Rod Martin Connolly

Sulfur stable isotopes separate producers in marine food-web analysis

Ecological applications of stable isotope analysis rely on different producers having distinct isotopic ratios to trace energy and nutrient transfer to consumers. Carbon (C) and nitrogen (N) are the usual elements analysed. We tested the hypothesis that producers unable to be separated using C and N would be separated by sulphur (S), by reviewing estuarine and marine food web studies using all three elements (total of 836 pairwise comparisons between producers). S had a wider range of values across all producers than C and N (S: 34.4, C: 23.3, N: 18.7‰), and a higher mean difference among producers (S: 9.3, C: 6.5, N: 3.3‰). We varied from 1 to 10‰ the distance producers must be apart to be considered separate. For each of these gap distances, S-separated producers tied on C and N in 40% or more of cases. Comparing the three elements individually, S had fewer tied pairs of producers for any gap distance than C or N. However, S also has higher within-producer variability. Statistical tests on simulated data showed that this higher variability caused S to be less effective than C for analysing differences among mean producer values, yet mixing models showed that S had the smallest confidence intervals around mean estimates of source contributions to consumers. We also examined the spatial and temporal scales over which S isotope signatures of the saltmarsh plant Spartina alterniflora varied. Differences between samples taken within tens of metres were smallest, but between samples hundreds of metres apart were as different as samples thousands of kilometres apart. The time between samples being taken did not influence S signatures. Overall, the use of S is recommended because it has a high probability of distinguishing the contribution of different producers to aquatic food webs. When two elements are employed, the combination of S and C separates more producers than any other combination.

True Value of Estuarine and Coastal Nurseries for Fish: Incorporating Complexity and Dynamics

Coastal ecosystems, such as estuaries, salt marshes, mangroves and seagrass meadows, comprise some of the world’s most productive and ecologically significant ecosystems. Currently, the predominant factor considered in valuing coastal wetlands as fish habitats is the contribution they make to offshore, adult fish stocks via ontogenetic migrations. However, the true value of coastal nurseries for fish is much more extensive, involving several additional, fundamentally important ecosystem processes. Overlooking these broader aspects when identifying and valuing habitats risks suboptimal conservation outcomes, especially given the intense competing human pressures on coastlines and the likelihood that protection will have to be focussed on specific locations rather than across broad sweeps of individual habitat types. We describe 10 key components of nursery habitat value grouped into three types: (1) connectivity and population dynamics (includes connectivity, ontogenetic migration and seascape migration), (2) ecological and ecophysiological factors (includes ecotone effects, ecophysiological factors, food/predation trade-offs and food webs) and (3) resource dynamics (includes resource availability, ontogenetic diet shifts and allochthonous inputs). By accounting for ecosystem complexities and spatial and temporal variation, these additional components offer a more comprehensive account of habitat value. We explicitly identify research needs and methods to support a broader assessment of nursery habitat value. We also explain how, by better synthesising results from existing research, some of the seemingly complex aspects of this broader view can be addressed efficiently.

Removal of seagrass canopy: effects on small fish and their prey

Abstract In an experiment in a southern Australian estuary, patches of seagrass canopy were removed to test the importance of the canopy to fish in areas where all other factors were known to be consistent with seagrass presence. The total number of fish was slightly lower in patches cleared of seagrass than in patches of undisturbed seagrass, but was not as low as in unvegetated patches. The benthic habitat was expected to be especially important to non-pelagic species, yet their numbers, and those of the most important commercial species, Sillaginodes punctata , were not lower in patches cleared of seagrass, despite being lower in unvegetated patches. The disturbance associated with removing seagrass was simulated and was not found to affect fish numbers. The diet of all fish caught consists mainly of invertebrates associated with the seagrass canopy and sediment surface (epifauna). Epifaunal abundance and production were highest in seagrass patches, lowest in unvegetated patches and intermediate in patches cleared of seagrass. Patterns of fish abundance did not provide evidence of the importance of seagrass canopy in attracting increased fish abundances compared with unvegetated areas but were consistent with a model stressing the importance of prey availability in the role seagrass plays as habitat for small fish.

Movement of carbon among estuarine habitats and its assimilation by invertebrates.

We measured the extent of movement of carbon and its assimilation by invertebrates among estuarine habitats by analysing carbon stable isotopes of invertebrates collected along transects crossing the boundary of two habitats. The habitats were dominated by autotrophs with distinct isotope values: (1) mudflats containing benthic microalgae (mean −22.6, SE 0.6‰) and (2) seagrass and its associated epiphytic algae (similar values, pooled mean −9.8, 0.5‰). Three species of invertebrates were analysed: a palaemonid shrimp, Macrobrachium intermedium, and two polychaete worms, Nephtys australiensis and Australonereis ehlersi. All species had a similar narrow range of isotope values (−9 to −14‰), and showed no statistically significant relationship between position along transect and isotope values. Animals were relying on carbon from seagrass meadows whether they were in seagrass or on mudflats hundreds of metres away. Particulate organic matter collected from superficial sediments along the transects had similar values to animals (mean −11.1, SE 1.3‰) and also showed no significant relationship with position. The isotope values of these relatively immobile invertebrates and the particulate detritus suggest that carbon moves from subtidal seagrass meadows to mudflats as particulate matter and is assimilated by invertebrates. This assimilation might be direct in the case of the detritivorous worm, A. ehlersi, but must be via invertebrate prey in the case of the carnivorous worm, N. australiensis and the scavenging shrimp, M. intermedium. The extent of movement of carbon among habitats, especially towards shallower habitats, is surprising since in theory, carbon is more likely to move offshore in situations such as the current study where habitats are in relatively open, unprotected waters.

Large-scale seagrass dieback in northern Spencer Gulf, South Australia

A major dieback of seagrass occurred in South Australia where 12,717 ha of intertidal and shallow subtidal seagrasses were lost along the north eastern coast of Spencer Gulf. This was a rapid decline, occurring toward the end of summer in January or early February of 1993. The extent and location of the dieback was mapped from aerial photographs taken before (1987) and after the event (1994). Eight habitat categories were represented including sand, dieback (severe and moderate) and seagrass (dense, intermediate and sparse). Of the total area of loss, the majority (8269 ha) was classified as severe dieback. It is significant that most of the dieback was previously dense seagrass (7523 ha) compared with the smaller areas of sparse (1044 ha) and intermediate (1600 ha) seagrass which were subsequently identified as dieback. Presence of seagrass remnants and data from previous surveys indicated that subtidal Amphibolis antarcticaand intertidal Zostera spp were the main species that died back. The pattern of the dieback, restricted to shallow subtidal and intertidal areas, in combination with extreme conditions associated with a hot El Nino summer, strongly suggest the loss resulted from environmental causes. Anthropogenic factors are unlikely as the sources of pollution along this sparsely populated coast cannot account for such a wide geographic impact or the pattern of loss. ©2000 Elsevier Science B.V. All rights reserved.

Seine nets and beam trawls compared by day and night for sampling fish and crustaceans in shallow seagrass habitat

Densities of nekton were estimated by comparing catch rates of two previously uncompared gear types, a beam trawl and a seine net, by day and night in a shallow seagrass (Zostera capricorni) habitat in Moreton Bay, Queensland, Australia. A total of 39,676 fish and crustaceans representing 42 species was caught. The catch rates of nekton were 1.4-68.1 times higher at night than in the day for 8 of the 17 common species, and were 1.4-9.2 times higher in seines than trawls for 11 of the common species. None of the common species had higher catch rates in the day than the night, or in the trawls than the seines. For some species there was no significant difference in catch rates amongst the sampling combinations. Night-time seine collections had a greater proportion of larger individuals than day and trawl samples. The differences in catch rates and size of nekton are probably a consequence of both gear avoidance and the movement of nekton out of seagrass during the day. Catch rates were estimated more accurately and precisely with the seine than the trawl, with higher catch rates at night. An analysis of the overall composition of the catch (based on presence/absence data) by multi-dimensional scaling separated the samples into four main groups: day-trawl, night-trawl, day-seine and night-seine. The results suggest that seine nets are a better choice for determining the relative proportion of species in a seagrass habitat, and estimating the density of most species. Such sampling should also be done by day and night, or by night alone.

Mechanisms and ecological role of carbon transfer within coastal seascapes

Worldwide, coastal systems provide some of the most productive habitats, which potentially influence a range of marine and terrestrial ecosystems through the transfer of nutrients and energy. Several reviews have examined aspects of connectivity within coastal seascapes, but the scope of those reviews has been limited to single systems or single vectors. We use the transfer of carbon to examine the processes of connectivity through multiple vectors in multiple ecosystems using four coastal seascapes as case studies. We discuss and compare the main vectors of carbon connecting different ecosystems, and then the natural and human‐induced factors that influence the magnitude of effect for those vectors on recipient systems. Vectors of carbon transfer can be grouped into two main categories: detrital particulate organic carbon (POC) and its associated dissolved organic and inorganic carbon (DOC/DIC) that are transported passively; and mobile consumers that transport carbon actively. High proportions of net primary production can be exported over meters to hundreds of kilometers from seagrass beds, algal reefs and mangroves as POC, with its export dependent on wind‐generated currents in the first two of these systems and tidal currents for the last. By contrast, saltmarshes export large quantities of DOC through tidal movement, while land run‐off plays a critical role in the transport of terrestrial POC and DOC into temperate fjords. Nekton actively transfers carbon across ecosystem boundaries through foraging movements, ontogenetic migrations, or ‘trophic relays’, into and out of seagrass beds, mangroves or saltmarshes. The magnitude of these vectors is influenced by: the hydrodynamics and geomorphology of the region; the characteristics of the carbon vector, such as their particle size and buoyancy; and for nekton, the extent and frequency of migrations between ecosystems. Through a risk‐assessment process, we have identified the most significant human disturbances that affect the integrity of connectivity among ecosystems. Loss of habitat, net primary production (NPP) and overfishing pose the greatest risks to carbon transfer in temperate saltmarsh and tropical estuaries, particularly through their effects on nekton abundance and movement. In comparison, habitat/NPP loss and climate change are likely to be the major risks to carbon transfer in temperate fjords and temperate open coasts through alteration in the amount of POC and/or DOC/DIC being transported. While we have highlighted the importance of these vectors in coastal seascapes, there is limited quantitative data on the effects of these vectors on recipient systems. It is only through quantifying those subsidies that we can effectively incorporate complex interactions into the management of the marine environment and its resources.

Differences in composition of small, motile invertebrate assemblages from seagrass and unvegetated habitats in a southern Australian estuary

Assemblages of small, motile invertebrates (epifauna) from eelgrass(Zostera muelleri) and unvegetated habitats in a shallow, marine-dominatedestuary were compared at five sampling periods over one year. Assemblagesbased on abundance and biomass of 21 taxa from the two habitats groupedseparately in multivariate analyses (MDS ordination), and these groupingswere shown to be significant using an analysis of similarities (ANOSIM)randomisation routine. Secondarily to habitat differences, weak influencesof water temperature and distance to open water, but not of salinity, weredetected at some periods. Abundance and biomass of key taxa and all speciescombined were higha in eelgrass than in unvegetated habitat. Cumaceans wereexceptional in being collected predominantly from unvegetated habitat. Totalepifaunal production and crustacean production estimated using twovariables, (1) the biomass of individuals of each size class, and (2) watertemperature, were also higher in eelgrass than in unvegetated habitat. Thehigher abundance in eelgrass of taxa such as amphipods, harpacticoidcopepods and polychaetes that are major components of the diets of smallfish is consistent with a model explaining higher fish numbers in eelgrassin terms of prey availability.

Local-scale mapping of benthic habitats to assess representation in a marine protected area

Macrobenthic habitat types were classified and mapped using a compact video array at 78 sites spaced 5 km apart in Moreton Bay, Australia. The area mapped was about 2400 km2 and extended from estuarine shallow subtidal waters to offshore areas to the 50-m isobath. Nine habitat types were recognised, with only one on hard substrate, and their representation within an existing marine protected area was assessed. Only two habitat types were represented in highly protected (no-take) zones, with less than 3% of the total area of each habitat type included. The habitat mapping characterised several habitat types not previously described in the area and located deep-water algal and soft coral reefs not previously reported. Seagrass beds were encountered in several locations where their occurrence was either unknown or had not previously been quantified. The study represents the most spatially comprehensive survey of epibenthos undertaken in Moreton Bay, with over 40 000 m2 sampled. Derived habitat maps provide a robust basis for inclusion of representative examples of all habitat types in marine protected area planning in, and adjacent to, Moreton Bay. The utility of video data to conduct a low-cost habitat survey over a comparatively large area was also demonstrated. The method used has potentially wide application for the survey and design of marine protected areas.

Land–Ocean Coupling of Carbon and Nitrogen Fluxes on Sandy Beaches

Rivers link oceans with the land, creating global hot spots of carbon processing in coastal seas. Coastlines around the world are dominated by sandy beaches, but beaches are unusual in that they are thought to rely almost exclusively on marine imports for food. No significant connections to terrestrial production having been demonstrated. By contrast, we isotopically traced carbon and nitrogen pathways leading to clams (Donax deltoides) on beaches. Clams from areas influenced by river plumes had significantly different isotope signatures (δ13C: −18.5 to −20.2‰; δ15N: 8.3–10.0‰) compared with clams remote from plumes (δ13C: −17.5 to −19.5‰; δ15N: 7.6–8.7‰), showing that terrestrial carbon and sewage, both delivered in river plumes, penetrate beach food webs. This is a novel mechanism of trophic subsidy in marine intertidal systems, linking the world’s largest shore ecosystem to continental watersheds. The same clams also carry pollution signatures of sewage discharged into rivers, demonstrating that coastal rivers connect ecosystems in unexpected ways and transfer contaminants across the land–ocean boundary. The links we demonstrate between terrigenous matter and the largest of all marine intertidal ecosystems are significant given the immense social, cultural, and economic values of beaches to humans and the predicted consequences of altered river discharge to coastal seas caused by global climate change.