Fine-scale planktonic habitat partitioning at a shelf-slope front revealed by a high-resolution imaging system

Ocean fronts represent productive regions of the ocean, but predator-prey interactions within these features are poorly understood partially due to the coarse-scale and biases of net-based sampling methods. We used the In Situ Ichthyoplankton Imaging System (ISIIS) to sample across a front near the Georges Bank shelf edge on two separate sampling days in August 2010. Salinity characterized the transition from shelf to slope water, with isopycnals sloping vertically, seaward, and shoaling at the thermocline. A frontal feature defined by the convergence of isopycnals and a surface temperature gradient was sampled inshore of the shallowest zone of the shelf-slope front. Zooplankton and larval fishes were abundant on the shelf side of the front and displayed taxon-dependent depth distributions but were rare in the slope waters. Supervised automated particle counting showed small particles with high solidity, verified to be zooplankton (copepods and appendicularians), aggregating near surface above the front. Salps were most abundant in zones of intermediate chlorophyll-a fluorescence, distinctly separate from high abundances of other grazers and found almost exclusively in colonial form (97.5%). Distributions of gelatinous zooplankton differed among taxa but tended to follow isopycnals. Fine-scale sampling revealed distinct habitat partitioning of various planktonic taxa, resulting from a balance of physical and biological drivers in relation to the front.Keywords: Shelf edge, Georges Bank, Fronts, Gelatinous, Zooplankton, Larval fishes, Diatom aggregates, Predator–prey, Salp

Larval fishes utilize Batesian mimicry as a survival strategy in the plankton

Marine teleost fishes often experience over 99% mortality in the early life stages (eggs and larvae), yet larval survival is essential to population sustainability. Marine fish larvae from a wide range of families display elaborate, delicate features that bear little resemblance to adult forms and hinder their swimming escape ability by increasing drag. Here, we systematically examine the criteria needed for Batesian mimicry to evolve as a survival strategy and present new evidence from in situ imaging technology and simulation modelling to support the hypothesis that many larval morphological features (particularly long, delicate fin rays) and behaviors evolved at least in part through Batesian mimicry of less palatable or noxious gelatinous zooplankton. Many of these organisms (e.g. hydromedusae, ctenophores, and siphonophores) are much more abundant than previously recognized. The high predation mortality during the larval phase provides strong potential for selection in favor of maintaining complex and metabolically costly features that mimic gelatinous zooplankton, provided that larger fishes, as selective visual predators, can occasionally be fooled. We conclude that recent advances in our understanding of mimicry combined with information obtained from plankton imaging supports the hypothesis that Batesian mimicry is a widespread survival strategy for larval fishes, which could have broad implications for fish population dynamics. However, further research is needed in the areas of predator cognition and larval fish behavior in the presence of different predators and models to elucidate the circumstances in which the larval fish mimicry hypothesis may apply.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Inter-Research. The published article can be found at: http://www.int-res.com/journals/meps/meps-home/Keywords: Survival, Plankton, Larval fish, Batesian mimicry, Recruitment, Gelatinous zooplankto

Ocean convergence and the dispersion of flotsam

Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km2 converged into a 60 × 60 m region within a week, a factor of more than 105 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s−1 and 0.01 ms−1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material

CALYPSO 2019 Cruise Report: field campaign in the Mediterranean

This cruise aimed to identify transport pathways from the surface into the interior ocean during the late winter in the Alborán sea between the Strait of Gibraltar (5°40’W) and the prime meridian. Theory and previous observations indicated that these pathways likely originated at strong fronts, such as the one that separates salty Mediterranean water and the fresher water in owing from the Atlantic. Our goal was to map such pathways and quantify their transport. Since the outcropping isopycnals at the front extend to the deepest depths during the late winter, we planned the cruise at the end of the Spring, prior to the onset of thermal stratification of the surface mixed layer.Funding was provided by the Office of Naval Research under Contract No. N000141613130

In situ ichthyoplankton imaging system ( I

Over the last two decades, there has been an accelerating advancement of acoustic and optical plankton samplers, opening many opportunities for fine‐scale studies of plankton distribution. To date, however, the imaging systems have been limited in the volume of water being sampled, thereby restricting their utility to quantifying highly abundant, small zooplankton like copepods, but not relatively rarer, larger ichthyo‐ and other meso‐zooplankton (e.g., larval decapods, salps, pteropods, ctenophores, etc.). Here we describe an imaging system, In situ ichthyoplankton imaging system (ISIIS), that is capable of In situ (i.e., noninvasive) sampling of sufficiently large volumes of water at very high resolution, allowing quantitative measurement of these rare plankton, while at the same time also recording the smaller more abundant taxa. Capitalizing on state‐of‐the‐art digital line scan cameras and high‐throughput computer data transfer and storage, combined with shadow photographic lighting techniques, we have designed and built a towed system capable of imaging at 68‐micron pixel resolution, yet with up to a 20‐cm depth of field (with a 14‐cm field of view). This system is coupled with various environmental sensors (e.g., CTD, fluorometer), enabling the evaluation of fine‐scale, taxon‐specific distributions in relation to environmental conditions. Field testing demonstrated high‐resolution imagery of plankters, while quantitatively imaging >70 L s−1 continuously for a 78‐min trial

In situ ichthyoplankton imaging system (ISIIS): system design and preliminary results

Over the last two decades, there has been an accelerating advancement of acoustic and optical plankton samplers, opening many opportunities for fine‐scale studies of plankton distribution. To date, however, the imaging systems have been limited in the volume of water being sampled, thereby restricting their utility to quantifying highly abundant, small zooplankton like copepods, but not relatively rarer, larger ichthyo‐ and other meso‐zooplankton (e.g., larval decapods, salps, pteropods, ctenophores, etc.). Here we describe an imaging system, In situ ichthyoplankton imaging system (ISIIS), that is capable of In situ (i.e., noninvasive) sampling of sufficiently large volumes of water at very high resolution, allowing quantitative measurement of these rare plankton, while at the same time also recording the smaller more abundant taxa. Capitalizing on state‐of‐the‐art digital line scan cameras and high‐throughput computer data transfer and storage, combined with shadow photographic lighting techniques, we have designed and built a towed system capable of imaging at 68‐micron pixel resolution, yet with up to a 20‐cm depth of field (with a 14‐cm field of view). This system is coupled with various environmental sensors (e.g., CTD, fluorometer), enabling the evaluation of fine‐scale, taxon‐specific distributions in relation to environmental conditions. Field testing demonstrated high‐resolution imagery of plankters, while quantitatively imaging >70 L s−1 continuously for a 78‐min trial

Larval Fishes Utilize Batesian Mimicry as a Survival Strategy in the Plankton

Marine teleost fishes often experience over 99% mortality in the early life stages (eggs and larvae), yet larval survival is essential to population sustainability. Marine fish larvae from a wide range of families display elaborate, delicate features that bear little resemblance to adult forms and hinder their swimming escape ability by increasing drag. Here, we systematically examine the criteria needed for Batesian mimicry to evolve as a survival strategy and present new evidence from in situ imaging technology and simulation modelling to support the hypothesis that many larval morphological features (particularly long, delicate fin rays) and behaviors evolved at least in part through Batesian mimicry of less palatable or noxious gelatinous zooplankton. Many of these organisms (e.g. hydromedusae, ctenophores, and siphonophores) are much more abundant than previously recognized. The high predation mortality during the larval phase provides strong potential for selection in favor of maintaining complex and metabolically costly features that mimic gelatinous zooplankton, provided that larger fishes, as selective visual predators, can occasionally be fooled. We conclude that recent advances in our understanding of mimicry combined with information obtained from plankton imaging supports the hypothesis that Batesian mimicry is a widespread survival strategy for larval fishes, which could have broad implications for fish population dynamics. However, further research is needed in the areas of predator cognition and larval fish behavior in the presence of different predators and models to elucidate the circumstances in which the larval fish mimicry hypothesis may apply

A Coupled Asymmetrical Multiple Opening Closing Net with Environmental Sampling System

Recruitment levels of fishes are potentially related to the abundance of larval fishes and their food source. A system that could allow for the concurrent investigation of fine-scale distribution of fish larvae and their potential prey could add significantly to the understanding of the early life history of marine fishes. A coupled Multiple Opening Closing Net and Environmental Sensing System (MOCNESS) that combines two sub-systems (1 m2 and 4 m2 net sets) working in synchronization was designed to answer these questions. The mesh size was different on each set of nets allowing the collection of a broad size range of organisms while optimizing the catch of larger fish larvae and eliminating unnecessary large samples of zooplankton. Moreover, the system eliminated the need to deploy separate MOCNESS using different mesh sizes, thus reducing ship time costs, and avoiding any aliasing associated with trying to sample the same water mass with separate nets fished sequentially. The system has been used at sea under varying weather conditions onboard the R/V F. G. Walton Smith and sampled adequately

Larval assemblages of large and medium-sized pelagic species in the Straits of Florida

Critical gaps in our understanding of the distributions, interactions, life histories and preferred habitats of large and medium-size pelagic fishes severely constrain the implementation of ecosystem-based, spatially structured fisheries management approaches. In particular, spawning distributions and the environmental characteristics associated with the early life stages are poorly documented. In this study, we consider the diversity, assemblages, and associated habitat of the larvae of large and medium-sized pelagic species collected during 2 years of monthly surveys across the Straits of Florida. In total, 36 taxa and 14,295 individuals were collected, with the highest diversity occurring during the summer and in the western, frontal region of the Florida Current. Only a few species (e.g. Thunnus obesus, T. alalunga, Tetrapturus pfluegeri) considered for this study were absent. Small scombrids (e.g. T. atlanticus, Katsuwonus pelamis, Auxis spp.) and gempylids dominated the catch and were orders of magnitude more abundant than many of the rare species (e.g. Thunnus thynnus, Kajikia albida). Both constrained (CCA) and unconstrained (NMDS) multivariate analyses revealed a number of species groupings including: (1) a summer Florida edge assemblage (e.g. Auxis spp., Euthynnus alleterattus, Istiophorus platypterus); (2) a summer offshore assemblage (e.g. Makaira nigricans, T. atlanticus, Ruvettus pretiosus, Lampris guttatus); (3) an ubiquitous assemblage (e.g. K. pelamis, Coryphaena hippurus, Xiphias gladius); and (4) a spring/winter assemblage that was widely dispersed in space (e.g. trachipterids). The primary environmental factors associated with these assemblages were sea-surface temperature (highest in summer-early fall), day length (highest in early summer), thermocline depth (shallowest on the Florida side) and fluorescence (highest on the Florida side). Overall, the results of this study provide insights into how a remarkable diversity of pelagic species spatially and temporally partition spawning within a region that is characterized by dynamic oceanography and strong habitat gradients

Fine-scale planktonic habitat partitioning at a shelf-slope front revealed by a high-resolution imaging system

Ocean fronts represent productive regions of the ocean, but predator–prey interactions within these features are poorly understood partially due to the coarse-scale and biases of net-based sampling methods. We used the In Situ Ichthyoplankton Imaging System (ISIIS) to sample across a front near the Georges Bank shelf edge on two separate sampling days in August 2010. Salinity characterized the transition from shelf to slope water, with isopycnals sloping vertically, seaward, and shoaling at the thermocline. A frontal feature defined by the convergence of isopycnals and a surface temperature gradient was sampled inshore of the shallowest zone of the shelf-slope front. Zooplankton and larval fishes were abundant on the shelf side of the front and displayed taxon-dependent depth distributions but were rare in the slope waters. Supervised automated particle counting showed small particles with high solidity, verified to be zooplankton (copepods and appendicularians), aggregating near surface above the front. Salps were most abundant in zones of intermediate chlorophyll-a fluorescence, distinctly separate from high abundances of other grazers and found almost exclusively in colonial form (97.5%). Distributions of gelatinous zooplankton differed among taxa but tended to follow isopycnals. Fine-scale sampling revealed distinct habitat partitioning of various planktonic taxa, resulting from a balance of physical and biological drivers in relation to the front. •Calculated a particle solidity metric to determine the dominant components of particle counts•Small zooplankton aggregated near the surface, especially in the vicinity of the front.•Larval fishes were abundant inshore and at the front but were extremely rare offshore.•Isopycnals appeared to limit the distribution of many zooplankton groups