Turbid Plume Research Articles

Enhancing Turbidity Predictions in Coastal Environments by Removing Obstructions from Unmanned Aerial Vehicle Multispectral Imagery Using Inpainting Techniques

High-resolution remote sensing of turbidity in the coastal environment with unmanned aerial vehicles (UAVs) can be adversely affected by the presence of obstructions of vessels and marine objects in images, which can introduce significant errors in turbidity modeling and predictions. This study evaluates the use of two deep-learning-based inpainting methods, namely, Decoupled Spatial–Temporal Transformer (DSTT) and Deep Image Prior (DIP), to recover the obstructed information. Aerial images of turbidity plumes in the coastal environment were first acquired using a UAV system with a multispectral sensor that included obstructions on the water surface at various obstruction percentages. The performance of the two inpainting models was then assessed through both qualitative and quantitative analyses of the inpainted data, focusing on the accuracy of turbidity retrieval. The results show that the DIP model performs well across a wide range of obstruction percentages from 10 to 70%. In comparison, the DSTT model produces good accuracy only with low percentages of less than 20% and performs poorly when the obstruction percentage increases.

Investigating sediment dynamics on a continental shelf mud patch under the influence of a macrotidal estuary: A numerical modeling analysis

Shelf mud patches represent major sinks for fine-grained particles on continental shelves, as well as for carbon and contaminants of continental origin. The West Gironde Mud Patch (WGMP) is an interesting example of such offshore marine systems as it is an active mud deposition area located offshore the Gironde estuarine mouth (France) at depths between 30 and 70 m. It is known to be the trap of fine particles coming from the estuary, but the contribution of this material to the total mass of the depocenter is poorly quantified. In addition, despite the economic and ecological issues at stake, the response of such subtidal sedimentary structure to the combination of tidal currents, waves, and river supply remain poorly understood. Thus, using a realistic 3-D hydrodynamic and mixed (mud/sand) sediment transport model, this study aims at investigating the sediment dynamics of the WGMP under different hydrometeorological conditions. The analysis of the residual fluxes at the estuarine mouth exhibited large discrepancies between the different sediment classes as well as for contrasted hydro-and meteorological conditions induced by different dominant transport mechanisms. During winter, the reinforced density gradients drive strong up-estuary baroclinic circulation at the bottom that dominates the sediment dynamics over the barotropic export of mud particles. The model also reproduced the signature of a subtidal mud accumulation area over the continental shelf around 30–40 m water depth, on the proximal side of the observed WGMP. On average over two years, 26% of the mud mass accumulating on the simulated subtidal mudflat comes from the estuary. The trapping efficiency of this mud patch is negatively correlated with the significant wave height. Moreover, due to the estuarine turbid plume being more concentrated and developed at the surface during high river discharge, the trapping efficiency of the mud body is enhanced compared to lower discharge. This study highlights the sensitivity of mud and sand fluxes to vertical and horizontal residual circulation, and points out the uncertainties associated with the simulation of short-term (i.e., years) fine particle deposits compared to long-term (i.e., centuries) sediment accumulation trends. In addition, these results show the primordial effects of both wave action and riverine sediment supply on the dynamics of such subtidal muddy structures, which raises concern about their fate facing climate change and human activities in the future.

Wind and river effects on a coastal current in Chatham Sound, British Columbia

Chatham Sound is an ecologically and economically important semi-enclosed basin in the northeast Pacific which has received only limited oceanographic attention. Here we investigate the surface circulation in the southern Sound by analyzing hourly current fields obtained from a high-frequency (HF) radar array, installed there in 2017 and still in operation, as well as other hydrographic and mooring measurements that allow us to characterize subsurface features at several specific locations during 2018 and 2019, and satellite imagery processed to obtain a measure of suspended particulate matter at high spatial resolution. We find that the surface circulation is dominated by the plume of the Skeena River, which flows into the southeast corner of the Sound. On average, this plume forms a northward flowing coastal current in the eastern half of the Sound, and is turbid during the freshet. The plume width changes only marginally even though the Skeena’s flow changes seasonally by a factor of about 7. However, the depth of the plume and its salinity is dramatically different during the early summer freshet compared to values in winter. Fastest plume currents appear near the offshore edges of the plume, even though the plume depth is greatest near the coast. We show that these features are consistent with those predicted from an analytical 1-1/2 layer buoyant coastal current model. Winds can accelerate or retard the coastal current; when strong enough the flow is arrested and turbid plume water fills the southern Sound.

Seasonal and interannual variations of suspended particulate matter in a West-African lagoon (Nokoué lagoon, Benin): Impact of rivers and wind

This study, based on five years of monthly in situ data collected from 2018 to 2022, examines the seasonal and interannual fluctuations of suspended particulate matter (SPM) concentration in Nokoué Lagoon, Benin. Seasonally, SPM exhibits significant variations primarily influenced by changes in river discharge. During low-flow periods (December to May), SPM concentrations are relatively low (<15 mg L-1) throughout the lagoon. During this time, slight temporal variations are correlated with wind energy and likely associated with wind-induced resuspension of sediments. This is confirmed by slightly higher concentrations of SPM in the bottom layers compared to the surface. Resuspension appears to be lower in the west than in the east, likely due to the increased presence of acadjas (brush parks) in the west, reducing fetch and wind intensity, thus decreasing resuspension. At the onset of the river flood period (July–August), associated with the West African monsoon, increased river flow generates a significant turbid plume extending from the northeast of the lagoon to the Cotonou channel, connecting the lagoon to the Atlantic Ocean. SPM levels then increase considerably (>100 mg L−1), with a pronounced SPM gradient from the western to eastern regions of the lagoon. The less dense freshwater laden with sediment from the rivers flows over the denser saline water of the lagoon, leading to slightly higher SPM concentrations in the surface layers. Between September and November, SPM concentration gradually decreases as river flows reach their peak values. Thus, on a seasonal scale, the relationships between SPM and river discharge show a temporal lag, resulting in a clockwise hysteresis cycle. This is explained by the early mobilization of fine sediments during rising river flows, followed by reduced sediment availability and dilution effects as the flood peaks. On an interannual scale, SPM variations are relatively low with slight temporal shifts observed in the formation and expansion of the turbid plume and peak SPM levels. The total SPM mass in the lagoon ranges from 0.2 to 0.3 × 104 tonnes during low-flow periods to 20-30 × 104 tonnes at the onset of flooding. We also discuss uncertainties associated with SPM determination, estimated at approximately 5–15%. This study leverages a unique database in West Africa and provides valuable insights into the hydro-sedimentary dynamics of Nokoué Lagoon.

The influence of flocculation in turbid plumes from mechanical and hydraulic dredging

The process of flocculation in dredge plumes was investigated by field measurements using Laser In-Situ Scattering and Transmissometry (LISST), Optical Back Scatter (OBS) sensors and an Acoustic Current Doppler Profiler (ADCP) during dredging campaigns in Port Curtis, Queensland (Australia). Suspended sediment characteristics differed significantly between natural ambient suspended sediment and sediment suspended by dredging as well as between sediment suspended by mechanical and hydraulic dredgers. LISST measurements enabled distinction of the suspended sediment by origin and showed that the majority of sediment particles naturally in suspension were aggregated flocs made up of multiple smaller particles. Primary particles in natural suspended sediment had a median particle size (d50) of 8 to 14 μm, while in-situ flocs had a d50 of 52 to 99 μm. There were both microflocs and macroflocs present in the natural suspended sediment, with the microflocs being dominant (representing 67%–80% of the flocs). Mechanical dredging resulted in the release of individual particles, microflocs and macroflocs, with the potential for a comparable percentage of macroflocs as in natural suspended sediment (11%–27% compared to 20%–33% in natural suspended sediment). Macroflocs released by mechanical dredging were likely to have originated from the seabed. During overflow from the hopper, the suspended sediment released from the hydraulic dredger had smaller flocs compared to the natural suspended sediment, with a reduction in the d50 of in-situ flocs from 91 to 67 μm and a reduction in the proportion of macroflocs from 32% to 24%. These results indicate that larger macroflocs present on the seabed had been broken up by the hydraulic dredging, with most of the suspended sediment remaining in the form of smaller microflocs, suggesting that the flocs were not completely disaggregated by the shear-stress generated during hydraulic dredging. Ongoing aggregation of suspended sediment was shown to occur in the sediment suspended by both dredging methods within 20–25 min of the plumes being generated. This study demonstrates the importance of considering flocculation in predictive dredge plume modelling studies to prevent significantly underestimating the settling velocity and substantially over-estimating the extent and intensity of dredge plumes if flocculation is not accounted for.

Remotely sensed seasonal dynamics of an estuarine plume in the coastal zone of El Rincón system (Southwestern Atlantic Ocean)

In this work we analyzed 18 years (2003–2021) of MODIS-Aqua daily images and multi-annual monthly mean turbidity maps to provide a comprehensive characterization of seasonal patterns of turbidity in the transitional area between the mouth of the Bahía Blanca Estuary and the adjacent inner shelf (El Rincón coastal system, Argentina). Field measurements collected in the study area were combined with MODIS data processed under different schemes of atmospheric correction before turbidity retrieval. Then, an Empirical Orthogonal function (EOF) analysis was used to assess the temporal patterns of turbidity and their regional variability over the inner shelf. Through a Generalized Linear Model (GLM), changes in the turbidity plume were further associated with environmental forcing (i.e. tides, winds and precipitation) controlling the seasonal dynamics. A strong gradient of decreasing turbidity from the northeast (close to the estuarine region) to the southwest (coastal region) was detected, while a major spatial pattern of annual variability, characterized by decreasing seasonality from northwest to southeast was defined. According to our results, the frequency of northwest wind direction and the impact of the wind mixing are considered as primary factors that shape the seasonal variability of turbidity with a strong winter dominance of largest plume events. The results found in this study are key to understanding the general dynamics of biological activity in the regional waters and the implications that climate variability might have on this complex ecosystem.

CoastalWQL: An Open-Source Tool for Drone-Based Mapping of Coastal Turbidity Using Push Broom Hyperspectral Imagery

Uncrewed-Aerial Vehicles (UAVs) and hyperspectral sensors are emerging as effective alternatives for monitoring water quality on-demand. However, image mosaicking for largely featureless coastal water surfaces or open seas has shown to be challenging. Another pertinent issue observed is the systematic image misalignment between adjacent flight lines due to the time delay between the UAV-borne sensor and the GNSS system. To overcome these challenges, this study introduces a workflow that entails a GPS-based image mosaicking method for push-broom hyperspectral images, together with a correction method to address the aforementioned systematic image misalignment. An open-source toolkit, CoastalWQL, was developed to facilitate the workflow, which includes essential pre-processing procedures for improving the image mosaic’s quality, such as radiometric correction, de-striping, sun glint correction, and object masking classification. For validation, UAV-based push-broom hyperspectral imaging surveys were conducted to monitor coastal turbidity in Singapore, and the implementation of CoastalWQL’s pre-processing workflow was evaluated at each step via turbidity retrieval. Overall, the results confirm that the image mosaicking of the push-broom hyperspectral imagery over featureless water surface using CoastalWQL with time delay correction enabled better localisation of the turbidity plume. Radiometric correction and de-striping were also found to be the most important pre-processing procedures, which improved turbidity prediction by 46.5%.

Long-term morphological and sedimentological changes caused by bottom trawling on the northern Catalan continental shelf (NW Mediterranean)

Bottom trawling is a fishing method that involves towing of nets along the seafloor to catch demersal species. The dragging of trawling gears along the seafloor results in scraping and ploughing of the surficial sediments, leading to the formation of turbid plumes of resuspended sediments and causing measurable changes in the seabed morphology. High-resolution multibeam data, side scan sonar, sediment grain size and vessel tracking data have been used to investigate the impact of bottom trawling on the seafloor morphology and surficial sediments of the northern Catalan continental shelf (NW Mediterranean), providing new insights into the impact of this anthropogenic activity on the seafloor. Multibeam data evidenced the occurrence of large-scale erosive features as a consequence of repeated scouring by fishing gears in localized areas. They are characterized by elongated (70-300 m wide and up to 8 km long) channelized areas of high backscatter with variable incision (from 0.2 m to 1.2 m). The spatial distribution of these morphologies shows a similar pattern to that observed in the fishing intensity, with maximum values in the areas of increased trawling intensity, corresponding to the main fishing grounds. Side scan sonar data also shows higher densities of trawl marks in these areas than in the surroundings. Sediment cores collected on these features show an upward-coarsening trend in the first 4-5 cm of the core, suggesting that part of the finer fraction resuspended by trawling is winnowed, increasing the sand content of the surface sediment. The identification of such large erosive morphologies in the main fishing grounds evidences that repeated trawling over the same fishing ground during decades can result in deep excavation of the seafloor, leading to permanent large-scale morphological changes. Furthermore, the evolution of these erosive features over a 13-year interval points towards long recovery periods after the cessation of trawling activities.

Detecting the magical yellow-blue demarcation off the Yellow River Estuary from the space

River deltas rank among the most ecologically and socio-economically valuable environments on the planet. The Yellow River Delta in China is not only home to young wetland ecosystem, but also to world-renowned tourism resources. One of these was the unique yellow-blue demarcation line (YBDL), formed in the Yellow River Estuary (YRE) as a result of the meeting of terrestrial sediment-laden river water and clear sea water, which had developed into a famous attraction and generated significant tourism revenues. Moreover, its distribution was of vital ecological and geomorphological importance due to association with the dynamics of estuarine turbidity plume. However, little is yet known about the dynamics and influencing factors of YBDL. In this study, we proposed an integrated method for extracting YBDL from satellite imagery and investigated the spatiotemporal distributions of YBDL during the flood season (June-October) from 2016 to 2022 using multi-source satellite imageries. The average relative error of our algorithm reached 10.53%. The results indicated that the YBDL started to frequently occur after 2018, mainly distributed within 10 km off the YRE, with two types of short straight and long curved shapes. July was the period with the highest frequency and most widespread distribution of YBDL. The riverine water and sediment discharge controlled 45.9% of the variations in YBDL length, and the extreme flooding, as well as the implementation of Water-Sediment Regulation Scheme, significantly increased the distribution extent of the YBDL. The morphology of river distributary mouths dominated the changes in the YBDL distribution centres, and the north distributary mouth experienced dramatic evolution in the last three years, with an increased YBDL distribution. Tidal currents could promote the development of YBDL at the front of the river mouth plumes while inhibiting the YBDL on the other side. This study is the first systematic investigation of the spatiotemporal evolutions and driving mechanisms of YBDL in the YRE, which could provide a reference for tourism planning and YRE National Park construction.

UAV-based remote sensing of turbidity in coastal environment for regulatory monitoring and assessment

The adoption of Unmanned Aerial Vehicle (UAV) remote sensing for the regulatory monitoring of turbidity plumes induced by land reclamation operations remains a difficult task. Compared to UAV remote sensing on ambient turbidity in estuaries and rivers, such monitoring of construction-induced turbidity plumes requires significantly higher spatial resolutions and accuracy as well as wider turbidity ranges with nonlinear reflectance. In this study, a pilot-scale deployment of UAV-based hyperspectral sensing is carried out for this objective, with specific new elements developed to overcome the challenges and minimise the uncertainties involved. In particular, Machine learning (ML) models for the turbidity determination were trained by the large dataset collected to better capture the non-linearity of the relationship between the water leaving reflectance and turbidity level. The models achieve a good accuracy with a R2 score of 0.75 that is deemed acceptable in view of the uncertainties associated with construction and land reclamation work.

Gravitational resedimentation as a fundamental process in filling fjords: Lessons from outcrops from a late Palaeozoic fjord in Namibia

AbstractThe stratigraphic architecture of fjords is complicated due to the delicate interplay between ice dynamics, sediment supply, relative sea‐level fluctuations and slope failures. Glaciogenic sediment is prone to failure and to be carried downslope to the fjord floor through the entire spectrum of mass movements and subaqueous density flows, as the unstable paraglacial submarine landscape moves towards stability. Palaeofjords formed by Gondwanan glaciers during the late Palaeozoic Ice Age contain a compelling record of gravitational resedimentation in fjord depositional systems. This paper showcases the geomorphology and depositional history of a glacial cycle in the Orutanda fjord in north‐western Namibia as an example of an overdeepened fjord basin fill dominated by products of subaqueous gravitational processes. During glaciation, the Orutanda glacier carved a 20 km long by 3.7 km wide glacial trough that embodies an overdeepened basin. Ice thickness during terminal glacial occupation of the fjord is estimated to had been up to 200 m based on the fjord geomorphology. The progressive retreat of the tidewater glacier, concomitant with marine flooding, increased accommodation space in the overdeepened basin during deglaciation. During this stage, proglacial sedimentation through iceberg rafting and settling of turbid plumes was outpaced by intense paraglacial downslope resedimentation of glacially‐transported debris. Successive failures from the fjord walls and downslope resedimentation resulted in coalescing debrite–turbidite lobes on the fjord floor. Slide deposits, composed entirely of deformed debrites and turbidites, indicate that these resedimented facies were prone to renewed mass wasting. As the Orutanda glacier melted, the fjord experienced the axial progradation of a fjord‐head delta registered only by turbidites and slide deposits derived from its collapse. The Orutanda fjord sheds light on the relevance of paraglacial mass wasting in overprinting glaciogenic deposits. This insight is key to understanding the role of glaciers versus non‐glacial processes in producing the glacial deep‐time record.

Relative importance of bacterivorous mixotrophs in an estuary‐coast environment

AbstractMixotrophic eukaryotes are important bacterivores in oligotrophic open oceans, but their significance as grazers in more nutrient‐rich waters is less clear. Here, we investigated the bacterivory partition between mixotrophs and heterotrophs in a productive, estuary‐influenced coastal region in the East China Sea. We found ubiquitous, actively feeding phytoplankton populations and taxa with mixotrophic potential by identifying ingestion of fluorescent prey surrogate and analyzing community 18S rRNA gene amplicons. Potential and active mixotrophs accounted for 10–63% of the total eukaryotic community and 17–69% of bacterivores observed, respectively, contributing 6–48% of estimated in situ bacterivory. The much higher mixotroph fitness outside of the turbid plume was potentially driven by increased light and decreased nutrient availability. Our results suggest that, although heterotrophs dominated overall in situ bacterivory, mixotrophs were abundant and important bacterivores in this low‐latitude mesotrophic coastal region.

NEW STANDARDS FOR MITIGATING DREDGING IMPACTS IN SENSITIVE HABITATS IN SAUDI ARABIA

We propose a systematic approach to monitoring and managing dredge operations based on industry best practice advice, tailored to the specific conditions in the Red Sea, including the following key elements: 1) Relocation of corals from the direct dredging footprint, 2) Development of a site and project-specific dredging environmental management plan, 3) Undertaking turbidity and coral monitoring, 4) Turbidity plume hindcast and forecast, 5) Adaptive management involving all key project stakeholders. The paper lists the above key elements and presents a case study from Sindalah Island where the system was successfully applied – the first time such an approach has been implemented in the Middle East region.

Comparison of high turbidity events: Sand nourishments and storm events on sandy beaches at the Baltic Sea, Germany

Coastal zones are permanently under stress by storms, cliff breaks, waves and currents. These events can cause the retreat of the coastline and are currently compensated by beach nourishments. The commonality of these sediment eroding and accumulating processes are turbidity plumes created in coastal waters, increasing the resuspension of particles and possibly nutrient concentrations. To compare sand nourishments and storm events, total suspended solids (TSS) and nutrient measurements were performed during winter season at the German Baltic Sea Coast. A pre-experiment showed that the TSS was homogenous through the water column. There were significant differences between both events, with sand nourishments exceeding the TSS of storms with the factor two. Due to the nourishment mechanism, the relative organic content was lower than during storm events. The nutrient uptake was unsteady. Therefore, sand nourishment can be considered to have more influence on the ecology of sand shores than storm events.

Detecting turbid plumes from satellite remote sensing: State-of-art thresholds and the novel PLUMES algorithm

Turbid coastal plumes carry sediments, nutrients, and pollutants. Satellite remote sensing is an effective tool for studying water quality parameters in these turbid plumes while covering a wide range of hydrological and meteorological conditions. However, determining boundaries of turbid coastal plumes poses a challenge. Traditionally, thresholds are the approach of choice for plume detection as they are simple to implement and offer fast processing (especially important for large datasets). However, thresholds are site-specific and need to be re-adjusted for different datasets or when meteorological and hydrodynamical conditions differ. This study compares state-of-the-art threshold approaches with a novel algorithm (PLUMES) for detecting turbid coastal plumes from satellite remote sensing, tested for Patos Lagoon, Brazil. PLUMES is a semi-supervised, and spatially explicit algorithm, and does not assume a unique plume boundary. Results show that the thresholds and PLUMES approach each provide advantages and limitations. Compared with thresholds, the PLUMES algorithm can differentiate both low or high turbidity plumes from the ambient background waters and limits detection of coastal resuspension while automatically retrieving metrics of detected plumes (e.g., area, mean intensity, core location). The study highlights the potential of the PLUMES algorithm for detecting turbid coastal plumes from satellite remote sensing products, which can have significantly positive implications for coastal management. However, PLUMES, despite its demonstrated effectiveness in this study, has not yet been applied to other study sites.

Insights for Sea Outfall Turbid Plume Monitoring with High-Spatial-Resolution Satellite Imagery Application in Portugal

Coastal municipalities and industries often discharge poorly treated wastewater into proximate marine and estuarine environments. The urban and/or effluent input can lead to eutrophication and lower water quality, as it holds high concentrations of nutrients and pollutants. One widely applied tool to increase effluent dispersion and direct it away from coastal areas, thus causing fewer impacts on human activities, is sea outfall. In Aveiro, Portugal, the São Jacinto sea outfall construction was completed in 1998; however, limited literature regarding the sea outfall’s monitoring using satellite data is available. The methodology in this study involved collecting four years’ worth (2016–2019) of satellite data to investigate visible traces of the interaction between the S. Jacinto sewage water mass and the Ria de Aveiro lagoon ecosystem using ocean color and spectral analysis, and producing new qualitative data regarding sea outfall plume dispersion monitoring through high-resolution Sentinel-2 imagery. The results showed a clear spectral signature of the sewage water mass, and a seasonal pattern was observed in which the plume was more evident in winter and autumn. Additionally, the coastal longshore current and the Aveiro lagoon’s runoff were able to restrict the marine outfall’s dispersion superficially. Ocean color data were revealed to be a factual and cost-effective tool to monitor the plume water. Finally, an exchange between the marine outfall water mass and Ria de Aveiro lagoon could happen in high tide under northern wind conditions. Therefore, it is important to monitor the water quality to ensure the coastal ecosystem’s good environmental state.

Controlling factors of phytoplankton distribution in the river–lake transition zone of a large lake

River–lake transition zones have been identified as major drivers of phytoplankton growth. With climate change reducing the frequency of complete lake overturns, it is expected that the Rhône River, the main tributary to Lake Geneva (France/Switzerland), will become the major source of nutrients for the lake euphotic zone. The river–lake transition zone was hence examined at the mouth of the Rhône River with the aim of understanding the complexities and controls of phytoplankton distribution in this specific deltaic ecosystem. Two field campaigns were carried out in which water samples were collected from longitudinal and transversal transects across the transition zone. These samples were analyzed for both nutrient and phytoplankton concentrations, while the fraction of Rhône River water in a lake sample was determined by the stable isotope composition of the water. The results indicate contributions in P and Si related to the Rhône intrusion into the lake. Furthermore, this river–lake transition zone appears to be a dynamic area that can locally present optimal conditions for phytoplankton growth. In early spring, a wind event broke the early and weak stratification of the lake, forcing the Rhône River and its turbidity plume to intrude deeper. Thus, this sharp drop of the turbidity within the euphotic zone allowed an increase in the phytoplankton biovolume of 44%. In early fall, outside of the turbid near field of the river mouth, the Rhône interflow, located just below the thermocline, promoted a local deep chlorophyll maximum.

Dark plumes of glacial meltwater affect vertical distribution of zooplankton in the Arctic

In polar regions, the release of glacial meltwater resulting in turbid plumes is expected to transform coastal waters with numerous consequences on the marine ecosystem. This study aimed to determine the influence of turbidity regimes on the vertical distribution of copepods together with their potential food (chlorophyll a fluorescence) and non-visual predators (gelatinous zooplankton). Hydrography, turbidity, suspended particulate matter and chlorophyll a were studied in July and August 2019 in West Spitsbergen waters (European Arctic). Fine-scale vertical distribution patterns of zooplankton were assessed by an optical counter (LOPC) and underwater camera (UVP) and verified by plankton nets. In waters with the shallow impact of dark plumes, Calanus spp. and gelatinous zooplankton were concentrated in the upper water layers, whereas in areas with a thick turbid layer, they were distributed evenly in the water column. However, chlorophyll a peaks were found to be restricted to the surface in the turbid waters and there were subsurface maxima in the shallow turbidity regime. Regardless of the region, the turbidity regime was a significant factor shaping the vertical distribution of Calanus spp. We speculate that similar trends might be observed in other rapidly emerging turbid ecosystems and urge that future plankton research should also include relatively simple turbidity measurements.

Influence of bottom and sand curtain on hydrodynamic performance of otter boards

Numerical simulation is an important method for calculating the hydrodynamic performance of otter boards used in sea floor trawling. Although such simulations have been explored in prior studies, the effects of the proximity of the otter boards to the seafloor and the plume of upward-drawn sediment during bottom trawling have largely been ignored. In this study, we assessed these factors. The results show that within the angles of attack used during normal operations, the effect of the seafloor bottom boundary of the flow field on the hydrodynamic performance of an otter board is obvious. We found that when the ratio of the distance between the bottom of an otter board and the surface boundary of the flow field to the chord length of the board exceeds 0.4, the influence of the bottom boundary of the flow field on the hydrodynamic performance of the board is negligible. For values of less than 0.4, the seafloor bottom boundary has an increasingly obvious impact on the hydrodynamic performance as this ratio decreases. We also found that the turbid plume of ocean floor sediment raised during bottom trawling has an obvious effect on the lift and resistance coefficients of an otter board at high angles of attack. At low angles, this effect on the lift-to-drag ratio is reversed and less obvious. The simulation results show that the optimal lift-to-drag ratio decreases with an increase in the sediment concentration; however, beyond a certain threshold, an increasing concentration of sediments was not found to have an obvious impact on the lift-to-drag ratio.

Using Optical Water-Type Classification in Data-Poor Water Quality Assessment: A Case Study in the Torres Strait

For many years, local communities have expressed concerns that turbid plume waters from the Fly River in Papua New Guinea may potentially deliver mine-derived contaminants to the Torres Strait, an ecologically and culturally unique area north of the Australian mainland. Information on suspended sediment transport and turbidity patterns are needed in this data-limited region to identify and manage downstream ecosystems that may be at risk of exposure from the Fly River runoff. This study used MODIS satellite time series and a colour-classification approach to map optical water types around the data-poor Gulf of Papua and Torres Strait region. The satellite data were supported by field data, including salinity and suspended sediment measurements, and used together in qualitative water quality assessments to evaluate the habitats that are likely exposed to Fly River discharge and/or derived sediments. It showed that the Fly River influence in the Torres Strait region is largely limited to the north-east corner of the Torres Strait. The drivers of turbidity vary between locations, and it is impossible to fully separate direct riverine plume influence from wave and tidally driven sediment resuspension in the satellite maps. However, results indicate that coastal habitats located as far east as Bramble Cay and west to Boigu Island are located in an area that is most likely exposed to the Fly River discharge within the region, directly or through sediment resuspension. The area that is the most likely exposed is a relatively small proportion of the Torres Strait region, but encompasses habitats of high ecological importance, including coral reefs and seagrass meadows. Satellite data showed that the period of highest risk of exposure was during the south-east trade wind season and complemented recent model simulations in the region over larger spatial and temporal frames. This study did not evaluate transboundary pollution or the ecological impact on local marine resources, but other recent studies suggest it is likely to be limited. However, this study did provide long-term, extensive but qualitative, baseline information needed to inform future ecological risk mapping and to support decision making about management priorities in the region. This is important for ensuring the protection of the Torres Strait ecosystems, given their importance to Torres Strait communities and turtle and dugong populations, and the Torres Strait’s connectivity with the Great Barrier Reef Marine Park.