Distribution and diel vertical migration of mesopelagic fishes in the Southern Sargasso Sea — observations through hydroacoustics and stratified catches

Vertical distribution patterns and relative abundance of mesopelagic fish species and other major taxonomic groups were investigated through vertically stratified trawl sampling and hydroacoustic analyses along the subtropical convergence zone from 52° W to 70° W in the oligotrophic Sargasso Sea. Persistent stationary layers and several migrating components of different scattering characteristics were detected. The results reveal varying vertical migration patterns, including different times of onset of diel vertical migration in different depths and a migrant pathway emerging daily from the lower deep scattering layer (DSL) at dusk and migrating through the upper DSL without affecting its composition. Fish species identification was made based on morphological characteristics and confirmed by genetic barcoding analyses of subsamples. In total, 5022 fish specimens from 27 families, 62 genera and 70 species were caught. In terms of relative abundance (A) and biomass (M), catches were dominated by species of the families Myctophidae (A=59.1%, M=47.4% of total fish catch) and Melamphaidae (A=22.5%, M=17.1%). Myctophidae and Stomiidae were the most species-rich families with 31 and 12 species, respectively. Catches at the two easternmost stations were dominated by Scopelogadus mizolepis and Nannobrachium cuprarium, while Bolinichthys photothorax and Ceratoscopelus warmingii were the most abundant species in catches from the two westernmost stations. This study provides insights into distribution and vertical migration behaviour of mesopelagic fish in the Sargasso Sea and adds to our understanding of the mesopelagic community in this large oceanic area.

Circadian Rhythm in Feeding Behavior of Daphnia dentifera

Circadian rhythms enable organisms to mediate their molecular and physiological processes with changes in their environment. Although feeding behavior directly affects within-organism processes, there are few examples of a circadian rhythm in this key behavior. Here, we show that Daphnia have a nocturnal circadian rhythm in feeding behavior that corresponds with their diel vertical migration (DVM), an important life history strategy for predator and UV avoidance. In addition, this feeding rhythm appears to be temperature compensated, which suggests that feeding behavior is robust to seasonal changes in water temperature. A circadian rhythm in feeding behavior can impact energetically demanding processes like metabolism and immunity, which may have drastic effects on susceptibility to disease, starvation risk, and ultimately, fitness.

Vertical Fine-Scale Distribution of Calanus sinicus in the Yellow Sea Cold Water Mass During the Over-Summering Process

Calanus sinicus, a temperate copepod with a lethal temperature >27°C, is one of the key species in Chinese coastal marine ecosystems. The C. sinicus population increases in spring and declines in early summer annually due to increasing water temperature. Numerous C. sinicus individuals then congregate in the Yellow Sea Cold Water Mass (YSCWM) and remain under the thermocline from early summer to early autumn. Development and reproduction is halted in this cold and foodless bottom water and they avoid ascending to the hot surface water, which is regarded as an over-summering strategy. Based on discrete water sampling approaches, previous studies demonstrated that higher chlorophyll a (Chl a) levels appeared in the mixed hot surface water layer; however, the subsurface chlorophyll a maximum layer (SCML) has seldom been described. In the present study, various probes and a visual plankton recorder (VPR) were used to determine the fine vertical distributions of environmental factors and C. sinicus. VPR observations showed the ecological responses in fine scale and indicated that few C. sinicus individuals ascend at night, the main population preferred to remain below the SCML all day long. The results demonstrated that a constant thin SCML existed in the YSCWM area, and that the SCML location coincided with or was beneath the thermocline and halocline layers, where the temperature was suitable for C. sinicus. The relationship between abundance and Chl a, showed the diel vertical migration trend of C. sinicus to feed at night in the YSCWM area. In addition to temperature as a main influencing factor, dissolved oxygen concentrations and column depth were also influencing factors. Therefore, in addition to avoiding high surface temperature, energy supplement may be an important driving force confining the diel vertical migration of C. sinicus in the Yellow Sea in summer.

Short term fluctuating temperature alleviates Daphnia stoichiometric constraints

In this study, we analysed how short term temperature fluctuation interacts with nutrient limitation in the vertical migrating Daphnia commutata. We hypothesize that short term (daily) temperature fluctuation will alleviate nutrient limitation. We carried out experiments analysing growth rates, phosphorus and RNA content of D. commutate grown under four different temperature regimes and two P-limited conditions. Our experiments showed that individuals grown under fluctuating temperature grew more than at the mean temperature. We estimated the expected sizes for the 15 °C treatment based on the Q10 and for the fluctuating temperature treatment. These expected sizes for both treatments resulted well below the observed ones. The P and RNA content of individuals grown at 10 °C were significantly higher than those at 20 °C, and when individuals grown at 10 °C were translocated to 20 °C they exerted an increased growth rate. Our results suggest that, under a regime of diel vertical migration, the temperature alternation would allow migrating organisms to alleviate the effect of severe nutrient limitation maintaining population growth. Under a scenario of global warming, where epilimnetic temperatures will increase, lake temperature will interact with nutrient limitation for consumers, but, organisms may be able to face these changes if they can still regularly move from a cold hypolimnion to a warmer epilimnion.

Metazoans, migrations, and the ocean's biological carbon pump

Diel vertical migration of fish and other metazoans actively transports organic carbon from the ocean surface to depth, contributing to the biological carbon pump. Here, we use a global vertical migration model to estimate global carbon fluxes and sequestration by fish and metazoans due to respiration, fecal pellets, and deadfalls. We estimate that fish and metazoans contribute 5.2 PgC/yr (2.1-8.8PgC/yr) to passive export out of the euphotic zone. Together with active transport, we estimate that fish are responsible for 20% (9-29%) of global carbon export, and 32% (18-43%) of oceanic carbon sequestration, with forage and deep-dwelling mesopelagic fish contributing the most. This essential ecosystem service could be at risk from unregulated fishing on the high seas.

Diel Vertical migration of marine organisms and the biological carbon pump

<p>Diel Vertical Migration (DVM) is a key feature of pelagic and mesopelagic ecosystems, mainly driven by predator-prey interactions along a time-varying vertical gradient of light. Marine organisms including meso-zooplankton and fish typically hide from visual predators at depth during daytime and migrate up at dusk to feed in productive near-surface waters during nighttime. Specific migration patterns, however, vary tremendously, for instance in terms of residency depth during day and night. In addition to environmental parameters such as light intensity and oxygen concentration, the migration pattern of each organism is intrinsically linked to the patterns of its conspecifics, its prey, and its predators through feedbacks that are hard to understand—but important to consider.</p><p>DVM not only affects trophic interactions, but also the biogeochemistry of the world’s oceans.  Organisms preying at the surface and actively migrating vertically transport carbon to depth, contributing to the biological carbon pump, and directly connecting surface production with mesopelagic and demersal ecosystems.</p><p>Here, we present a method based on a game-theoretic trait-based mechanistic model that enables the optimal DVM patterns for all organisms in a food-web to be computed simultaneously. The results are used to investigate the contributions of the different food-web pathways to the active component of the biological carbon pump. We apply the method to a modern pelagic food-web (comprised of meso- and macro-zooplankton, forage fish, mesopelagic fish, large pelagic fish and gelatinous organisms), shedding light on the direct effects that different trophic levels can have on the DVM behaviours of each other. The model is run on a global scale to assess the carbon export mediated by different functional groups, through fecal pellet production, carcasses sinking and respiration.</p><p>Finally, the model output is coupled to an ocean inverse circulation model to assess the carbon sequestration potential of the different export pathways. Results indicate that the carbon sequestration mediated by fish is much more important than presently recognised in global assessments of the biological carbon pump. The work we present relates to contemporary ecosystems, but we also explain how it can be adapted to fit any pelagic food-web structure to assess the contribution of the active biological pump to the global carbon cycle in past ecosystems.</p>