Zostera Capricorni Research Articles

The value of estuarine producers to fisheries: A case study of Richmond River Estuary.

Nutrient input from estuarine producers underpins coastal fisheries production and knowing which producers are the most responsible for fish diet helps effectively protect and restore coastal ecosystems. Focussing on the Richmond River in Australia as a case study, we sampled the main estuarine producers and estimated their proportional contributions of nutritional input to seven commercially important fisheries species using Bayesian isotope mixing models. We valued the dietary input of estuarine producers to the commercial fisheries by combining dietary contribution estimates with total annual catch data from commercial fishers. A conservative estimate is that estuarine producers in the Richmond River Estuary contribute at least 82 725kg (78%) of the total annual catch of the seven commercially important fish with an estimated annual value of $AU 450 117. Sea mullet and Mud crab contributed 95% of the total catch, and 93% of the total value assigned to estuarine producers. The two highest valued estuarine producers were tidal marsh (Juncus kraussii) $AU 82 432 and seagrass (Zostera capricorni) $AU 65 423. This study demonstrates the substantial role of estuarine producers to commercial fisheries production and the fisheries economy more broadly. With large areas of estuarine producers under threat globally from land clearing for agriculture, aquaculture and urbanisation, the results presented here provide evidence to support the value of coastal habitats and benefits of their preservation and restoration.

Comparison of mangrove (Avicennia marina) metal tissue concentrations to ambient sediment with an extensive range of contaminant levels in a highly-modified estuary (Sydney estuary, Australia)

Statistically distinct ‘high’, ‘moderate’ and ‘low’ metal contamination positions were distinguished across five Sydney estuary embayments for total sedimentary metal concentrations of Ni, Pb and Zn. While statistically distinct total sedimentary As, Cd, Co and Cu concentrations were indicated for the ‘low’ position samples. Mangrove (Avicennia marina) pneumatophore tissue metal concentrations from ‘high’ position trees were statistically distinct for Cd, Ni, Pb and Zn, while root tissue samples from ‘low’ position trees were statistically distinct for As, Cd and Zn. Outcomes from CAP, simple linear regression and DISTLM modelling indicated sedimentary metals were predominantly assimilated into mangrove root tissue, with smaller uptake into pneumatophore tissue. A comparison of floristic tissue metal concentrations indicated mangrove (A. marina) root tissue assimilated higher sedimentary metal concentrations than leaf and root tissue of seagrasses (Zostera capricorni and Halophila ovalis), which had relatively higher metal concentrations than those from pneumatophore and leaf mangrove tissue.

Trophic transfer of metals in a seagrass food web: Bioaccumulation of essential and non-essential metals

Metal concentrations are reported for a seagrass ecosystem receiving industrial inputs. δ13C and δ15N isotope ratios were used to establish trophic links. Copper concentrations (dry mass) ranged from <0.01 μg/g in fish species to 570 μg/g (μ = 49 ± SD = 90 μg/g) in the oyster Saccostrea glomerata. Zinc concentrations ranged from 0.6 μg/g in the seagrass Zostera capricorni to 10,800 μg/g in the mud oyster Ostrea angasi (μ = 434 ± 1390 μg/g). Cadmium concentrations ranged from <0.01 μg/g in fish species to 268 μg/g in Ostrea angasi (μ = 6 ± 25 μg/g). Lead concentrations ranged from <0.01 μg/g for most fish species to 20 μg/g in polychaetes (μ = 2 ± 3 μg/g). Biomagnification of metals did not occur. Organisms that fed on particulate organic matter and benthic microalgae had higher metal concentrations than those that fed on detritus. Species physiology also played an important role in the bioaccumulation of metals.

Metal concentrations in seagrass (Halophila ovalis) tissue and ambient sediment in a highly modified estuarine environment (Sydney estuary, Australia)

Research into sediment-seagrass tissue metal relationships has been undertaken in Sydney estuary due to the recognized role contamination plays in threats to seagrass health. Seagrass (Halophila ovalis) leaf tissue concentrations are elevated in Cu, Pb and Zn and contain the highest reported root Cr concentrations. Seagrass metal concentrations were significantly different between species H. ovalis and Zostera capricorni; between root and leaf tissue; and between sampling locations. Greatest tissue enrichment was for Pb, however metals were not enriched in seagrass relative to surficial sediment. Fine and total sediment metal concentrations were temporally consistent between collection years 2013/15, whereas root tissue metals changed between years and sites and leaf metal contents were temporally inconsistent. Extractable metal concentrations in fine sediment (<62.5 μm) showed moderate significant correlation with root tissue and a weak significant relationship with leaf tissue, whereas total sediment metal showed no such relationships. Management implications are provided.

The relationship between metal concentrations in seagrass (Zostera capricorni) tissue and ambient fine sediment in modified and near-pristine estuaries (Sydney estuaries, Australia)

Seagrass (Zostera capricorni) tissue metal concentrations in Sydney estuary are some of the highest reported for urban environments. A strong declining metal (As, Co, Cr, Cu, Mn, Pb and Zn) concentration gradient in fine sediment from highly modified to the near-pristine estuarine environments was matched by a concurrent and statistically significant, moderately strong, relationship with declining metal concentrations for leaf, rhizomes and root tissue types. Leaf tissue more consistently expressed metal concentration uptake from sediment. Rhizome tissue contained the highest concentrations for all metals, except Mn, while leaf concentrations were higher for Cr and Zn and roots were more elevated in Cr and Pb. Tissue Cr, Pb and Zn were close to background for reference sites for leaves. Maximum tissue enrichment over background was 25, 19, 47 and 8 times for Cr, Cu, Pb and Zn. Bio-sediment accumulation factors were generally <1 for all metals.

Feeding ecology of the seagrass-grazing nerite Smaragdia souverbiana (Montrouzier, 1863) in subtropical seagrass beds of eastern Australia

By amalgamating all seagrass-associated grazing invertebrates into an epiphyte-feeding guild, the currently accepted model of seagrass trophic dynamics ignores the diverse range of invertebrates that feed directly on, and do considerable damage to, seagrasses. Of the wide range of invertebrates documented to damage seagrass directly, the gastropod genus Smaragdia has adaptations and ecology that suggests it could be a specialized seagrass-feeding group, of which at least two species are known preferentially to consume seagrass. This paper investigated the dietary associations of Smaragdia souverbiana, one of the most widely distributed but least studied species of the genus, in the subtropical eastern Australian part of its range. Using field-based assessments of grazing damage and targeted laboratory feeding trials, we assessed the dietary associations, digestive ability and feeding preferences of S. souverbiana with local seagrasses (Halophila ovalis, Zostera capricorni and Cymodocea serrulata). We found that this species consumed and damaged all available species, but showed a strong preference for the most abundant and moderately digestible Z. capricorni. Although it avoided seagrass bearing a high epiphyte load in a laboratory context, considerable amounts of epiphytic material were found in the faeces of field-caught individuals. Grazing and digestibility of seagrass cells was higher in Z. capricorni and H. ovalis, and the former was preferred when both were available. This study adds to the growing body of literature demonstrating that S. souverbiana - and potentially many other grazing invertebrates - cause considerable damage to seagrasses directly, rather than targeting epiphytes.

Resilience of Zostera muelleri seagrass to small‐scale disturbances: the relative importance of asexual versus sexual recovery

Resilience is the ability of an ecosystem to recover from disturbance without loss of essential function. Seagrass ecosystems are key marine and estuarine habitats that are under threat from a variety of natural and anthropogenic disturbances. The ability of these ecosystems to recovery from disturbance will to a large extent depend on the internsity and scale of the disturbance, and the relative importance of sexual versus asexual reproduction within populations. Here, we investigated the resilience of Zostera muelleri seagrass (Syn. Zostera capricorni) to small-scale disturbances at four locations in Lake Macquarie – Australia's largest coastal lake – and monitored recovery over a 65-week period. Resilience of Z. muelleri varied significantly with disturbance intensity; Z. muelleri recovered rapidly (within 2 weeks) from low-intensity disturbance (shoot loss), and rates of recovery appeared related to initial shoot length. Recovery via rhizome encroachment (asexual regeneration) from high-intensity disturbance (loss of entire plant) varied among locations, ranging from 18-35 weeks, whereas the ability to recover was apparently lost (at least within the time frame of this study) when recovery depended on sexual regeneration, suggesting that seeds do not provide a mechanism of recovery against intense small-scale disturbances. The lack of sexual recruits into disturbed sites is surprising as our initial surveys of genotypic diversity (using nine polymorphic microsatellite loci) at these location indicate that populations are maintained by a mix of sexual and asexual reproduction (genotypic diversity [R] varied from 0.24 to 0.44), and populations consisted of a mosaic of genotypes with on average 3.6 unique multilocus genotypes per 300 mm diameter plot. We therefore conclude that Z. muelleri populations within Lake Macquarie rely on clonal growth to recover from small-scale disturbances and that ongoing sexual recruitment by seeds into established seagrass beds (as opposed to bare areas arising from disturbance) must be the mechanism responsible for maintaining the observed mixed genetic composition of Z. muelleri seagrass meadows.

Effectiveness of habitat classes as surrogates for biodiversity in marine reserve planning

AbstractSurrogates are used in marine conservation planning when there is limited information on the distribution of biodiversity, and representation of species and assemblage diversity are conservation goals. With prior confirmation of their relationship to spatial variation in biodiversity, habitat classification schemes are a potentially useful surrogate.Polychaetes can comprise over one‐third of species of benthic infaunal assemblages, they are the most frequent and abundant marine metazoans in benthic environments, and they are a reliable surrogate for other macrobenthic taxa.It was tested whether polychaete biodiversity differed among six estuarine habitat classes defined for conservation planning in the Port Stephens–Great Lakes Marine Park, New South Wales, Australia: subtidal sand, mud, muddy sand, and seagrass beds comprisingPosidonia australis,Zostera capricorniand mixedPosidonia/Zostera. Polychaetes were sampled from replicate sites in each habitat and differences among habitat classes in species richness, abundance, and assemblage structure were examined. Several environmental variables, known to be important determinants of polychaete distribution, were also quantified at each site.Ninety‐five species of polychaetes (belonging to 35 families) were identified. Species richness and abundance did not differ among the habitat classes. Polychaete assemblages of subtidal sand differed from assemblages in both mud and muddy sand, however, assemblages in all other habitats were not different. A combination of some of the measured environmental variables (distance to the estuary entrance, depth, sediment grain size) was a more important determinant of assemblage variation than the habitat classes.Using these predictors, an alternative habitat classification scheme to the scheme currently utilized in marine park planning is proposed.This study demonstrates the critical importance of testing assumptions about surrogacy and an approach for refining surrogates.Copyright © 2013 John Wiley &amp; Sons, Ltd.

Strong consequences of diet choice in a talitrid amphipod consuming seagrass and algal wrack

Deposits of detached macrophytes, known as wrack, are a common feature on shore lines and can represent an important subsidy of organic material from subtidal systems to low productivity intertidal and terrestrial systems. On beaches, these support high densities of consumers that have an important role in wrack decomposition. The feeding behavior of wrack consumers is poorly known relative to the marine herbivores that consume marine macrophytes in situ. To understand how feeding behavior relates to the quality of wrack for an abundant wrack consumer, the talitrid amphipod Notorchestia sp., we test for habitat preferences, differences in feeding rates, and survival among four species of macroalgae and seagrasses in an estuary in New South Wales, Australia. Notorchestia displayed strong preferences for Sargassum sp. and Zostera capricorni as habitat, but consumed only Sargassum in feeding experiments, and only this alga supported high survival in a longer term performance assay. The differences in food quality, as measured by survival over 30 days, did not translate to differences in the abundance of amphipods colonizing each food resource in the field. Our results suggest that feeding by Notorchestia will result in the rapid loss of Sargassum in the wrack, and that other consumers or microbial degradation may be more important in the decomposition of seagrass.

Replacement of native seagrass with invasive algal detritus: impacts to estuarine sediment communities

Biological invasions modify the quality and supply of detrital subsidies to aquatic and terrestrial ecosystems. Where the invader has very different traits to native species, major changes in associated consumer communities may result, as a consequence of differences in their nutritional value and effects on the sedimentary habitat. We assessed how the replacement of seagrasses with the invasive alga Caulerpa taxifolia in modified Australian estuaries influences invertebrate communities of mudflats that are subsidized by detritus from submerged aquatic vegetation. Two months after experimental enrichment of sediments with high (60 g dry weight per 0.25 m2 plot) or low (30 g dry weight) quantities of either non-native C. taxifolia or native Posidonia australis or Zostera capricorni detritus, there were positive effects of detrital addition on invertebrate abundance that occurred irrespective of the resource added. By 4 months after addition, however, detritus from invasive C. taxifolia had produced effects on benthic communities that could not be replicated by detritus from either of the native seagrasses. Plots receiving the high loading of C. taxifolia detritus contained fewer invertebrates than plots of the other treatments, perhaps due to the induction of sediment hypoxia. The pattern, however, reversed at low detrital loading, with the plots receiving 30 g of C. taxifolia containing more invertebrates and more taxa than the other plots, presumably due to the greater resource availability for detritivores. Our results demonstrate that replacement of native seagrass with invasive algal detritus can have large impacts on sediment-dwelling communities.

Carbon self‐utilization may assist Caulerpa taxifolia invasion

Additions of 13C‐labeled macroalgal detritus (Caulerpa taxifolia) and seagrass detritus (Zostera capricorni) to a vegetated intertidal mudflat in subtropical Australia provided insight into the mechanisms and ecosystem effects of C. taxifolia invasions in seagrass beds. Despite the high lability typical of macroalgae, carbon from seagrass detritus was removed from sediments and transferred to benthic compartments (microphytobenthos, bacteria, mud whelks, and live C. taxifolia and Z. capricorni) at a faster rate than carbon from macroalgal detritus. This preference was more pronounced for live Z. capricorni than live C. taxifolia. Whereas rates of dissolved inorganic carbon production and utilization were similar for seagrass and macroalgal detritus, higher fluxes of dissolved organic carbon (DOC) from macroalgal detritus early in the experiment reflected the lower utilization of this carbon pool. Although both seagrass and algae can use DOC as a source of carbon, caulerpenyne may comprise part of the DOC pool leached from C. taxifolia detritus during early degradation. This compound can have allelopathic effects on seagrass and may, therefore, be inaccessible to Z. capricorni. The availability of an additional carbon source for C. taxifolia, generated from its own detritus, represents a competitive advantage for this invasive macroalga over existing seagrass, particularly because seagrass productivity can be carbon‐limited. A similar mechanism may exist for other species of invasive macroalga, particularly those that produce toxic secondary metabolites.

Differences in benthic metabolism, nutrient fluxes, and denitrification in Caulerpa taxifolia communities compared to uninvaded bare sediment and seagrass (Zostera capricorni) habitats

Benthic metabolism, nutrient fluxes, and denitrification were measured in Caulerpa taxifolia communities that had recently invaded (between 2003 and 2008) bare sediments following the loss of Zostera capricorni (prior to 2003), and in adjacent Z. capricorni communities and bare sediments, to determine the likely changes in ecosystem function following C. taxifolia colonization. Areas colonized by C. taxifolia had higher gross primary production (GPP), net primary production (NPP), and dark benthic N2 effluxes than adjacent bare sediments, but lower dark benthic NH+4 effluxes. C. taxifolia invasion of bare sediments had a beneficial role in removing excess nitrogen from the system through enhanced denitrification. In contrast, areas occupied by C. taxifolia had similar GPP and NPP relative to Z. capricorni beds and similar benthic N2 and NH+4 effluxes. Thus, C. taxifolia appears to play a comparable role to Z. capricorni in terms of some ecosystem functions, although the degree of similarity is likely to be affected by the relative densities of the two macrophytes. However, replacement of Z. capricorni communities by C. taxifolia appears to significantly alter the quantity and quality (higher ratio) of benthic dissolved organic carbon and dissolved organic nitrogen fluxes.

Representative measurement of two-dimensional reactive phosphate distributions and co-distributed iron(II) and sulfide in seagrass sediment porewaters

The high degree of heterogeneity within sediments can make interpreting one-dimensional measurements difficult. The recent development and use of in situ techniques that measure two-dimensional distributions of porewater solutes have facilitated investigation of the role of spatial heterogeneity in sediment biogeochemistry. A colourimetric diffusive equilibration in thin films method has been developed that allows two-dimensional, high-resolution measurement of reactive phosphate in sediment porewaters. A method detection limit of 0.22 μM, an effective upper limit of ∼1000 μM and relative standard deviations typically below 5% were achieved. This method was evaluated by deployment in seagrass ( Zostera capricorni) colonised sediments, as part of combined probes with similar colourimetric methods for sulfide and iron(II). The two-dimensional, high resolution distributions obtained provide a highly representative measurement of the co-distributions of porewater solutes, allowing heterogeneous features and biogeochemical processes to be observed and interpreted. Microniches of high phosphate concentration >100 μM were observed throughout the distributions and were interpreted to be due to localised zones of rapid organic matter mineralisation, possibly using electron acceptors other than iron(III) oxyhydroxides ( e.g. aerobic respiration) as often they did not correspond with microniches of higher Fe(II) concentration.

Arsenic distribution and species in two Zostera capricorni seagrass ecosystems, New South Wales, Australia

Environmental context Arsenic concentrations and species were determined in seagrass ecosystems where the food web was established using carbon and nitrogen isotopes. There was a clear increase in the proportion of arsenobetaine in tissues of higher trophic level organisms, which is attributed to an increasing arsenobetaine content of the diet and the more efficient assimilation and retention of arsenobetaine over other arsenic species. The results provide an explanation for the prominence of arsenobetaine in higher marine animals. Abstract Arsenic concentrations and species were compared in biota from two Zostera capricorni ecosystems. Mean arsenic concentrations were not significantly different for non‐vegetative sediment, rhizosphere sediment, Z. capricorni blades, roots, rhizomes, epiphytes, amphipods, polychaetes, molluscs, crustaceans and fish, but were significantly different in detritus. Sediments and plant tissues contained mostly inorganic arsenic and PO4–arsenoriboside. Detritus contained mostly PO4–arsenoriboside. Fish tissues contained predominately arsenobetaine. Other animals had lower proportions of arsenobetaine and variable quantities of minor arsenic species. Bioconcentration but not biomagnification of arsenic is occurring with no evidence of arsenic hyper accumulation. The proportion of arsenobetaine increases through the food web and is attributed to a shift from a mixed diet at lower trophic levels to animals containing mostly arsenobetaine at higher trophic levels and the more efficient retention of arsenobetaine, compared to other arsenic species.

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.

Temporal changes in the estuarine benthic fauna of Towra Point, Botany Bay.

The estuarine wetlands in the Towra Point region of Botany Bay, New South Wales, represent one of the few large remaining such systems near Sydney. There are probably the major contributor of organic material to the food web within Botany Bay and provide habitats and nursery areas for many important fish and invertebrates species of the Bay. Botany Bay has been converted to a major port facility to serve Sydney. As well as heavy shipping usage (and its associated dredging activities), it is also fringed by considerable industrial development and a major airport complex. Research doe on the estuarine flora of Towra Point includes a major study on the physical and biological factors that control the structure of the mangrove and saltmarsh plant communities (Clarke and Hannon 1967, 1969, 1970, 1971; Kratochvil et al. 1973), distribution and biomass determinations of the seagrasses Posidonia australis (Larkum 1976; West and Larkum 1979) and Zostera capricorni (Larkum, Collett and Williams, in preparation) and comparisons of the epiphytic algae communities on both species of seagrass (May et al. 1978).

The seagrasses of New South Wales estuaries and embayments

This article outlines some important functions of seagrasses and some aspects concerned with the distribution, biology and ecology of the New South Wales seagrasses. Introduction Plants of the marine environment can be separated into a number of categories. The most basic division considers whether or not the plant is an angiosperm – the so-called ‘higher’ plants which produce flowers and have a greater structural complexity than the algae. Marine angiosperms are more commonly referred to as seagrasses and, although they are all monocotyledons, they are related only in appearance to the terrestrial grasses. Seagrasses generally inhabit soft sediments, from sand to mud, within sheltered shallow waters of estuaries, bays, lagoons and lakes. World-wide there are about fifty-two species of seagrasses. Twenty-five of these occur in Australian waters, and six are found in the New South Wales coastline. These are Posidonia australis, Zostera capricorni, Zostera muelleri, Heterozostera tasmanica, Haliphilia ovalis, and Halophilia decipiens. Seagrasses are strictly defined as angiosperms which are able to complete their life cycle in seawater. This definition excludes another genus of aquatic angiosperms which will be treated in this review (Ruppia). This genus, which is more often found in brackish waters, has three species in New South Wales, Ruppia maritima, Ruppia megacarpa and Ruppia polycarpa. The latter two species have water-surface pollination and are thus usually restricted to shallow, still waters. Ruppia species are also sometimes found in non-marine brackish waters (Sainty and Jacobs 1981).

Diversity and distribution of fishes in an intermittently open coastal lagoon at Shellharbour, New South Wales

A study was undertaken to examine the diversity and distribution of fish in an intermittently open coastal lagoon proposed for replacement by a marina development at Shellharbour, New South Wales. Eighteen sites, consisting of seagrass communities (mainly Zostera capricorni) and bare mud and sand substrata, were sampled by seine net at least once a week during both day and night over a three month period. Of the 985 fish caught, the most abundant species were sand mullet (Myxus eloes an ngates), tamar river goby (Afurcagobius tamarensis), yellowfin bream (Acanthopagrus australis) and mosquito fish (Gambusia holbrooki). Species diversity was found to increase after lagoon opening. Many more of the 27 species caught were foind in the more saline habitats (25sp.) than in the more brackish upland habitats (13sp.). more species were caught during the day (24sp.) than at night (16sp.). catches of juvenile (20-50mm) Myxus elongates increased significantly after lagoon opening. Catches of Afurcagobius tamarensis and Acanthopagrus australis also increased after lagoon opening but this was probably due to high turbidity and changes in estuary morphology rather than salinity. The sizes of the fishes caught suggest that the lagoon acts as a nursery area, although some adult fish were also found.

The seagrasses of Lake Illawarra, New South Wales.

Ecological studies on seagrasses and seagrass communities in eastern Australia date back only as far as those of Wood (1959a, b). The most detailed studies in New South Wales have been made on the central coast lakes, Lake Macquarie (see King 1986, for references) and Tuggerah Lakes (see King & Holland 1986). Detailed studies in those lakes over five years (King & Hodgson 1986) showed that in terms of the area occupied by seagrasses, species composition of the communities, and the biomass of the eelgrass Zostera capricorni Ascherson, there are marked fluctuations which cannot be correlated easily with seasonal or environmental changes. Such variation appears to be inherent in the nature of seagrass communities but has not often been properly documented. In some cases aerial photographs can be used to give an overview of the (recent) history of the distribution of seagrass beds but without concurrent field survey there is no information on species present or biomass. The biota of Lake Illawarra is of special interest, not only because it is in a region of rapid urbanisation with associated recreational use, but also because of proposals to refurbish and upgrade the existing Tallawarra Power Station. Lake Illawarra is already subject to environmental problems, especially eutrophication which has resulted in excessive algal growth (LIMC 1986b), but while this is a major immediate problem the most severe long-term concern is with the infilling of the lake (LIMC 1986a). Lake Illawarra has, as a consequence, been the subject of a range of intensive studies most of which are summarized in the ‘Environmental Audit’ (SPCC 1986).

Larval settlement preference of a native bivalve: the influence of an invasive alga versus native substrata

Recruitment patterns of marine invertebrates are strongly influenced by the habitat preference of larvae as larvae can choose to settle (or not) in response to positive or negative cues. High abundances of recruits of the native infaunal bivalve Anadara trapezia occur in the invasive alga Caulerpa taxifolia compared to native habitats. Using controlled laboratory experiments, A. trapezia larval habitat preference was investigated through exposure to available native (adult A. trapezia, Zostera capricorni and sediment from unvegetated areas) and invasive (C. taxifolia and sediments from C. taxifolia) substrata that co-occur in estuaries invaded by C. taxifolia in New South Wales, Australia. When exposed to all substrata, larval settlement was significantly higher on adults compared to all other substrata except Z. capricorni. Although settlement to C. taxifolia was low, lar- vae did not reject it as settlement surface. In pairwise comparisons, larval settlement was always higher on adults compared to all other substrata, although differences were only significant com- pared to C. taxifolia and unvegetated sediments. There was no difference in settlement when larvae were exposed to Z. capricorni and C. taxifolia. When offered a single substratum, larval settlement was significantly higher on adults and Z. capricorni compared to all remaining substrata. Manipula- tions of shells of adults indicated that larvae may be responding positively to biofilms on the surface of shells. The data indicate that A. trapezia larvae prefer to settle on adults and, while they do not pre- fer C. taxifolia, they do not reject it as a settlement surface. Therefore, C. taxifolia may serve as a sink habitat for A. trapezia.