Journal of Marine Systems最新文献

Species identification remains crucial for interpreting acoustic backscatter delivered by active acoustic methodologies. The study took place in a Marine Protected Area where highly restricted areas were present such as no take zones. We used an innovative methodology coupling split-beam and multibeam echosounders to detect and classify monospecific fish shoals (i.e. schools or aggregations). Species identifications were realised by underwater visual censuses made by scientific divers. Two experimental protocols, where the divers gave the identifications instantaneously thanks to a communication wireframe, were tested: three roving scuba divers locating the shoals or a towed scuba diver directly behind the vessel. Energy responses, 3-D morphological, shape indexes and spatial descriptive variables of multiple independent samples of 4 observed fish species shoals (Atherina sp., Boops boops, Chromis chromis and Spicara maena) were calculated from the acoustic data. According to their behaviour and feeding strategy, significant differences in the acoustic variables were found between species. The combined use of acoustic data from both echosounders significantly improved the fish species classification. They were well discriminated using a Linear Discriminant Analysis (LDA), including for B. boops, C. chromis and S. maena, which were all observed in aggregations. Finally, we used this LDA model to allocate species to unknown shoals monitored by acoustics methods in the studied site, highlighting the interest of our methodology to predict bentho-pelagic and pelagic fish distributions in shallow waters. We suggest that these acoustic methods to discriminate fish species could provide valuable insights for marine management and decision-making.

This study aimed to evaluate the spatial distribution and seasonal variability of zooplankton, mainly copepod groups, along the Beagle Channel (BC), an interoceanic passage at the southernmost tip of South America. We focused on the Argentine part of the BC, which comprises an inner sector under a strong continental influence and an outer sector influenced by offshore sub-Antarctic waters, separated by the narrow Mackinlay Strait. We analyzed zooplankton samples collected with 67 μm- and 200 μm-mesh nets in the inner and outer sectors of the BC during different seasons from 2014 to 2018, along with oceanographic parameters, total chlorophyll a (Chl a) concentration, and microbial community composition. Results showed a seasonal pattern, where zooplankton abundance was highest during spring and summer, with the largest-sized calanoid copepods being dominant in the inner sector in spring and in the outer sector in summer, and lowest in autumn and winter, small calanoid copepods being dominant in winter. Calanoid and cyclopoid copepodites and copepod nauplii were more abundant in spring and summer than in the other seasons. Conversely, adults of both groups were predominant in autumn and winter. The variables that better explained the abundance of the main copepod groups were Chl a concentration, picoeukaryotes abundance, and, to a lesser extent, water column stratification. Our findings suggest the existence of two seasonal patterns in the mid-eastern BC: a classical herbivore food web in spring-summer under high phytoplankton biomass, where the presence of larger copepods is associated with higher trophic levels, and a microbial food web in autumn-winter, with low phytoplankton biomass, characterized by great dominance of picoplankton that supports the grazing of the smallest copepods.

Ecopath mass-balanced models are widely-used tools to address various challenges in the understanding and protection of ecosystems. To track the continuing improvements in data and the evolving environment (climate change, anthropic pressure), new models are regularly being developed. In this study, we built a Gulf of Lion Ecopath model, focused on the continental shelf, featuring enhanced representation of benthic invertebrates and a realistic assessment of catches, and which takes into account the significant changes observed after 2008–2009 in the trophic structure of this ecosystem as well as related changes in fisheries activities. The model is composed of 68 functional groups, including 6 primary producers, discards and detritus, 27 invertebrate groups, 31 fish groups, dolphins and seabirds. New datasets were taken into account for biomasses, as well as for diets. P/B and Q/B parameters were calculated to include the most recent and geographically closest data. Model results highlight a food web diagram, ranging over 5 trophic levels and placing Prionace glauca, Squalus acanthias and dolphins as top predators. The mixed trophic impact analysis showed that the groups with the highest accumulated negative impacts are, in decreasing order, benthic trawls, nets and carnivorous echinoderms. The groups with the highest accumulated positive impacts are, in decreasing order, detritus, microphytoplankton and nanoplankton. The flux analysis shows that a major part of the flows occurs at trophic level 2 with 35.1% of the model total throughput and 43.8% of the total biomass. The catches have a mean trophic level of 3.47, higher than in previous studies, reflecting the changes in the fisheries activities.

The presence of harmful microalgae in aquaculture sites represents a risk for production and human health, so surveillance of these species is relevant for early warning of harmful algal blooms. Here we examine the monitoring results of potentially toxic phytoplankton in Peruvian coastal sites associated with the cultivation of Argopecten purpuratus and natural banks of other mollusks. We evaluated the density, frequency, temporal and spatial distribution of these species, as well as blooms occurred from 2011 to 2019. Results showed that the most abundant species was Heterosigma akashiwo, and the most frequent ones were Pseudo-nitzschia delicatisima and seriata complexes. The majority of blooms were caused by H. akashiwo (21 events) and Dinophysis acuminata (19 events). Canonical correspondence analysis revealed the preference of dinoflagellates to warmer sea surface temperatures and more stable conditions (indicated by low wind velocities) than Pseudo-nitzschia species. Generalized additive models (GAMs) indicated that wind velocities > 3.7 m s⁻¹ and El Niño conditions favor Pseudo-nitzschia species while warm temperatures (> 20 °C), weaker winds (< 3.7 m s⁻¹), and cold periods associated with La Niña promoted higher densities of D. acuminata complex. A recent increasing trend in the density of D. acuminata was observed. This information is useful to understand the dynamics of potentially toxic species in upwelling regions.

This study evaluated fish fauna using aqueous environmental DNA (eDNA), sedimentary eDNA, and bottom-trawl catch survey data, based on 2-year field samplings in Tokyo Bay, central Japan. Aqueous eDNA detected the highest number of fish species, followed by sedimentary eDNA and then catch surveys. Due to the prevalence of hypoxia in the inner and central areas during summer and the influence of open ocean water in the mouth area during autumn, the number and composition of fish species detected by each data type by area (inner, central, and mouth areas) significantly differed depending on the season. Among the dominant species in Tokyo Bay, pelagic and inshore fishes were more frequently detected by eDNA (both in water and sediment) than by catch surveys. In contrast, cartilaginous and benthic fishes inhabiting sandy or muddy bottoms tended to be recorded by catch surveys rather than by eDNA. For non-dominant species, eDNA was more sensitive than catch surveys; occasional migratory fishes were more likely to be detected by aqueous eDNA, while small, rare fish species inhabiting rocky reef areas tended to be detected by sedimentary eDNA. Thus, information from aqueous eDNA, sedimentary eDNA, and catch survey samples could complement each other to improve the accuracy and representativeness of fish fauna monitoring.

Off the central-southern Chilean coast (35°–38°S), the Gulf of Arauco is one of Chile's largest semi-enclosed coastal areas. It hosts industrialized activities within a highly productive zone of the Southern Humboldt Current System. One of the principal hydrodynamical forcings of the region is the Biobio River, whose discharge significantly influences coastal dynamics in the Gulf. The present work aims to study the impact of the Biobio River freshwater discharge on the circulation patterns in the Gulf of Arauco, using a high-resolution interannual simulation of the period 2013–2018. The simulation includes monthly interannual discharge from the significant four rivers for the study zone (Mataquito, Maule, Biobío, and Itata). The focus is primarily on wintertime (June–September), the highest freshwater discharges period. The modeled temperature and salinity fields were consistent with in-situ observations, presenting a moderate bias. The water masses highlighted in the TS diagrams, the temperature time series, and especially the currents near the Biobio River mouth were well represented in the simulation. It was found that the Biobio River strongly impacted the circulation in the Gulf of Arauco, intensifying the currents and causing a notable salinity decrease. Offshore zonal currents were intensified west of the Biobio River mouth, whereas southward alongshore currents were enhanced, especially during August. The influence of the Biobio River in the Gulf of Arauco was closely related to discharge strength. A strong relation between predominant southward downwelling-favorable wind stress and meridional currents was found, probably due to the formation of a buoyant coastal current strengthened by the wind-driven current during winter. Finally, surface buoyant waters associated with the river discharge generated a strong baroclinic zonal pressure gradient equilibrating the sheared meridional flow and enhancing the meridional ageostrophic pressure gradient, Reynolds stress, and near-surface vertical mixing of momentum.

The concentration of suspended sediment was found to impact the characteristics and dynamics of oil spills, as indicated by the rate of formation of oil particle aggregates (OPA). The transport and fate of oil spills in the Zhoushan Archipelago were simulated by using a combined hydro-sediment-oil spill model. We studied the impacts of suspended sediment on oil spills in macro-tidal turbid archipelagic seas. The model results indicated that the formation of OPAs is affected by the concentration of suspended sediment, energy dissipation rate, oil density, and hydrodynamics (winds and tides). An increase in oil density accelerated the formation of suspended buoyant OPAs but suppressed the formation of negatively buoyant OPAs. Winds and tides affected the rate of OPA formation by changing the energy dissipation rate and the concentration of suspended sediment, respectively. Oil spill dispersion is impacted by winds, tides, and oil spill location, through the changes in the formation of both suspended buoyant OPAs and negatively buoyant OPAs. Although strong winds promote the formation of OPAs and tend to reduce oil content they also potentially enlarge the oil film (depending on tidal currents). OPAs form more readily in oil spills in main tidal channels, likely due to the high concentration of suspended sediment and high degree of mixing generated by strong currents. The findings of this study provide theoretical foundations for the studies of oil spills and coastal management in turbid coastal zones.

The Beagle Channel (BC) is a subantarctic passage connecting the Pacific and Atlantic oceans at latitude ∼55°S. Along its tortuous path, the channel defines particular environments of prominent ecological value that are under study from a variety of scientific fields, while the main physical features that form the basis for these ecosystems are still critically understudied. In this work, historical data series of currents measured with surface drifters and moored current meters as well as sea-level data measured by gauge stations and pressure transducers, are analyzed. These are used to describe the propagation of the tidal wave and the general water motion along the BC. Astronomical amplitudes and phases of the main tidal components, computed from historical and recent sea-level data series obtained at the BC and gathered from global numerical models, are integrally analyzed to describe the propagation of a single, progressive tidal wave in the channel and in the surrounding ocean area, in contrast with previous assumptions that considered two tidal waves advancing in opposite directions. The relative narrowness of the channel, which is roughly oriented in a west-east direction, favors currents with a dominant zonal component. Such fact is exacerbated at and near topographic constrictions such as Mackinlay Strait, where a substantial acceleration of the flow is also observed. Superimposed on the mixed semidiurnal tidal regime, there is a remarkably persistent residual surface flow in an eastwards direction along the BC, likely contributing to the regional transport of water and properties from the Pacific Ocean to the Atlantic Patagonian Shelf. The existence of a semi-permanent sea-level tilt between the Pacific and Atlantic sides of the BC, is proposed as the main driver for that along-channel current. However, inversions of the residual current (i.e., westwards flow) do occur below the depths of major topographic constrictions, likely promoting recirculation and high residence times in the middle BC west of Mackinlay Strait.

The use of acoustic scattering models provide estimates of single target echoes that allow acousticians to convert acoustic information into biologically meaningful measures. The literature on organisms’ target strength is extensive but is mainly focused on commercial stocks of small pelagic fishes and zooplankton species. A few models of swimbladdered fishes of the mesopelagic zone are also available. However, deep species of the lower mesopelagic and bathypelagic zones tend to have regressed swimbladders or lack one. These habitats have low numerical densities and thus single target studies and angle variation are of particular relevance. Cyclothone spp, the most abundant fishes in the planet and a major constituent of the biomass in the bathypelagic zone, possess gas-filled swimbladders in the upper mesopelagic zone and in larvae stages of all species, but deeper species gradually fill their swimbladder with age. They thus change from a gas-bearing acoustic scattering to a fluid like type. This study applies the Kirchoff Ray Mode (KRM) model based on real fish body shapes of Cyclothone individuals derived from photographs of organisms captured along the year in the Bay of Biscay in order to obtain target strength (TS) of these individuals. Width versus standard length (SL) values fitted the following equation: width=0.01+0.02*SL. Estimated TS values in the Rayleigh zone at broadside had significant linear correlations with SL that can be employed as an approximation of their scattering (TS = 35*log${}_{10}$(SL) − 119, TS = 35*log${}_{10}$(SL) − 106 and TS = 35*log${}_{10}$(SL) − 97 at 18, 38 and 70 kHz respectively). TS at 120 and 200 kHz were not significantly correlated with standard length. Changes in fish body sound speed and density values highly vary the TS level. Assuming neutral buoyancy (body density close to surrounding seawater density), mean TS values were located at −91, −85, −78, −77, −80 dB at 18, 38 and 70, 120 and 200 kHz respectively. TS changes with orientation were also considered depicting important variations in echo level as well as in TS spectra. This study provides relevant information on the acoustic characteristics of lower mesopelagic and bathypelagic Cyclothone species that can be employed to better infer knowledge from acoustic recordings in those areas.

Selecting the appropriate in-situ and satellite matchups is a critical task for evaluating and application of ocean color products in optically complex coastal waters. This article investigates the performance of five single-sensor and two merged multi-sensors Chl-a products in the Persian Gulf. In this study, bio-optical and Chl-a measurements collected from 531 stations in the northern Persian Gulf from 2008 to 2019 were used. In-situ samples were initially controlled to filter out inappropriate datasets. Results indicated that surface measurements may not be representative of satellite-derived Chl-a, and satellite estimations were mostly consistent with Chl-a concentrations in the first optical depth (Chl_opt). Statistical analysis showed that all satellite-derived Chl-a products overestimated the Chl_opt by 48%–170%, in which Chl-a from Neural-Network and OC5 algorithms yielded the best agreement. Afterwards, the QC tests were designed based on the remote sensing reflectance (R_rs) at 555 nm as a proxy of SPM, R_rs(412)/R_rs(443) as a proxy of CDOM, and R_rs(560)/R_rs(490) as a proxy of Chl-a, to select the matchups with the least interference of Colored Dissolved Organic Matter (CDOM) and Suspended Particulate Matter (SPM) on Chl-a. The correlation between Chl_opt and satellite-derived Chl-a improved significantly after applying the QC tests (R² = 0.80–0.89). The valid pixels were determined using matching the results of QC tests and standard level-2 quality flags. Finally, statistical calibration was used to create the final quality controlled calibrated Chl-a maps, using regression coefficients obtained from training/validation exercises and bootstrapping-like techniques applied to the quality-controlled datasets. The findings showed that in turbid coastal waters, standard ocean color Chl-a products cannot be validated only by performing statistical methods, and quality control experiments are necessary for their applications.