Sediment Plumes Research Articles

Experimental mining plumes and ocean warming trigger stress in a deep pelagic jellyfish

The deep pelagic ocean is increasingly subjected to human-induced environmental change. While pelagic animals provide important ecosystem functions including climate regulation, species-specific responses to stressors remain poorly documented. Here, we investigate the effects of simulated ocean warming and sediment plumes on the cosmopolitan deep-sea jellyfish Periphylla periphylla, combining insights gained from physiology, gene expression and changes in associated microbiota. Metabolic demand was elevated following a 4 °C rise in temperature, promoting genes related to innate immunity but suppressing aerobic respiration. Suspended sediment plumes provoked the most acute and energetically costly response through the production of excess mucus (at ≥17 mg L−1), while inducing genes related to aerobic respiration and wound repair (at ≥167 mg L−1). Microbial symbionts appeared to be unaffected by both stressors, with mucus production maintaining microbial community composition. If these responses are representative for other gelatinous fauna, an abundant component of pelagic ecosystems, the effects of planned exploitation of seafloor resources may impair deep pelagic biodiversity and ecosystem functioning.

How to obtain clear images from in-trawl cameras near the seabed? A case study from the Barents Sea demersal fishing grounds

Underwater camera systems are commonly used for monitoring fish and fishing gear behaviours. More recently, camera systems have been applied to scientific trawl surveys for improved spatial resolution and less invasive sampling and to commercial fisheries for better catch control and reduced by-catch. A challenge when using cameras in demersal trawls is poor image clarity due to the door and ground gear generated sediment plume. In this study we have measured the height of the sediment plume produced by a large commercial trawl in the Barents Sea using acoustic methods and investigated its effect on in-trawl camera image clarity. The trawl extension was lengthened, and additional buoyancy added to lift the camera system in the aft end of the trawl. The camera system was tested at increasing heights above seabed until no sediment plume was visible in the images. Based on the acoustic data the sediment plume was measured to be on average 4–5 m (SD 1.7 m) above sea floor. Image clarity improved significantly as the camera system clearance from seabed increased from 4 to 11 m. No effect of sediment type on image clarity was identified. The trawl modifications did not affect the trawl’s opening geometry or bottom contact. However, the increased length and angle of the under panel aft in the trawl and in the extension appears to have resulted in reduced water flow and may influence the passage and retention of fish. The feasibility of using camera systems in demersal trawls and this and other solutions for obtaining clear images are discussed.

Deep-sea mining: using hyperbaric conditions to study the impact of sediment plumes in the subtidal clam Spisula solida.

With the growing interest to exploit mineral resources in the deep-sea, there is the need to establish guidelines and frameworks to support hazard and risk assessment schemes. The present study used a subtidal species of filter-feeding bivalve, the clam Spisula solida, as a proxy to better understand the impacts of sediment plumes in marine organisms under hyperbaric conditions. Four concentrations of suspended sediments (0g/L, 1g/L, 2g/L, and 4g/L) were used in a mixture with different grain sizes at 4 Bar for 96h. Functional (filtration rate-FR) and biochemical endpoints (catalase-CAT, glutathione s-transferase-GST, and lipid peroxidation-LPO) were analyzed in the gonads, digestive gland, and gills of S. solida after a 96-h exposure at 4 Bar (the natural limit of the species vertical distribution). The FR showed a decreasing trend with the increasing sediment concentrations (significant effects at 2 and 4g/L). Additionally, significant changes were observed for some of the tested oxidative stress biomarkers, which were concentration and tissue-dependent, i.e., CAT activity was significantly elevated in gills (1g/L treatment), and GST was decreased in digestive gland (1g/L treatment). Overall, the results show that suspended sediments, at 2 and 4g/L, have negative functional impacts in the bivalve S. solida providing additional insights to improve hazard assessment of deep-sea mining. These findings represent a step forward to ensure the mitigation of the potential negative effects of deep-sea resource exploitation.

Adaptation of a risk-based framework for evaluating indirect effects of dredging on sensitive habitats near federal navigation channels: An application of the framework to coral reefs at Honolulu Harbor, Hawai'i.

In major harbors and ports in the United States and its territories, the US Army Corps of Engineers maintains federal navigation channels in proximity to coral reefs (e.g., Honolulu Harbor, HI; Miami Harbor, FL; Apra Harbor, Guam) and other sensitive habitats. To effectively predict potential adverse impacts from dredging activities near these sensitive habitats, a holistic approach to improve understanding of the pressures on these habitats is needed to foster a more complete prediction of risk drivers. To achieve this, risk-based frameworks that account for the full range of natural and anthropogenic impacts need to be adapted and applied specifically for assessing and managing indirect dredging impacts on sensitive environments. In this article, we address this need by incorporating a drivers-pressures-stressors-condition-response (DPSCR4 ) conceptual framework to broaden a comprehensive conceptual model of the coupled human-ecological system. To help understand these complex interactions, DPSCR4 was applied to evaluate dredging and other unrelated environmental pressures (e.g., terrestrial runoff) in a proof-of-concept dredging project in Honolulu Harbor, Hawai'i, USA, with a focus on the indirect effects of dredge plumes. Particle tracking models and risk-based tools were used to evaluate sediment resuspended during a hypothetical mechanical dredging activity near sensitive coral habitats. Stoplight indicators were developed to predict indirect sediment plume impacts on coral and then compared to exposure modeling results. The strengths and limitations of the approach are presented and the incorporation of the risk framework into environmental management decisions is discussed. Integr Environ Assess Manag 2024;20:547-561. Published 2023. This article is a U.S. Government work and is in the public domain in the USA.

The role of waves in the resuspension and transport of fine sediment and mine tailings from the Fundão Dam failure, Doce River, Brazil

In one of the world's worst mining disasters, after flowing 670 km down the Doce River, about 20 million m3 of tailings reached the coast of Brazil in November 2015. The occurrence of persistent northerly winds favored the detachment of the river plume from the coastline and its displacement southward. However, observations of the sediment plume along the coast during the year following the breach, using aerial photographs, indicated that the plume is more extensive and persistent in a northerly direction. Previous studies have shown that although the most persistent winds are from the northern sector (70%), there is a greater incidence of waves from the southern sector (62%), especially with greater heights and periods. In this research, the effect of gravity waves on the transport of sediments is investigated, and, therefore, tailings from the Doce River and deposited on the adjacent inner continental shelf, mixed with the fluvial mud deposit also are investigated. Two mechanisms were investigated: 1) the effect of waves on sediment resuspension and their combination with tidal and wind forcings, using a two-dimensions in the horizontal directions (2DH) numerical model within the DELFT3D software, and 2) the capture of the fluvial plume by littoral drift, calculated by means of analytical models using data from wave propagation modeling. Both mechanisms were studied for 2019, also considering two major meteorological events in the region. Both effects were relevant for the northward dispersion of fine sediment, highlighting the effect of waves on the potential for northward transport of tailings towards the Abrolhos Bank.

ASSESSING WATER QUALITY CHANGES CAUSED BY DREDGING OPERATIONS IN SANTOS ESTUARY, SÃO PAULO, BRAZIL

Dredging activity may cause sediment resuspension, altering water quality regarding to dissolved oxygen and conductivity (measured in the field), turbidity, total suspended solids and nutrients (such as phosphorus and total organic carbon), analyzed in the laboratory. In this work ten sampling campaigns were performed in four areas of Santos harbor’s navigation channel, with different grain sizes, in order to monitor along one hour, the variation of these parameters in the sediment plume formed due to dredging activity. A previous campaign was carried out before dredging activities. This study showed that despite of variations in the results of the investigated parameters, these variations are of small magnitude and of short duration.

A Numerical Investigation of the Dynamic Interaction between the Deep-Sea Mining Vehicle and Sediment Plumes Based on a Small-Scale Analysis

The discharge of sediment plumes, which occurs mainly in the two depth zones, has a critical impact on assessing the deep-sea environment. Therefore, it is necessary to establish the corresponding physical oceanography for the evolution of these sediment plumes. For a more accurate evolution estimation of the plumes, the model in this research is concerned with the dynamic interaction between the deep-sea mining vehicle (DSMV) and the sediment plumes on small scales (t ≤ 2 s), contributing to a focus on the vital physical mechanics of controlling the extent of these plumes. The sediment concentration and particle trajectories of the plume emissions were determined using the Lagrangian discrete phase model (DPM). The results show that (1) the wake structure of the DSMV wraps the plume vortex discharged from the rear of the vehicle and inhibits the lateral diffusion of the plume, (2) the length of the entire wake (Lw) increases exponentially as the relative discharge velocity of the plume (U*) increases, where U* is defined as the dimensionless difference between the traveling velocity of the DSMV and the discharge velocity of the plume, and (3) at the same traveling speed of the DSMV and U* less than 0.75, the dispersion of the sediment particles in the early discharge stage of the plume does not vary with the plume discharge rate. This will be beneficial for the more accurate monitoring of ecological changes in deep-sea mining activities and provide theoretical guidance for the green design of DSMVs.

Effects of sediment pulses on the deep-sea coral Goniocorella dumosa

ABSTRACT Seabed-contact activities operating in the deep sea can generate sediment plumes that pose varying levels of threat to benthic fauna. Corals are important components of deep-sea ecosystems and can be particularly sensitive to elevated suspended sediment concentrations. In this study, we exposed colony fragments of the New Zealand deep-sea scleractinian Goniocorella dumosa to four-day pulses of four target sediment concentrations: 0 mg l−1 (representing control conditions) and 45, 102 and 435 mg l−1 (targeting concentrations expected from mining and trawling disturbance). All coral fragments survived. Oxygen consumption rates were not affected by treatments and time. No visible detrimental effects on coral health were noted after the first pulse of sediment exposure. However, both a loss of coenosarc and instances of polyp mortality were observed on fragments exposed to suspended sediments during the following sediment pulses. This observed decline in coral health over time indicates that G. dumosa could cope with sediment disturbance from human activities that disturb the seafloor for periods of up to four days, but that repeated, or prolonged sediment exposure could cause a deterioration in the health condition of this species. This hypothesis should be further investigated in following studies.

Oceanic bottom mixed layer in the Clarion-Clipperton Zone: potential influence on deep-seabed mining plume dispersal

The oceanic bottom mixed layer (BML) is a well mixed, weakly stratified, turbulent boundary layer. Adjacent to the seabed, the BML is of intrinsic importance for studying ocean mixing, energy dissipation, particle cycling and sediment-water interactions. While deep-seabed mining of polymetallic nodules is anticipated to commence in the Clarion-Clipperton Zone (CCZ) of the northeastern tropical Pacific Ocean, knowledge gaps regarding the form of the BML and its potentially key influence on the dispersal of sediment plumes generated by deep-seabed mining activities are yet to be addressed. Here, we report recent field observations from the German mining licence area in the CCZ that characterise the structure and variability of the BML locally. Quasi-uniform profiles of potential temperature extending from the seafloor reveal the presence of a spatially and temporally variable BML with an average local thickness of approximately 250 m. Deep horizontal currents in the region have a mean speed of 3.5 cm s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document} and a maximum speed of 12 cm s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document} at 18.63 ms above bottom over an 11 month record. The near-bottom currents initially have a net southeastward flow, followed by westward and southward flows with the development of complex, anticyclonic flow patterns. Theoretical predictions and historical data show broad consistency with mean BML thickness but cannot explain the observed heterogeneity of local BML thickness. We postulate that deep pressure anomalies induced by passing surface mesoscale eddies and abyssal thermal fronts could affect BML thickness, in addition to local topographic effects. A simplified transport model is then used to study the influence of the BML on the interplay between turbulent diffusion and sediment settling in the transport of deep-seabed mining induced sediment plumes. Over a range of realistic parameter values, the effects of BML on plume evolution can vary significantly, highlighting that resolving the BML will be a crucial step for accurate numerical modelling of plume dispersal.

Sediment plume simulation from bottom-trawled fishery and deposition effects on rhodoliths and deep-water corals from Campos basin, Brazil

Extensive rhodoliths and deep-water coral has been described along the continental shelf and slope of the Campos Basin, but such environments can be highly vulnerable to the bottom trawling fishing. This study applied the MOHID software to simulate the sediment resuspension during the bottom trawling operation and to evaluate possible impacts of sediment deposition onto rhodoliths and deep-water coral. The sediment transport model was validated by field measurements of a bottom trawling operation and presented good accuracy. The results indicated that sediment resuspension by a double-rig trawling induced a 0.13 mm sediment layer deposition onto the rhodoliths from the continental shelf which has potential deleterious effects considered that such thickness is higher than the threshold proposed by the literature. On the contrary, a sediment resuspension/deposition by a single-rig trawling has no impact in the deep-water corals from the continental slope based on the thin layer deposition that was lower than the proposed tolerance limits.

Sediment Erosion Generated by a Coandă-Effect-Based Polymetallic-Nodule Collector

To date, hydraulic collection is the most widely considered technology in polymetallic-nodule mining, since there is no direct contact between hydraulic collectors and ocean floor. To construct a hydraulic collector that results in the least sediment disturbance, it is critical to develop an insightful understanding of the interaction between the collector and sediment bed. To this end, we conducted a set of small-scale experiments in which several operational conditions were tested, delivering the first quantitative data for sediment erosion resulting from a hydraulic collector driving over a sand bed. This paper presents and discusses the experimental results and observations. It is found that the collector’s forward velocity is inversely proportional to the bed-sediment erosion depth, since the bed is exposed to the flow for a longer time when the collector drives slower and vice versa. In contrast, an increased jet velocity leads to a larger erosion depth. Furthermore, when the collector underside is nearer to the sediment bed, a larger sediment layer is exposed to the water flow, resulting in a larger erosion depth. Finally, the experimental results show that collector water jets strike the sediment bed under an inclined angle, destabilizing the upper sediment layer and consequently dragging sediment particles along toward the collection duct and behind the collector head. This study improves the predictability of sediment erosion created by Coandă-effect-based collectors, which is a crucial asset to optimize the collector design and decrease the extent of the associated sediment plumes.

Spatio-temporal analysis of sediment plumes formed by mussel fisheries and aquaculture in the western Wadden Sea

The Wadden Sea is a highly dynamic tidal system and an important area for a variety of species, including wild and cultivated mussels. Increased suspended sediment concentrations may negatively affect ecosystem functioning across trophic levels; hence, insight into the effects of anthropogenic activities is of importance. The aim of this study was to investigate the scale of sediment resuspension associated with commercial mussel cultivation activities. Turbidity prior to and during fisheries and harvest was investigated by in situ measurements with towed and fixed sensors. Four case studies were selected to represent maximum disturbance. For one case study, sediment resuspension was further investigated by analysis of satellite images. The in situ measurements showed that turbidity is enhanced by mussel fisheries and harvest but that effects are limited in time and space. Turbidity enhancement during fisheries was several-fold higher than that during harvest, whereas the spatial scale of both activities was limited to their direct vicinity. In contrast to the in situ study, the satellite image analysis identified a spatial magnitude that extended over a somewhat larger spatial scale, in the form of a dark water plume, and further revealed that this plume consisted of high concentrations of coloured dissolved organic matter (CDOM), rather than suspended particles. Along with turbidity, CDOM thus seems an important consideration for determining light attenuation in a water body affected by anthropogenic seafloor disturbance. As we presented case studies that were expected to represent maximum impact, we conclude that there is no specific need for further in-depth studies that allow upscaling to ecosystem-wide effects.

A Numerical Investigation of Dynamic Interaction between the Deep-Sea Mining Vehicle and Sediment Plumes Based on a Small-Scale Analysis

A Numerical Investigation of Dynamic Interaction between the Deep-Sea Mining Vehicle and Sediment Plumes Based on a Small-Scale Analysis

Воздействие отработанных месторождений россыпной платины на речную систему р. Вывенки (Камчатский край)

Geological exploration and mining operations have been conducted in the Vyvenka River basin on the Seinav-Galmoenan platinum deposit since 1994. The majority of the explored and prospective reserves are located in the valleys of spawning salmon tributaries, raising concerns about the environmental impact of mining activities. Despite the cessation of platinum mining in the 2010s, there has been a continuing impact on the river system in recent years, with extensive sediment plumes forming along the length of the rivers. This research aims to assess the dynamics of disturbed lands and determine the scale of sediment export into the river network from the territory of the ore mining sites of Vetvisty, Penisty, Ledyanoy, Yuzhny, and Levtyrinivayam. Field and satellite data were combined to analyze the suspended sediment discharge regime and the main sediment sources, i.e., the riverbank deformations along the mining sites. As of 2021, the total area of disturbed lands in the Vyvenka basin due to open-cast platinum mining is estimated to be 19.2 km2 corresponding to 0.15 % of the area of the Vyvenka River basin. Up to 75 % of the disturbed area consists of bare lands, making them prone to surface erosion. The largest amounts of sediment influx from the mining-affected river basins were observed in 2018 during the unevenly high freshet. The increased sediment discharge was due to the overflow of sedimentation ponds in certain areas. The study results indicate a prolonged anthropogenic impact on river ecosystems, manifested in the change of hydrological regime, activation of channel deformations, and the increased sediment influx to the river network from ore mining sites.

Provenance of the Middle Miocene Barreiras Fan Delta in the Onshore Pará-Maranhão Basin, Northern Brazil

The demise of the carbonate platform in the Brazilian Equatorial Margin has been related to the progradation of massive siliciclastic inflow from the Cratonic Amazon River during the middle Miocene. The sediment plume did not influence the eastern portion of this drainage, including the Bragantina Platform andthe Marajó Basin, the onshore Pará-Maranhão Basin. The provenance signatures of these tectonic segments have been demonstrated in siliciclastic coastal deposits of the Barreiras Formation, whose progradation caused the suppression of the Oligocene-Miocene carbonate platform. Our provenance study is based on clast morphology, quartz cathodoluminescence, and heavy minerals of mainly Barreiras alluvial-fluvial deposits, using new and published facies data. This study opens a window of opportunity to investigate, for the first time, the proximal facies combined with paleocurrents exposed only in this area of Brazil. The restricted occurrence of proximal facies indicates an exceptional potential for preservation related to subsiding regions generated by reactivated faults in the Gurupi Belt. Provenance data indicate the strong influence of metamorphic source rocks with minor volcanic and plutonic contributions from the southeastern and southern of the Bragantina Platform and eastern Marajo Basin. Respectively, two heavy minerals assemblages were found: 1) staurolite, metamorphic quartz, and angular clasts; and 2) abundant zircon, kyanite, tourmaline, mixed quartz (metamorphic, plutonic, and volcanic), and rounded clasts. The assemblages are associated with São Luís Craton, mainly crystalline rocks from Gurupi Belt. In addition, the proximal facies of the Bragantina Platform provide rich staurolite concentration and angular clasts indicating near source area; the uplifted Marajó Basin during the middle Miocene contributed with reworked sediments containing rounded clasts and ultrastable minerals related to Silurian-Cretaceous rocks. This work complements the previous paleoenvironmental reconstruction for the Barreiras deposits, including the most robust proximal facies interpretation contributing to understanding sediment dispersion during the middle Miocene, reinforcing previously defined source areas.

Monitoring of a sediment plume produced by a deep-sea mining test in shallow water, Málaga Bight, Alboran Sea (southwestern Mediterranean Sea)

In this study different experimental designs for monitoring of sediment plumes produced by deep-sea mining are presented. Plumes of sediment stirred up from the seabed by mining machines are considered to represent a major environmental pressure which may extend far beyond the actual mining area. Two industry field tests with the scaled mining vehicle Apollo II of Royal IHC conducted in a relatively shallow setting offshore southern Spain provided valuable insights for anticipated monitoring of nodule mining activities in the deep Pacific. Although the tests were performed in only 300 m water depth, much less than the depth where future deep-sea mining will take place, the weakly stratified bottom water, tide-dominated near-bed currents with mean magnitude of around 5–10 cm s−1, and gently sloping seabed covered with fine muddy sediment provide a good analogue to operational conditions in the deep sea. The plume of suspended sediment mobilised by the mining vehicle was monitored with turbidity sensors deployed on a ship-operated CTD system and on a static array of moored sensors and monitored visually using a ship-operated ROV. It was found that the generated sediment plume extended no >2 m above the seabed close to the source (<100 m) but increased in height at greater distance. Furthermore, turbidity values decreased rapidly with increasing distance to the source. Even though plume monitoring suffered interference from bottom trawling activities in neighbouring areas, a distinct turbidity signal generated by the mining equipment could still be distinguished above background turbidity at 350 m away from the source. From the experience gained in shallow water, recommendations are made on how a combination of sensors operated from moving and moored platforms may be a suitable and successful strategy for monitoring man-made sediment plumes in the deep sea.

Simulation Study on the Sediment Dispersion during Deep-Sea Nodule Harvesting

During the harvesting of polymetallic nodules on the seabed, the sediment plume due to disturbance on the seabed impacts the benthic ecosystem. A numerical simulation based on the SPH (smooth particle hydrodynamics) method is used to estimate the time and length scale of the plume impact near the seabed during a small-scale harvesting process. The simulation result considerably agrees with the one from the lab-scale water-channel experiment. It is found that, in the sediment plume, the traced sub-plume with iso-surface of lower sediment concentration travels a longer distance, and spends a longer time to achieve the stable state. Moreover, with the increase of the releasing rate of the disturbed sediment, the sub-plume spreads over greater distance, which also needs more time to achieve the stable state.

Sedimentation and Erosion Patterns of the Lena River Anabranching Channel

Lena River is one of the largest “pristine” undammed river systems in the World. In the middle and low (including delta) 1500 km course of the Lena main stem river forms complex anabranching patterns which are affected by continuous permafrost, degradation of the frozen ground and changes in vegetation (taiga and tundra). This study provides a high-resolution assessment of sediment behavior along this reach. Comprehensive hydrological field studies along the anabranching channel located in the middle, low and delta courses of the Lena River were performed from 2016 to 2022 including acoustic Doppler current profiler (ADCP) discharge measurements and sediment transport estimates by gravimetric analyses of sediment concentration data and surrogate measurements (optical by turbidity meters and acoustic by ADCP techniques). These data were used to construct regional relationships between suspended sediment concentrations (SSC, mg/L), turbidity (T, NTU) and backscatter intensity (BI, dB) values applicable for the conditions of the Lena River. Further, field data sets were used to calibrate the seasonal relationships between Landsat reflectance intensities and field surface sediment concentration data. Robust empirical models were derived between the field surface sediment concentration and surface reflectance data for various hydrological seasons. Based on the integration of in situ monitoring and remote sensing data we revealed significant discrepancies in the spatial and seasonal patterns of the suspended sediment transport between various anabranching reaches of the river system. In the middle course of the Lena River, due to inundation of vegetated banks and islands, a downward decrease in sediment concentrations is observed along the anabranching channel during peak flows. Bed and lateral scour during low water seasons effects average increase in sediment load along the anabranching channels, even though a significant (up to 30%) decline in SSC occurs within the particular reaches of the main channel. Deposition patterns are typical for the secondary channels. The anabranching channel that was influenced by the largest tributaries (Aldan and Viluy) is characterized by the sediment plumes which dominate the spatial and temporal sediment distribution. Finally, in the distributary system of the Lena delta, sediment transport is mostly increased downwards, predominantly under higher discharges and along main distributary channels due to permafrost-dominated bank degradation.

Remote sensing and buoy based monitoring of chlorophyll a in the Yangtze Estuary reveals nutrient-limited status dynamics: A case study of typhoon

Nutrient concentrations and its limited statuses can affect phytoplankton community structure and marine primary productivity. Identifying limiting nutrients under different ocean conditions, the causes, and the nature of such limitations, has important implications for understanding ecosystem changes and interpretation of in situ data. However, there is still a lack of retrieval methods for nutrients in seawater, especially in coastal waters. We propose to characterize the spatiotemporal variability of phytoplankton and nutrient-limited status in the surface ocean with diurnal valuevs of chlorophyll concentration. In this paper, the sediment fronts and plume fronts are used as a reference to select the relative and absolute nutrient-limited status regions. The variance of the six times per day (9:30-14:30) is calculated to represent the diurnal variation of chlorophyll, and the diurnal variation and concentration of chlorophyll combined are used to analyze nutrient-limited status. The results indicate that the diurnal variation of chlorophyll is greater after the typhoon, and the nutrient-limited status is different following each typhoon passing by. The in situ data shows that the highest chlorophyll a concentration reached 20.7 mg/m3 after the typhoon in August 2011.

Modelling of sediment plume associated with the capital dredging for sustainable mangrove ecosystem in the Old Mangalore Port, Karnataka, India

Old Mangalore port is located at the Confluence of River Gurpur and Netravathi in the Mangalore District, Karnataka. The Ministry of Shipping has promoted the modernisation of ports, including the Old Mangalore port, under the Sagarmala Project. The Old Mangalore port’s navigational channel is approximately 3.2 km long and 80 m wide. It is proposed to deepen the existing approach channel to 7 m to allow medium-capacity vessels access to the port. The expected quantity of dredging is estimated to be 1160000 m3. We investigated the sediment spill plume movement of the dredging/dumping operations at Old Mangalore port using FVCOM (Finite Volume Community Ocean Model). Analysis showed that the spill plume was moved towards the offshore direction during the wet season and is primarily dictated by the prevailing currents and freshwater influx from the rivers. During the dry season (minimal discharge), the plume spreads approximately 8 km to the upstream regions and is mainly controlled by tidal currents. The study also revealed that the dumping point at a distance of 18–20 km offshore (depth > 20 m) is ideal for the optimum settling of the sediments for both seasons. The capital dredging of a navigational channel and construction of a new coastal berth is a one-time operation. Any siltation of the river mouth during the dry season will likely be flushed out in the subsequent wet season. We identified 12 types of mangrove species in the vicinity of the dredging area, and mangroves near the Old Mangalore port may be more exposed to the dredging activities impacts. Spillage during the dredging produces the sediment plume, and its transport to the mangrove region may choke the root system, thereby reducing oxygen availability. Hence, the study recommends that dredging operations can be carried out at any season but restricted to the ebb tide, thereby preventing the movement of sediment plumes to the mangrove region and thus conserving the mangrove ecosystem.