Fine-scale vertical structure of sound-scattering layers over an east border upwelling system and its relationship to pelagic habitat characteristics

Understanding the relationship between sound-scattering layers (SSLs) and pelagic habitat characteristics is a substantial step to apprehend ecosystem dynamics. SSLs are detected on echo sounders representing aggregated marine pelagic organisms. In this study, SSL characteristics of zooplankton and micronekton were identified during an upwelling event in two contrasting areas of the Senegalese continental shelf. Here a cold upwelling-influenced inshore area was sharply separated by a strong thermal boundary from a deeper, warmer, stratified offshore area. Mean SSL thickness and SSL vertical depth increased with the shelf depth. The thickest and deepest SSLs were observed in the offshore part of the shelf. Hence, zooplankton and micronekton seem to occur more frequently in stratified water conditions rather than in fresh upwelled water. Diel vertical and horizontal migrations of SSLs were observed in the study area. Diel period and physicochemical water characteristics influenced SSL depth and SSL thickness. Although chlorophyll-a concentration insignificantly affected SSL characteristics, the peak of chlorophyll a was always located above or in the middle of the SSLs, regularly matching with the peak of SSL biomass. Such observations indicate trophic relationships, suggesting SSLs to be mainly composed of phytoplanktivorous zooplankton and micronekton. Despite local hypoxia, below 30 m depth, distribution patterns of SSLs indicate no vertical migration boundary. The results increase the understanding of the spatial organization of mid-trophic species and migration patterns of zooplankton and micronekton, and they will also improve dispersal models for organisms in upwelling regions.publishedVersio

Micronektonic acoustic density variations along Benguela Current Large Marine Ecosystem continental shelf from 1994 to 2001

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018The Benguela Current Large Marine Ecosystem (BCLME) is situated along the coast of south-western Africa, stretching from Tombua (Angola) in the north (16°N, 11°W) southwards to the east of the Cape of Good Hope (South Africa) (29°S, 17°E) and includes the study zone, from 17°S, 9°E to 31°S, 17°E. In this work, we focused on the Namibian continental shelf where fishing activities are mostly due to industrial fishing. Acoustic data were recorded with a 38 kHz echosounder, from 10 to 500 m depth over 8 surveys totalling 46 302 nmi from 1994 to 2001. To get homogenous data (i) only off-upwelling season surveys (October to June) were studied and (ii) only continental shelf data were considered (10-150 m). The mean volume backscattering strength (Sv in dB) was used as a micronektonic biomass proxy to assess its spatial inter-annual variability. Diel transition periods were removed from analyses to avoid micronektonic density changes bias due to diel vertical migrations. Data were echointegrated at a spatial resolution of 0,1 nmi*1 m depth using the Matecho tool. (i) On horizontal dimension, the variability in annual micronektonic densities was assessed using the mean Sv value for each 0,1 nmi Elementary Sample Unit (ESU). Then, hot and cold spots were computed from the combined analysis of the spatial correlation and the Moran's I index of these values. (ii) On vertical dimension, the change of micronektonic spatial structure between day and night was assessed using the mean Sv value for each 1 m depth step. The inter-annual variability inside the eastern boundary upwelling ecosystem of the BCLME was analysed. (i) No significant change in micronektonic density was observed over the study period. Mean micronektonic acoustic density values observed were lower than in other African Atlantic large marine ecosystems. (ii) Hot and cold spots were spatially stable over time. Further analysis of physico-chemical parameters should improve the understanding of this pattern. (iii) A different vertical structure was reported between day and night, suggesting a migration from bottom to surface at dusk, as in the well-known diel vertical migrations. In perspective, physical processes occurring in the water column from turbulence to mesoscale activities should be considered in future studies

Echo level segmentation on echo-integration of fisheries acoustics data

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018In fisheries acoustics the analysis of data usually often concern biomass assessment mainly for small pelagic fish stocks using the well-known echointergration approach. Other can concern the analysis of single fish using their target strength (TS in dB) and more seldom analysis can also be done with the fish school descriptors using e.g. shoal extraction method (Movies+, Ifremer Software). In the framework of the Preface project we have focused on the micronektonic layers observed by scientific echosounder. Matecho, a friendly automatized processing method to extract information and perform echo-integration, fish shoal extraction and also performs a segmentation, on each zone of a cruise with a constant twilight, of the echointegrated echogram from an echo level threshold fixed by user to extract micronektonic layers in the water column. Here we describe this methodology which allows an accurate description of the spatial organisation and structuration of the marine ecosystem. The process is based on three main steps which consist in : (i) adjust the echo level threshold in dB, (ii) the extraction of the echoes inside each contours and the calculation of the layer descriptors, (iii) and then the correction of the extraction. Finally the echo segmentation, setup to extract micronektonic sound scattered layer, allows to get 34 layers descriptors, e.g., minimum/maximum depth (m), geographical position in 3D, maximum depth width (m), duration of the layer, surface covered by the layer, mean volume backscattering strength 'Sv' (dB re 1 m-1)': mean nautical area scattered coefficient 'Sa' (or NASC m2 nmi-2), to characterise their spatial position in the water column and acoustics properties. Moreover, a second class of descriptors, classified by elementary sampling unit (ESU), are estimated e.g. number of layer per ESU, layer depth per ESU. An innovative descriptor is also computed using this methodological approach: the water column fulling rate per layer and per ESU. Both classes of descriptors are then available for ecological studies

Effect of environmental variables on the structure of micronectonic layers over the Senegalese continental shelf

ICAWA : International Conference AWA, Mindelo, CPV, 13-/11/2017 - 17/11/2017The micronectonic organisms aggregate at specific depths and occur as scattering layer on echosounder records. They constitute an important component in the marine food web in direct contact with primary producers. We characterized the Senegalese water masses of the 'Petite côte' on physicochemical and biological criteria using an in situ data set collected during an acoustics survey. Then we described at fine scale spatial and temporal variation of micronektonic layers in relation with their environment. Two areas with different characteristics have been discriminated: the upwelling's cell area and the upwelling's offshore area more stratified, warm and sharply separated from the other area by a strong thermal boundary. The spatio-temporal variation of scattering layer's thickness of micronectonic is strongly influenced by depth and the time of the day. The continental shelf scattering layer's thickness increases with depth, but no variation is reported in longitudinal plane. In both areas nocturnal layers are thicker and deeper than diurnal ones. The hydrological structure of the water column also influence the micronectonic scattering layer. The scattering layer requires "stable" physical conditions which support vertical stratification. In the upwelling's area cell, the chlorophyll-a (CHL) concentration is correlated to scattering layer thickness during night time. In the upwelling's offshore area, sea temperature, water density and oxygen have a significant effect on the scattering layer's thickness during the nightime. However, during the daytime, CHL has a significant effect on the scattering layer's thickness. This correlation between CHL and scattering layer thickness in the upwelling's offshore area during day could be explained by an inverse diel vertical migration of a micronectonic group. On this basis we assume that trophic relationship between phytoplankton and micronecton operate during the day at the surface in this area

Micronektonic acoustic density variations in Guinea Current Large Marine Ecosystem continental shelf from 1999 to 2006

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018The Guinea Current Large Marine Ecosystem (GCLME) extends from Bissagos Island (Guinea Bissau) in the north (11°N, 16°W) to Cape Lopez (Gabon) in the south (0°S, 8°E) and includes the study zone, from 4°N, 8°W to 6°N, 3°E. Acoustic data were recorded with a 38 kHz echosounder, from 10 to 500 m depth over 7 surveys, 6 were selected here, totalling 16 618 nmi from 1999 to 2006. To get homogenous data, (i) only off-upwelling season surveys (April to September) were studied and (ii) only continental shelf data were considered (10-150 m). The mean volume backscattering strength (Sv in dB) was used as a micronektonic biomass proxy to assess its spatial inter-annual variability. Diel transition periods were removed from analyses to avoid micronektonic density changes bias due to diel vertical migrations. Data were echointegrated at a spatial resolution of 0,1 nmi*1 m depth using Matecho tool (Perrot et al., 2018). (i) On horizontal dimension, the variability in annual micronektonic densities was assessed using the mean Sv value for each 0,1 nmi Elementary Sample Unit (ESU). (ii) On vertical dimension, the water column variation (%) filled by micronektonic acoustic layer (filling rate) across years was estimated using a linear regression and the change of micronektonic spatial structure between day and night was assessed using the mean Sv value for each 1 m depth step. GCLME have a narrow continental shelf vs. other African Atlantic LMEs. No significant change of micronektonic biomass proxy has been observed from 1999 to 2006 in this study (Fig. 1). As expected, a difference is observed in the vertical micronektonic acoustic density between day and night (Fig. 3). However, there is a paradoxical process, indeed there is an increase in Sv during nighttime. Two hypotheses are proposed: (i) the increase in density could be explained by an offshore horizontal diel migration or (ii) a very high contribution of the micronektonic density occurring in surface (0-10 m) is suspected, which corresponds to the blind zone of the research vessel. According to the new descriptor “water column filling rate”, a significant change in the system is reported. Indeed there is an increase in five years. Future investigations should focus on this interesting phenomenon, which could be link to an effect of global change. We have to take care that such increase could also inform on a major change in the trophic web in this part of the GCLME. These findings can be interpreted as an early warning signal and encourage for future study

The effect of oceanographic factors on micronektonic acoustic density in the three African Atlantic large marine ecosystems

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018The interest of modelling the effect of oceanographic factors on micronektonic acoustic densities and its variability is relevant in the context of climate change to better understand the environmental processes controlling ecosystem productivity. Ultimately for the stakeholders, we plan to forecast changes induced by climate change effects and study inter annual variability. Satellite data have been processed using the same time steps as the time series of fisheries acoustic surveys carried out by the R/V Dr. Fridtjof Nansen along the Atlantic African coasts, overlapping three Large Marine Ecosystems. The observed split at Cape Blanc (21°N) separates the coastal upwelling into a strong and stable dynamic upwelling, and a highly seasonal one. Because of the highly non-linear nature of the relationships the BRT modelling accounts for a considerably higher part of the environmental variability, compared to classic multivariate approaches. Environmental data are extracted from daily series of AVHRR (SST), MODIS (SST and Chl-a) and others at spatial resolution between 4 and 25 km. Boosted Regression Tree classification is well suited to show the importance of the large scale environmental variability, despite a limited set of variables. It is interesting to note that the inter-annual variability is not significant in the model, showing that the underlying environmental forcing is associated with relatively stable processes. The structural variables, i.e., bathymetry and distance to the coast, consistently explain a large part of the variability. SST has a minor influence in the north (consistently cold and windy) and a pronounced effect in the south where seasonality is high and variable. Especially in Senegal and Guinea, the detrimental effects of the coastal upwelling (mostly offshore drifts due to strong winds) are strongly attenuated by the wider continental shelf which favour retention processes. The next step will be to couple our results with climate projections to forecast major changes in African coastal systems as the micronektonic compartment is essential at mid-trophic level in all marine ecosystems. Considering the oceanographic factors relative influence, and under the assumption of similar warming in the three Atlantic African LMEs, a stronger ecosystem perturbation is expected in BBCLME, then in the CCLME and particularly when comparing the southern part of the CCLME vs North part. In all LME i.e. including GCLME, the oceanographic factors relative influence get a significant role confirming the important changes expected due to climate change on the ecosystems and thus in the fisheries

Characterization of micronektonic spatial structure using ecosystemic acoustics descriptors applied in three Atlantic African Large Marine Ecosystems

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018Using the segmentation algorithm within Matecho (Perrot et al., 2018) we are able to deliver 15 descriptors to characterize the acoustic micronektonic layers in the water column. Even if the species composition is not known, these descriptors which are obtained using the same methodology allow for comparison between ecosystems and to study inter-annual variability. Some of these descriptors are new and others are based on the ones usually used to characterize pelagic fish schools using echointegration per shoal (Weill et al., 1993). In this work we will focus on the new ones and show some application cases in the three Atlantic African Large Marine Ecosystems, to monitor potential perturbations due to global change. All layer descriptors are estimated per layer and per elementary sampling unit of 0.1 nautical miles (ESU) with an accuracy of 1 meter depth. In this study we present four classes of descriptors: spatial (e.g. altitude, mean depth, minimal depth); morphological (e.g. width, ESU number, filling rate of water column); acoustic (e.g. mean volume backscattering strength Sv (dB)) and the layer number per ESU. In this study we focus on the original descriptors: (i) Filling rate of the water column (%): this indicator is based on the calculation of the width of the micronektonic layer vs. the local bottom depth. (ii) Filling rate contribution of first layer (%): this indicator shows the contribution of the first layer (the closest layer of surface) in the global filling rate. It is computed by dividing the filling rate of first layer by the filling rate of all layers. (iii) Number of layers: this indicator is calculated for each ESU, giving the number of layers in this water column. The descriptors have been computed over more than 1 million of ESUs, 992 737 in the CCLME, 166 183 in the GCLME and 462 807 in the BCLME. Such descriptors allow classification of micronekton layers and appear relevant to monitor changes in the ecosystem. Next step will be to use multifrequency or even wide-band data to improve the quality of descriptors. They were efficiently applied to study diel vertical behaviour as well as the effect of water mass characteristics on the spatial structure of the layers. In future applications it should help in the classification of the layers per functional group as well as to improve our knowledge on ecosystem organization and functioning

Comparative analysis of Diel Vertical migration between three Atlantic African large marine ecosystems

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018Diel vertical migration (DVM) of micronekton is a behavioural mechanism driven by a trade-off between predator avoidance and access to prey. This trade-off is controlled by environmental forcing that can lead to changes of DVM pattern under changing environmental conditions. Time series of hydro acoustic surveys between 1995 – 2015 of three large Atlantic ecosystems (Canary Current - CCLME, Guinea Current - GCLME, and Benguela Current - BCLME) were analysed to calculate DVM patterns based on volume backscattering strength (Sv). DVM related descriptors (n=15) were calculated for areas according to bathymetric definitions (shelf = 10 – 150 m bottom depth, slope = 150 – 500 m bottom depth, and plain > 500 m bottom depth). Typical DVM I pattern, with micronekton descending during daytime and ascending during night-time, were observed on the slope and plain in all three ecosystems, but not on the shelf with only negative day-night values in the CCLME and BCLME. Lower daytime Sv values during the day compared to night-time suggest either less dense patches of micronekton leading to negative day-night differences in the CCLME and GCLME or insufficient measurements of certain depth strata (e.g., 0 – 10 m surface). Only a few significant and different DVM descriptors suggest a change in the CCLME and the GCLME in the last 20 years. All other insignificant descriptors assume natural variability in large Atlantic ecosystems. Our results provide insight into inter-annual variability in micronekton DVM patterns

Unsupervised functional classification applied on high resolution oceanographic data in Canaries Current Large Marine Ecosystem : toward fine scale analysis

ICAWA : International Conference AWA, Lanzarote, ESP, 17-/04/2018 - 20/04/2018The understand of the fine scale process occurring in the ocean needs high resolution data and ad hoc analysis approach to improve the knowledge of ecosystem functioning. During an international survey carry out in 2014'AWA’ on-board the research vessel Thalassa (Ifremer, Brest) along the coast of Mauritania and Senegal we have used simultaneously multifrequency scientific echosounder and a Scanfish, both system allow a continuous acquisition of high- quality data at high spatial and temporal resolution over long distance. The functional data analyses have recently raising in serval field of statistics and appear to be well suited for the analysis of this dataset. In fact such data has spatial-functional nature and may be considered as observations of a stochastic process X in space of continuous functions over an interval T. Let X1(t), X2(t),.., Xn(t), t T, be the collection of n observations from X. First, we study an eventual horizontally or vertically variation of the acoustic intensity, we consider for a given frequency (here 200 kHz) and one vessel radial the two cases: vertical and horizontal variations of the acoustic intensity. Unsupervised functional classification used, shows a horizontal and vertical variation of acoustic intensity for a given frequency and a given radial. The approach can led to scrutinized at fine scale the processes occurring in three dimensions in the pelagic environment. The statistical functional classification applied to this case study appears powerful, ad hoc for ecological studies of marine ecosystem and will be extend to model the spatial structuration of the pelagic ecosystem according to the physcio-chemical parameters of the water mass which will allow to improve the forecast of the effect of the environment on marine ecosystem organization