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      Digitise This! A Quick and Easy Remote Sensing Method to Monitor the Daily Extent of Dredge Plumes

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          Abstract

          Technological advancements in remote sensing and GIS have improved natural resource managers’ abilities to monitor large-scale disturbances. In a time where many processes are heading towards automation, this study has regressed to simple techniques to bridge a gap found in the advancement of technology. The near-daily monitoring of dredge plume extent is common practice using Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and associated algorithms to predict the total suspended solids (TSS) concentration in the surface waters originating from floods and dredge plumes. Unfortunately, these methods cannot determine the difference between dredge plume and benthic features in shallow, clear water. This case study at Barrow Island, Western Australia, uses hand digitising to demonstrate the ability of human interpretation to determine this difference with a level of confidence and compares the method to contemporary TSS methods. Hand digitising was quick, cheap and required very little training of staff to complete. Results of ANOSIM R statistics show remote sensing derived TSS provided similar spatial results if they were thresholded to at least 3 mg L −1. However, remote sensing derived TSS consistently provided false-positive readings of shallow benthic features as Plume with a threshold up to TSS of 6 mg L −1, and began providing false-negatives (excluding actual plume) at a threshold as low as 4 mg L −1. Semi-automated processes that estimate plume concentration and distinguish between plumes and shallow benthic features without the arbitrary nature of human interpretation would be preferred as a plume monitoring method. However, at this stage, the hand digitising method is very useful and is more accurate at determining plume boundaries over shallow benthic features and is accessible to all levels of management with basic training.

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          Most cited references 5

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          Absorption spectrum (380-700 nm) of pure water. II. Integrating cavity measurements.

          Definitive data on the absorption spectrum of pure water from 380 to 700 nm have been obtained with an integrating cavity technique. The results are in good agreement with those recently obtained by our group with a completely independent photothermal technique. As before, we find that the absorption in the blue is significantly lower than had previously been generally believed and that the absorption minimum is at a significantly shorter wavelength, i.e., 0.0044 ? 0.0006 m(-1) at 418 nm. Several spectroscopic features have been identified in the visible spectrum to our knowledge for the first time.
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            Environmental impacts of dredging on seagrasses: a review.

            Main potential impacts on seagrasses from dredging and sand mining include physical removal and/or burial of vegetation and effects of increased turbidity and sedimentation. For seagrasses, the critical threshold for turbidity and sedimentation, as well as the duration that seagrasses can survive periods of high turbidity or excessive sedimentation vary greatly among species. Larger, slow-growing climax species with substantial carbohydrate reserves show greater resilience to such events than smaller opportunistic species, but the latter display much faster post-dredging recovery when water quality conditions return to their original state. A review of 45 case studies worldwide, accounting for a total loss of 21,023 ha of seagrass vegetation due to dredging, is indicative of the scale of the impact of dredging on seagrasses. In recent years, tighter control in the form of strict regulations, proper enforcement and monitoring, and mitigating measures together with proper impact assessment and development of new environmental dredging techniques help to prevent or minimize adverse impacts on seagrasses. Costs of such measures are difficult to estimate, but seem negligible in comparison with costs of seagrass restoration programmes, which are typically small-scale in approach and often have limited success. Copying of dredging criteria used in one geographic area to a dredging operation in another may in some cases lead to exaggerated limitations resulting in unnecessary costs and delays in dredging operations, or in other cases could prove damaging to seagrass ecosystems. Meaningful criteria to limit the extent and turbidity of dredging plumes and their effects will always require site-specific evaluations and should take into account the natural variability of local background turbidity.
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              Mapping the pollutants in surface riverine flood plume waters in the Great Barrier Reef, Australia.

              The extent of flood plume water over a 10 year period was mapped using quasi-true colour imagery and used to calculate long-term frequency of occurrence of the plumes. The proportional contribution of riverine loads of dissolved inorganic nitrogen, total suspended sediments and Photosystem-II herbicides from each catchment was used to scale the surface exposure maps for each pollutant. A classification procedure was also applied to satellite imagery (only Wet Tropics region) during 11 flood events (2000-2010) through processing of level-2 ocean colour products to discriminate the changing characteristics across three water types: "primary plume water", characterised by high TSS values; "secondary plume water", characterised by high phytoplankton production as measured by elevated chlorophyll-a (chl-a), and "tertiary plume water", characterised by elevated coloured dissolved and detrital matter (CDOM+D). This classification is a first step to characterise flood plumes. Copyright © 2012 Elsevier Ltd. All rights reserved.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2012
                11 December 2012
                : 7
                : 12
                Affiliations
                [1 ]Department of Environment and Conservation, Kensington, Western Australia, Australia
                [2 ]Oceans Institute, University of Western Australia, Crawley, Western Australia, Australia
                [3 ]Remote Sensing and Satellite Research Group, Department of Imaging and Applied Physics, Curtin University, Bentley, Western Australia, Australia
                NASA Jet Propulsion Laboratory, United States of America
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Conceived and designed the experiments: RDE KLM SNF GS PF MB. Performed the experiments: RDE KLM DM GS MB. Analyzed the data: KM RDE PF LIWM MB JAYM BGH. Contributed reagents/materials/analysis tools: KM RDE PF LIWM MB JAYM GS BGH. Wrote the paper: RDE KLM SNF JAYM. Contributed to the first draft and to the corrections of the manuscript: KM RDE SNF PF LIWM MB JAYM DM GS BGH.

                Article
                PONE-D-12-16834
                10.1371/journal.pone.0051668
                3519868
                23240055

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                Page count
                Pages: 10
                Funding
                This project was funded as part of the Dredging Audit and Surveillance Program by the Gorgon Joint Venture as part of the environmental offsets. The Gorgon project is a joint venture of the Australian subsidiaries of Chevron, Exxonmobil, Shell, Osaka Gas, Tokyo Gas and Chubu Electric Power. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Computer Science
                Geoinformatics
                Remote Sensing Imagery
                Earth Sciences
                Environmental Sciences
                Environmental Geology
                Marine and Aquatic Sciences
                Marine Geology
                Sediment
                Oceanography
                Physical Oceanography
                Coastal Ecology
                Marine Ecology
                Marine Monitoring
                Water Quality

                Uncategorized

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