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EARTH SCIENCE | BIOSPHERE | ECOSYSTEMS | MARINE ECOSYSTEMS | COASTAL | KELP FOREST

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  • 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.

  • NESP Marine Biodiversity Hub Project E7. Results from the outplanting of lab-selected and cultivated warm-adapted genotypes of giant kelp (Macrocystis pyrifera), at two trial restoration sites. A third restoration trial site had no surviving kelp, so those data were not included here. Data and details from lab-selection experiments can be found in the associated dataset - "NESP Marine Hub Project E7 - Macrocystis pyrifera thermal tolerance testing" https://metadata.imas.utas.edu.au/geonetwork/srv/eng/catalog.search#/metadata/0b91d7fd-7d29-452f-954a-78cf75151035

  • -- Layton et al. Chemical microenvironments within macroalgal assemblages: implications for the inhibition of kelp recruitment by turf algae. Limnology & Oceanography. DOI:10.1002/lno.11138 -- Kelp forests around the world are under increasing pressure from anthropogenic stressors. A widespread consequence is that in many places, complex and highly productive kelp habitats have been replaced by structurally simple and less productive turf algae habitats. Turf algae habitats resist re-establishment of kelp via recruitment inhibition; however little is known about the specific mechanisms involved. One potential factor is the chemical environment within the turf algae and into which kelp propagules settle and develop. Using laboratory trials, we illustrate that the chemical microenvironment (O2 concentration and pH) 0.0–50 mm above the benthos within four multispecies macroalgal assemblages (including a turf-sediment assemblage and an Ecklonia radiata kelp-dominated assemblage) are characterised by elevated O2 and pH relative to the surrounding seawater. Notably however, O2 and pH were significantly higher within turf-sediment assemblages than in kelp-dominated assemblages, and at levels that have previously been demonstrated to impair the photosynthetic or physiological capacity of kelp propagules. Field observations of the experimental assemblages confirmed that recruitment of kelp was significantly lower into treatments with dense turf algae than in the kelp-dominated assemblages. We demonstrate differences between the chemical microenvironments of kelp and turf algae assemblages that correlate with differences in kelp recruitment, highlighting how degradation of kelp habitats might result in the persistence of turf algae habitats and the localised absence of kelp.

  • Intraspecific variation in the thermal tolerance of microscopic giant kelp (Macrocystis pyrifera) sporophytes was tested using a common garden experiment, where 49 unique family-lines were raised under four different water temperatures (12, 16, 20, and 24°C). The unique family-lines were taken from ongoing giant kelp gametophyte cultures held at IMAS, and represented F1 offspring from seven 'selfed' individuals collected from 6 sites across ~250km in Tasmania, Australia, in addition to a site-level cross from each of the sites, and a panmictic cross using the 42 pure family lines. Survivorship of the selected warm-adapted family-lines after outplanting trials at restoration sites can be found here with the associated dataset "NESP Marine Hub Project E7 outplanted kelp survivorship". https://metadata.imas.utas.edu.au/geonetwork/srv/eng/catalog.search#/metadata/908afd8c-cc7a-4ea3-a87e-4497ae8da87a

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    This data set consists of a scored time-series of Autonomous Underwater Vehicle (AUV) images from the Bicheno region on the east coast of Tasmania. Surveys were conducted between 2011 and 2016 within the Governor Island Marine Reserve and nearby sites outside the reserve. Governor Island was surveyed in 2011, 2013, 2014 and 2016. The outside sites of Trap Reef, Cape Lodi and Butlers Point were surveyed in 2011, 2013 and 2016. Imagery across all surveys was scored for the presence of Centrostephanus rodgersii urchin barrens across rocky reef at each site. Prior to analysis the data was subsetted to every fifth image to avoid overlapping images. The data set also contains depth information for each image and a measure of rugosity (Vector Rugosity Measure) computed in ArcGIS software from a one metre resolution bathymetric map covering the survey sites. Analysis was conducted to examine the trend in the presence of barrens through time and to compare the occurrence of barrens inside the Governor Island Marine Reserve with sites outside the reserve. A spatio-temporal model incorporating both spatial and temporal correlation in the time-series of data was used. This data set contains the scored data used in the analysis. Further details of the methods used and results are contained in the following article. Please cite any use of the data or code by citing this article: Perkins NR, Hosack GR, Foster SD, Monk J, Barrett NS (2020) Monitoring the resilience of a no-take marine reserve to a range extending species using benthic imagery. PLOS ONE 15(8): e0237257. https://doi.org/10.1371/journal.pone.0237257

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    Data accompanying Layton et al. 2019, Resilience and stability of kelp forests: the importance of patch dynamics and environment-engineer feedbacks. PLOS ONE. To explore how resilience and stability of kelp habitats is influenced by this habitat degradation, we created an array of patch reefs of various sizes and supporting adult Ecklonia radiata kelp transplanted at different densities. This enabled testing of how sub-canopy abiotic conditions change with reductions in patch size and adult kelp density, and how this influenced demographic processes of microscopic and macroscopic juvenile kelp.

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    The Victorian Benthic Habitats - Western Port Bay (CBICS) is a synthesis of all existing benthic habitat characterisations of the embayment which have been reclassified to conform to the Combined Biotope Classification Scheme (CBiCS). Base layers for the synthesised dataset were sourced from data provided by: Marine and Freshwater Resources Institute, Queenscliff, Victoria Institute for Sustainability and Innovation, Victoria University, Melbourne. Parks Victoria, Victorian Government Deakin University, Victoria Department of Environment, Land, Water and Planning, Victorian Government

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    The Victorian Benthic Habitats - Port Phillip Bay (CBICS) is a synthesis of all existing benthic habitat characterisations of the embayment which have been reclassified to conform to the Combined Biotope Classification Scheme (CBiCS). Base information for the synthesised dataset were sourced from data provided by: Marine and Freshwater Resources Institute, Queenscliff, Victoria Institute for Sustainability and Innovation, Victoria University, Melbourne. Parks Victoria, Victorian Government Deakin University, Victoria Department of Environment, Land, Water and Planning, Victorian Government

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    The Victorian Benthic Habitats - Gippsland Lakes (CBICS) is a synthesis of all existing benthic habitat characterisations of the Gippsland Lakes Region which have been reclassified to conform to the Combined Biotope Classification Scheme (CBiCS). The study area for this layer is defined as Jack Smith Lake in the west to Mallacoota in the east.