Wilson Inlet Research Articles

Significance of Fluvial Sediment Supply in Coastline Modelling at Tidal Inlets

The sediment budget associated with future coastline change in the vicinity of tidal inlets consists of four components; sea level rise-driven landward movement of the coastline (i.e., the Bruun effect), basin infilling effect due to sea level rise-induced increase in accommodation space, basin volume change due to variation in river discharge, and coastline change caused by change in fluvial sediment supply. These four components are affected by climate change and/or anthropogenic impacts. Despite this understanding, holistic modelling techniques that account for all the aforementioned processes under both climate change and anthropogenic influences are lacking. This manuscript presents the applications of a newly-developed reduced complexity modelling approach that accounts for both climate change and anthropogenically-driven impacts on future coastline changes. Modelled results corresponding to the year 2100 indicate considerable coastline recessions at Wilson Inlet (152 m) and the Swan River system (168 m) in Australia and Tu Hien Inlet (305 m) and Thuan An Inlet (148 m) in Vietnam. These results demonstrate that coastline models should incorporate both climate change and anthropogenic impacts to quantify future changes in fluvial sediment supply to coasts to achieve better estimates of total coastline changes at tidal inlets. Omission of these impacts is one of the major drawbacks in all the existing coastline models that simulate future coastline changes at tidal inlets. A comparison of these modelled future coastline changes with the predictions made by a relevant existing modelling technique (Scale Aggregated Model for Inlet-interrupted Coasts (SMIC)) indicates that the latter method overestimates total coastline recessions at the Swan River system, and the Tu Hien and Thuan An Inlets by 7%, 10%, and 30%, respectively, underlining the significance of integrating both climate change and anthropogenic impacts to assess future coastline changes at tidal inlets.

The use of benthic macroinvertebrates to establish a benchmark for evaluating the environmental quality of microtidal, temperate southern hemisphere estuaries

Establishment of a benchmark against which deleterious changes to an estuary can be evaluated requires validating that it has not been subjected to detrimental anthropogenic perturbations and then identifying the biological features which are indicative of a pristine condition and can thus be employed as indicators for detecting and monitoring departures from the natural state. The characteristics of the benthic macroinvertebrate fauna of an essentially pristine, seasonally-open estuary in Western Australia (Broke Inlet) have been determined and compared with those previously recorded for a nearby eutrophic, seasonally-open estuary (Wilson Inlet). Density was far lower in Broke than Wilson. Compositions differed radically at all taxonomic levels, with polychaetes contributing less, and crustaceans more, to the abundance in Broke. Average taxonomic distinctness was greater for Broke than both Wilson and 16 other temperate southern hemisphere estuaries, whereas the reverse was true for variation in taxonomic distinctness, emphasizing that Broke Inlet is pristine.

Implications of the divergent use of a suite of estuaries by two exploited marine fish species

Biological characteristics of the marine species King George whiting Sillaginodes punctatus and Australian herring Arripis georgianus in three seasonally open estuaries (Broke, Irwin and Wilson Inlets), one permanently open estuary (Oyster Harbour) and one normally closed estuary (Wellstead Estuary) on the south coast of Western Australia have been determined and compared. Sillaginodes punctatus enters the seasonally and permanently open estuaries early in life and reaches total lengths (L(T)) >280 mm at which it can be legally retained and thus contributes to commercial and recreational fisheries in these systems. This sillaginid almost invariably emigrates from these estuaries before reaching its typical size at maturity (L(T50)) and does not return after spawning in marine waters. In contrast, virtually all female A. georgianus (≥ 98%) in the three seasonally open estuaries and the majority in the normally closed (89·5%) and permanently open estuaries (83%) exceeded the L(T50) of this species at maturity, reflecting the fact that the nursery areas of this species are predominantly located much further to the east. Although adult females of A. georgianus in seasonally open and normally closed estuaries had developed mature ovaries by autumn, at which time they were prevented from migrating to the sea by closure of the estuary mouths, this species did not spawn in those estuaries. The oocytes in their ovaries were undergoing extensive atresia, a process that had been incipient prior to oocyte maturation. As the adult females of A. georgianus in the permanently open Oyster Harbour at this time all possessed resting gonads, i.e. their oocytes were all previtellogenic, the adults that were present in that estuary earlier and were destined to spawn in autumn must have emigrated from that permanently open estuary to their marine spawning areas prior to the onset of gonadal recrudescence. The body masses at length of A. georgianus, which were almost invariably higher in summer and autumn than in winter and spring, were greater in the very productive environments of the seasonally open and normally closed estuaries than in the less productive and essentially marine environment of Oyster Harbour and coastal marine waters. In general, the same pattern of differences between water bodies was exhibited by the growth of A. georgianus and by the more restricted data for body mass at L(T) and growth of S. punctatus. Despite an increase in anthropogenic activities in Wilson Inlet over the last two decades, the growth of both species was very similar to that recorded 20 years earlier. The fisheries implications of the results for the two species are discussed.

Intra-annual Variability in Primary Producer Groups and Nitrogen Dynamics in an Intermittently Closed Estuary Exposed to Mediterranean Climate

Nutrient dynamics in estuaries are temporally variable in response to changing physical–chemical conditions and biogeochemical processes involving primary producer groups such as phytoplankton, microphytobenthos, seagrass and macroalgae. In order to reveal intra-annual changes in the biomass of primary producer groups and associated changes in estuarine nutrient dynamics, we developed a box model, coupling water inflows and outflows and nitrogen dynamics in Wilson Inlet, a large, central-basin-dominated, intermittently closed estuary exposed to a Mediterranean climate in Western Australia. The model is calibrated and validated with monitoring data, aquatic plant biomass estimates and biogeochemical rate measurements. Macrophytes and their microalgal epiphytes appear to rapidly assimilate nutrients from the first flush from the catchment in winter, but this buffer capacity then ceases, and a phytoplankton ‘bloom’ develops in response to subsequent river runoff events in spring. Significant amounts of bioavailable nitrogen are exported to the ocean because phytoplankton predominance occurs while the sand bar is breached. Surface sediments play a key role for nitrogen dynamics: In late spring to autumn, high light availability at the sediment surface stimulates high primary production by microphytobenthos, leading to reduced benthic ammonium fluxes particularly in the deep basin. Microphytobenthos contributes about 60% of annual whole-system primary production. Despite high benthic primary production, nitrogen release from sediments is the biggest nitrogen source to the estuary.

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.

Organic matter sources in an enclosed coastal inlet assessed using lipid biomarkers and stable isotopes

The sources of organic matter (OM) in surface sediments from Wilson Inlet, an enclosed and seasonally barred inlet in Western Australia, have been determined using a combination of lipid biomarkers (including fatty acids, sterols, phytol, long chain alcohols, alkyl diols, tetrahymanol and C 32 hopanol) and stable carbon and nitrogen isotope data for bulk OM and δ 13C data for n-alkanols. The organic composition of sediments from the inflowing rivers was used as a proxy for terrestrial inputs of higher plant OM, since these sediments contained only small amounts of carbon from microalgae (mostly diatoms), bacteria and cyanobacteria. The relative proportions of “marine” and “terrestrial” OM in the sediments could be estimated using the stable carbon isotope signature of the bulk organic carbon. The sediments from Wilson Inlet showed similar lipid compositions and isotope values, and the sterol and fatty acid distributions showed the complexity typical of marine ecosystems. Relative contributions from algal (phytoplankton and microphytobenthos), bacterial, terrestrial plants and seagrass sources were estimated using biomarker/organic carbon ratios in the different source terms. Microalgae and bacteria are the most important source of OM within the inlet (over 80% in most sediments) and most of the algal material originates from the water column, except in sandy shallow sediments at the eastern end of the inlet where a contribution from microphytobenthos can be discerned. The distributions, abundances and δ 13C isotope values of long chain n-alkanols proved to be useful in quantifying the contributions from terrestrial plants and the seagrass Ruppia megacarpa. The latter is a relatively small contributor (< 10%) to the OM in the sediments, despite the presence of large seagrass beds around the edges of the inlet and the presumed importance of seagrass as a source of carbon in the foodwebs of the inlet. Nonetheless, seagrass is a significant contributor of the “higher plant” marker 24-ethylcholesterol (sitosterol) in the sediments.

Inter‐ and intraspecific partitioning of food resources by six large and abundant fish species in a seasonally open estuary

This study has tested the hypotheses that the dietary compositions of the six large and abundant fish species in the 46 km2 basin of a seasonally open estuary (Wilson Inlet) will differ significantly, change with increasing body size and vary with water depth and season. These species comprise four marine species, the sea mullet Mugil cephalus, yellow‐eye mullet Aldrichetta forsteri, King George whiting Sillaginodes punctata and Australian herring Arripis georgiana, and populations of two species that are confined to the estuary, the cobbler Cnidoglanis macrocephalus and the southern blue‐spotted flathead Platycephalus speculator. Non‐metric multidimensional ordination and associated tests showed that, overall, the dietary composition was influenced to a far greater extent by the species of fish than by either water depth or season. At even a relatively broad level of taxonomic discrimination, the dietary compositions of each pair of species were significantly different, except for those of A. georgiana v. P. speculator, and even in that case those of their larger individuals differed. Differences among the diets of the six species, which represented five families, are related to differences in feeding morphology and location within the water column. Mugil cephalus ingested almost exclusively sediment and fine organic material. Although A. forsteri and S. punctata both consumed substantial amounts of polychaetes (including nereids, capitellids and orbiniids), the former ingested relatively larger volumes of small crustaceans (copepods) and relatively lower volumes of large crustaceans (the carid Palaemonetes australis), nemertines and bivalve siphons. The diets of A. georgiana and P. speculator were both typified by the consistent and high contributions of large crustaceans and fishes. Among fish prey, however, A. georgiana focused to a far greater extent on the small clupeid Engraulis australis, whereas P. speculator consumed a wider range of teleosts, which included species of atherinid and goby. Although the diets of A. forsteri and C. macrocephalus both contained substantial amounts of coarse organic material, the latter differed markedly from those of all other species by the frequent presence of mytilids (mainly Xenostrobus spp.) and other bivalves (especially Tellina deltoidalis and Irus crenata). The diets of each species except M. cephalus changed with increasing body size and, for comparable size classes, did not differ between shallow and deeper waters. The partitioning of food resources within and among the six large and abundant fish species found in Wilson Inlet would reduce the potential for competition for these resources and help account for the large numbers of those species in this seasonally open estuary.

Addressing off-site nutrient pollution through conventional management actions: a modelling case study

The ecology of estuaries on the south coast of Western Australia has been disrupted by increased nutrient and sediment discharge from predominantly rural catchments. A range of conventional management actions are available to reduce nutrient loss at source, and it is important to evaluate possible reductions, and costs, so that limited funds can be targeted to realise the greatest moderation of nutrient loss. A lumped landuse nutrient generation rate model was developed for four catchments (Wilson Inlet, Oyster Harbour, Torbay Inlet and Princess Royal Harbour) near Albany Western Australia and the output compared with existing monitoring data. The nutrient moderating effects of six conventional management actions (perennial pastures; vegetated stream buffers; minimum tillage, effective fertiliser use; stock control and water management; and effluent management) and their associated costs were implemented at different levels in the model. These were used to determine the extent to which these actions could address offsite nutrient pollution, and the cost of doing so. Management actions were implemented in three major scenarios representing the status quo (current nutrient reduction efforts through volunteerism), the highest possible implementation of each action (perhaps indicating mandated change) and the most cost effective set of actions. In each catchment dominated by diffuse nutrient sources, current nutrient reduction efforts amounted to about 10%, whilst the highest possible reductions were of the order of25–30% above this. In the point source dominated catchment current nutrient reduction efforts amounted to about 40%, with an additional 40% possible. The most cost effective scenarios reduced nitrogen more than phosphorus. Under the most cost effective scenarios, it was estimated that the net cost of management actions over 10 years was budget positive, resulting in a net benefit to the land managers involved. This suggests that there are limited economic barriers to the adoption of these conventional management actions, and on this basis current nutrient reduction efforts could be significantly increased. However, predicted reductions from the implementation of conventional management actions may not be sufficient to arrest estuarine decline.

The Seasonal Closure of Tidal Inlets: Causes and Effects

Seasonally open tidal inlets usually occur in micro-tidal, wave-dominated coastal environments where strong seasonal variations of streamflow and wave climate are experienced. These inlets are closed to the ocean for a number of months every year due to the formation of a sand bar across the entrance. Because the estuaries connected to these inlets commonly serve as harbours or recreational facilities, the annual inlet closure is undesirable in most cases. Hence, there is increased interest in understanding the processes governing inlet closure and the hydrodynamics and exchange processes of these estuaries. This study was undertaken with the objectives of: (a) identifying the morphodynamic processes governing seasonal inlet closure, and (b) determining the effect of the seasonal closure of the inlet on the hydrodynamic characteristics of the estuary/lagoon connected to the inlet. Wilson Inlet, a typical seasonally open tidal inlet located along the southwestern coast of Australia, was selected as a case study. The inlet is usually closed to the ocean for 6–7 months every year.Field experiments and numerical models were employed to examine inlet morphodynamics and estuarine hydrodynamics. As cross-shore processes, which the field data indicated to be important, have not been included in coastal area models to date, a quasi-3D morphodynamic model, which includes longshore and cross-shore processes, was developed to determine processes governing seasonal inlet closure. Application of the model to Wilson Inlet clearly indicates that onshore sediment transport during summer is the cause of inlet closure. Further applications show that summer streamflow and storm events also exert significant control on inlet closure depending on the intensity and timing of the event. The results of the estuarine hydrodynamic study indicate that the hydrodynamic regime of seasonally open estuaries changes distinctively when the inlet state varies from “open” to “closed.”

NUTRIENT LIMITATION OF PHYTOPLANKTON IN A SEASONALLY OPEN BAR‐BUILT ESTUARY: WILSON INLET, WESTERN AUSTRALIA

The potential for nutrient limitation to affect the phytoplankton community was assessed from 1997 to 1998 in Wilson Inlet, Western Australia. Samples of the endemic phytoplankton community were assayed by giving them nutrient mixes deficient in one of the following: nitrogen, phosphate, silicate, iron, trace metals, or vitamins. When ranked from those treatments with the most potential to limit phytoplankton biomass to those with less potential, the bioassay results suggested that N&gt;P&gt;&gt;Si&gt;Fe, and trace metals and vitamins were never potentially limiting. In summer and autumn, the bioassay data suggested that nitrogen and phosphorus were the nutrients with greatest potential to constrain phytoplankton biomass. Though the concentrations of soluble nutrients were high in winter, bioassays indicated that the phytoplankton community was potentially nutrient limited. Physical conditions such as high flow rate, greater turbidity, lower temperature, and greater light attenuation contributed to lower phytoplankton growth rates during winter. The bioassay data indicated that the phytoplankton biomass was least likely to be constrained by dissolved inorganic nutrients in spring than any other time of the year. In spring the endemic phytoplankton community responded to salinity‐stratification induced sediment‐nutrient release with a marked increase of biomass. Nitrogen was the nutrient with greatest potential to limit the phytoplankton biomass during spring. The bioassay data conflicted with the dissolved inorganic nitrogen to phosphorus molar ratios, which suggested that P was more likely to limit the phytoplankton biomass during spring. The discrepancy between the two data sets was probably caused by inaccuracies in measuring the concentration of dissolved inorganic phosphorus in spring. Therefore, the results from the current study suggest that the Redfield paradigm, based on the ratio of dissolved inorganic nutrient concentration, provided an inadequate description of phytoplankton nutrient limitation in Wilson Inlet.

The role of benthic vegetation as a sink for elevated inputs of ammonium and nitrate in a mesotrophic estuary

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 219:99-107 (2001) - doi:10.3354/meps219099 The role of benthic vegetation as a sink for elevated inputs of ammonium and nitrate in a mesotrophic estuary Bernard J. Dudley*, Anna M. E. Gahnström, Diana I. Walker Botany Department, The University of Western Australia, Perth Western Australia 6907, Australia *E-mail: bdudley@cyllene.uwa.edu.au ABSTRACT: Benthic vegetation plays an important role in determining the fate of nitrogen inputs to estuaries, thus influencing their degree of eutrophication. This study investigated the role of benthic vegetation as a sink for anthropogenic inputs of nitrate and ammonium into Wilson Inlet, a mesotrophic estuary in southwestern Australia. The dominant aquatic angiosperm in Wilson Inlet is Ruppia megacarpa Mason. We examined: (1) whether R. megacarpa leaves remove inorganic N (as nitrate and/or ammonium) from the water column, despite the presence of a layer of epiphytes; (2) whether the macrophyte and its epiphytes are equally important in the removal of inorganic N from the water column; and (3) whether inorganic N taken up by leaves is translocated to other plant parts. We added inorganic 15N nitrogen, as ammonium, nitrate, or both, to aquaria containing intact cores of sediment, R. megacarpa and attached epiphytes, and unfiltered estuary water. We measured depletion of nitrogen species from the water column and incorporation of 15N into components of the core. Epiphytes removed more nitrate and ammonium from the water column than R. megacarpa, despite having 25% of the biomass of the macrophyte. Maximum rates of nitrate uptake were 4.6 (for epiphytes) and 2.0 µmol h-1 g-1 DW (for R. megacarpa), and maximium rates of ammonium uptake were 35 (for epiphytes) and 23 µmol h-1 g-1 DW (for R. megacarpa). The presence of ammonium reduced rates of nitrate uptake, indicating that benthic vegetation prefers ammonium as a nitrogen source. Using mass spectrometry, we recovered between 37 and 45% of the added 15N nitrogen. The remainder was transformed to either organic nitrogen in the water column by algal epiphytes or nitrogen gas via coupled nitrification-denitrification in the sediment. This experiment indicates ecosystem-scale responses to dissolved inorganic nitrogen which would not have been observable from experiments conducted with isolated plants. Benthic vegetation in Wilson Inlet removes nitrate and ammonium quickly from the water column. Depending on water mixing, it may reduce transient increases in the concentration of these nutrients to background levels within 30 h. This process may be responsible for maintaining low water-column concentrations and reducing the likelihood of algal blooms. KEY WORDS: Benthic vegetation · Eutrophication · Estuary · 15N · Nutrient sink Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 219. Online publication date: September 10, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Sensitivity of transects across a depth gradient for measuring changes in aerial coverage and abundance of Ruppia megacarpa Mason

Effective management of estuarine systems that contain submerged aquatic macrophytes, requires knowledge of whether macrophyte populations are stable, increasing or decreasing in terms of aerial coverage and abundance. This study established three transects within Wilson Inlet and monitored them five times during 1996 for percent cover of Ruppia megacarpa and maximum depth limits of these meadows. Secchi depth data collected weekly in the Inlet for water quality monitoring were used for comparison. Percentage cover of R. megacarpa varied along the length of the transects and also between sampling times. This variation was partially explainable by variation in light availability, but was affected by other factors within the system. Overall sites and times, the maximum depth limit of R. megacarpa was at approximately 24% of incident irradience. This value is high in comparison to other submerged macrophytes and suggests that R. megacarpa is not limited by light. Its distribution may be limited by sediment type, with the seagrass being confined to coarser sediments in the Inlet. It was concluded that R. megacarpa depth distribution in Wilson Inlet is not currently limited by light, and so a minimum acceptable Secchi depth cannot be recommended. Repeated measurement of permanent transects for percent cover and canopy height, replicated within each meadow and along each transect, is recommended.

The seasonal closure of tidal inlets: Wilson Inlet—a case study

The seasonal closure of tidal inlets is a common and important coastal phenomena. However, studies which have been specifically geared to identify processes governing seasonal inlet closure are almost non-existent. Hence, this study was undertaken to gain insight into processes governing seasonal inlet closure. To determine the processes governing this phenomenon, Wilson Inlet, Western Australia, a typical seasonally open tidal inlet is taken as a case study. The study comprised of a field experiment over the summer of 1995, and a numerical modeling exercise employing a morphodynamic model. Results of the field study imply that longshore processes may not be the cause of inlet closure, but that onshore sediment transport due to persistent swell wave conditions in summer may govern seasonal closure of the inlet. Application of a morphodynamic model, which includes both cross-shore and longshore processes, to Wilson Inlet conclusively shows that seasonal closure of the inlet is due to onshore sediment transport under typical summer conditions. The effects of summer streamflow and storm events, which are not uncommon, are also examined using the morphodynamic model. The effect of both streamflow and storm events on the `open duration' of the inlet is shown to be dependent on the intensity and timing of the event.

Abundance ofRuppia megacarpaMason in a Seasonally Variable Estuary

The aim of this paper was to assess how much of the variation in abundance ofRuppia megacarpawithin Wilson Inlet, Western Australia, could be attributed to variations in conductivity, turbidity, depth, dissolved oxygen, pH and temperature of the water column (hydrological variables).R. megacarpawas collected from six sites within Wilson Inlet, a bar built estuary in southwestern Australia. Hydrological variables were measured and seasonal variation assessed using non-metric Multi-Dimensional Scaling (non-metric MDS). Canonical Correlation Analysis (CCA) was used to determine correlations betweenRuppiaabundance and these hydrological variables, for the same time period and with a lag time of 2, 4 and 6 months. Strong seasonal patterns in hydrological variables in Wilson Inlet were related to seasonal rain and the annual winter opening of the channel to the ocean by the local council. Conductivity, turbidity and depth were related to approximately 40% of the variation inR. megacarpaabundance. Morphological plasticity is hypothesized to be an important component of the unexplained 60% of variation inR. megacarpaabundance. Strongest relationships betweenR. megacarpaabundance and individual hydrological variables were over different time periods: turbidity was directly related; depth with a lag of 2 months; and conductivity with a lag of 4 months. This study confirms that in a variable estuarine system there is a strong relationship between hydrological and biological variables.

Influence of Water Depth, Season, Habitat and Estuary Location on the Macrobenthic Fauna Of a Seasonally Closed Estuary

The macrobenthic invertebrates in the shallow (&lt;0·5 m), medium-depth (0·75–1·0 m) and deep (2·0–2·5 m) waters of the lower and upper regions of the large basin of the seasonally closed Wilson Inlet were sampled, using corers, in each season between the winters of 1989 and 1990. The fauna was dominated by capitellid, spionid and nereid polychaetes and mytilid, venerid and tellinid molluscs. Almost 75% of the total numbers were contributed by the polychaetes Heteromastus filiformis (Claparède) (30·2%), Capitella capitata (Fabricius) (17·4%), Prionospio cirrifera (Wirén) (7·9%) and Boccardia chilensis (Blake &amp; Woodwick) (5·4%), and the gastropod Hydrococcus brazieri (Tenison Woods) (12·4%). The numbers of species recorded in Wilson Inlet (41) were relatively very low for an estuary. This presumably reflects (i) the low salinities, which would restrict colonization by stenohaline marine species, (ii) the prevention of recruitment from the sea by the closure of the estuary mouth during the summer, at which time many macrobenthic species are producing pelagic larvae, (iii) the very limited tidal movements, and thus mechanisms for transporting larvae from the sea, when the estuary mouth is open, and (iv) the lack of a pronounced salinity gradient and the small variation in sediment composition along the estuary. Characteristics (ii–iv), together with the dispersal of pelagic larvae within the basin by wind mixing, would also account for the fact that faunal composition was influenced less by the region within the basin than by water depth. The densities of species, such as Ceratonereis aequisetis (Augener), Capitella capitata and H. brazieri, were greater in the shallow and medium depths than in deeper water, while the reverse pertained with P. cirrifera. Since densities of the first three species are correlated with the biomass of Ruppia megacarpa Mason, the greater amounts of this aquatic macrophyte in the shallow and medium-depth waters may thereby provide more food and/or protection from predators in those waters than would be the case in deep water. In contrast, P. cirrifera, which is a detritivore, would benefit from the greater proportion of particulate organic material in the deep waters. The greater habitat complexity in the shallow and medium-depths, that is provided by the greater amounts of R. megacarpa and shell debris in those waters, was associated with a greater species richness, diversity, density and biomass of benthic invertebrates. The composition of the fauna underwent cyclical changes, which were related, in part, to the patterns of recruitment of certain major taxa following spawning.

Composition, Distribution and Seasonal Abundance of Zooplankton in a Shallow, Seasonally Closed Estuary in Temperate Australia

Nets of 53- and 500-μm mesh sizes were used to collect zooplankton and fish larvae, respectively, from the lower, middle and upper regions of the large basin and saline reaches of a major tributary river of the seasonally closed Wilson Inlet in each month between July 1988 and June 1989. Copepods, of which approximately two-thirds were naupliar stages, contributed 52% to the total number of zooplankton. The dominant copepods were Oithona simplexand Acartia simplex. Copepod nauplii was the most abundant single group, reaching a density of 680 768 m −3in one region in January and having a mean monthly density for the whole system of 147 166 m −3. Other numerically important taxa were the tintinnid ciliates Tintinnopsissp. a, Helicostomellasp. and Favellasp., the rotifer Synchaetacf. baltica, and the meroplanktonic stages of molluscs and polychaetes. The densities of the 10 most abundant taxa did not exhibit sharply defined seasonal peaks, except for S.cf. baltica, which peaked in summer. The densities of the other abundant taxa remained high for between 5 and 10 months between late winter and the early winter of the following year. The mean monthly density of zooplankton was very high (447 238 m −3), presumably reflecting a combination of the use of a fine-mesh net, the high nutrient loading of this system and the relative stability of the water mass within the estuary basin that results from a restricted exchange of water with the sea. Although the density of the total zooplankton community reached a maximum in mid-summer, densities were also high in all but the mid-winter to early spring months, when salinities and temperatures were at their lowest. The zooplankton community in Wilson Inlet changed progressively throughout the year as temperature and salinity also changed. The zooplankton assemblages in the lower, middle and upper basin at any one time were similar, as were the salinities, but they differed from those in the river, where salinities were usually lower. Although there was only a limited exchange of water between the basin and the ocean, and salinities in the basin never exceeded 29, the main species in the zooplankton were euryhaline marine rather than estuarine. The seasonal densities of zooplankton and fish larvae were correlated in both the middle basin and Denmark River. Since even copepod nauplii, the main prey of fish larvae, were typically at least 20 000 times more abundant than fish larvae, it is unlikely that the zooplankton food supply of fish larvae in Wilson Inlet was limiting.

Composition of the fish fauna of a permanently open estuary on the southern coast of Australia, and comparisons with a nearby seasonally closed estuary

The fish faunas of the outer basin (Nornalup Inlet), inner basin (Walpole Inlet) and saline region of the main tributary (Frankland River) of the permanently open Nornalup-Walpole Estuary on the southern coast of Western Australia, were sampled bimonthly for a year using seine and gill nets, and also during a further two months by the former method. Although the Nornalup-Walpole Estuary is permanently open, the catches of fish in its shallows were dominated (98.4%) by estuarine-spawning species, thereby paralleling the situation in the nearby and seasonally closed Wilson Inlet. In contrast, larger representatives of several marine species were present in appreciable numbers in the offshore, deeper waters of both of these estuaries. The delayed recruitment of marine species into these estuaries apparently reflects the distance that the juveniles of these species have to travel from the areas where they are believed predominantly to spawn. The larger representatives of marine species made a greater contribution to the fish faunas of the offshore, deeper waters in the Nornalup-Walpole Estuary than in Wilson Inlet (64.5 vs 36.9%) and, unlike the situation in the latter estuary, they included five species of elasmobranchs, two of which (Mustelus antarcticus and Myliobatis australis) were relatively abundant. Classification and ordination of the combined data for both estuaries demonstrated that the composition of the fish fauna in the offshore, deeper waters of the outer basin of the Nornalup-Walpole Estuary was particularly distinct, with some marine species being restricted to these waters. This is presumably related both to the presence of a permanently open entrance channel and the relatively deep waters found in Nornalup Inlet, which allow the ready exchange of water between the sea and estuary and the maintenance of high salinities in the deeper regions of the outer basin for much of the year. The fish faunas in Walpole Inlet and the tributaries of both the Nornalup-Walpole Estuary and Wilson Inlet were more similar to each other than they were to those in the more seawards end of either estuary. This similarity reflects the apparent preference of certain teleosts, such as the estuarine species Acanthopagrus butcheri and the marine species Mugil cephalus and Aldrichetta forsteri for reduced salinities and/or features associated with riverine environments.

The larval fish assemblage of the Nornalup-Walpole Estuary, a permanently open estuary on the southern coast of Western Australia

Fish larvae were sampled in the entrance channel and in the two basins of the permanently open Nornalup-Walpole Estuary, on the southern coast of Western Australia, in each month between October 1989 and September 1990. Sampling yielded a total of 39 068 larvae belonging to 36 species and 23 families, with the engraulidid Engraulis australis (56.7%) and the gobies Pseudogobius olorum (24.4%) and Favonigobius lateralis (15.0%) being the most abundant species. Most of the larvae were caught between November and March, with the concentrations of the most abundant species reaching peaks between January and March, when water temperatures had reached 21-24�C. In terms of number of larvae, the larval fish assemblage in the basins was dominated by species that spawn within the estuary, with the larvae of these species contributing ≥98.7% to the totals at the basin sites. Although the larvae of 26 marine species were caught in the entrance channel, these were either rare or absent in the basins, except for those of the terapontid Pelates sexlineatus, which were moderately abundant in the outer basin. The fact that the larvae of most of these marine species were at the preflexion stage, and that all but three of those species had never been previously recorded as either juveniles or adults within the system, indicates that they were passively transported from outside the estuary. The absence of larvae of most of the marine teleosts that are abundant in the basins of the Nornalup-Walpole Estuary parallels the situation in the nearby and seasonally closed Wilson Inlet.

The significance of length at sexual maturity, mesh size and closed fishing waters to the commercial fishery for the catfish Cnidoglanis macrocephalus in Australian estuaries

The total lengths and numbers of Cnidoglanis macrocephalus caught in the closed and open fishing waters of Wilson Inlet, using a gill net cornrising a wide range of mesh sizes, were recorded. These data were then compared with those obtained for the commercial fishery. The results strongly indicate that (I) the density and percentage contribution of large fish are lower in open than in closed fishing waters, (II) female fish were more readily caught than male fish and (III) the minimum legal mesh size and minimum legal length for C. macrocephalus should be increased to prevent fish being caught before they reach sexual maturity.

Reproductive biology and larval morphology of the marine plotosid Cnidoglanis macrocephalus (Teleostei) in a seasonally closed Australian estuary

Monthly trends shown by gonadosomatic indices, the prevalence of the different gonadal stages, and the size distribution of the oocytes, indicate that the large marine and commercially important plotosid Cnidoglanis macrocephalus spawns in Wilson Inlet between October and January. The conclusion that spawning occurs within this seasonally closed estuary was confirmed by the presence of males in large nests and by the capture of newly-hatched, yolk sac larvae from one of those nests. The fact that C. macrocephalus, which is also widely distributed in coastal marine waters throughout much of southern Australia, can spawn within Wilson Inlet would be of particular value to this species in those periods when closure of the estuary would preclude a seawards spawning migration. Sexual maturity is size dependent, with spawning rarely occurring before fish have reached a total length of 425 mm. Sexual maturity was attained by a few fish at the end of their second year, by several at the end of their third year and by most, if not all fish, at the end of their fourth year. Comparisons with data for the more northern and permanently open Swan Estuary indicate that C. macrocephalus also spawns within that system and that the spawning time of this species is related to water temperature. The adult male guards the larvae under its pelvic fins in burrows. The larvae increased in total length from 29 mm just after hatching to 43 mm in the 17–18 days after capture, during which time their yolk sac was resorbed. Details are given of the morphology, morphometrics, meristics and pigmentation of larval C. macrocephalus. In comparison with the larvae of three other plotosid genera, the larva of C. macrocephalus is far larger in size and more developed at hatching and takes a shorter time to transform into a juvenile.