Ecosystem Function

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  • Antarctic krill (Euphausia superba) are a keystone species in the Southern Ocean, but little is known about how they will respond to climate change. Ocean acidification, caused by sequestration of carbon dioxide into ocean surface waters (pCO2), is known to alter the lipid biochemistry of some organisms. This can have cascading effects up the food chain. In a year-long laboratory experiment adult krill were exposed to ambient seawater pCO2 levels (400 μatm), elevated pCO2 levels that mimicked near-future ocean acidification (1000, 1500 and 2000 μatm) and an extreme pCO2 level (4000 μatm). The laboratory light regime mimicked the seasonal Southern Ocean photoperiod and krill received a constant food supply. Total lipid mass (mg g -1 DM) of adult krill was unaffected by near-future levels of seawater pCO2. Fatty acid composition (%) and fatty acid ratios associated with immune responses and cell membrane fluidity were also unaffected by near-future pCO2, apart from an increase in 18:3n-3/18:2n-6 ratios in krill in 1500 μatm pCO2 in winter and spring. Extreme pCO2 had no effect on krill lipid biochemistry during summer. During winter and spring, krill in extreme pCO2 had elevated levels of omega-6 fatty acids (up to 1.2% increase in 18:2n-6, up to 0.8% increase in 20:4n-6 and lower 18:3n-3/18:2n-6 and 20:5n-3/20:4n-6 ratios), and showed evidence of increased membrane fluidity (up to three-fold increase in phospholipid/sterol ratios). These results indicate that the lipid biochemistry of adult krill is robust to near-future ocean acidification.

  • Antarctic krill (Euphausia superba) have a keystone role in the Southern Ocean, as the primary prey of Antarctic predators. Any decreases in krill abundance could result in a major ecological regime shift, but there is currently limited information on how climate change may affect krill. Increasing anthropogenic carbon dioxide (CO2) emissions are causing ocean acidification, as absorption of atmospheric CO2 in seawater alters ocean chemistry. Ocean acidification increases mortality and negatively affects physiological functioning in some marine invertebrates, and is predicted to occur most rapidly at high latitudes. Here we show that, in the laboratory, adult krill are able to survive, grow, store fat, mature, and maintain respiration rates when exposed to near-future ocean acidification (1000 – 2000 μatm pCO2) for one year. Despite differences in seawater pCO2 incubation conditions, adult krill are able to actively maintain the acid-base balance of their body fluids in near-future pCO2, which enhances their resilience to ocean acidification.

  • The fatty acid content and composition of the Antarctic krill Euphausia superba Dana, 1850 were investigated using samples collected by a commercial fishing vessel. This dataset allowed comparison between seasons, years (2013–2016), and different fishing locations. Quantities of omega 3 fatty acids 20:5n-3 and 22:6n-3 (mg/g dry mass; DM) were highest in autumn and decreased through winter to reach a spring low. Quantities of the flagellate marker 18:4n-3 and diatom marker 16:1n-7c were variable and did not display the same seasonal fluctuations. In summer, krill had high percentages (% total fatty acids) of 20:5n-3 and 22:6n-3, total PUFA, and low 18:1n-9c/18:1n-7c ratios, indicating a more herbivorous diet. Krill became more omnivorous from autumn to spring, indicated by increasing ratios of 18:1n-9c/18:1n-7c and percentages of Σ 20:1 + 22:1 isomers. Bacterial fatty acids (Σ C15 + C17 + C19 isomers) were minor components year-round (0.9–1.8 %). Seasonal levels of herbivory and omnivory differed between years, and levels of specific fatty acid ratios differed between fishing locations. The fatty acid 18:4n-3 was a major driver of variability in krill fatty acid composition, with no obvious seasonal driver. This is the first study to report krill fatty acid data during all four seasons over consecutive years. This large-scale study highlights the value of using fisheries samples to examine seasonal and annual fluctuations in krill diet and condition.

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    Annotations of Centrostephanus rogersii sea urchin barrens derived from towed video at selected key abalone blocks along the east coast of Tasmania. The purpose of the study was to examine the patch dynamics of urchin barrens and to provide validation for the identification of urchin barrens from multibeam surveys.

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    Fatty acid analysis is a powerful tool in food web research for estimating dietary sources in marine predators. However, the utility of fatty acids as dietary indicators from whole lipid samples, rather than from separate lipid classes, has been questioned. Samples are often collected at a single time point, precluding seasonal dietary comparisons. We investigated variations in the fatty acid composition of structural (phospholipids) and storage lipids (triacylglycerols) of Antarctic krill (Euphausia superba) using fisheries samples obtained over one year. Seasonal variation was observed in fatty acid biomarkers within triacylglycerol and phospholipid fractions of krill. Fatty acids in krill triacylglycerols (thought to better represent recent diet), reflected omnivorous feeding with highest percentages of flagellate biomarkers (18:4n-3) in summer, and diatom biomarkers (16:1n-7c) in autumn, winter and spring. Carnivory biomarkers (∑ 20:1 + 22:1 and 18:1n-9c/18:1n-7c) in krill were greater in autumn. Phospholipid fatty acids were less variable and higher in 20:5n-3 and 22:6n-3, which are essential components of cell membranes. Sterol composition did not yield detailed dietary information, but percentages of the major krill sterol, cholesterol, were significantly higher in winter and spring compared with summer and autumn. Unexpectedly, 18:4n-3 and copepod markers ∑ 20:1 + 22:1 were not strongly associated with the triacylglycerol fraction during some seasons. Krill may mobilise 18:4n-3 to phospholipids for conversion to long chain polyunsaturated fatty acids, which would have implications for its role as a dietary biomarker. For the first time, we demonstrate the dynamic seasonal relationship between specific biomarkers and krill lipid classes.

  • The capacity of wetlands to mitigate greenhouse gas (GHG) emissions is the sum of two services–the protection of vulnerable organic stocks from remineralisation, and the capacity to sequester GHGs relative to their anthropogenic replacements. Organic carbon accumulation (CA) down through the sediment column is often taken as the measure of sequestration because of its capacity to record long-term variability and trends. However, we demonstrate that: i) CA is not equivalent to sequestration as net ecosystem production (NEP) for open systems; it requires the subtraction of the initial deposition rate of labile allochthonous carbon sources; ii) CA also requires subtraction of intrinsically allochthonous recalcitrants down through the sediment column, and together with subtraction of autochthonous recalcitrants from organic stock services; iii) CA as a climatic mitigation service also requires a diagenetic correction, as the annual deposition of labile organic carbon continues to remineralise over the long-term; and iv) preserving of a wetland has a significantly greater mitigation potential than restoring one. To address the above concerns, a global diagenetic solution is proposed, applied, and tested for a tropical seagrass and mangrove. As expected traditional CA estimates were disproportionately larger than their respective cal. NEPs and together with stocks fell within the ranges reported in the literature, with a final carbon accreditation highly dependent on the choice of their anthropogenic replacements. The review demonstrates that mitigation concepts and measurements for natural carbon sequestration solutions require re-evaluation to avoid GHG emissions above their capacity or reduce the ability to fulfil emission targets.

  • The spatial extent of the long spined sea urchin, Centrostephanus rodgersii, was estimated by divers using underwater visual census methods to survey rocky reef habitats at 13 regions along Tasmania's east coast (between Eddystone Point and Recherche Bay). Within each region 3 subsites were surveyed, and within each subsite 4 belt transects were surveyed. Divers recorded depth, percent substrata type, percent C. rodgersii barrens habitat; abundance of urchins (C. rodgersii and H. erythrogramma), rock lobster (J. edwardsii) and abalone (H. rubra). Divers also recorded algal cover (estimated), C. rodgersii barrens and substratum type using set categories (see below).

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    Annotations of canopy forming seaweed derived from towed video at selected key abalone blocks along the east coast of Tasmania. The purpose of the study was to examine the patch dynamics of seaweeds and urchin barrens and to provide validation for the identification of urchin barrens from multibeam surveys.

  • Biogenic marine habitats are increasingly threatened by a multitude of human impacts, and temperate coasts in particular are exposed to progressively more intense and frequent anthropogenic stressors. In this study, the single and multiple effects of the urban stressors of nutrification and sedimentation on kelp bed communities were examined within Australia’s largest urbanised embayment (Port Phillip Bay, Victoria). Within this system, grazing by sea urchins (Heliocidaris erythrogramma) plays an important role in structuring reef communities by overgrazing kelp beds and maintaining an alternative and stable urchin barrens state. It is therefore important to explore the effects of urban stressors on kelp bed dynamics related to urchin abundance, and test the relative strengths of bottom-up and / or physical drivers (e.g. elevated nutrients and sediment) versus top-down (e.g. urchin grazing) forces on kelp bed community structure. The interactions of these drivers were assessed to determine whether their combination has synergistic, antagonistic, or additive effects on kelp beds. It was found that kelp responds positively to nutrient enhancement, but when combined with enhanced abundance of grazing sea urchins, the local positive effect of nutrient enhancement is overwhelmed by the negative effect of increased herbivory. Turf-forming algae behaved very differently, showing no detectable response to nutrification, yet showing a positive response to urchins, apparently mediated by overgrazing of canopy-forming algae that limit turf development. No direct effects of enhanced sediment load (at twice the ambient load) were found on intact kelp beds. Collectively, the results demonstrate that the ‘top-down’ control of urchin grazing locally overwhelms the positive ‘bottom-up’ effect of nutrient enhancement, and that intact kelp beds demonstrate resilience to direct impacts of urban stressors.

  • Sea urchins have the capacity to destructively overgraze kelp beds and cause a wholesale shift to an alternative and stable ‘urchin barren’ state. However, their destructive grazing behaviour can be highly labile and contingent on behavioural shifts at the individual and local population level. Changes in supply of allochthonous food sources, i.e. availability of drift-kelp, is often suggested as a proximate trigger of change in sea urchin grazing behaviour, yet field tests of this hypothesis are rare. Here we conduct a suite of in situ behavioural surveys and manipulative experiments within kelp beds and on urchin barrens to examine foraging movements and evidence for a behavioural switch to an overgrazing mode by the Australian sea urchin Heliocidaris erythrogramma (Echinometridae). Tracking of urchins using time-lapse photography revealed urchin foraging to broadly conform to a random-walk-model within both kelp beds and on barren grounds, while at the individual level there was a tendency towards local ‘homing’ to proximate crevices. However, consistent with locally observed ‘mobile feeding fronts’ that can develop at the barrens-kelp interface, urchins were experimentally inducible to show directional movement toward newly available kelp. Furthermore, field assays revealed urchin grazing rates to be high on both simulated drift-kelp and attached kelp thalli on barren grounds, however drift-kelp but not attached kelp was consumed at high rates within kelp beds. Time-lapse tracking of urchin foraging before/ after the controlled addition of drift-kelp on barrens revealed a reduction in foraging movement across the reef surface when drift-kelp was captured. Collectively results indicate that the availability of drift-kelp is a pivotal trigger in determining urchin feeding modes, which is demonstrably passive and cryptic in the presence of a ready supply of drift-kelp. Recovery of kelp beds therefore appears possible if a sustained influx of drift-kelp was to inundate urchin barrens, particularly on reefs where local urchin densities and where grazing pressure is close to the threshold enabling kelp bed recovery.