C MacLeod

Effects of shellfish farming on the benthic environment

The benthic environment under and near three shellfish farms in Tasmania, Australia, which had had a relatively high level of production over many years was investigated. Benthic samples were collected along transects which ran across the farms, generally from 100 m upstream to 100 m downstream. Sediment deposition, redox values, sediment sulphide concentrations, organic carbon content and water turbidity levels near the bottom were significantly different between the farms but not between sites outside the farm, at the boundary and sites within the farm. Video recordings at one farm showed dense coverage of fine filamentous algae and patchy bacterial mats directly under some longlines and this algae is thought to have fallen off the mussel longlines. At another farm dense beds of seagrass were observed in the videos both under trays of oysters and outside the farm. The benthic infauna did not show clear signs of organic enrichment, and neither univariate nor multivariate measures of benthic infauna were significantly different between sites inside and outside the farm, although they were different between farms. It was concluded from these results that shellfish farming is having little impact, and much less than salmon farming, on the benthic environment in Tasmania. Thus extensive monitoring of shellfish farms would appear to be not necessary.

Detection of organic enrichment near finfish net-pens by sediment profile imaging at SCUBA-accessible depths

Sediment profile images (SPI) of cores collected by SCUBA diver were obtained using a modified Hargrave corer from fish farm sites in the Bay of Fundy, Canada and southeastern Tasmania, Australia. Shipboard and land based photography were used to obtain the SPI with a tripod mounted digital camera and image analysis by commercially available software. Computer images were analyzed to determine the variables used by Nilsson and Rosenberg [Mar. Ecol., Prog. Ser. 197 (2000) 139], modified to account for non-equilibrium conditions, to assess successional stages of organic enrichment. To validate the method, we concurrently sampled macrofaunal species composition and abundance and measured profiles of redox potentials and total sulphides by ion analysis. In each case, the null hypothesis that sediments collected directly under an active salmon net-pen were indistinguishable from a nearby reference site was rejected. The SPI method can successfully detect organic enrichment where impacts occur in soft sediments in geographically diverse locations.

Novel observations of benthic enrichment in contrasting flow regimes with implications for marine farm monitoring and management

We examine macrofaunal and physico-chemical responses to organic enrichment beneath salmon farms in contrasting flow environments, and reveal pronounced flow-related differences in the magnitude and spatial extent of effects. Total macrofaunal abundances at high flow sites were nearly an order of magnitude greater than at comparable low flow sites, representing a significant benthic biomass. These very high abundances occurred in conjunction with moderate-to-high species richness, and were evident in the absence of appreciable organic matter accumulation. Biological responses to increasing sulfide were variable; however a significant biological threshold was evident at 1500 μM. Macrofaunal responses at high flow sites differed substantially from the Pearson-Rosenberg model. The atypical ecological conditions were attributed to (i) limited accumulation of fine sediments, (ii) maintenance of aerobic conditions in near-surface sediments, and (iii) an abundant food supply. Thus, enhanced resilience to organic waste at well-flushed sites appears related to both biological and physical processes.

Influence of a burrowing, metal-tolerant polychaete on benthic metabolism, denitrification and nitrogen regeneration in contaminated estuarine sediments

We investigated the effects of the burrowing cirratulid polychaete Cirriformia filigera (Delle Chiaje, 1828) on benthic respiration and nitrogen regeneration in metal-contaminated estuarine sediments using laboratory mesocosms. C. filigera is a dominant component of assemblages in the most severely contaminated sediments within the Derwent estuary, southern Australia. In the presence of C. filigera sediment O2 consumption doubled, with approximately 55% of this increase due to their respiration and the remaining 45% attributable to oxidation reactions and increased microbial respiration associated with burrow walls. Combined NO3 and NO2 fluxes were unaffected. The addition of labile organic matter did not affect benthic fluxes, in the presence or absence of C. filigera, presumably due to the short timeframe of the experiment and naturally enriched test sediments. The results suggest that a combination of tolerance and burrowing activity enables this species to provide an ecosystem service in the removal of N from contaminated sites.

Long term trends of Hg uptake in resident fish from a polluted estuary.

Mercury contamination of fish is dependent upon a systems ability to transform inorganic Hg into biologically available forms; however, fish biometrics also play an important role. To assess long term trends in Hg concentrations in sand flathead (Platycephalus bassensis) a polynomial model, corrected for fish length, was used to evaluate temporal trends and spatial variability, while growth rates were estimated using the Von Bertalanffy length-at-age model. Hg concentrations showed no decrease over time, and generally remained near recommended consumption levels (0.5 mg kg(-1)). Previously reported spatial differences in Hg concentrations were not supported by the data once the models were corrected for fish length. Growth rate variation accounted for a large part of the previously published spatial differences. These results suggest that inclusion of fish biometrics is necessary to facilitate an accurate interpretation of spatial and temporal trends of contaminant concentrations in long term estuarine and marine monitoring programs.

Spatial variability in selenium and mercury interactions in a key recreational fish species: Implications for human health and environmental monitoring

Seleniums (Se) protective effects against mercury (Hg) toxicity have been demonstrated; however, this is seldom considered in health assessments, where dietary exposure is still evaluated by Hg concentration alone. Se:Hg ratios and selenium health benefit values (Se HBVs) offer a more comprehensive seafood safety model. Here we describe total mercury (THg), methylmercury (MeHg) and Se concentrations in fish from a Hg-polluted estuary. Spatial variation in THg, MeHg and Se was evident, though all regions maintained Se:Hg ratio values >1. Se HBV varied between regions and in one region mean negative values (-5.17) were evident. This study provides the first evidence that quoting a single all-encompassing Se HBV is not appropriate when species demonstrate strong site fidelity. It highlights the need for research into Se-Hg relationships in environments with established Hg pollution and reinforces the assertion that Se concentration be considered in assessments of human health risk to Hg exposure.

Quantification of the impacts of finfish aquaculture and bioremediation capacity of integrated multi-trophic aquaculture using a 3D estuary model

Reduced water quality is a potential outcome from intensive finfish aquaculture. Integrated multi-trophic aquaculture (IMTA) can mitigate the negative effects of dissolved nutrients emanating from fish farms by harvesting species that extract nutrients grown at adjacent sites. In this study, a coupled 3D hydrodynamic, sediment, and biogeochemical model was used to simulate an idealized temperate test estuary. A macroalgal-based IMTA model was applied within the estuarine model, to examine the spatial pattern of phytoplankton production arising from increasing levels of finfish aquaculture and the capacity of Macrocystis pyrifera to bioremediate the impacts of nutrification. Through increasing fish farm waste loads of dissolved inorganic nitrogen (DIN), the water quality in the estuarine model was forced into a “poor water quality” classification as determined by annual mean concentration of chlorophyll. Primary production was greatest in the northern section of the estuary due to circular water motion set up by a region of freshwater influence (ROFI). A nonlinear increase in annual phytoplankton biomass was simulated (under elevated finfish loads) due to the occurrence of an additional autumn phytoplankton bloom under elevated fish farm nutrient loads. IMTA scenario results demonstrated a strong spatial variability in the capacity of M. pyrifera-based IMTA to reduce water column chlorophyll concentration. Siting macroalgae farms next to those finfish farms situated in areas of high natural phytoplankton production resulted in a “good water quality” classification for the whole system. This demonstration of the use of IMTA to improve system wide water quality is valuable for regional planners and managers as it provides an analysis and quantification of a method to achieve estuarine health and economic benefit.

Technology selection - the impact of economic risk on decision making

ABSTRACT Developing an understanding of economic variance (risk) is critical when evaluating alternative aquaculture production technologies. This article assesses the efficacy of employing a quantitative stochastic analysis technique to support technology selection decision making by undertaking a case study investment assessment of three alternative production expansion strategies (offshore sea-pens, land-based RAS growout and larger post-smolt) for the Tasmanian salmon industry. Results demonstrate that salmon aquaculture is undertaken with considerable underlying levels of economic risk, expansion offshore probably represents the lowest initial capital investment and greatest economic return, and that levels of financial uncertainty increase with land-based RAS production. The study highlights stochastic modeling provides significant “added-value” over single-point deterministic analysis and that developing an appreciation of the input variability is a key component in critically evaluating alternative production technologies.

Macrophytes as bioindicators of heavy metal pollution in estuarine and coastal environments

The Derwent estuary, in Tasmania (Australia), is highly contaminated with heavy metals with significant levels in both sediments and benthic fauna. However, little is known about metal content in benthic primary producers. We characterized metal content (Arsenic, Cadmium, Copper, Lead, Selenium and Zinc) in twelve species of macrophyte, including red, green, and brown algae, and seagrasses, from the Derwent. The metals, arsenic, copper, lead, and Zinc were detected in all of the macrophytes assessed, but the levels differed between species. Seagrasses accumulated the highest concentrations of all metals; with Zn levels being particularly high in the seagrass Ruppia megacarpa (from the upper Estuary) and Pb was detected in Zostera muelleri (from the middle estuary). Ulva australis was ubiquitous throughout the middle-lower estuary and accumulated Zn in relatively high concentrations. The findings suggest that analysis of multiple species may be necessary for a comprehensive understanding of estuary-wide metal pollution.

Progress in integrating natural and social science in marine ecosystem-based management research

Climate change, in combination with population growth, is placing increasing pressure on the world’s oceans and their resources. This is threatening sustainability and societal wellbeing. Responding to these complex and synergistic challenges requires holistic management arrangements. To this end, ecosystem-based management (EBM) promises much by recognising the need to manage the ecosystem in its entirety, including the human dimensions. However, operationalisation of EBM in the marine environment has been slow. One reason may be a lack of the inter-disciplinary science required to address complex social–ecological marine systems. In the present paper, we synthesise the collective experience of the authors to explore progress in integrating natural and social sciences in marine EBM research, illustrating actual and potential contributions. We identify informal barriers to and incentives for this type of research. We find that the integration of natural and social science has progressed at most stages of the marine EBM cycle; however, practitioners do not yet have the capacity to address all of the problems that have led to the call for inter-disciplinary research. In addition, we assess how we can support the next generation of researchers to undertake the effective inter-disciplinary research required to assist with operationalising marine EBM, particularly in a changing climate.