A statistical algorithm for estimating chlorophyll concentration in the New Caledonian lagoon

Spatial and temporal dynamics of phytoplankton biomass and water turbidity can provide crucial information about the function, health and vulnerability of lagoon ecosystems (coral reefs, sea grasses, etc.). A statistical algorithm is proposed to estimate chlorophyll-a concentration ([chl-a]) in optically complex waters of the New Caledonian lagoon from MODIS-derived remote-sensing reflectance (R-rs). The algorithm is developed via supervised learning on match-ups gathered from 2002 to 2010. The best performance is obtained by combining two models, selected according to the ratio of R-rs in spectral bands centered on 488 and 555 nm: a log-linear model for low [chl-a] (AFLC) and a support vector machine (SVM) model or a classic model (OC3) for high [chl-a]. The log-linear model is developed based on SVM regression analysis. This approach outperforms the classical OC3 approach, especially in shallow waters, with a root mean squared error 30% lower. The proposed algorithm enables more accurate assessments of [chl-a] and its variability in this typical oligo- to meso-trophic tropical lagoon, from shallow coastal waters and nearby reefs to deeper waters and in the open ocean

Evaluation of Satellite Retrievals of Chlorophyll-a in the Arabian Gulf

The Arabian Gulf is a highly turbid, shallow sedimentary basin whose coastal areas have been classified as optically complex Case II waters (where ocean colour sensors have been proved to be unreliable). Yet, there is no such study assessing the performance and quality of satellite ocean-colour datasets in relation to ground truth data in the Gulf. Here, using a unique set of in situ Chlorophyll-a measurements (Chl-a; an index of phytoplankton biomass), collected from 24 locations in four transects in the central Gulf over six recent research cruises (2015–2016), we evaluated the performance of VIIRS and other merged satellite datasets, for the first time in the region. A highly significant relationship was found (r = 0.795, p < 0.001), though a clear overestimation in satellite-derived Chl-a concentrations is evident. Regardless of this constant overestimation, the remotely sensed Chl-a observations illustrated adequately the seasonal cycles. Due to the optically complex environment, the first optical depth was calculated to be on average 6–10 m depth, and thus the satellite signal is not capturing the deep chlorophyll maximum (DCM at ~25 m). Overall, the ocean colour sensors’ performance was comparable to other Case II waters in other regions, supporting the use of satellite ocean colour in the Gulf. Yet, the development of a regional-tuned algorithm is needed to account for the unique environmental conditions of the Gulf, and ultimately provide a better estimation of surface Chl-a in the region

Remote Sensing of the Aquatic Environments

The book highlights recent research efforts in the monitoring of aquatic districts with remote sensing observations and proximal sensing technology integrated with laboratory measurements. Optical satellite imagery gathered at spatial resolutions down to few meters has been used for quantitative estimations of harmful algal bloom extent and Chl-a mapping, as well as winds and currents from SAR acquisitions. The knowledge and understanding gained from this book can be used for the sustainable management of bodies of water across our planet

Ocean remote sensing techniques and applications: a review (Part II)

As discussed in the first part of this review paper, Remote Sensing (RS) systems are great tools to study various oceanographic parameters. Part I of this study described different passive and active RS systems and six applications of RS in ocean studies, including Ocean Surface Wind (OSW), Ocean Surface Current (OSC), Ocean Wave Height (OWH), Sea Level (SL), Ocean Tide (OT), and Ship Detection (SD). In Part II, the remaining nine important applications of RS systems for ocean environments, including Iceberg, Sea Ice (SI), Sea Surface temperature (SST), Ocean Surface Salinity (OSS), Ocean Color (OC), Ocean Chlorophyll (OCh), Ocean Oil Spill (OOS), Underwater Ocean, and Fishery are comprehensively reviewed and discussed. For each application, the applicable RS systems, their advantages and disadvantages, various RS and Machine Learning (ML) techniques, and several case studies are discussed.Peer ReviewedPostprint (published version

Hydrodynamique et transport de particules en suspension dans le lagon sud-ouest de Nouvelle-Calédonie

This thesis participates to a study that has been lead for several years by IRD which aims increased knowledge of hydrodynamics and transport of dissolved substances and particles on the South-West Lagoon of New Caledonia (SLNC). This work stands on field measurements and numerical modelling.The first step of this thesis is an attempt to synthesise the great amount of data produced by a numerical hydrodynamic model (MARS3D). Indexes having the dimension of time and related to the renewal of water masses were computed from numerical tools applied to the hydrodynamic model. The computation methods, the significance and the application of these indexes to the Southwest Lagoon of New-Caledonia (SLNC) are exposed. Examples ofapplication of these indexes aiming to quantify the influence of hydrodynamics on biological processes are shown.Out of flooding periods, the re-suspension of freshly deposited sediments is the main source of suspended particles on the SLNC. The combined actions of waves and currents induce a bottom shear stress that is responsible for particle re-suspension. In order to access the wave field characteristics, a wave model (WavewatchIII) was implemented over the SLNC. It was validated by in situ directional measurements of the wave field.The last part of this thesis focuses on the determination of physical properties of suspended particles such as particle size distributions, density, and fall velocity, are compulsory to model particle transport. This work stands on in situ measurements and laboratory experiences performed with laser particle size analyser. Results emphasise the importance of bioaggregation in a coral reef ecosystem.Cette thèse s’inscrit dans le cadre d’une étude conduite depuis plusieurs années par l’IRD qui vise à mieux connaître les processus de l’hydrodynamique et du transport d’éléments dissous et de particules dans le Lagon Sud-Ouest de Nouvelle Calédonie (SLNC). Les travaux présentés s’appuient sur des mesures de terrain et la modélisation numérique.Le premier volet de la thèse vise à synthétiser l’abondante information produite par un modèle numérique hydrodynamique (MARS3D). Des indices relatifs au renouvellement des masses d’eau et ayant la dimension d’un temps, communément appelés temps de résidence, sont élaborés à partir d’outils numériques. La signification de ces indices, leur mode de calcul et leur application au SLNC sont exposés. Plusieurs applications mettent en évidence les effets de l’hydrodynamique sur des processus biologiques et biochimiques.Hors période de crue, les sédiments fraîchement déposés constituent la principale source de particules en suspension. Les tensions de cisaillement provoquées par les effets combinés de la houle et du courant provoquent leur remise en suspension. Pour les calculer en tout point, il est nécessaire de simuler précisément le champ de vagues. La seconde partie de la thèse y est consacrée. Pour cela, le modèle de houle WaveWatchIII est adapté au SLNC et validé par des mesures directionnelles de vagues.Le troisième et dernier volet de la thèse est focalisé sur la détermination des propriétés physiques des particules en suspension. Ces paramètres (concentration par classe granulométrique, densité, vitesse de chute) sont nécessaires pour modéliser le transport desparticules. Ce travail s’appuie sur des mesures in situ et des expériences en laboratoire réalisées à l’aide d’un granulomètre laser

Socio-ecological drivers of fish biomass on coral reefs: the importance of accessibility, protection and key species

Coral reefs have the greatest biodiversity of any ecosystem on the planet and support ecosystem goods and services to million people who depend directly on them for food, economic income, coastal protection and cultural values. There is a clear consensus that accessibility through road networks and infrastructure expansion is a main driver of ecosystem conditions, with the most accessible resources being most at risk. Yet to date measuring the extent to which coral reefs are accessible to humans is strictly limited to examining the linear distance between fishing grounds and markets or ports. However, linear distance ignores ragged coastlines, road networks and other features that can affect the time required to reach fishing grounds from a human settlement. This thesis presents a double challenge: (i) developing new metrics of accessibility that account for seascape heterogeneity to better assess human impacts on coral reefs; and (ii) evaluating the importance of coral reef accessibility, in interactions with their management, to explain variations of fish biomass. First, I estimated the travel time between any given coral reef and human populations and markets based on the friction distance which is related to transport surfaces (paved road, dirt road, water) influencing transportation costs and the effective reach from human settlements. I found that travel time is a strong predictor of fish biomass. Second, using a downscaling of the travel time approach I illustrated how market proximity can affect the behavior of fishermen and, ultimately, trigger changes in marine resource exploitation in North-Western Madagascar. Market access appears as a critical step toward a long-term management of coral reef fisheries. Third, travel time was used to build a human gravity index, defined as human population divided by the squared travel time, to better assess the level of human pressure on any reef of the world. Then, gravity was used to assess the effectiveness of marine reserves given the level of human pressure. The results highlighted critical ecological trade-offs in conservation since reserves with moderate-to-high human impacts provide substantial gains for fish biomass while only reserves located where human impacts are low can support populations of top predators like sharks which are otherwise absent from coral reefs. Fourth, I developed a new Community-Wide Scan (CWS) approach to identify fish species that significantly contribute, beyond the socio-environmental and species richness effects, to fish biomass and coral cover on Indo-Pacific reefs. Among about 400 fishes, I identified only a limited set of species (51), belonging to various functional groups and evolutionary lineages, which promote biomass and coral cover; such key species making tractable conservation targets. Within the context of global changes and biodiversity loss, the thesis challenges the sustainable and efficient management of coral reef socio-ecological systems with accessibility being the cornerstone but also the main danger in a near future where roads will expand and coastal human populations will grow

Copernicus Ocean State Report, issue 6

The 6th issue of the Copernicus OSR incorporates a large range of topics for the blue, white and green ocean for all European regional seas, and the global ocean over 1993–2020 with a special focus on 2020