Influence of site,season and year on contributions made by marine,estuarine, diadromous and freshwater species to the fish fauna of a temperate Australian estuary

Catches obtained at regular intervals by beach seining, gill netting and otter trawling at ten, four and six sites, respectively, have been used to determine the contribution of the different species and life-cycle categories of fish to the ichthyofauna of the large Swan Estuary in temperate south-western Australia between February 1977 and December 1981. These data were also examined to investigate the influence of site, season and year on the densities of the more abundant species. A total of 630 803 fish, representing 36 families and 71 species, were caught in the shallows using beach seines during this 5 yr study. Although the majority of these species were marine teleosts that were caught infrequently (marine stragglers), representatives of 7 of the 15 most abundant species were marine teleosts which entered the estuary regularly, and in large numbers (marine estuarine-opportunists). Of the remaining 8 most abundant species in the shallows, 7 completed their life cycle within the estuary (estuarine species) and 1 (Nematalosa vlaminghi) was anadromous, feeding for a period at sea and spawning in the upper reaches of the estuary. The contribution of individuals of the marine estuarine-opportunist category to catches in the shallows declined from nearly 95% in the lower estuary, to 17% in the middle estuary and 6% in the upper estuary. The estuarine and anadromous groups made a considerable contribution to the catches in both the middle and upper estuaries. By contrast, the contribution of freshwater species was small and even in the upper estuary accounted for only 0.2% of the catch. Site within the estuary generally influenced the catches of individual species to a greater extent than either season or year, or the interactions between these factors. When seasonal effects were strong, they could be related to summer spawning migrations into the upper estuary (Nematalosa vlaminghi, Amniataba caudavittatus), spring immigrations into the lower estuary (Mugil cephalus), or winter movements into deeper and more saline waters (Apogon rueppellii). Annual variations in the density of Torquigener pleurogramma were related to marked annual differences in the recruitment of the 0+age class.     

The fish fauna of a seasonally closed Australian estuary. Is the prevalence of estuarine-spawning species high?

The fish at sites located throughout the large, seasonally closed Wilson Inlet, on the southern coast of Western Australia, were sampled bimonthly between September 1987 and April 1989. Seine nets were used to sample nearshore shallow waters, while gill nets were employed in slightly more offshore and deeper waters. Twenty species were recorded in the shallows, of which the three species of atherinid and the three species of goby comprised >97% of the total catch. In terms of number of individuals, the 27 species recorded in gill nets in the deeper waters were dominated by Cnidoglanis macrocephalus and Platycephalus speculator, and to a lesser extent Engraulis australis, Aldrichetta forsteri, Sillaginodes punctata and Arripis georgianus. Fifty-five percent of the species recorded in the nearshore shallow waters and 18% of those in offshore deeper waters spawn within Wilson Inlet; these species contributed 98.5 and 63.0%, respectively, to the total catches in those waters. Classification and ordination showed that the composition of the fauna in the shallows was similar at all sites throughout the large basin and did not change conspicuously with season. However, the composition of catches taken in offshore waters differed between the lower part of the basin and the middle and upper regions of the basin, which in turn differed from those in the saline reaches of a tributary river. The four diagnostic species of the lower estuary were all marine species, while the three diagnostic species in the river included a marine species (Mugil cephalus) that often penetrates far upstream in other systems, and a species which was confined to the rivers (Acanthopagrus butcheri). The composition of the fish fauna in the offshore waters of the lower estuary between the middle of spring and middle of autumn was different in 1987/1988 (when the estuary mouth was open for only the first two months of that period) from that in 1988/1989 (when the mouth was open for the whole of that period). This difference is related to the greater number of marine species that were retained in the first of these years, when the estuary was open to the ocean for only a short period. The greater retention of marine species in 1987/1988 than in 1988/1989 probably reflects a far lower level of freshwater flusing and/or a less marked decline in salinity.     

Spatial and seasonal differences in the fish fauna in the shallows of a large Australian estuary

Samples of fish were collected by beach seine throughout the shallow waters of the large Peel-Harvey estuarine system (south-western Australia) in the wet (June to November) and dry periods (December to May) between August 1979 and July 1981. The number of species, density and biomass declined with distance from the estuary mouth and rose with increasing temperature and salinity. Both classification and ordination distinguished the faunal composition of the saline reaches of the rivers from that of the narrow Entrance Channel and two large basins (Peel Inlet and Harvey Estuary). Classification also separated the fauna of the riverine group into wet- and dry-period components, and divided samples taken in the Entrance Channel from those in the basins. Differences between the faunal composition of the Peel Inlet and its tributary rivers were related to differences in salinity regime. The riverine fauna was subjected to much more variable and lower minimum salinities. Species characteristic of the rivers included teleosts such as Atherinosoma wallacei and Amniataba caudavittatus, which are estuarine sensu stricto in southwestern Australia, the semi-anadromous Nematalosa vlaminghi and juveniles of the marine Mugil cephalus. The species diagnostic of the wet periods in the rivers were the estuarine species A. wallacei and Favonigobius suppositus, while the dry periods were characterised by the marine species Atherinomorus ogilbyi and Sillago schomburgkii. Marine species also characterised the Entrance Channel (Favonigobius lateralis, Sillago bassensis), whereas the indicators in Peel Inlet and Harvey Estuary were Hyporhamphus regularis and Apogon rueppellii, both of which can pass through the whole of their life cycle in estuarine as well as marine environments.     

River inflow, estuarine salinity, and Carolina wolfberry fruit abundance: linking abiotic drivers to Whooping Crane food

The supply of freshwater to estuarine ecosystems is a critical factor in maintaining the overall health and organization of coastal marshes. Specifically along the Texas Gulf coast, the coupled effects of decreased freshwater inflows to the estuary and natural processes (e.g., precipitation, wind, and tides) can exert significant salt-stress on coastal marsh vegetation. In this project we sought to quantitatively link the inflow of freshwater to the estuary (San Antonio Bay) with Aransas National Wildlife Refuge (ANWR) coastal marsh salinity and assess the influence of salinity and inundation on Carolina wolfberry (Lycium carolinianum Walt.) phenology (leaf and fruit abundance). The Carolina wolfberry is one of the more common high marsh plant species found at ANWR and has been shown to be a key food source for endangered Whooping Cranes which inhabit the coastal marshes of the ANWR each fall/winter. Results from our study show that periods of decreased freshwater inflows to the estuary correlated with increased marsh salinity at the ANWR. Wolfberry plants at ANWR marsh sites displayed increased fruit abundance during years which had lower mean summer time salinity (June, July, and August) in San Antonio Bay; conversely, during years of increased bay salinity during the same summertime months, wolfberry plants showed decreased fruit abundance. Through the continued validation of the relationship between inflows and coastal marsh salinity, we hope to provide additional insight into how wolfberry phenology varies inter-annually across both salinity and inundation regimes and how freshwater inflows may affect food availability for the endangered Whooping Crane.     

Influence of environmental variables on the fish fauna of the deeper waters of a large Australian estuary

Fish were collected by gill nets from the deeper waters of the Entrance Channel, basins and rivers of the large Peel-Harvey estuarine system (south-western Australia) in the wet (June to November) and dry (December to May) periods between August 1979 and July 1981. Simple-regression analysis showed that the number of species, abundance and biomass of fish in the rivers rose with increases in the salinity and temperature of both the surface and bottom of the water column. No such significant correlations were found in the Entrance Channel andbasins (Peel Inlet and Harvey Estuary), where salinity changes were far less marked. The number of species at sites throughout the estuary was inversely correlated with distance from the estuary mouth. Multiple-regression equations showed that, compared with the other environmental variables tested, bottom salinity had a greater influence on the nunber of species and abundance both in the rivers and in the system as a whole. These results indicate that salinity has a greater effect on the fauna in the deeper waters than in the shallows (cf. Loneragan et al., 1986). The larger fish which characterise the deeper waters may thus be less tolerant to low salinities than the smaller fish typically found in the shallows. Both classification and ordination separated the faunal composition of the rivers from those of the Entrance Channel and basins. The fauna of the two narrow and deeper sites in the rivers separated into wet- and dry-period components. Differences between the faunal composition of the riverine regions and those of the Entrance Channel and basins have been related to the much more variable and lower minimum salinities in the rivers. Species characteristic of the rivers included Amniataba caudavittatus, which is estuarine sensu stricto in south-western Australia, the semianadromous Nematalosa vlaminghi and the highly euryhaline Mugil cephalus. The indicator species for the Entrance Channel and basins were all marine species (Cnidoglanis macrocephalus, Hyporhamphus melanochir, Gerres subfasciatus and Pomatomus saltator).     

Ichthyofaunas of a temperate estuary and adjacent marine embayment. Implications regarding choice of nursery area and influence of environmental changes

A fine-mesh seine net was used at regular intervals to collect fishes from the entrance channel and basin of the Blackwood River Estuary (south-western Australia), from Deadwater Lagoon, which is joined to the entrance channel by a narrow and shallow water-course and thus constitutes part of this estuary, and from Flinders Bay into which the estuary discharges. Sampling was at six-weekly intervals between February and December 1994. The juveniles of some marine species, such as Pelates sexlineatus, Rhabdosargus sarba and Aldrichetta forsteri, were either found only in the estuary or were in far higher densities in the estuary than in Flinders Bay. In contrast, the juveniles of some other marine species, such as Sillago schomburgkii, were relatively abundant in both environments, while others such as S. bassensis, Pelsartia humeralis, Lesueurina platycephala and Spratelloides robustus were either far more abundant in Flinders Bay or entirely restricted to this marine embayment. The various marine species found in inshore waters thus apparently vary considerably in their “preference” for estuaries as nursery areas. Although some marine species were abundant in the shallows of the estuary, the fish fauna of these waters was dominated by the estuarine-spawning species Leptatherina wallacei, Favonigobius lateralis, L. presbyteroides and Atherinosoma elongata. The above regional differences help account␣for the very marked difference that was found between the compositions of the shallow-water␣ichthyofaunas of Flinders Bay and each of the three estuarine regions. The ichthyofaunal compositions of the basin and channel underwent pronounced changes during winter, when freshwater discharge increased markedly and salinities in the estuary thus declined precipitously. This faunal change was mainly attributable to the emigration of marine stragglers, a reduction in the densities of marine estuarine-opportunist species such as Pelates sexlineatus and R. sarba, and the immigration of large numbers of both young 0+ Aldrichetti forsteri from the sea and of L. wallacei from the river. Although most of the above species were also abundant in Deadwater Lagoon, the ichthyofaunal composition of this region did not undergo the same seasonal changes, presumably due to the lack of riverine input and thus the maintenance of relatively high salinities throughout the year. The number of marine straggler species was much lower in Deadwater Lagoon than in the estuary basin, reflecting a far more restricted tidal exchange with the entrance channel. However, the overall density of fishes was far higher in Deadwater Lagoon than in the estuary basin or entrance channel, due mainly to the far higher densities of the estuarine species Atherinosoma elongata and L. wallacei and of the 0+ age class of the marine species R. sarba. The high densities of certain species in Deadwater Lagoon are assumed to be related, at least in part, to the high level of productivity and protection that is provided by the presence of patches of Ruppia megacarpa, an aquatic angiosperm that was not present in the estuary basin or entrance channel. Received: 3 December 1996 / Accepted: 19 December 1996     

Protracted estuarine phase in the life cycle of the marine pufferfish Torquigener pleurogramma

The present study was undertaken to elucidate the way in which the Swan Estuary in south-western Australia is used by the common blowfish Torquigener pleurogramma, a representative of the abundant and widely distributed family Tetraodontidae. T. pleurogramm were collected by beach seine and otter trawl from the Swan Estuary between February 1977 and December 1980 and between May 1984 and February 1986. While T. pleurogramma feeds on a wide variety of organisms in the estuary, the main components of its diet are polychaetes and amphipods for fish <130 mm and bivalve molluscs for larger fish. Numbers of blowfish were inversely correlated with water depth, with densities on the banks (water depth <1.5 m) sometimes reaching 5 fish m^-2, and tended to be greater at night than during the day. The density of T. pleurogramma in the shallows was positively correlated with salinity and inversely correlated with distance from the estuary mouth. Numbers increased greatly in the latter half of 1980 and 1985 as a result of the recruitment of large numbers of the 0+ age class (i.e., fish in their first year of life). Blowfish were represented by seven age classes in the estuary and attained a maximum size of 230 mm (220 g). By the end of their first and second years of life, fish had reached approximately 90 mm (14 g) and 125 mm (39 g), respectively. Sexual maturity was generally not reached until the end of the second year of life. The presence of higher gonadosomatic indices and more mature gonads in fish collected just outside than within the estuary indicate that T. pleurogramma leaves the estuary before spawning. Comparisons between lengthfrequency data, allied with information on the prevalence and intensity of gill parasites, indicate that assemblages in estuarine and neighbouring inshore waters remain distinct for many months and that growth within the estuary is faster than in inshore marine environments.     

The ichthyofaunal composition of the Elbe Estuary: an analysis in space and time

A stow net (90 m² opening) was used, at regular intervals between July 1989 and June 1993, to collect fish over ca. 4 h on both the flood and ebb tides at five sites distributed at regularly spaced distances throughout the Elbe Estuary. Overall, 53.4% of the species were marine, and some of these were abundant, e.g.Clupea harengus, Pleuronectes flesus, Pomatoschistus minutus, Merlangius merlangus and Sprattus sprattus. However, in terms of number of individuals, such species contributed only 9.1%, compared with 90.0% by anadromous species, which was predominantly due to exceptionally high catches of Osmerus eperlanus. The ichthyofaunal compositions on the flood and ebb tides were similar, even downstream where salinities change markedly during each tidal cycle. Although this is probably due in part to groups of fish being swept first upstream on the flood tide and then back downstream on the ebb tide, several species clearly tended to occupy particular regions of the estuary. Each year the species composition changed sequentially from the most downstream site (max. salinity=ca. 31.4‰) to the most upstream site (max. salinity=ca. 1.5‰), mainly as a result of a sequential decline in the number and abundance of marine species and a progressive increase in the contributions of both anadromous and freshwater species. However, the marine species diagnostic of differences in the compositions among regions varied between years. Thus, for example, the diagnostic species for the downstream sites were C. harengus and M. merlangus in 1990/1991 and S. sprattus in 1992/1993, reflecting differences in the recruitment strengths of these marine species in the two years. The ichthyofaunal composition exhibited pronounced annual cyclical changes in each region and thus occurred irrespective of whether or not the salinity in a region underwent pronounced changes during the year. This cyclicity was attributable to intra-annual variations in the times of recruitment of certain marine species, particularly in the downstream region, and to seasonal migrations of diadromous species into each region, as well as to the movements of freshwater species into the upstream region in winter and early spring. Received: 18 November 1999 / Accepted: 25 October 2000     

Statistical modeling of seasonal and environmental influences on the population dynamics of an estuarine fish community

A statistical modeling study was performed on the population fluctuations of the 15 commonest fish species frequenting the tidal Scheldt estuary in Belgium. These included marine juvenile and seasonal visitors, estuarine residents and diadromous fish species that were recorded on the filter screens of a power plant cooling-water intake between September 1991 and April 2001. The species population abundance was regressed against a candidate set of 6 environmental variables and 13 instrumental variables, accounting for seasonality and long-term trends present in the data. Population abundances of the different species were, in general, best described by seasonal variables. Seasonal components contributed, on average, up to 63.8% of the variance explained by the models. Ten species were found to show a slightly negative, though significant, trend over the period of the survey. Most models also included at least one environmental variable, and 25.4% of the explained variance could be attributed to environmental fluctuations. Of all physico-chemical variables, dissolved oxygen was the most important predictor of fish abundance, suggesting that the estuary suffered from poor water quality during the survey. Temperature, salinity, freshwater flow, suspended solids and chlorophyll a concentrations were minor determinants of fish abundance.Communicated by O. Kinne, Oldendorf/Luhe     

Reproductive cycles in two populations of the Pacific sand dollar Dendraster excentricus

East Pacific swimming crabs are poorly known ecologically. We trawled samples along 4 estuarine-continental shelf transects to provide information on the population structure and species composition of portunids along environmental stress gradients. Portunids comprised almost 99% of the benthic crabs. Callinectes toxotes dominates the freshest estuarine areas, but is replaced by C. arcuatus in estuary mouth and shallow-shelf samples. Deeper shelf samples are dominated by Euphylax robustus and Portunus sper. Great numbers of E. dovii, a pelagic species, were taken on the shelf; their size distribution implies intraspecific competition. Evidence is given that differences in salinity tolerance account for differential landward limits, but competition, predation by fishes, increased fouling and decreased available food exclude eurytopic species from physicochemically equable shelf waters.     

The relative abundance and feeding habits of the grey mullet species occurring in an Estuary in Sierra Leone,West Africa

The occurrence and relative frequency of the 5 grey mullet species found in the Black Johnson Estuary, Sierra Leone, have been observed by sampling throughout the year. Examination of stomach contents and scales has also been made. Salinity within the estuary is maximum during the dry season and minimum at the height of the rains. The organic content of the sediment reaches a peak towards the end of the rains, but is low both at the end of the dry season and during the rains. Mullet tend to be most abundant as the rains are finishing and at the beginning of the dry season when the salinity is rising and the organic matter in the sediment is maximum. The mullet encountered in the estuary were Liza falcipinnis (Cuvier and Valenciennes), L. dumerilii hoefleri (Steindachner), Mugil cephalus ashanteensis Bleeker, M. curema Cuvier and Valenciennes and L. grandisquamis (Cuvier and Valenciennes). L. falcipinnis and L. dumerilii hoefleri penetrated furthest up the estuary and have been caught in freshwater some distance up rivers. All species feed principally on the detritus and algae in the estuarine sediment. There is some seasonal variation in the composition of the diet. Comparative data on the organic content of the estuarine sediment and the sand in the stomach of the mullet are given. A field experiment with L. falcipinnis suggests an assimilation efficiency of 52% for this species. Well-marked rings have been found upon the scales of four species, and their possible significance and use are discussed.     

Comparison of zooplankton community of different areas in Changjiang River estuary北大核心CSCD

To compare the zooplankton communities in the South Branch, North Branch, North Channel and offshore of the Changjiang (Yangtze River) estuary, two cruises were carried out in the autumn of 2009 and the spring of 2010. A total of 27 species of zooplankton were identified in the spring, including 17 in the offshore, 11 in the south branch, 9 in the north branch, and only 5 in the North Channel. In autumn, altogether 71 species were collected, including 6 in the south branch, 11 in the north branch, 8 species in the North Channel area and 63 in the offshore. Either in the spring or autumn, the average species numbers of the offshore were the highest. Additionally, species numbers of the offshore were significantly higher than that in other areas (P < 0.05) for both seasons. There were no significant differences between the species number of the South Branch and the North Branch (P > 0.05). The species number and diversity index of the inner area were lower than that of the offshore. The average diversity index of offshore area was still the highest in both spring (1.69) and autumn (3.73). In spring, the diversity index of the North Channel was the lowest among four areas, while in autumn, that of the South Branch was the lowest (0.49). In spring, Sinocalanus sinensis was the dominant species of the South Branch, North'Branch and North Channel (> 50% of the total collection by number). The dominant species of offshore area were Tortanus vermiculus and Labidocera euchaeta. While in autumn, the dominant species were different for each area. In the South Branch, the most important species was S. dorrii (93.55% of the total collection by number). The main species was S. dorrii in the South Branch area, S. stenellus and T. vermiculusin in the North Branch and Subeucalanus subcrassus in the offshore (< 30% of the total collection by number). Numbers of species and diversity index seemed to vary among different areas of Changjiang estuary. The underline mechanisms of the regional variation in species number were mainly the salinity and suspended sediment concentrations. Generally speaking, zooplankton was more abundant in the polyhaline water. Diversity index was largely affected by both species number and abundance percentage. The zooplankton community was mainly influenced by the Changjiang diluted water and offshore water mass through affecting the salinity. In autumn, the Kuroshio flowed northward carrying polyhaline water, and brought subtropical species to the offshore of the Changjiang estuary. Since the North Channel is located in the estuarine turbidity zone, suspended sediment concentration of that area is higher than the other areas, which might limit the distribution of some species. To sum up, salinity is the most important factor causing the difference of zooplankton community among different areas in the Changjiang estuary. Besides, water masses and estuarine turbidity also influence the zooplankton community.     

Utilisation of mangroves and seagrasses by fishes in the Negombo Estuary,Sri Lanka

In two years of sampling, 10 287 fishes belonging to 62 species (39 families) were collected from mangroves and seagrasses of the Negombo Estuary. Of the species collected, 70% were marine, 27% estuarine and 3% freshwater. Aplocheilus melastigma, Ambassis dayi, Etroplus suratensis, E. maculatus, Bathygobius fuscus and Arothron immaculatus constituted 90% of the catch. Sixteen species were caught only from mangroves, 6 only from seagrasses, and 40 from both. The number of species, individuals, biomass and diversity were higher in seagrasses than in mangroves. Gobies and some siganids moved to the mangrove habitat as they grew, but cichlids, triacanthids, some eleotrids and tetraodontids preferred seagrasses. The number of species and individuals caught increased at the beginning of the north-east monsoon when the rainfall was low. The number of species and individual numbers were positively correlated with salinity and dissolved oxygen and negatively correlated with water temperature. Of the 33 species examined, seagrasses formed >25% of the gut content in juvenile Lates calcarifer, Apogon thermalis, E. suratensis, E. maculatus, Siganus javus, Butis butis and Bathygobius fuscus. Mangrove litter was the main gut content of the family Syngnathidae. E. suratensis, E. maculatus and S. javus from seagrasses contained more filamentous algae than seagrasses in their stomachs. S. javus collected from mangroves also contained more seagrasses in their stomachs than mangrove litter, indicating that the association of juvenile fishes with mangroves or seagrasses may not be obligate in estuaries subject to low tidal fluctuations.     

Spatial segregation amongst goby species within an Australian estuary,with a comparison of the diets and salinity tolerance of the two most abundant species

The present study was undertaken to determine whether the various species of gobies that are found within the large Swan Estuary in south-western Australia are segregated within that system, and to attempt to determine the basis for any differences in their spatial distributions. The Swan Estuary comprises a long entrance channel (lower estuary), two wide basins (middle estuary) and the saline reaches of the tributary rivers (upper estuary). A total of 26232 gobies, representing seven species, was collected using a 3 mm-mesh seine net at 15 sites throughout this estuary on at least one occasion monthly over seven consecutive seasons between September 1983 and March 1985. Favonigobius lateralis and Pseudogobius olorum contributed 47.0 and 47.8%, respectively, to the total catch of gobies at all sites. The densities of each species at each site were used to determine the relative contribution of each species to the gobiid fauna at each of the sites in the lower, middle and upper estuary. Comparisons of these data with those published on the distribution and abundance of gobiid larvae confirmed that F. lateralis, which was found predominantly in the lower estuary, is a marine species that spawns in high salinities near the estuary mouth or in inshore coastal waters. In contrast, the life cycle of P. olorum and Papillogobius punctatus are typically completed within the saline reaches of the upper estuary, and that of Arenigobius bifrenatus within both this region and parts of the middle estuary where the substrate is particularly soft. Afurcagobius suppositus also spawns in this area, as well as in fresh water. Tridentiger trigonocephalus, represented by only eight individuals, is an introduced, marine species that was found mainly in the lower estuary. A single representative of the marine species Callogobius depressus was caught. The relatively low numbers of gobies caught in the middle estuary, where they contributed only about 3.5% to the total number of all gobies at all sites, may represent an aversion to the presence of rougher waters in the large basins. Circumstantial evidence suggests that the sandy substrate and consistently high salinities found in the lower estuary are preferred by F. lateralis, whereas the silty surface to the substrate and lower salinities of the upper estuary are preferred by Pseudogobius olorum. Densities of three of the four most abundant species were higher in either spring or summer than in winter, reflecting the influx of 0 + recruits, and possibly also the tendency for species in estuaries to congregate in the shallows during the warmer periods of the year. F. lateralis fed mainly on polychaetes and crustaceans, whereas P. olorum ingested predominantly algae, reflecting differences in mouth morphology and feeding behaviour, rather than the type of food available.     

Turbulence measurements in a small subtropical estuary under king tide conditions

In natural waterways and estuaries, the understanding of turbulent mixing is critical to the knowledge of sediment transport, stormwater runoff during flood events, and release of nutrient-rich wastewater into ecosystems. In the present study, some field measurements were conducted in a small subtropical estuary with micro-tidal range and semi-diurnal tides during king tide conditions: i.e., the tidal range was the largest for both 2009 and 2010. The turbulent velocity measurements were performed continuously at high-frequency (50Hz) for 60?h. Two acoustic Doppler velocimeters (ADVs) were sampled simultaneously in the middle estuarine zone, and a third ADV was deployed in the upper estuary for 12?h only. The results provided an unique characterisation of the turbulence in both middle and upper estuarine zones under the king tide conditions. The present observations showed some marked differences between king tide and neap tide conditions. During the king tide conditions, the tidal forcing was the dominant water exchange and circulation mechanism in the estuary. In contrast, the long-term oscillations linked with internal and external resonance played a major role in the turbulent mixing during neap tides. The data set showed further that the upper estuarine zone was drastically less affected by the spring tide range: the flow motion remained slow, but the turbulent velocity data were affected by the propagation of a transient front during the very early flood tide motion at the sampling site.     

Vegetation succession of low estuarine marshes is affected by distance to navigation channel and changes in water level

Climate change and engineering activities have modified the hydrology and morphology of estuaries. However, the potential effects of these modifications on vegetation succession in estuarine marshes are still poorly understood. Therefore, we studied temporal changes in tidal habitats of the Elbe estuary over a period of 30 years. We compared vegetation maps from 1980 to 2010 and calculated the change in area of habitats with respect to three salinity and three elevational zones. To analyze the direction of the temporal change, we differentiated between progressive and regressive succession. By using regression tree models (conditional inference trees), we identified the most influential factors determining progressive or regressive succession of low marshes. The total area of the estuarine tidal marshes at the Elbe increased by 2 % from 1980 to 2010, but changes were unequal among the salinity zones. In the salt and brackish zones, the area covered by high marshes increased substantially but decreased in the tidal freshwater zone, while that covered by low marshes decreased in all the salinity zones. Additionally, we determined high persistence of tidal flats and high marshes, whereas only 19 to 28 % of the low marshes found in 1980 remained in 2010. In salt and brackish marshes, more than two-thirds of the area that had been identified as low marshes in 1980 had progressively developed into high marshes. In contrast, 44 % of the area of low marshes in tidal freshwater marshes showed regressive succession back into tidal flats. The distance to the navigation channel was the main factor determining successional direction in salt and brackish marshes. Here, greater proximity to the channel was correlated with higher risk of regressive succession. In tidal freshwater marshes, we identified both the distance to the navigation channel and the situation on the river shore (i.e. inner bank, outer bank or straight bank) as the main factors for marsh succession. Here, considerable engineering activities in the channel had simultaneously decreased the mean low water level and increased the mean high water level between 1980 and 2010, which led to an increase in tidal amplitude. It is quite likely that these changes negatively modified marsh distribution, increased regressive succession and, thus, lowered the quality of tidal freshwater marshes.     

Hydrodynamic and ecological assessment of nearshore restoration: A modeling study

Along the Pacific Northwest coast, much of the estuarine habitat has been lost over the last century to agricultural land use, residential and commercial development, and transportation corridors. As a result, many of the ecological processes and functions have been disrupted. To protect and improve these coastal habitats that are vital to aquatic species, many projects are currently underway to restore estuarine and coastal ecosystems through dike breaches, setbacks, and removals. Understanding site-specific information on physical processes is critical for improving the success of such restoration actions. In this study, a three-dimensional hydrodynamic model was developed to simulate estuarine processes in the Stillaguamish River estuary, where restoration of a 160-acre parcel through dike setback has been proposed. The model was calibrated to observed tide, current, and salinity data for existing conditions and applied to simulate the hydrodynamic responses to two restoration alternatives. Model results were then combined with biophysical data to predict habitat responses within the restoration footprint. Results showed that the proposed dike removal would result in desired tidal flushing and conditions that would support four habitat types on the restoration footprint. At the estuary scale, restoration would substantially increase the proportion of area flushed with freshwater (<5 ppt) at flood tide. Potential implications of predicted changes in salinity and flow dynamics are discussed relative to the distribution of tidal marsh habitat.     

Causes of Extirpations in the Wadden Sea, an Estuarine Area in The Netherlands

Abstract: The various causes of extirpation of marine and estuarine species were investigated for the Dutch Wadden Sea to gain an impression of their relative importance. I obtained data from geological, archaeological, historical, and biological publications. At least 10 species of algae, 10 invertebrates, 13 fishes, 5 birds, and 4 marine mammals became extinct during the past 2000 years. Habitat destruction played a part in at least 26 cases, overexploitation in at least 17, and pollution in at least 3. Causes of the disappearance of two species are unknown. Introduction of exotic marine species was not involved in any of the extirpations. I compared this pattern of extinction to that of worldwide extinctions of marine and estuarine species, which have been attributed to overexploitation, habitat destruction, and in two cases to the introduction of terrestrial predators to the breeding habitats of marine birds. The results imply that conservation of marine species should address overexploitation and habitat destruction as the main causes of extinction.     

Dispersal-mediated coexistence of mud snails (Hydrobiidae) in an estuary

The distribution of the mud snails Hydrobia ventrosa (Montagu), H. neglecta Muus, and Potamopyrgus jenkinsi (Smith) was studied in the Kysing Fjord estuary, Denmark, from November 1977 to February 1979. The hydrobiids show habitat selection with regard to salinity but coexist in large areas of the estuary: P. jenkinsi at the innermost parts of the estuary together with H. ventrosa, with the latter species extending its distribution further to the mouth of the estuary, where H. neglecta dominates. H. ventrosa and H. neglecta are the only species that have permanent populations in the estuary. P. jenkinsi dies out in the estuary in the winter, but recolonizes there from a small river in the spring and builds up a population in the innermost parts of the estuary during the summer. H. ventrosa and H. neglecta coexist in the middle part of the estuary due to continued dispersal into this area.     

Environmental variables and fisheries diversity of the Naaf River Estuary,Bangladesh

The Naaf River estuary is one of the large estuaries in the Bangladesh coastal region not to have been affected by extensive human disturbance. This research provides information about the fisheries diversity status by Estuarine Set Bag Net (ESBN) sampling relation to physicochemical variables in both spatio-temporal scales. About 25 km of the lower estuary was divided into six zones for sample collection by considering the accessibility and availability of the ESBN operation, fish landing centers and location of the fishing villages. In total 48 samples have been analyzed which were taken throughout March to October 2006. To quantify the species diversity, all fisheries data were analyzed by using EstimateS and EcoSim software which accounts the different diversity indices viz., species richness, Shannon–Wiener diversity Index, Dominance and Evenness index. The research results demonstrate that the Naaf River estuary is a habitat of 161 (species richness, Sobs = 161, Choa 1 = 162 ± 2.34, ACE = 161.73) different species which belong to 98 fin fishes, 23 shrimps and prawns, 13 crabs, 11 molluscs, 3 echinoderms, 4 other crustaceans; while 9 remain unidentified. Results on the aquatic environment, mainly salinity and turbidity were found to have a major influence on their occurrence and distribution. All the findings indicated that the Naaf River estuary is a highly productive system and provides a favourable environment for large variety of estuarine species assemblages.