EARTH SCIENCE | OCEANS | MARINE SEDIMENTS
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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.
Coccolithophore assemblages of a 9 000 year old marine sediment core from a climate hotspot in Tasmania, southeast Australia
Predicting phytoplankton impacts in response to a changing climate on Tasmania's east coast is presently based on short-term plankton data sets (~75 years). However, given the vital contribution coccolithophores make to oceanic carbon pumps, it is crucial to understand longer-term assemblage trends better. Here, we expand the archive of calcareous nannoplankton in southeast Australian waters by analysing coccolithophore microfossils in a ~2.68m long marine sediment core from the climate hotspot of Maria Island, Tasmania, using polarising light and scanning electron microscopy techniques in combination with analysis of sedimentary ancient DNA (sedaDNA). Coccolith assemblages at this site represent the complex interplay between the East Australian Current, Subantarctic incursions, and the hydrodynamics driving Subtropical Front positioning. Microfossil analysis revealed a shift from a cold to warm-adapted assemblage ~8 200 years BP, expressed by a striking transition from assemblages dominated by the cold-water species Gephyrocapsa muellerae to warmer water species Emiliania huxleyi. This transition compares with similar occurrences in the literature at ~11 000 years reported in the Southern Ocean and 12 000 years in the Tasman Sea, reflecting a broad shift of the Subtropical Front. E. huxleyi microfossils displayed the highest relative abundance, but less abundant larger taxa (including Calcidiscus, Coccolithus, Helicosphaera) accounted for >50% of coccolith CaCO3 sequestration, indicating large densely calcified species do the 'heavy-lifting' in terms of carbon cycling within mixed coccolithophore assemblages. Analysis of sedaDNA showed coccolithophores comprised the largest number of eukaryote molecular sequences recovered (~44%), far exceeding diatoms and dinoflagellates.
IMAS/CSIRO undertook a multibeam mapping campaign in eastern and Southern Tasmania to map shelf waters of the Freycinet, Huon and Tasman Fracture Marine Parks and several reference areas for the Tasman Fracture Park, including waters around Pedra Brancha and South-west Cape. The dataset includes a post-processed transit along the mid-shelf i=of Western Tasmania. The dataset includes raw mutibeam outputs and post-processed data, including Caris Files, xyz data and geotiffs. A data report for this has been produced by CSIRO. The study was intended to increase knowledge of the distribution of habitats within the SE Australian Australian Marine Park network, and at nearby reference areas with similar habitat. This information is required to underpin subsequent biological monitoring of key habitats within the AMP network, and to contrast the observations within parks with nearby fished locations to determine the extent that changes in biological communities are driven by natural vs anthropogenic pressures.
This dataset has been superseded by https://metadata.imas.utas.edu.au/geonetwork/srv/eng/catalog.search#/metadata/0145df96-3847-474b-8b63-a66f0e03ff54 (Victorian Statewide Marine Habitat Map 2023). The Victorian seabed habitat map documents the distribution of broad benthic habitat types in Victorian Coastal Waters to the State’s 3 nautical mile jurisdictional limit. The map was created using a top-down modelling process whereby habitat descriptors were assigned using seafloor structure and biological information derived from multibeam sonar (Victorian Marine Habitat Mapping Project), bathymetric LiDAR (Future Coasts program) and observations from underwater video. Identification of benthic biota, to the lowest discernible taxonomic level, and substrate characteristics were recorded according to the Victorian Towed Video Classification scheme (Ierodiaconou et al. 2007).