Micronekton Biomass Research Articles

Global characterization of modelled micronekton in biophysically defined provinces

Micronekton are the mid-trophic level of the ecosystem and contribute to active carbon export to the deep ocean through diel vertical migrations. Better characterization of micronekton functional groups depending on relationships to environmental variables is useful for the management of marine resources, the conservation of biodiversity and a better understanding of climate change impacts. For this purpose, regionalization of global ocean into homogeneous provinces is an approach that is generating increasing interest. However, published regionalizations efforts (i) derived from environmental forcings, that do not specifically focus on micronekton and (ii) derived from acoustic backscatter, which do not allow direct estimates of micronekton biomass. Here, we propose to fill the gap between biophysical regionalizations and micronekton biomass. We notably defined biophysical biomes using global environmental variables known to affect micronekton: temperature of the epipelagic layer, temperature stratification, and net primary production (NPP). Six biophysical biomes were defined with a clustering method. A characterization of these biophysical biomes with simulated micronekton from the SEAPODYM-LMTL model displayed biome-specific relationships between biomass and the environmental variables used in the clustering (i.e. biomasses mostly structured by NPP and temperature). Biophysical biomes also displayed specific vertical structures suggested by modelled micronekton functional groups ratios. Then, a validation of biophysical biomes’ boundaries was performed to identify potential vertical structure reorganization in acoustic backscattering response from adjacent biomes. The regionalization identified homogeneous areas in terms of acoustic vertical structure, which were also different between adjacent biomes. Finally, a comparison with another biomes’ definition computed from micronekton biomasses suggested that environmental variables can account for only some of the variability of the micronekton structures.

Temporal and spatial variations in squid jigging catch efficiency in the Oyashio Extension region

AbstractThe abundance of fishery resources significantly impacts the ratio of fishing effort to harvest. However, traditional statistics of fishery catches or commonly used catch per unit effort (CPUE) metrics cannot accurately capture the complexity of resource abundance in the ocean. To address this issue, we propose here a novel approach that integrates the actual fishery catch from vessel logs with fishing duration obtained through automatic identification system (AIS) positioning. This combined analysis eliminates confounding factors and introduces a novel metric called “catch efficiency (CE)” to evaluate fishing operations more accurately, thereby reflecting resource abundance in a more reasonable way. In this study, we focus on the CE of squid in the Oyashio Extension region in the Northwestern Pacific. Our analysis reveals significant temporal and spatial variations of CE, manifesting in both intensity and distribution patterns. Moreover, our findings establish a close relationship between CE and background water mass distribution, chlorophyll‐a concentration, and micronekton biomass. This implies that the resource abundance of squid can be inferred by considering the varying environmental factors within the fishing area.

Using long‐term data series to design adequate protected areas that ensure the conservation of inconspicuous small petrel species

Abstract Marine protected areas (MPAs) are widely used tools for conservation and management. Their correct delimitation is challenging, especially when the target species are small, elusive and inconspicuous, as little data are generally available to adequately assess their distribution at sea. Therefore, currently designated MPAs may not effectively cover key areas for small seabirds, particularly during migration and wintering seasons. We used ensemble species distribution models (ESDMs) on a 15‐year time‐series data set of at‐sea census along the Atlantic Iberian arc to predict the potential distribution of the smallest European seabird, the European storm‐petrel (Hydrobates pelagicus), and compare it with official marine special protection areas (SPAs). Occurrence of European storm‐petrel was related to shifts in sea surface temperature, and to small distances from the coast over the continental shelf. Most relevant area for the species in the Atlantic Iberian arc was west‐central Portugal to north‐western coast of the Iberian Peninsula, with an additional key area in the Gulf of Cádiz. Both zones host significant SPAs, but they inadequately cover key areas for European storm‐petrels. Our findings support extending marine SPAs in the Atlantic Iberian arc to ensure their effective protection. The distribution of the species expands over the years, varying in both size and location. These changes might be attributed to dynamic oceanographic variables, such as sea surface temperature and biomass of micronekton, which seem to play a significant role in their foraging behaviour. Synthesis and applications. Our study highlights the importance of analysing long time series and ESDMs to design adequate protected areas, which ensure the conservation of small and highly mobile species such as storm petrels. Our results should be considered by decision‐makers to prioritise and update marine protected areas, while incorporating the dynamic nature of the ocean within an ecosystem‐based approach.

Species distribution modeling of deep‐diving cetaceans

AbstractSpecies distribution models (SDMs) have been developed and extensively validated for diverse cetaceans within the California Current Ecosystem off the West Coast of the United States. These studies have recognized the challenges associated with developing robust models for deep‐diving cetaceans—sperm whales and beaked whales—thus limiting the accuracy of predictions for management and ecological understanding. In this study, we explore whether additional biologically relevant predictor variables can improve models for deep‐divers. These variables are related to the oxygen minimum layer and phytoplankton and micronekton biomass and could influence prey availability for cetacean top predators. We found that the addition of these variables improved the performance of SDMs for sperm whales, as well as for some more common baleen whale and dolphin species, but that the accuracy of deep‐diver models was nevertheless poor. The sightings data sets for deep‐diving cetaceans have small sample sizes compared to other cetaceans, and sightings are distributed nearly randomly across the study area and model domain. These factors hinder the development of useful environmentally driven models of spatial distribution.

Near-island enhancement in mesopelagic micronekton assemblages off Hawaiʻi

The west coast of Hawaiʻi Island hosts elevated primary production compared to offshore waters and an abundance of large pelagic animals for reasons that are not entirely understood. Here we show that the nearshore environment off the west coast of Hawaiʻi exhibits an increased biomass and abundance of mesopelagic micronekton. Acoustic surveys from this study and prior work show a higher nautical area scattering coefficient (as a proxy for biomass) in a deep non-migratory layer in nearshore sites compared to offshore sites that is persistent over five years. Cobb trawl samples taken at the depths of the deep scattering layer (∼450 to ∼550 m) in 2016 and 2017 showed 1.3 to 2.2 times higher biomass and 2.7 times higher abundance nearshore (∼4 km from shore) compared to offshore (∼24 km from shore). Fishes dominated the trawl catches and a large fraction of the nearshore enhancements were due to Sternoptychidae and Serrivomeridae across both years. In contrast, Melamphaeidae consistently were more abundant and had greater biomass offshore. This deep scattering assemblage contrasts with the mesopelagic boundary layer assemblage by being nonmigratory and taxonomically different in composition. These nearshore mesopelagic enhancements could occur as the result of increased nearshore food supplies deriving from Island Mass Effect enhanced primary production or from advection and concentration in a complex flow environment in the lee of the island. Regardless of mechanism, this temporally persistent, high biomass, largely nonmigratory layer of mesopelagic micronekton is different from the better known migratory mesopelagic boundary layer assemblage and may provide food to deeper diving marine mammals and pelagic fishes possibly explaining the aggregation of large pelagic animals in this region.

Spatiotemporal variability of micronekton at two central North Pacific Fronts

The North Pacific Subtropical Frontal Zone (STFZ) seasonally aggregates economically important fish and protected species. The aggregation of top predators is hypothesized to be a response to convergent flow at the prominent thermohaline Subtropical Front (STF) in the STFZ and a sharp northward increase in primary productivity, the Transition Zone Chlorophyll Front (TZCF), which is thought to link primary productivity to top predators via secondary and tertiary consumers. Given existing data gaps in our knowledge on forage biomass, distribution, and composition in the area, characteristics of micronekton, forage for top predators, were investigated using in situ multi-frequency active acoustics from three springtime shipboard surveys conducted along the 158°W meridional. The effects of STF and TZCF on micronekton was accessed using in situ CTD profiles. Results of this study showed a significant positive effect of the STF on micronekton biomass. The acoustic data implied that the STF also acted as a boundary for the distribution of micronekton with differing taxonomic composition from south to its north. The TZCF, as well as Chl-a concentrations, did not show a significant effect on micronekton relative biomass or composition that might be due to effects of larger-scale variability masked in the in situ data. Contrary to expectation, significantly higher relative micronekton biomass was associated with higher temperatures, the mechanisms of which still need to be determined.

Seamount effects on micronekton at a subtropical central Pacific seamount

Seamounts are globally ubiquitous features with potential for increased biodiversity and biomass, including those of economically important fish. Although their ecological and economical importance is well known, the mechanisms for supporting seamount-associated communities are not well understood. In this study, the effects of an intermediate depth seamount (Cross Seamount) on the micronekton communities, forage for economically important bigeye tuna, are investigated. Relative biomass and composition estimates were calculated from multi-frequency active acoustic data from surveys over 3 years. Mean micronekton biomass was significantly higher than in the ambient environment and its composition differed over the flanks and plateau of Cross Seamount. The effects of the seamount extended ∼3.5 km away from the plateau's edge, possibly further below 400 m depth at the flanks. Micronekton occupied the water column from the surface to the 400 m deep plateau with dense aggregations immediately over the bottom at night. During the day, these micronekton migrated both horizontally and downward, occupying depths of 500–700 m, preferably along the upstream flank of the seamount. Descending micronekton from near-surface waters appeared to be temporarily blocked by the topography before swimming below the plateau at the flanks. Mechanisms supporting the increase in micronekton biomass are uncertain, although hydrographic data support topographic trapping of zooplankton and the existence of transient or semi-permanent Taylor caps.

Long-term dynamics of forage base and food supply for nekton in the upper epipelagic layer of the western Bering Sea. Part 1. Composition and abundance of zooplankton and small-sized nekton

An updated concept for the state, dynamics, and production of plankton communities in the upper epipelagic layer of the western Bering Sea is presented based on the timeseries for 1986–2020. The zooplankton biomass in summer exceeded the biomass in fall season in 1.3–2.1 times for the layer of 0–50 m and in 1.1–1.8 times for the layer of 0–200 m, mostly because of decreasing abundance of copepods and chaetognaths that was not compensated by slight increase of the euphausiids and amphipods biomass. Interannual variations were higher and reached 2–3 times and 4–5 times, respectively. Abnormal blooming of certain ecological groups of zooplankton (either warm-water or cold-water) occurred in the anomalous years. Species structure of zooplankton community varied in dependence on oceanographic conditions, generally toward higher abundance in warmer environments. In spite of considerable impact of thermal regime, this dependence was not close and even was absent in some cases that indicated a complex organization of zooplankton communities subjected to influence of many environmental factors. Mean total biomass of large-sized zooplankton and micronekton (prey for large-sized nekton) in the upper epipelagic layer of the western Bering Sea is estimated as 41 . 106 t in summer and 24 . 106 t in fall season, its total production as 101 . 106 t and 67 . 106 t, respectively. In summer, production of non-predatory zooplankton (phyto- and euryphages) prevailed over the predatory zooplankton production, with exception of 2009 and 2013. On the contrary, production of zoophages prevailed in autumn due to successive seasonal changes in the epipelagic plankton communities. These modern data on biomass and production of the zooplankton communities indicate significant reserves of food resources for fish and squids in the deep-water Commander Basin, western part of the Aleutian Basin, and in the area at Cape Navarin.

Macrozooplankton and micronekton diversity and associated carbon vertical patterns and fluxes under distinct productive conditions around the Kerguelen Islands

Mesopelagic communities are characterized by a large biomass of diverse macrozooplankton and micronekton (MM) performing diel vertical migration (DVM) connecting the surface to the deeper ocean and contributing to biogeochemical fluxes. In the Southern Ocean, a prominent High Nutrient Low Chlorophyll (HNLC) and low carbon export region, the contribution of MM to the vertical carbon flux of the biological pump remains largely unknown. Furthermore, few studies have investigated MM communities and vertical flux in naturally iron fertilized areas associated with shallow bathymetry. In this study, we assessed the MM community diversity, abundance and biomass in the Kerguelen Island region, including two stations in the HNLC region upstream of the islands, and two stations in naturally iron fertilized areas, one on the Plateau, and one downstream of the Plateau. The MM community was examined using a combination of trawl sampling and acoustic measurements at 18 and 38 kHz from the surface to 800 m. A conspicuous three-layer vertical system was observed in all areas - a shallow scattering layer, SSL, between 10 and 200 m; mid-depth scattering layer, MSL, between 200 and 500 m; deep scattering layer, DSL, between 500 and 800 m - but communities differing among stations. While salps ( Salpa thompsoni ) dominated the biomass at the productive Kerguelen Plateau and the downstream station, they were scarce in the HNLC upstream area. In addition, crustaceans (mainly Euphausia vallentini and Themisto gaudichaudii ) were particularly abundant over the Plateau, representing a large, although varying, carbon stock in the 0–500 m water layer. Mesopelagic fish were prominent below 400 m where they formed permanent or migrant layers accounting for the main source of carbon biomass. Through these spatial and temporal sources of variability, complex patterns of the MM vertical distribution and associated carbon content were identified. The total carbon flux mediated by migratory myctophids at the four stations was quantified. While this flux was likely underestimated, this study identified the main components and mechanisms of active carbon export in the region and how they are modulated by complex topography and land mass effects. • Macrozooplankton and micronekton communities are described in the Kerguelen area using trawl and acoustic • Despite different assemblages, a consistent three-layers system occurred in the first 800 m over the Kerguelen seascape • Salps dominated global biomass in productive areas but not in HNLC region while crustaceans were abundant on the Plateau • Myctophids dominated below 400 m as permanent or migrant layers accounting for the main source of carbon biomass • We estimate total carbon flux mediated by migratory myctophids in the contrasting productive areas

Ecological Trap or Favorable Habitat? First Evidence That Immature Sea Turtles May Survive at Their Range-Limits in the North-East Atlantic

Unusual environmental events can push marine animals outside their physiological tolerances through changes in trophic and/or thermal conditions. Such events typically increase the risk of stranding. Rescue Centers offer a unique opportunity to report animals in distress and satellite track rehabilitated individuals to identify potential new habitats and support an effective conservation of these endangered species. By combining sightings (1988–2020) and tracking data (2008–2020) collected along the French Atlantic and English Channel coasts, our study assessed if the Bay of Biscay is an ecological trap or a favorable habitat for immature sea turtles. The largest tracked individuals migrated westward to pelagic waters, likely toward their natal beaches, while smaller individuals remained within the Bay of Biscay (BoB) and crossed colder (mean: 17.8 ± 3.0°C) but more productive waters. The turtles’ directions differed from the ones of ocean currents, excluding a passive advection to these unexpected habitats. Although the BoB might be thermally unsuitable in winter, the higher micronekton biomass predicted in this region could offer a productive foraging habitat for immature turtles. However, the majority of the sightings referred to individuals stranded alive (75%), suggesting this area could also act as an ecological trap for the smallest individuals that are mostly reported in winter suffering cold-stunning. Assumed to be outside the species range, our results reveal a potential foraging ground in the North-East Atlantic for these young turtles, confirming the crucial role of the rehabilitation centers and the need to continue prioritizing conservation of these endangered species, particularly vulnerable at this stage and at such temperate latitudes.

Dynamics of currents and biological scattering layers around Senghor Seamount, a shallow seamount inside a tropical Northeast Atlantic eddy corridor

Biophysical interactions of the mesopelagic fauna and hydrodynamic features were investigated at a shallow, tall seamount in the tropical Northeast Atlantic based on data from three inter-disciplinary surveys collected between 2009 and 2015. Senghor Seamount is located close to the equator in a unique but underexplored oceanographic setting, where classical seamount effects are expected to be small. Shipboard Acoustic Doppler Current Profiler (ADCP) and remote sensing data revealed flow phenomena strongly varying over different temporal and spatial scales. The quasi-steady, longer period flow at the seamount was largely unchanged by flow-topography interaction with the exception of a persistent and recurring shadow zone of weaker currents downstream of the summit. Senghor Seamount is exposed to energetic mesoscale eddy activity in surrounding waters with potentially large physical and biological implications. At higher frequencies, tidal currents interacting with the seamount generated large-amplitude internal waves propagating horizontally and vertically away from the summit. Analysis of acoustic backscatter data and micronekton biomass displayed prominent near-surface night time and deep (>400 m) day time scattering layers associated with the diurnal vertical migration of the mesopelagic fauna. An intense and previously unreported aggregation of acoustic scatterers was detected in the depth range 150–300 m inside a radius of 3 km along the upper slopes of the seamount summit. ADCP measurements over a full semi-diurnal tidal cycle suggest that biophysical interactions were present in near-seafloor waters around the upper seamount as evidenced by plankton distributions that were modified by topographic blocking during descent and aggregation in waters characterised by internal waves and enhanced tidally-driven energy dissipation.

Population demographics and growth rate of Salpa thompsoni on the Kerguelen Plateau

This study aimed to obtain the first estimates of S. thompsoni population dynamics and growth rates over the Kerguelen Plateau (Southern Indian Ocean). Micronekton, including salps, were repeatedly sampled during late summer to early autumn (26th February – 15th March 2018) at contrasting hydrological stations on the Kerguelen Plateau in the southern proximity of Kerguelen Islands. At two stations, S. thompsoni made up almost half of the micronekton biomass. Environmental conditions were important in determining the density and development of S. thompsoni populations. Growth rates (0.5–7.0% d−1) were higher than previously reported from the Antarctic Peninsula (0.3–4.6% d−1) but lower than near the Antarctic Polar Front (APF; 3.7–20.7% d−1). Despite warm surface waters (4–5 °C), low chlorophyll a concentrations restricted the salp populations from growing as fast as populations near the APF. Because the Kerguelen Plateau region deflects a branch of warm water southward towards Antarctica, more studies of S. thompsoni population dynamics across multiple seasons are needed to fully understand their importance over the Kerguelen Plateau and their invasion potential into higher latitudes.

Structure and functioning of four North Atlantic ecosystems - A comparative study

The epi- and mesopelagic ecosystems of four sub-polar ocean basins, the Labrador, Irminger, Iceland and Norwegian seas, were surveyed during two legs from Bergen, Norway, to Nuuk, Greenland, and back to Bergen. The survey was conducted from 1 May to 14 June, and major results were published in five papers (Drinkwater et al., Naustvoll et al., Strand et al., Melle et al., this issue, and Klevjer et al., this issue a, this issue b). In the present paper, the structures of the ecosystem are reviewed, and aspects of the functioning of the ecosystems examined, focusing on a comparison of trophic relationships in the four basins. In many ways, the ecosystems are similar, which is not surprising since they are located at similar latitudes and share many hydrographic characteristics, like input of both warm and saline Atlantic water, as well as cold and less saline Arctic water. Literature review suggests that total annual primary production is intermediate in the eastern basins and peaks in the Labrador Sea, while the Irminger Sea is the most oligotrophic sea. This was not reflected in the measurements of different trophic levels taken during the cruise. The potential new production was estimated to be higher in the Irminger Sea than in the eastern basins, and while the biomass of mesozooplankton was similar across basins, the biomass of mesopelagic micronekton was about one order of magnitude higher in the western basins, and peaked in the Irminger Sea, where literature suggests annual primary production is at its lowest. The eastern basins hold huge stocks of pelagic planktivore fish stocks like herring, mackerel and blue whiting, none of which are abundant in the western seas. As both epipelagic nekton and mesopelagic micronekton primarily feed on the mesozooplankton, there is likely competitive interactions between the epipelagic and mesopelagic, but we're currently unable to explain the estimated ~1 order of magnitude difference in micronekton standing stock. The results obtained during the survey highlight that even if some aspects of pelagic ecosystems are well understood, we currently do not understand overall pelagic energy flow in the North Atlantic.

Editorial: Zooplankton and Nekton: Gatekeepers of the Biological Pump

Editorial on the Research Topic Zooplankton and Nekton: Gatekeepers of the Biological Pump Zooplankton and nekton organisms create and destroy particles in manifold ways. They feed on the diverse components of the plankton community and on detrital matter. They disaggregate these components, but also repackage them into fecal pellets. Zooplankton and nekton thereby contributes to the attenuation, but also to the export of vertically settling particles. Many zooplankton and nekton organisms also ascend to the surface layer of the ocean at dusk to feed during the dark hours, and return to midwater at the break of dawn. This diurnal vertical migration (DVM) shuttles organic matter from the surface ocean to deeper layers, where it is metabolized and excreted. This active flux (as opposed to the passive flux of sinking particles) can contribute substantially to the biological pump, the downward export of carbon and nutrients into the oceans interior. Due to their multiple roles in oceanic particle dynamics, zooplankton and nekton organisms can actually be considered the gatekeepers of the biological pump. Several articles in this Research Topic deal with the contribution of zooplankton and nekton-mediated active flux to the total export of organic matter. Using biomass and enzyme transport system (ETS) assessments of respiratory flux for both mesozooplankton and micronekton communities, Hernandez-Leon et al. estimated the total active transport of carbon (respiration, excretion, mortality, and egestion) along a transect in the Atlantic from the Canary Islands to Brazil. They found that active flux by these communities ranged from 25 to 80% of the total particulate organic carbon export at 150 m depth and that the importance of active flux increased with increasing surface productivity. Kwong et al. compared biomass, diel vertical migration, and active flux of mesozooplankton and micronekton across a range of mesoscale eddy structures along the east-coast of Australia during winter and spring. They found that although all eddy regimes had similar integrated biomass of mesozooplakton and micronekton, the organisms in the individual eddies had different migratory behavior, which resulted in contrasting importance of active flux. Kiko et al. assessed the impact of mesozooplankton DVM on elemental cycling at three stations in the Eastern Tropical North Atlantic. They found that approximately 31 to 41% of the total nitrogen loss from the upper 200 m of the water column was attributable to DVM mediated fluxes. They also suggest that gut flux-the flux created by migrators when they evacuate their gut at DVM-depth-and migrator mortality at DVM-depth contribute to an Intermediate Particle Maximum. In their study conducted in the Peruvian upwelling system (which features a severe midwater oxygen minimum zone), Kiko and Hauss concluded that the metabolic suppression

Influence of oceanic conditions in the energy transfer efficiency estimation of a micronekton model

Abstract. Micronekton – small marine pelagic organisms around 1–10 cm in size – are a key component of the ocean ecosystem, as they constitute the main source of forage for all larger predators. Moreover, the mesopelagic component of micronekton that undergoes diel vertical migration (DVM) likely plays a key role in the transfer and storage of CO2 in the deep ocean: this is known as the “biological pump”. SEAPODYM-MTL is a spatially explicit dynamical model of micronekton. It simulates six functional groups of vertically migrant (DVM) and nonmigrant (no DVM) micronekton, in the epipelagic and mesopelagic layers. Coefficients of energy transfer efficiency between primary production and each group are unknown, but they are essential as they control the production of micronekton biomass. Since these coefficients are not directly measurable, a data assimilation method is used to estimate them. In this study, Observing System Simulation Experiments (OSSEs) are used at a global scale to explore the response of oceanic regions regarding energy transfer coefficient estimation. In our experiments, we obtained different results for spatially distinct sampling regions based on their prevailing ocean conditions. According to our study, ideal sampling areas are warm and productive waters associated with weak surface currents like the eastern side of tropical oceans. These regions are found to reduce the error of estimated coefficients by 20 % compared to cold and more dynamic sampling regions.

Micronekton distribution in the southwest Pacific (New Caledonia) inferred from shipboard-ADCP backscatter data

Acoustic data are invaluable information sources for characterizing the distribution and abundance of mid-trophic-level organisms (micronekton). These organisms play a pivotal role in the ecosystem as prey of top predators and as predators of low-trophic-level organisms. Although shipboard-ADCP (acoustic Doppler current profiler) acoustic backscatter signal intensity cannot provide an absolute biomass estimate, it may be a useful proxy to investigate variability in the distribution and relative density of micronekton. This study used acoustic recordings data spread across 19 years (1999–2017) from 54 ADCP cruises in New Caledonia's subtropical EEZ (exclusive economic zone) to assess seasonal and interannual variabilities and spatial distribution of micronekton. The dataset was composed of two different ADCPs: 150 kHz for the first period, followed by 75 kHz for more recent years. We examined the 20–120 m averaged scattering layer. Using the few cruises with concurrent EK60 measurements, we proposed that the backscatter from the ADCPs and 70 kHz EK60 were sufficiently closely linked to allow the use of the backscatter signal from the ADCPs in a combined dataset over the full time series. We then designed a GAMM (generalized additive mixed model) model that takes into account the two ADCP devices as well as temporal variability. After accounting for the effect of the devices, we showed that the acoustic signal was mainly driven by diel vertical migration, season, year, and ENSO (El Niño-Southern Oscillation). In a second step, a consensus model between two statistical approaches (GAMM and SVM) (support vector machine) was constructed, linking the nighttime 20–120 m backscatter to the oceanographic and geographic environment. This model showed that sea surface temperature was the main factor driving backscatter variability in the EEZ, with intensified backscatter during the austral summer (December to May) in the northern part of the EEZ. We showed that acoustic density differed significantly, spatially and temporally from micronekton biomass predicted for the same period by the SEAPODYM-MTL (mid-trophic level) ecosystem model. The seasonal cycle given by ADCP data lagged behind the SEAPODYM-MTL seasonal cycle by around three months. Reasons to explain these differences and further needs in observation and modeling were explored in the discussion. In addition to providing new insights for micronekton dynamics in this EEZ (i.e., the science needed for ecosystem-based fisheries management), the data should help improve our ability to model this key trophic component.

Mesozooplankton and Micronekton Active Carbon Transport in Contrasting Eddies

Mesozooplankton (June 2015 and September 2017) and micronekton (September 2017) were sampled along the eastern coast of Australia. Depth stratified mesozooplankton and micronekton were collected using a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) and an International Young Gadoid Pelagic Trawl (IYGPT) equipped with an opening/closing codend. Sampling was undertaken at the center and edge of a frontal cold-core eddy (F-CCE center and edge) in 2015, and at the center of a cold-core eddy (B-CCE) and two warm-core eddies (R-WCE and WCE) in 2017. We assess the diel vertical structure, biomass and downward active carbon transport by mesozooplankton and micronekton in eddies. Total water column mesozooplankton and micronekton biomass did not vary substantially across water masses, while the extent and depth of diel vertical migration did. Using in-situ measurements of temperature and measurements of mesozooplankton and micronekton abundance and biomass, we estimated the contribution of respiration, dissolved organic carbon (DOC) excretion, gut flux and mortality to total downward active carbon transport in each water mass. Overall, active carbon transport by mesozooplankton and micronekton below the mixed layer varied substantially across water masses. We corrected estimates of micronekton migratory biomass and active carbon transport assuming 50% net efficiency. Specifically, inIn the R-WCE mesozooplankton remained within the mixed layer during the day and night; only 50% of the total micronekton population migrated below the mixed layer contributing to carbon transport, equating to 1.52.89 mg C m-2 d-1. Mesozooplankton actively transported 16.1 and 8.0 mg C m-2 d-1 at the F-CCE Center and Edge, respectively. Mesozooplankton and micronekton active carbon transport in the B-CCE were 5.4 and 0.40.74 mg C m-2 d-1, and in the WCE 88 and 6.713.4 mg C m-2 d-1. Differences in carbon export were dependent on food availability, temperature, time spent migrating and mixed layer depth. These findings suggest that under certain conditions mesoscale eddies can act as important carbon sinks.

Micronekton biomass distribution, improved estimates across four north Atlantic basins

Distribution of micronekton was investigated during early summer of 2013, using data from a cruise covering the central parts of four north Atlantic basins, the Norwegian Sea (NS), Iceland Sea (ICS), Irminger Sea (IRS), and Labrador Sea (LS). Continuous underway acoustics mapped vertical and horizontal distributions, and trawl sampling provided data on biomass and taxonomic composition. The hull mounted acoustics and trawl catches suggested that, among the four basins, biomass of epipelagic, larger nektonic species (>20 cm length) during the cruise was highest in the NS and ICS basins, while mesopelagic non-gelatinous micronekton biomass peaked in the IRS and LS basins. Biomass of Scyphozoa was also about 1 order of magnitude higher in IRS and LS compared to ICS and NS. In ICS and NS, crustaceans made up about 50% of total non-gelatinous micronekton biomass, with fish making up less than 20% of total biomass. In contrast, fish constituted more than 60% of non-gelatinous biomass of catches in IRS and LS. In catches from ICS and NS the myctophid Benthosema glaciale dominated the catches, whereas bathylagids, gonostomatids, barracudinas and stomiids contributed to the high biomass densities of fish in IRS and LS. In addition to the differences in biomass between the basins, the acoustic measurements suggested gradients within the north-eastern basins, and large differences in vertical distribution of biomass between the basins during the cruise.

Diel vertical migration of zooplankton and micronekton on the northern slope of the South China Sea observed by a moored ADCP

Acoustic Doppler current profiler (ADCP) echoes can be transformed into mean volume backscattering strength (MVBS), which is positively proportional to the biomass of zooplankton and micronekton. A mooring with an upward-looking 75-kHz ADCP was deployed on the northern slope of the South China Sea to monitor diel vertical migration (DVM) and vertical distributions of zooplankton and micronekton under general conditions and extreme conditions such as typhoons. DVM occurs throughout the year, with the maximum migrating speed reaching 9.0 cm s−1. It had some cyclic variation, which was in response to marine environment and astronomical influence. Light initiates the ascents and descents of the migrators, and migration intensity may be proportional to the solar altitude. Neap-spring tidal cycle and full moon phase influences are prominent, and the peak at 112 d may be driven by variation of currents. In addition, 270 m is the demarcation between upward and downward migrations; some organisms living below this layer, especially zooplankton and micronekton, migrate to the surface to obtain nutrients and energy; other marine mesopelagic organisms also survive in this manner. Super and severe typhoons reduced vertical migration, having less influence on the deep scattering layer. As Super Typhoon Rammasun passed by the mooring station, current speed increases and temperature decreases were synchronous with changes in the deep scattering layer; the migrators swam downward to evade the influence of a higher-speed current rather than swam upward to compensate for the decrease of temperature.

Variation in Zooplankton and Micro-Nekton Biomass in Response to Seawater Temperature Changes in the Central Bering Sea during Summer

Variation in Zooplankton and Micro-Nekton Biomass in Response to Seawater Temperature Changes in the Central Bering Sea during Summer