Unvegetated Sediments Research Articles

The Influence of Vegetation on Microbial Enzyme Activity and Bacterial Community Structure in Freshwater Constructed Wetland Sediments

Microorganisms play important roles in wetland ecosystems, but little is known about the influence of wetland plants on microbial community structure and activity. A greenhouse experiment was conducted to study the short-term influence of wetland vegetation on the sediment microbial community. Mesocosms were either planted with Juncus effusus, Carex lurida, or Dichanthelium acuminatum var. acuminatum or remained unvegetated. After eight weeks, sediment samples were taken and assayed for the activity of five microbial extracellular enzymes associated with carbon, nitrogen, and phosphorus cycling. β-1,4-glucosidase, phosphatase, and N-acetylglucosaminidase exhibited similar activity for all vegetation treatments, while the activity of the phenolic-degrading enzymes phenol oxidase and peroxidase was higher in sediments with no vegetation. Denaturing gradient gel electrophoresis and sequencing of partial 16S rRNA genes indicate differences in the sediment bacterial community associated with each plant regime. Acidobacteria, Firmicutes and Proteobacteria were the dominant phyla, although unvegetated sediments contained proportionally fewer Firmicutes and Alphaproteobacteria. This study provides insights into the structure of wetland bacterial communities and suggests that vegetation can influence both bacterial community structure and specific enzyme activity in wetland sediments. Moreover, these influences can occur over a relatively short time and could occur within just a few months of vegetation changes.

Submerged macrophytes modify bacterial community composition in sediments in a large, shallow, freshwater lake

Submerged aquatic macrophytes are an important part of the lacustrine ecosystem. In this study, the bacterial community compositions in the rhizosphere sediments from three kinds of submerged macrophytes (Ceratophyllum demersum, Potamogeton crispus, and Vallisneria natans) were investigated to determine whether submerged macrophytes could drive the variation of bacterial community in the eutrophic Taihu Lake, China. Molecular techniques, including terminal restriction fragment length polymorphism (T-RFLP) of PCR-amplified 16S rRNA gene and clone libraries, were employed to analyze the bacterial community compositions. Remarkable differences of the T-RFLP patterns were observed among the different samples, and the results of LIBSHUFF analysis also confirmed that the bacterial community compositions in the rhizosphere sediments of three kinds of submerged macrophytes were statistically different from that of the unvegetated sediment. Acidobacteria, Deltaproteobacteria, and Betaproteobacteria were the dominant bacterial groups in the rhizosphere sediments of Ceratophyllum demersum, Potamogeton crispus, and Vallisneria natans, respectively, accounting for 15.38%, 29.03%, and 18.00% of the total bacterial abundances. Our study demonstrated that submerged macrophytes could influence the bacterial community compositions in their rhizosphere sediments, suggesting that macrophytes have an effect on the cycling and transportation of nutrients in the freshwater lake ecosystem.

Impact of aquaculture on benthic virus–prokaryote interactions in the Mediterranean Sea

We investigated the effects of organic enrichment due to the biodeposition from fish farms on benthic prokaryotic and viral abundance and production, viral-induced prokaryotic mortality, enzymatic activities and bacterial diversity. We compared four areas across the Mediterranean Sea, from Cyprus to Spain, and two different habitats: sediments covered by the seagrass Posidonia oceanica and soft-bottom unvegetated sediments. In several cases, the sediments beneath the cages showed higher prokaryotic and viral abundance and production, and higher rates of organic matter decomposition. However, the differences between impact and control sediments were not consistent at all regions and habitats. Benthic bacterial diversity was always lower below the cages, where high viral-induced bacterial mortality rates were also observed. The δ- and γ-Proteobacteria dominated in both impacted and control sediments, but the relative importance of sulphate-reducing δ-Proteobacteria increased beneath the cages. We conclude that aquaculture can have a significant impact on benthic prokaryotes and viruses, by stimulating prokaryotic metabolism and viral infections, reducing bacterial diversity and altering assemblage composition. However, these impacts vary depending upon the sediment type and the habitat characteristics.

Higher nitrate-reducer diversity in macrophyte-colonized compared to unvegetated freshwater sediment

Freshwater macrophytes stimulate rhizosphere-associated coupled nitrification–denitrification and are therefore likely to influence the community composition and abundance of rhizosphere-associated denitrifiers and nitrate reducers. Using the narG gene, which encodes the catalytic subunit of the membrane-bound nitrate reductase, as a molecular marker, the community composition and relative abundance of nitrate-reducing bacteria were compared in the rhizosphere of the freshwater macrophyte species Littorella uniflora and Myriophyllum alterniflorum to nitrate-reducing communities in unvegetated sediment. Microsensor analysis indicated a higher availability of oxygen in the rhizosphere compared to unvegetated sediment, with a stronger release of oxygen from the roots of L. uniflora compared to M. alterniflorum. Comparison of narG clone libraries between samples revealed a higher diversity of narG phylotypes in association with the macrophyte rhizospheres compared to unvegetated sediment. Quantitative PCR targeting narG- and 16S rRNA-encoding genes pointed to a selective enrichment of narG gene copies in the rhizosphere. The results suggested that the microenvironment of macrophyte rhizospheres, characterized by the release of oxygen and labile organic carbon from the root system, had a stimulating effect on the diversity and relative abundance of rhizosphere-associated nitrate reducers.

Ragworm fatty acid profiles reveals habitat and trophic interactions with halophytes and with mercury

The present study aimed to assess if ragworm fatty acids (FA) profiles could be used to discriminate their spatial distribution in an historically mercury-contaminated estuarine environment, i.e., if it was possible to differentiate ragworms present in salt marsh sediments surrounding plant roots and rhizomes (rhizosediment) from adjacent unvegetated sediment. Additionally, we also tried to determine if ragworms differed in mercury content and if these values could also be used to identify the habitat they occur in. Results show that, within the same area, ragworms can be distinguished using FA profiles and that in halophyte rhizosediment ragworms display more than twice the levels of alpha-linolenic acid (18:3n-3). The ratio cis-vaccenic/oleic acids (18:1n-7/18:ln-9) in ragworms suggests higher carnivory in unvegetated sediments. Our study indicates that ragworm FA profiles can be used to identify their habitat, their trophic interaction with halophytes and reveal a spatially contrasting feeding behaviour, which also reflects mercury accumulation.

Optimisation of baited remote underwater video sampling designs for estuarine fish assemblages

Baited remote underwater video stations (BRUVS) are used for monitoring fish assemblages and assessing management effectiveness in reef environments but are infrequently used in estuaries. A review of the BRUVS literature found that most adopted sampling designs from other studies were rarely designed from pilot studies. This potentially compromises their value for monitoring natural and anthropogenic variation. The aims of this study were: (i) to assess the suitability of BRUVS for sampling fishes in estuarine habitats (seagrass beds and unvegetated sediments) and (ii) to develop an optimal and cost effective sampling methodology for each habitat. Fishes in both habitats were sampled independently using BRUVS with soak times of 30, 60, 90min (n=4). Thirty five species of fishes were recorded including 18 species of economic importance. Mean number of species, mean total Max N and mean Max N of species did not differ among soak times. Precision was generally greater in seagrass and in both habitats it improved with increasing soak time. Bootstrapping revealed that greater improvements in precision occurred from increasing soak time rather than increasing replication. A sampling design with n=5 replicates of 90min soak time was optimal for most variables. This sampling effort is greater than many current applications of BRUVS. The results highlight the importance of pilot studies to optimise sampling methods and develop cost effective and statistically-robust monitoring programs.

Invasive ecosystem engineer selects for different phenotypes of an associated native species

Invasive habitat-forming ecosystem engineers modify the abiotic environment and thus represent a major perturbation to many ecosystems. Because native species often persist in these invaded habitats but have no shared history with the ecosystem engineer, the engineer may impose novel selective pressure on native species. In this study, we used a phenotypic selection framework to determine whether an invasive habitat-forming ecosystem engineer (the seaweed Caulerpa taxifolia) selects for different phenotypes of a common co-occurring native species (the bivalve Anadara trapezia). Compared to unvegetated habitat, Caulerpa habitat has lower water flow, lower dissolved oxygen, and sediments are more silty and anoxic. We determined the performance consequences of variation in key functional traits that may be affected by these abiotic changes (shell morphology, gill mass, and palp mass) for Anadara transplanted into Caulerpa and unvegetated habitat. Both linear and nonlinear performance gradients in Anadara differed between habitats, and these gradients were stronger in Caulerpa compared to unvegetated sediment. Moreover, in Caulerpa alternate phenotypes performed well, and these phenotypes were different from the dominant phenotype in unvegetated sediment. By demonstrating that phenotype-performance gradients differ between habitats, we have highlighted a role for Caulerpa as an agent of selection on native species.

Prokaryotes in salt marsh sediments of Ria de Aveiro: Effects of halophyte vegetation on abundance and diversity

The aim of this study was to investigate the influence of monospecific colonization of sediment stands by Spartina maritima or Halimione portulacoides on benthic prokaryote assemblages in a salt marsh located in Ria de Aveiro (Portugal). The distribution of Bacteria, Archaea and sulfate-reducing bacteria (SRB) in sediments with monospecific plant stands and in unvegetated sediments was characterized by Fluorescence In Situ Hybridization (FISH). Total prokaryote abundance (0.4 × 109–1.7 × 109 cells gdw−1) was highest in sediments from the surface layer. The domain Bacteria comprised approximately 40% of total prokaryote communities with the highest percentages occurring in the surface layer. Archaeal cells corresponded to an average of 25% of total prokaryote population, with higher abundance in the vegetation banks, and displaying homogeneous vertical distribution. The relative abundance of SRB represented approximately 3% of total 4′, 6-diamidino-2-phenylindole dihydrochloride (DAPI) stained cells at unvegetated sediment and H. portulacoides stand and 7% at S. maritima stand. Headspace solid-phase microextraction (HS-SPME) combined with Comprehensive Two-Dimensional Gas Chromatography–Time-of-Flight Mass Spectrometry (GC × GC–ToFMS) was used to analyse the volatile and semi-volatile fraction of root exudates. A total of 171 compounds were identified and Principal Component Analysis showed a clear separation between the chemical composition (volatile and semi-volatile organic compounds) of the exudates of the two plants.The patterns of vertical distribution and differences in the proportion of SRB and Archaea in the prokaryote communities developing in sediments colonized by Spartina maritima or Halimione portulacoides suggest the existence of plant-specific interactions between halophyte vegetation and estuarine sediment bacteria in Ria de Aveiro salt marshes, exerted via sediment lithology and root-derived exudates.

Recovery trajectories during state change from bare sediment to eelgrass dominance

Seagrasses are important foundation species in shallow coastal ecosystems that pro- vide critical ecosystem services including stabilizing sediment, sequestering carbon and nutrients, and providing habitat and an energy source for a diverse fauna. We followed the recovery of functional (primary productivity, carbon and nitrogen sequestration, sediment deposition) and structural (shoot density, biomass, plant morphometrics) attributes of Zostera marina (eelgrass) meadows in replicate large plots (0.2 to 0.4 ha) restored by seeding in successive years, resulting in a chrono sequence of sites from 0 (unvegetated) to 9 yr since seeding. Shoot density was the structural metric that changed most significantly, with an initial 4 yr lag, and a rapid, linear increase in plots 6 to 9 yr after seeding. Changes in Z. marina aerial productivity, sediment organic content, and exchangeable ammonium showed a similar trend with an initial 4 yr lag period before differences were observed from initial bare sediment conditions. After 9 yr, Z. marina meadows had 20× higher rates of areal productivity than 1 to 3 yr old meadows, double the organic matter and exchangeable ammonium concentrations, 3× more carbon and 4× more nitro- gen, and had accumulated and retained finer particles than bare, unvegetated sediments. These results demonstrate the reinstatement of key ecosystem services with successful large-scale restoration, although none of the parameters reached an asymptote after 9 yr, indicating that at least a decade is required for these attributes to be fully restored, even in an area with high habitat suitability. Survivorship along a depth gradient showed that ~1.6 m (mean sea level) is the maxi- mum depth limit for Z. marina, which matches the 'tipping point' for survival predicted for this sys- tem from a non-linear hydro dynamic/seagrass growth model.

Acceleration of Nonylphenol and 4-tert-Octylphenol Degradation in Sediment by Phragmites australis and Associated Rhizosphere Bacteria

We investigated biodegradation of technical nonylphenol (tNP) in Phragmites australis rhizosphere sediment by conducting degradation experiments using sediments spiked with tNP. Accelerated tNP removal was observed in P. australis rhizosphere sediment, whereas tNP persisted in unvegetated sediment without plants and in autoclaved sediment with sterile plants, suggesting that the accelerated tNP removal resulted largely from tNP biodegradation by rhizosphere bacteria. Three bacterial strains, Stenotrophomonas sp. strain IT-1 and Sphingobium spp. strains IT-4 and IT-5, isolated from the rhizosphere were capable of utilizing tNP and 4-tert-octylphenol as a sole carbon source via type II ipso-substitution. Oxygen from P. australis roots, by creating highly oxygenated conditions in the sediment, stimulated cell growth and the tNP-degrading activity of the three strains. Moreover, organic compounds from P. australis roots functioned as carbon and energy sources for two strains, IT-4 and IT-5, supporting cell growth and tNP-degrading activity. Thus, P. australis roots elevated the cell growth and tNP-degrading activity of the three bacterial strains, leading to accelerated tNP removal. These results demonstrate that rhizoremediation of tNP-contaminated sediments using P. australis can be an effective strategy.

Seasonal fluxes of O 2, DIC and CH 4 in sediments with Vallisneria spiralis: indications for radial oxygen loss

Seasonal fluxes of dissolved oxygen, inorganic carbon and methane were measured in microcosms containing vegetated ( Vallisneria spiralis L.) and unvegetated sediments under controlled laboratory conditions. We tested if measured fluxes were affected by a moderate (6% as loss on ignition, LOI) and an elevated (10%) organic matter content (OM) in sediments. Microcosms were set up with plants and sediments collected from two riverine sites, upstream (moderate OM load) and downstream (elevated OM load) of a wastewater treatment plant. Light and dark fluxes were measured and V. spiralis net primary production and respiration rates were calculated. Unvegetated sediments were always net heterotrophic and behaved as methane sources to the water column, with significantly higher CH 4 release during summer from sediment with elevated OM load. Vegetated sediments were always net autotrophic with attenuated or negative CH 4 fluxes, suggesting the occurrence of processes within the rhizosphere that inhibit methane production or favor its oxidation. Vegetated sediments had an unbalanced O 2 to DIC stoichiometry, with average photosynthetic quotients varying between 0.30 and 0.68, significantly below one. The missing oxygen amount varied seasonally, with a minimum in the summer coinciding with the highest water temperature, but was not dependent upon the two OM levels. Overall these results suggest that V. spiralis is likely to transport a significant proportion of photosynthetically produced oxygen to the rhizosphere.

Accelerated biodegradation of pyrene and benzo[ a]pyrene in the Phragmites australis rhizosphere by bacteria–root exudate interactions

We investigated the biodegradation of pyrene and benzo[ a]pyrene in Phragmites australis rhizosphere sediment. We collected P. australis plants, rhizosphere sediments, and unvegetated sediments from natural aquatic sites and conducted degradation experiments using sediments spiked with pyrene or benzo[ a]pyrene. Accelerated removal of pyrene and benzo[ a]pyrene was observed in P. australis rhizosphere sediments with plants, whereas both compounds persisted in unvegetated sediments without plants and in autoclaved rhizosphere sediments with sterilized plants, suggesting that the accelerated removal resulted largely from biodegradation by rhizosphere bacteria. Initial densities of pyrene-utilizing bacteria were substantially higher in the rhizosphere than in unvegetated sediments, but benzo[ a]pyrene-utilizing bacteria were not detected in rhizosphere sediments. Mycobacterium gilvum strains isolated from rhizosphere sediments utilized pyrene aerobically as a sole carbon source and were able to degrade benzo[ a]pyrene when induced with pyrene. Phragmites australis root exudates containing phenolic compounds supported growth as a carbon source for the one Mycobacterium strain tested, and induced benzo[ a]pyrene-degrading activity of the strain. The stimulatory effect on benzo[ a]pyrene biodegradation and the amounts of phenolic compounds in root exudates increased when P. australis was exposed to pyrene. Our results show that Mycobacterium–root exudate interactions can accelerate biodegradation of pyrene and benzo[ a]pyrene in P. australis rhizosphere sediments.

Dissolved oxygen fluxes and ecosystem metabolism in an eelgrass (Zostera marina) meadow measured with the eddy correlation technique

Dissolved oxygen (DO) fluxes were measured by eddy correlation to estimate net ecosystem metabolism (NEM) during summer in a restored eelgrass (Zostera marina) meadow and a nearby, unvegetated sediment. This technique measures benthic fluxes under true in situ light and hydrodynamic conditions, integrates over a large area (typically > 100 m2), and captures short‐term variations. DO fluxes measured through eight 24‐h periods showed pronounced temporal variation driven by light and local hydrodynamics on multiple scales: hour‐to‐hour, within each daily cycle, and between deployments. The magnitude of variation between hours during single deployments equaled that between deployments, indicating that short‐term variation must be included for metabolism estimates to be accurate. DO flux variability was significantly correlated to mean current velocity for the seagrass site and to significant wave height for the unvegetated site. Fluxes measured in low‐flow conditions analogous to many chamber and core incubations underestimated those measured in higher‐flow conditions typical of in situ conditions by a factor of 2–6. Rates of gross primary production (GPP), respiration (R), and NEM varied substantially between individual deployments, reflecting variations in light and hydrodynamic conditions, and daily values of GPP and R for individual deployments were tightly linked. Average daily NEM of the seagrass site was higher than that of the unvegetated site; the seagrass site was in metabolic balance, and the unvegetated site showed a tendency toward net heterotrophy during this midsummer period.

Relationships between heavy metals distribution and organic matter cycling in mangrove sediments (Conception Bay, New Caledonia)

Mangroves of New Caledonia act as a buffer between a lagoon of more than 20,000 km 2 and the Island, which suffers intense processes of erosion resulting from urbanization and natural resources exploitation. This preliminary study aims at determining how heavy metals are distributed in mangrove sediments and pore-waters in relationship with their organic content. To reach our goal, a series of 50-cm deep cores were collected in the mangrove of Conception Bay. The various coring sites are representative of live forests ( Avicennia marina, Rhizophora stylosa), dead forest, and intertidal unvegetated area. The ranges of concentrations in sediments were the following (μmol g −1): Cu (0.08 to 0.51), Co (0.01 to 0.38), Ni (0.03 to 3.55), Cr (0.36 to 3.11), Zn (0.68 to 2.36), Mn (1.13 to 5.0) and Fe (22.64 to 721.69). Heavy metals distribution within sediments and pore-water appear to result from diagenetic processes linked to OM decomposition. Beneath the dead Avicennia forest and in the unvegetated sediments, the higher the organic content, the higher the metal concentration in the solid phase. Beneath living mangrove stands, despite a higher organic content, as well as higher sulphur content, heavy metals concentrations were not higher than those measured in the other sampling sites. Beneath these forests, redox conditions were mainly controlled by the length of waterlogging and the activity of root system. We suggest that, because of the specificity of the Avicennia root system and its position in the intertidal zone, heavy metals are more bioavailable and potentially more mobile than beneath Rhizophora stand.

Small-scale effect of intertidal seagrass (Zostera muelleri) on meiofaunal abundance, biomass, and nematode community structure

Seagrass beds are common features of coastal ecosystems worldwide, and their associated infauna are often more productive and diverse than in unvegetated habitats. Little is known, however, about the ecology of meiofaunal communities living in seagrass sediments. We compared the abundance and biomass of sediment meiofauna inside and outside an intertidalZostera muelleribed in southern New Zealand to assess the impact of seagrass cover on meiofaunal distribution. Nematode community structure, diversity, and feeding groups were also compared between habitats and sediment depths (0–2, 2–5 and 5–10 cm) to evaluate the effect of seagrass on nematode communities. Meiofaunal biomass was significantly higher inside than outside theZ. muelleribed, but secondary productivity inside the bed is likely to have been limited by the availability of labile organic matter. There were significant differences in nematode community structure between unvegetated, sparsely vegetated, and densely vegetated sites (102 m scale), as well as between sediment depths (cm scale). No significant differences were found in depth-integrated (0–10 cm) nematode diversity between sites, but vertical gradients in diversity differed between vegetated and unvegetated sites. Epistrate feeders were the most common feeding group in unvegetated sediments whereas most feeding groups were common inside the seagrass bed. Findings from this study indicate that seagrass beds can have a marked impact on infaunal structure and function over small spatial scales through their effect on sediment characteristics and organic matter input. Some unexpected trends observed in the present study, i.e. low meiofaunal biomass at the vegetated sites, and lower abundance of copepods inside than outside the seagrass bed, suggest that the nature of seagrass–invertebrate interactions may depend on habitat characteristics and the identity and ecology of species considered.

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.

Short term changes in pore water chemistry in river sediments during the early colonization by Vallisneria spiralis

This study aims to investigate the effects of benthic vegetation colonization on pore water chemistry in river sediments with different organic matter and nutrient loadings. Shoots of Vallisneria spiralis L., surface sediments and water were collected in March 2009 from two fluvial reaches, upstream (U) and downstream (D) an urban wastewater treatment plant. Laboratory microcosms were created with homogenised sediments and macrophytes from each sampling site, half with bare sediments (UB and DB) and half with transplanted shoots of V. spiralis (UV and DV). Microcosms were then incubated over 25 days in two tanks with water from U and D. Approximately every 4 days, three microcosms from each treatment (UB, DB, UV and DV) were terminated, and pore water was analysed for Eh, pH, O2, CO2, CH4, Fe2+, PO43−, NH4+, NO3− and DRSi. Significant effects of both vegetation and wastewater loadings were found. Vegetated sediments of both sites, especially UV, displayed significantly higher Eh and O2 and significantly lower dissolved CH4, NH4+ and DRSi in pore water compared to bare sediments. At site D, despite an elevated nitrate availability, pore water NH4+ was the preferred N-source for V. spiralis uptake. Unvegetated sediments downstream the sewage plant (DB) exhibited the lowest redox potential and CH4, NH4+, PO43− and Fe2+ accumulation in the pore water. Overall, results from this study suggest that early colonisation by rooted macrophytes affects pore water chemistry towards more oxidized conditions, along with nutrient retention within sediments, which are related to the uptake capacity and oxygen release by roots. They also suggest an elevated physiological plasticity for V. spiralis, as higher organic matter content and lower redox potential in downstream sediments did not affect its functions of ecosystem engineer as benthic metabolism regulator.

Occurrence and significance of copepod resting egg accumulation in seagrass sediments

The resting eggs produced by some species of marine calanoid copepods have been observed to accumulate on the seafloor in areas of high deposition and low resuspension. Seagrass beds are known to be environments that promote the accumulation of fine sestonic particles by inhibiting resuspension but have not been previously investigated as possible reservoirs for copepod resting eggs. Field sampling on a shallow reef in the northern Gulf of Mexico over 3 yr revealed that viable resting eggs of the copepod Acartia tonsa were significantly more abundant in seagrass-colo- nized sediment than in adjacent unvegetated sediment. Egg abundance differences between the 2 environment types occurred throughout each year. Differences tended to be greatest during the sum- mer when the seagrass canopy was highest. These results suggest that the unique effects seagrass blades and rhizomes have on sediment stabilization may make seagrass beds important accumula- tion sites for resting copepod eggs in shallow areas subject to frequent disturbance. Seagrass-medi- ated resuspension events could therefore influence the population dynamics of some copepod spe- cies and may be essential for local copepod populations reliant on recruitment from benthic eggs. The effects that seagrass presence may have on the benthic distribution of resting stages produced by other species of zooplankton and phytoplankton should be evaluated, and the possible effects of resting stage accumulation in seagrass on plankton communities and whole ecosystems should be considered.

Native species behaviour mitigates the impact of habitat-forming invasive seaweed

Habitat-forming invasive species cause large, novel changes to the abiotic environment. These changes may elicit important behavioural responses in native fauna, yet little is known about mechanisms driving this behaviour and how such trait-mediated responses influence the fitness of native species. Low dissolved oxygen is a key abiotic change created by the habitat-forming invasive seaweed, Caulerpa taxifolia, which influences an important behavioural response (burrowing depth) in the native infaunal bivalve Anadara trapezia. In Caulerpa-colonised areas, Anadara often emerged completely from the sediment, and we experimentally demonstrate that water column hypoxia beneath the Caulerpa canopy is the mechanism instigating this "pop-up" behaviour. Importantly, pop-up in Caulerpa allowed similar survivorship to that in unvegetated sediment; however, when we prevented Anadara from popping-up, they suffered >50% mortality in just 1 month. Our findings not only highlight the substantial environmental alteration by Caulerpa, but also an important role for the behaviour of native species in mitigating the effects of habitat-forming invasive species.

Botany Bay Benthos: Status and management

The questions to be considered in this symposium are: 1. How distinctive is Botany Bay in terms of processes and biota? 2. How has it been modified by human activity? 3. Is what is left worth saving? And 4. If so, what ought to be the objectives of future management and how might 5. management be implemented? This paper addresses questions 1 and 2 in the context of the macrozoobenthos of unvegetated sediments, omits question 3 (which is largely values-based) and, concerning question 4, addresses some general issues. The emphasis in question 1 is on biota rather than processes.