycle is complex, incorporating a marine phase of development during the larval stages. Breeding peaks are in summer and first-stage larvae are released into the plankton at night. Maximum release activity and export to the marine environment follow a semi-lunar cycle synchronized to the time when high water in the estuary is crepuscular. This occurs after peak spring tidal amplitude. Estuarine reinvasion by postlarvae is also nocturnal, and maximum return occurs after neap's when low water at sea occurs around sunset. Rhythmic cycles of larval export and postlarval estuarine reinvasion are therefore asynchronous during the lunar cycle and are best explained by the timing of the change in light intensity relative to high and low water respectively. If maximum activity rhythms of Stage 1 and postlarvae are independent of tidal amplitude, then timing of maximum release and reinvasion during the lunar cycle would alter as the time of sunset shifts between solstices. Much of southern Africa experiences a semi-arid type climate and most estuaries close off from the sea for varying periods owing to sandbar development across tidal inlets. Larvae do not metamorphose if trapped in estuaries and recruitment ceases. Thus, mudprawn populations are directly affected by tidal inlet dynamics. In extreme cases populations become locally extinct if inlets remain closed for extended periods.     

Ecological zonation of the hyperhaline estuary of the Casamance River (Senegal): Foraminifera,zooplankton and abiotic variables

Due to a long-lasting drought afflicting the Sahel, the Casamance River has been transformed into a hyperhaline estuary, with salinities up to 170% at a distance of 210 km from the sea. Foraminifera and zooplankton populations both show a marked decrease in the number of species in increasingly confined water, the distribution of species being closely related to the evolution of abiotic variables. Our three-prong study allowed us to identify six zones, upwards from the sea. The uppermost ones are characterized by drastic conditions which considerably reduce the number of species. Therefore, the populations of Foraminifera, zooplankton and even fish become oligo or mono specific. The Casamance River appears to fit quite well into the general rules concerning hyperhaline environments. However, it shows some peculiar features which are: the large dimensions of the hyperhaline estuary (over 230 km long and over 5 km wide in the lower course); peak salinities among the highest known for a permanently open estuary (up to 170%); and water remaining trapped inside the river for several years.     

A preliminary study of the movements of yellow and silver eels, Anguilla japonica, in the estuary of the Fukui River, Japan, as revealed by acoustic tracking

The behavior and movements of yellow and silver phase Japanese eels were observed using acoustic telemetry in the Fukui River estuary and the adjacent waters of Tachibana Bay, Tokushima Prefecture, Japan. The eels were tagged with ultrasonic transmitters and released in the bay, about 300 m from the river mouth in August and November, 1999. All four yellow eels released at the river mouth in August returned to the river. All eels swam further upstream and each stopped at similar locations as the others, which were possibly used as refuges. Each refuge appeared to be a relatively small area (less than 10 m) adjacent to a series of concrete blocks along the shore (100–300 m). These areas were repeatedly utilized by all the yellow eels tracked during the study. The yellow eels spent most of their time in these refuges during daytime and moved predominantly at night. In contrast, a silver eel released in November demonstrated rapid movement towards the sea without stopping after release.