Ecotone or Ecocline: Ecological Boundaries in Estuaries

Two main ecological boundaries, ecotone and ecocline, have been defined in landscape ecology. At this scale, the estuary represents a boundary between rivers and the sea, but there has been no attempt to fit empirical data for estuaries to these boundary models. An extensive data set from the Thames estuary was analysed using multivariate techniques and species-range analysis, in order to investigate whether the ecocline or the ecotone model was most relevant to this estuary. Data for periods of high and low freshwater flow allowed the impact of large-scale fluctuations implicit in both models to be determined.A continuum of assemblages existed along the salinity gradient from freshwater river to the North Sea, with shifts in the ranges of organisms apparent in response to changes in freshwater flow. This pattern closely fits an ecocline model. However, the estuary differs from previously defined ecoclines in having two overlapping gradients in the major stressor: from river to mid-estuary for freshwater species and from sea to mid-estuary for marine species. We propose, therefore, that the estuary represents a two-ecocline model, with fauna inhabiting the mid-estuary being either freshwater or marine species at the edge of their range, rather than ‘ true estuarine organisms ’. This allows a redefinition of the Remane diagram, with estuarine species removed, and supports previous arguments that brackish-water species do not exist. Such two-ecocline models may also exist in other marine systems, such as rocky shores.     

The Gambia River estuary: A reference point for estuarine fish assemblages studies in West Africa

The Gambia River is one of the last aquatic ecosystems in West Africa that has not yet been affected by strong environmental changes and human disturbances. In contrast to the neighbouring Casamance and Sine Saloum estuaries, the Gambia estuary is free of major climatic perturbation and remains a “normal” estuary, with a salinity range from freshwater to 39. The present paper aims to study the spatial and seasonal variability of fish assemblages in this estuary in terms of bio-ecological categories and of their relation with some environmental variables. Four surveys were conducted, from June 2001 to April 2002, in order to cover the major hydroclimatic events, at 44 sampling sites along the lower, intermediate and upper zones of the Gambia estuary (up to 220 km). Fish assemblages were sampled using a purse seine net, fish were identified to species level and environmental variables such as water depth, transparency, salinity, temperature and percentage oxygen saturation were measured. The main spatial structure of the fish assemblages and its seasonal changes were first studied using the STATIS-CoA multitable method. The combination of fish assemblages and environmental variables was then analysed using the STATICO method, designed for the simultaneous analysis of paired ecological tables. A total of 67 species were observed, belonging to all bio-ecological categories characterizing West African estuaries. The marine component of the community was largely dominant throughout the estuary, while the freshwater component was permanently observed only in the upstream zone. The main spatial structure was a longitudinal gradient contrasting marine and freshwater affinity assemblages, with strong seasonal variations. The most complete gradient was observed in December, at the beginning of the dry and cool season, while in June, at the end of the dry and warm season, there was the least structured gradient. The role of salinity, always correlated with temperature, was emphasized, while turbidity appeared to be another important factor. Oxygen and depth did not play a major role at the estuary scale. The relative importance of the bio-ecological categories varied according to the season and the distance to sea. Stable fish assemblages were observed in the lower zone at the end of the dry season, in the upper zone during the flood and in the middle zone throughout the year. In some situations, a relative inadequacy between fish assemblages and their environment was noticed. The present study contributes to the definition of the functioning of a “normal” West African estuary, the Gambia estuary, with balanced effects of marine and freshwater influences and the presence of all bio-ecological categories. The Gambia estuary can therefore be considered to be a reference ecosystem for further comparisons with other tropical estuarine ecosystems, subjected to natural or artificial perturbations.     

The characteristics of the nematode faunas in subtidal sediments of a large microtidal estuary and nearshore coastal waters differ markedly

The present study examines traditional paradigms regarding the differences between faunas in estuaries vs coastal waters. The ecological characteristics of the free-living nematode faunas of nearshore, subtidal sediments in downstream and upstream areas of the large, microtidal Swan River Estuary are compared with those similarly recorded seasonally in subtidal sediments along an adjacent part of the coast of temperate south-western Australia. Overall, the nematode species richness recorded in the upstream (38) and downstream estuarine areas (58) and from throughout the estuary (61) were substantially less than in marine waters (75). In addition, the value for Simpson’s diversity index was marginally less in the estuary and the dominance of the most abundant species greater. In contrast, the mean nematode species richness and diversity in individual cores followed the reverse trend, reflecting a combination of less variability among the species compositions and far greater densities in the cores from estuarine sediments. Furthermore, the mean density (numbers 10 cm⁻²) was far higher in both upstream (341) and downstream (903) areas of the estuary than in marine waters (87). Although the compositions of the assemblages in upstream and downstream estuarine areas differed markedly from each other at the species, genus and family levels, these differences were less pronounced than those between either of these areas and marine waters. The trophic compositions at the moderately sheltered and fully exposed marine sites differed from that in both areas of the estuary, whereas that at the most sheltered marine site was similar to that in the downstream estuarine area, with both containing substantial proportions of epistrate-grazing species. The variations among the species richness, diversity, densities and taxonomic and trophic compositions of nematode assemblages in the sediments of the two estuarine areas and nearby marine waters appear to reflect differences in 1) salinity regimes, 2) extents of exposure to wave action and its related effects and 3) amounts and types of food available to nematodes.     

The influence of catchment management on salinity,nutrient stochiometry and phytoplankton biomass of Eastern Cape estuaries,South Africa

Three estuaries with differing catchment use and freshwater input were investigated in terms of their nutrient status, phytoplankton biomass, freshwater inflow and salinity between 1993 and 1995. The nutrients analysed include phosphate, nitrate, nitrite, ammonia and total particulate nitrogen. All the parameters were investigated for their relationship with land-use and freshwater abstraction. The Kromme River catchment area is relatively pristine, the river is impounded for ca. 133% of its mean annual runoff, and consequently, freshwater input into the estuary is only episodic. Nutrient and chlorophyll-a concentrations are low, but become elevated when freshwater does reach the estuary. The Geelhoutboom tributary contributes nutrients to the Kromme estuary during high freshwater inflow conditions, but is not a viable nutrient contributor during low flow conditions. Freshwater abstraction from the Swartkops River catchment is limited, and it is characterised by urbanisation and industrial development. The Swartkops River was the main source of phosphate in the estuary, whereas other small tributaries along the estuary were additional point sources for nitrate, ammonia and nitrite. The third system, the Sundays estuary, has no tributaries or other point sources except the Sundays River, where the catchment is extensively used for agriculture and freshwater input relatively high. The phytoplankton biomass (in terms of chlorophyll-a) was highest in the Sundays estuary, although phosphate concentrations were as low as in the Kromme estuary. Trends over time indicated a decrease in phosphate concentrations and showed variations for inorganic dissolved nitrogen concentrations since the previous 15 years in all the three estuaries. Nutrient stochiometry had changed in favour of inorganic dissolved nitrogen.     

The impact of river flooding and high flow on the demersal fish assemblages of the freshwater-dominated Great Fish Estuary,South Africa

Freshwater inflow has a strong impact on the biological, chemical and physical characteristics of estuaries, which in turn affect the distribution and abundance of estuarine organisms. Increased climatic variability associated with climate change is predicated to modify precipitation patterns, which will likely intensify floods in estuaries. The demersal fish assemblage of the freshwater-dominated Great Fish Estuary, South Africa, was sampled using beam trawls, monthly, from December 2013 to November 2014. The first six months of the study were characterised by river flooding and high flow, with estuarine conditions found only in the mouth region; this was followed by six months of intermediate flow, with estuarine conditions recorded up to 10 km from the mouth. River flooding and subsequent reduced salinity resulted in a decrease in species richness and abundances of fishes in the estuary, with only two estuarine species (Glossogobius callidus and Psammogobius knysnaensis) and one marine migrant (Solea turbynei) recorded following river flooding (201 m³ s^?1), in January 2014. The greatest species richness and abundances among both marine and estuarine fishes were recorded during intermediate flow conditions. We conclude that although freshwater inflow into estuaries is important for the nursery function of these systems, flooding—especially in freshwater-dominated estuaries—may cause a temporary decline in the abundance of most marine and estuarine fish species, including important bentho-pelagic marine migrant fishery species, such as Argyrosomus japonicus and Pomadasys commersonnii.     

Dry Season Salinity Changes in Arid Estuaries Fringed by Mangroves and Saltflats

Mangrove swamps and hypersaline saltflats fringe many estuaries in dry tropical climates, especially in Northern Australia. For most of the year these estuaries receive zero riverine freshwater input and thus, after the wet season, a steady increase in salinity occurs. In some locations the estuary becomes fully inverse, i.e. the salinity increases monotonically from the mouth to the head. In other locations, a salinity maximum zone separates the sea from low salinity water that persists at the head of the estuary throughout the dry season. Field data from five estuaries indicate that in short estuaries where a large area of saltflats and mangroves extends over the whole length of the estuary, the estuary becomes completely inverse with salinity rising to 55 within a couple of months. The evaporation and evapotranspiration over the saltflats and mangroves cause this rapid increase in salinity. Longer estuaries where a large area of salt flat exists only close to the mouth do not become completely hypersaline for the whole length of the estuary by the end of the dry season. A salinity-maximum is generated close to the river mouth but salinities of less than 10 persist in the upper reaches of the estuary until the end of the dry season, even though the estuary does not receive any further freshwater input. A simple analytical expression is presented that reproduces the changes in salinities in the estuaries studied. This model can be used to predict the formation of hypersaline conditions in other mangrove and saltflat fringed estuaries where freshwater flow is negligible.     

Changes in phytoplankton diversity and community composition along the salinity gradient of the Schelde estuary (Belgium/The Netherlands)

Phytoplankton diversity and community composition were studied along the Schelde river–estuary–coastal zone continuum during the summer of 2003. DCA analysis indicated a gradual compositional turnover of the phytoplankton community within the estuary. GAM modelling of species response curves along the estuarine gradient was used to identify taxa that had their population maximum in the river, the coastal zone or within the estuary. Taxa that had their population maximum within the estuary did not form a homogenous community but comprised species with different salinity optima and rather restricted salinity tolerances. The observed changes in community composition along the estuarine transition zone correspond more closely to an ecocline than to an ecotone model. Despite the fact that few taxa had their population maximum at or near the salinity gradient, alpha diversity did not display a minimum around the salinity gradient. This lack of a diversity minimum within the estuary was ascribed to an important contribution of taxa of riverine or coastal origin to alpha diversity within the estuary contributed. On average 55% of the alpha diversity in the estuarine samples was due to riverine or coastal taxa. Beta diversity displayed a clear maximum around the salinity gradient. For planktonic organisms which are subject to mass effects, beta diversity is probably a better indicator for the impact of the salinity gradient on diversity in estuaries than alpha diversity.     

The effect of a storm surge event on the macrophytes of a temporarily open/closed estuary,South Africa

Temporarily open/closed estuaries typically open to the sea due to freshwater inflow coupled with storm surge events. In September 2008, in the absence of freshwater inflow, the mouth of the East Kleinemonde Estuary breached in response to a storm surge. The mouth of the estuary closed the following day at a high level. Marine overwash events following the breach introduced large volumes of saline water into the estuary and raised the water level by 0.07–0.33 m. Salinity was significantly higher in the 15 month closed phase after the breach (31 ± 0.9) compared to 21.9 ± 0.9 in the closed brackish phase before the breach. The historical average salinity for the estuary during a closed period is 23–25. The increase in salinity has reduced submerged macrophytes Ruppia cirrhosa and Chara vulgaris cover by 38.1%. Macroalgal cover of species such as Dictyota dichotoma, Caulacanthus ustulatus, Codium tenue and Ulva spp. have increased by 7.9%. The saline high water levels have also significantly reduced supratidal salt marsh cover by 15.2%, and reed and sedge cover by 19.7%. Loss of these habitats may result in bank destabilisation and erosion. This is the first record of an extended saline period in the 15 years the estuary has been monitored. Sea level rise in association with climate change, together with localised freshwater inflow reduction is likely to result in an increase in marine overwash events. The frequency and duration of closed saline periods are likely to increase in this type of estuary. A loss of submerged macrophytes may have significant impacts on faunal composition and abundance and on the subsequent functioning of temporarily open/closed estuaries. This has serious ecological implications since these estuaries represent 70% of the different types of estuaries found in South Africa.     

Meiobenthic communities of seagrass beds (Zostera capricorni) and unvegetated sediments along the coast of New South Wales,Australia

Seagrass beds have higher biomass, abundance, diversity and productivity of benthic organisms than unvegetated sediments. However, to date most studies have analysed only the macrofaunal component and ignored the abundant meiofauna present in seagrass meadows. This study was designed to test if meiobenthic communities, especially the free-living nematodes, differed between seagrass beds and unvegetated sediments. Sediment samples from beds of the eelgrass Zostera capricorni and nearby unvegetated sediments were collected in three estuaries along the coast of New South Wales, Australia. Results showed that sediments below the seagrass were finer, with a higher content of organic material and were less oxygenated than sediments without seagrass. Univariate measures of the fauna (i.e. abundance, diversity and taxa richness of total meiofauna and nematode assemblages) did not differ between vegetated and unvegetated sediments. However multivariate analysis of meiofaunal higher taxa showed significant differences between the two habitats, largely due to the presence and absence of certain taxa. Amphipods, tanaidacea, ostracods, hydrozoans and isopods occurred mainly in unvegetated sediments, while kinorhyncs, polychaetes, gastrotrichs and turbellarians were more abundant in vegetated sediments. Regarding the nematode assemblages, 32.4% of the species were restricted to Z. capricorni and 25% only occurred in unvegetated sediments, this suggests that each habitat is characterized by a particular suite of species. Epistrate feeding nematodes were more abundant in seagrass beds, and it is suggested that they graze on the microphytobenthos which accumulates underneath the seagrass. Most of the genera that characterized these estuarine unvegetated sediments are also commonly found on exposed sandy beaches. This may be explained by the fact that Australian estuaries have very little input of freshwater and experience marine conditions for most of the year. This study demonstrates that the seagrass and unvegetated sediments have discrete meiofaunal communities, with little overlap in species composition.     

Meiobenthos assemblages in the mekong estuarine system with special focus on free-living marine nematodes

Meiobenthos assemblages in eight estuaries of the Mekong river system were investigated in August 2008 (from the Cua Tieu estuary to the Tran De estuary). In each estuary, one sampling station was established for meiobenthos sampling. Twelve major taxa of meiobenthos were recorded in this estuarine system, including Nematoda, Copepoda, Turbellaria, Polychaeta, Oligochaeta, Tardigrada, Bivalvia, Ostracoda, Amphipoda, Cumacea, Gastrotricha, Gastropoda, and Crustacean Nauplii larvae. The densities of the meiobenthos range from 581 to 3168 inds/10 cm². Nematodes always occupy the highest numbers with a percentage ranging from 64–99%. There are 135 nematode genera recorded in this study with the following as dominant genera Desmodora, Leptolaimus, Halalaimus, Thalassomonhystera, Theristus, Daptonema, Rhynchonema, Parodontophora, and Oncholaimus. Although the biodiversity of the meiobenthos at higher taxa level is not high compared to other marine environments, the estimates of nematode biodiversity at the genus level indicates high values. The increase in number of genera with increasing sampling intensity illustrate that the diversity is underestimated and would have been higher if the authors had considered a larger number of individuals, more replicates per station, and more sampling stations.     

Influence of Wind on Dynamics and Flushing of Shallow Estuaries

Observations of two small estuaries in Cape Cod, U.S.A. indicate large variations in salinity structure that are forced by variations in along-estuary wind stress. During onshore winds, the estuarine circulation is reduced, and the along-estuary salinity gradient increases as freshwater accumulates. During offshore winds, the surface outflow is enhanced, freshwater is flushed out of the estuary, and the along-estuary salinity gradient becomes weak. Constrictions block the wind-induced flushing, resulting in strong salinity fronts across the constrictions. The residence time of one of the estuaries varies by more than a factor of three in response to variations in wind-induced flushing. The other estuary has little variation of flushing associated with winds, due to a constriction at the mouth that inhibits the wind-induced exchange. The strong influence of winds on the flushing of these estuaries is due in part to their shallow depths, which accentuates the influence of wind stress relative to the effects of the horizontal density gradient. In addition, the residence times of the estuaries are comparable to the time scale of wind forcing, allowing large changes in water properties during wind events.     

Reproduction of the intertidal barnacle Balanus glandula along an estuarine gradient

The barnacle Balanus glandula is predominantly an open coast species in the Northeast Pacific. However, B. glandula densely inhabits estuaries where environmental conditions such as salinity and temperature drastically differ from the open coast. The increased variability of environmental conditions within an estuary can potentially affect spatial patterns of reproduction in B. glandula . I examined gonad maturity, reproductive periodicity and fecundity, and then calculated reproductive output in B. glandula populations distributed along an estuarine gradient. Results indicated that reproductive output of this intertidal barnacle decreased four times over a spatial scale of kilometers, as a transition occurred from a marine to a freshwater habitat along an estuarine gradient. Additionally, a higher proportion of the population had well-developed gonads in the oceanic end of the estuary compared to the riverine end. These results indicate how reproductive pattern can significantly vary over a spatial scale of kilometers, resulting in site-specific contributions of offspring to the larval pool.     

Distinctive copepod community of the estuarine turbidity maximum: comparative observations in three macrotidal estuaries (Chikugo, Midori, and Kuma Rivers), southwestern Japan

Copepods are considered to be a vital component connecting the unique macrotidal environment to the high productivity and high biodiversity of the Ariake Sea. To examine the spatiotemporal succession of copepod communities, we conducted monthly sampling (vertical hauls of a 100-μm mesh plankton net) in three neighboring macrotidal estuaries between 2005 and 2006. Irrespective of the season, three copepod communities were recognized in relation to the relatively long gradients of salinity and turbidity along the Chikugo and Midori River estuaries. The oligohaline community (salinity 1–10) was observed at higher turbidities (>100 NTU), whereas the freshwater (salinity <1) and meso/polyhaline (salinity >10) communities were associated with lower turbidities (<100 NTU). The oligohaline calanoid Sinocalanus sinensis occurred only in the Chikugo River estuary, maintaining a large biomass (dry weight >10 mg m^?3) in or close to the well-developed estuarine turbidity maximum (ETM) throughout the year. In the Midori River estuary, the oligohaline community lacked S. sinensis and showed a minimum biomass during winter (<10 mg m^?3). In both estuaries, the freshwater community always remained at a small biomass (<1 mg m^?3), whereas the meso/polyhaline community showed marked seasonal changes in biomass (0.1–657 mg m^?3). The prevalence of higher salinities allowed only the meso/polyhaline community to occur in the Kuma River estuary. In summary, S. sinensis characterized the copepod community distinctive of the well-developed ETM, potentially serving as an important link to higher trophic levels during winter when copepods are scarce in other areas.     

Diatom succession and silicon removal from freshwater in estuarine mixing zones: From experiment to modelling

Estuaries act as filters for land derived material reducing the river input to the coastal zone. Silicon (Si) removal from freshwater which is tightly linked to the growth of diatoms was studied in the estuarine mixing zone where the mixing of freshwater and seawater results in a salinity gradient. Three planktonic diatom species with different origin and salinity tolerance were grown in an artificial salinity gradient. Salinity stress and nutrient depletion led to a specific succession of the three diatoms along the salinity gradient. When available light was increased, diatoms reached higher biomass and the Si removal from water column was more efficient along the mixing. From this experiment, a conceptual model of Si transformations and removal from freshwater was build and applied to an idealized stratified estuary. Sensitivity analysis with varying initial conditions and parameter values pointed transit time of freshwater in the estuary, freshwater and seawater mixing rate and river turbidity as important interactive factors influencing Si removal from freshwater. Other factors like the total amount and the salinity tolerance of diatoms in the upstream river were shown to significantly affect riverine Si removal from the surface layer of an estuary. Finally it appears that Si removal from freshwater in estuarine mixing zones proceeds in two ways: a first rapid death and sedimentation of planktonic stenohaline diatoms imported from the river and second, the growth and subsequent settling of planktonic euryhaline diatoms of either freshwater or marine origin.     

Comparisons between the characteristics of ichthyofaunas in nearshore waters of five estuaries with varying degrees of connectivity with the ocean

The characteristics of the fish faunas in nearshore, shallow (<1.2 m) waters of the basins of estuaries along the same coastline, but which were open to the ocean for varying periods, have been determined and compared. The fish faunas of the permanently-open Oyster Harbour, the seasonally-open Broke, Irwin and Wilson inlets and the normally-closed Wellstead Estuary on the south coast of Western Australia were sampled by seine net seasonally for 2 years. Irrespective of the frequency and duration that the estuary mouth was open, the ichthyofauna of each estuary was numerically dominated by three atherinid species and three gobiid species (92.9–99.7%), each of which completes its life cycle within these estuaries. The ichthyofaunal compositions of each estuary differed significantly, however, from that of each other estuary. These differences were largely attributable to the relative abundances of the above six species varying between estuaries, which, in turn, reflected differences in such factors as estuary mouth status, macrophyte cover and salinity. For example, Favonigobius lateralis and Leptatherina presbyteroides, which are also represented by marine populations, were most abundant in the permanently-open estuary (Oyster Harbour), which, in terms of substrate and salinity, most closely resembled the nearshore marine environment. In contrast, Leptatherina wallacei made its greatest contribution in the only estuary to exhibit a protracted period of greatly reduced salinities, which is consistent with its distribution in permanently-open estuaries on the lower west coast of Australia, while Atherinosoma elongata and Pseudogobius olorum were particularly numerous in estuaries containing dense stands of the seagrass Ruppia megacarpa. Marine species made the greatest contribution to species richness in the permanently-open estuary and least in the normally-closed estuary. Species richness was greatest in summer and least in winter in each estuary, but differed markedly between years only in Wilson Inlet. Density of fishes was greatest in the most eutrophic estuary (Wellstead Estuary) and least in the most oligotrophic estuary (Broke Inlet) and only underwent marked seasonal variations in Wilson Inlet and Wellstead Estuary, in which densities fell to their minima in winter. Ichthyofaunal composition varied between years in the Broke and Wilson inlets and Wellstead Estuary, in which there was little or no connection with the ocean in one of those years. Species composition underwent progressive seasonal changes throughout the year in Wellstead Estuary, due to the abundance of certain species peaking at different times of the year.     

Comparison of,and the effects of altered freshwater inflow on,fish assemblages of two contrasting South African estuaries: the cool-temperate Olifants and the warm-temperate Breede

This study compares the fish assemblages of the permanently open cool-temperate Olifants and warm-temperate Breede estuaries on the west and south coasts of South Africa respectively, and their responses to altered freshwater flows. Both estuaries have experienced a >35% reduction in mean annual runoff (MAR) from a historical reference condition to the present day with a >60% reduction possible under future flow scenarios. With the exception of species that have both marine and estuarine breeding populations, the Olifants Estuary fish assemblage has experienced an overall 20% decrease in abundance from reference (pristine state) to the present day and will gradually decline to 55% of reference with a predicted future 60% reduction in MAR. Consequently, future reductions in flow are likely to result in the Olifants Estuary progressing towards a low biomass, low diversity, marine-dominated system. In contrast, reduced freshwater flows in the Breede Estuary are likely to experience an overall reduction in the abundance of species that breed only in estuaries, and in freshwater and catadromous species. Collectively, entirely estuarine-dependent fish will increase in abundance, but considered individually some important exploited species such as Argyrosomus japonicus and Pomadasys commersonnii will collapse to 50% of historical numbers once there has been a 64% reduction in MAR. Overall, fish abundance in the estuary has increased by 6% from reference to the present day and is likely to increase to 115% of reference with future reductions in flow. Some species with a preference for fresh and brackish water will be all but lost from the system, but overall diversity is likely to increase with the range expansion of warm-temperate and subtropical marine species westward. In all, the fish assemblage of the Breede Estuary will experience a gradual change from a relatively high-diversity, low-abundance, freshwater rich system under historical flow conditions to a high diversity, high-abundance, marine-dominated system with future reductions in flow.     

The concept of an estuary: A definition that incorporates systems which can become closed to the ocean and hypersaline

The majority of the definitions of estuaries have been based on the characteristics of estuaries in temperate regions of the northern hemisphere. As previously pointed out (Day, 1980), such definitions do not take into account such features as periodic closure of their mouths and hypersaline conditions during dry periods, which characterise many estuaries in southern Africa and south-western Australia. There is also ambiguity as to whether an estuary sensu stricto must be fed by a river. The following definition was developed to encompass the main characteristics of all estuaries: An estuary is a partially enclosed coastal body of water that is either permanently or periodically open to the sea and which receives at least periodic discharge from a river(s), and thus, while its salinity is typically less than that of natural sea water and varies temporally and along its length, it can become hypersaline in regions when evaporative water loss is high and freshwater and tidal inputs are negligible. Estuaries are thus regarded as unique ecosystems, which, in the case of fishes, for example, are occupied by species that collectively represent a particular suite of guilds.     

Dynamics of recruitment of larval and juvenile Cape stumpnose Rhabdosargus holubi (Teleostei: Sparidae) into the Swartkops and Sundays estuaries,South Africa

Recruitment of early life stages into estuaries is an integral part of the life cycle of many marine fish species. Although estuaries are naturally environmentally dynamic, they also are subject to anthropogenic disturbances, including land use and climate change, which may affect recruitment. Rhabdosargus holubi is an endemic marine-spawning species predominantly associated with freshwater-rich estuaries which serve as nursery areas for postflexion larvae and juveniles. This study assessed the effect of environmental variables on the dynamics of recruitment of R. holubi larvae and juveniles into the Swartkops and Sundays estuaries, South Africa. Over a period of two years, fyke nets were set at each estuary mouth to monitor movement into the estuaries at each tidal phase over a 24 h cycle during two sampling sessions per season. Rhabdosargus holubi larvae recruited into estuaries primarily in summer and autumn and during the ebb tide at night, while juvenile movements showed no pattern. Salinity, turbidity and temperature were significantly important factors affecting R. holubi recruitment, with pH having no significant effect. Turbidity and salinity are affected by rainfall and freshwater abstraction which may thus influence recruitment. Rhabdosargus holubi is dependent on estuaries, so a combination of future changes in turbidity, salinity and temperature due to predicted climate change may place populations at risk.     

Sea ice,hydrological, and biological processes in the Churchill River estuary region,Hudson Bay

A conceptual scheme for the transition from winter to spring is developed for a small Arctic estuary (Churchill River, Hudson Bay) using hydrological, meteorological and oceanographic data together with models of the landfast ice. Observations within the Churchill River estuary and away from the direct influence of the river plume (Button Bay), between March and May 2005, show that both sea ice (production and melt) and river water influence the region's freshwater budget. In Button Bay, ice production in the flaw lead or polynya of NW Hudson Bay result in salinization through winter until the end of March, followed by a gradual freshening of the water column through April–May. In the Churchill Estuary, conditions varied abruptly throughout winter–spring depending on the physical interaction among river discharge, the seasonal landfast ice, and the rubble zone along the seaward margin of the landfast ice. Until late May, the rubble zone partially impounded river discharge, influencing the surface salinity, stratification, flushing time, and distribution and abundance of nutrients in the estuary. The river discharge, in turn, advanced and enhanced sea ice ablation in the estuary by delivering sensible heat. Weak stratification, the supply of riverine nitrogen and silicate, and a relatively long flushing time (∼ 6 days) in the period preceding melt may have briefly favoured phytoplankton production in the estuary when conditions were still poor in the surrounding coastal environment. However, in late May, the peak flow and breakdown of the ice-rubble zone around the estuary brought abrupt changes, including increased stratification and turbidity, reduced marine and freshwater nutrient supply, a shorter flushing time, and the release of the freshwater pool into the interior ocean. These conditions suppressed phytoplankton productivity while enhancing the inventory of particulate organic matter delivered by the river. The physical and biological changes observed in this study highlight the variability and instability of small frozen estuaries during winter–spring transition, which implies sensitivity to climate change.     

Horizontal distribution and population dynamics of the dominant mysid Hyperacanthomysis longirostris along a temperate macrotidal estuary (Chikugo River estuary, Japan)

The estuarine turbidity maximum (ETM) that develops in the lower salinity areas of macrotidal estuaries has been considered as an important nursery for many fish species. Mysids are one of the dominant organisms in the ETM, serving as a key food source for juvenile fish. To investigate the horizontal distribution and population dynamics of dominant mysids in relation to the fluctuation of physical conditions (temperature, salinity, turbidity, and freshwater discharge), we conducted monthly sampling (hauls of a ring net in the surface water) along the macrotidal Chikugo River estuary in Japan from May 2005 to December 2006. Hyperacanthomysis longirostris was the dominant mysid in the estuary, usually showing peaks of density and biomass in or close to the ETM (salinity 1–10). In addition, intra-specific differences (life-cycle stage, sex, and size) in horizontal distribution were found along the estuary. Larger males and females, particularly gravid females, were distributed upstream from the center of distribution where juveniles were overwhelmingly dominant. Juveniles increased in size toward the sea in marked contrast with males and females. The findings suggest a possible system of population maintenance within the estuary; gravid females release juveniles in the upper estuary, juveniles grow during downstream transport, young males and females mature during the upstream migration. Density and biomass were primarily controlled by seasonal changes of temperature, being high at intermediate temperatures (ca. 15–25 °C in late spring and fall) and being low at the extreme temperatures (ca. 10 °C in midwinter and 30 °C in midsummer). High density (up to 666 ind. m⁻³) and biomass (up to 168 mg dry weight m⁻³) of H. longirostris were considered to be comparable with those of copepods in the estuary.