Quantifying connectivity between mesophotic and shallow coral larvae in Okinawa Island, Japan: a quadruple nested high-resolution modeling study

Coral bleaching has recently been occurring extensively across the world’s oceans, primarily because of high water temperatures. Mesophotic corals that inhabit depths of approximately 30–150 m are expected to survive bleaching events and reseed shallow water corals afterward. In Okinawa, Japan, mesophotic coral ecosystems have been reported to serve as a refuge for preserving the genotypic diversity of bleaching-sensitive corals. The connectivity of larval populations among different habitats is a key element that determines the area to be conserved in desirable coral ecosystems. Because coral larvae are largely transported passively by ambient oceanic currents, particularly in the horizontal direction, numerical ocean circulation models greatly help to quantify connectivity with detailed spatiotemporal network structures. The present study aimed to quantify the short-distance connectivity of shallow and mesophotic coral larvae in reef areas on the northwest coast of Okinawa Island. To this end, a quadruple nested high-resolution synoptic ocean model at a lateral spatial grid resolution of 50 m was developed, which was capable of realizing detailed coastal currents influenced by complex nearshore topography, and coupled with an offline 3-D Lagrangian particle-tracking model. After validating the developed model, short-distance horizontal coral connectivity across reef areas on the northwest coast was successfully evaluated. The alongshore lateral connectivity had apparent asymmetry caused by depth-dependent horizontal currents, whereas the larvae spawned at shallow and mesophotic depths were reachable to each other. Such across-depth larval dispersal was attributable to the mixed-layer depth in the spawning period, viz., the boreal spring, which approximately coincides with the boundary between shallow and mesophotic coral, leading to the intensive vertical exchange of virtual larvae

Eddy-induced transport of the Kuroshio warm water around the Ryukyu Islands in the East China Sea

In this study, an oceanic downscaling model in a double-nested configuration was used to investigate the role played by the Kuroshio warm current in preserving and maintaining biological diversity in the coral coasts around the Ryukyu Islands (Japan). A comparison of the modeled data demonstrated that the innermost submesoscale eddy-resolving model successfully reproduced the synoptic and mesoscale oceanic structures even without data assimilation. The Kuroshio flows on the shelf break of the East China Sea approximately 150–200 km from the islands; therefore, eddy-induced transient processes are essential to the lateral transport of material within the strip between the Kuroshio and the islands. The model indicated an evident predominance of submesoscale anticyclonic eddies over cyclonic eddies near the surface of this strip. An energy conversion analysis relevant to the eddy-generation mechanisms revealed that a combination of both the shear instability due to the Kuroshio and the topography and baroclinic instability around the Kuroshio front jointly provoke these near-surface anticyclonic eddies, as well as the subsurface cyclonic eddies that are shed around the shelf break. Both surface and subsurface eddies fit within the submesoscale, and they are energized more as the grid resolution of the model is increased. An eddy heat flux (EHF) analysis was performed with decomposition into the divergent (dEHF) and rotational (rEHF) components. The rEHF vectors appeared along the temperature variance contours by following the Kuroshio, whereas the dEHF properly measured the transverse transport normal to the Kuroshio\u27s path. The diagnostic EHF analysis demonstrated that an asymmetric dEHF occurs within the surface mixed layer, which promotes eastward transport toward the islands. Conversely, below the mixed layer, a negative dEHF tongue is formed that promotes the subsurface westward warm water transport

Influences of the Kuroshio on Interisland Remote Connectivity of Corals Across the Nansei Archipelago in the East China Sea

For the preservation and protection of coral habitats along the Nansei Archipelago in the East China Sea, a submesoscale eddy‐resolving synoptic ocean model was developed based on the Regional Oceanic Modeling System coupled with a 3‐D Lagrangian particle tracking model. Millions of neutrally buoyant particles representing coral spawn and larvae were released from 19 major islands and one lagoon every spring from 2012 to 2015. The model results were compared to satellite data, in situ observation, and surface drifters to confirm reasonable agreement. The connectivity matrix across the archipelago was quantified using Lagrangian probability density functions of the modeled particle displacement. Most particles remained near the release areas, while some traveled long distances by the northeastward drifting Kuroshio, leading to notable interisland coral transport across the archipelago that promotes interisland connectivity. A possible mechanism was examined by analyzing the transition from coastal to pelagic transport of the particles released from the Yaeyama Islands, the southernmost area of the archipelago. The Kuroshio trapped the particles released from the northern coast of the islands with considerable temporal variability in the entrainment rate. By contrast, particles released from the southern coast are markedly affected by the eastward current around the release sites, which significantly reduces their entrainment in the Kuroshio and, thus, long‐distance transport. Some entrained particles were expelled abruptly from the Kuroshio, trapped by the southwestward drifting Kuroshio Counter Current developed between the Kuroshio and the archipelago, and subsequently transported eastward to the islands

Identification of coral spawn source areas around Sekisei Lagoon for recovery and poleward habitat migration by using a particle-tracking model

A massive coral bleaching event occurred in 2016 in the interior of Japan\u27s largest coral lagoon, the Sekisei Lagoon, located in the Kuroshio upstream region in southwestern Japan. Recovery of the coral lagoon will require the influx of coral spawn and larvae; therefore, it is important to identify and conserve source sites. A surface-particle-tracking simulation of coral spawn and larvae was used to identify source areas of coral spawn outside of the Sekisei Lagoon for potential recovery of the interior lagoon. The northern coastal zone of Iriomote Island, including Hatoma Island, was identified as a major source area. Hatoma Island was also identified as a key source for the Kuroshio downstream region and for aiding the poleward migration of coral habitat under ongoing global climate change, making it one of the most important source areas in the Nansei Archipelago

Dynamics of bacterial populations under the feast-famine cycles

Bacterial populations in natural conditions are expected to experience stochastic environmental fluctuations, and in addition, environments are affected by bacterial activities since they consume substrates and excrete various chemicals. We here study possible outcomes of population dynamics and evolution under the repeated cycle of substrate-rich conditions and starvation, called the "feast-famine cycle", by a simple stochastic model with the trade-off relationship between the growth rate and the growth yield or the death rate. In the model, the feast (substrate-rich) period is led by a stochastic substrate addition event, while the famine (starvation) period is evoked because bacteria use the supplied substrate. Under the repeated feast-famine cycle, the bacterial population tends to increase the growth rate, even though that tends to decrease the total population size due to the trade-off. Analysis of the model shows that the ratio between the growth rate and the death rate becomes the effective fitness of the population. Hence, the functional form of the trade-off between the growth and death rate determines if the bacterial population eventually goes extinct as an evolutionary consequence. We then show that the increase of the added substrate in the feast period can drive the extinction faster. Overall, the model sheds light on non-trivial possible outcomes under repeated feast-famine cycles

When to wake up? The optimal waking-up strategies for starvation-induced persistence.

Prolonged lag time can be induced by starvation contributing to the antibiotic tolerance of bacteria. We analyze the optimal lag time to survive and grow the iterative and stochastic application of antibiotics. A simple model shows that the optimal lag time can exhibit a discontinuous transition when the severeness of the antibiotic application, such as the probability to be exposed the antibiotic, the death rate under the exposure, and the duration of the exposure, is increased. This suggests the possibility of reducing tolerant bacteria by controlled usage of antibiotics application. When the bacterial populations are able to have two phenotypes with different lag times, the fraction of the second phenotype that has different lag time shows a continuous transition. We then present a generic framework to investigate the optimal lag time distribution for total population fitness for a given distribution of the antibiotic application duration. The obtained optimal distributions have multiple peaks for a wide range of the antibiotic application duration distributions, including the case where the latter is monotonically decreasing. The analysis supports the advantage in evolving multiple, possibly discrete phenotypes in lag time for bacterial long-term fitness

Effects of the Submesoscale Anticyclonic Eddies Induced by Kuroshio in East China Sea

ABSTRACT For maintaining biological diversity in the coral coasts around Ryukyu Islands, Japan, a role played by the adjacent Kuroshio warm current is anticipated to be necessary for larval and nutrient transport. In order to understand dynamics and mixing between Kuroshio and the islands, we develop a detailed ocean downscaling model around Ryukyu Islands in a doubly nested configuration using ROMS at horizontal resolutions down to 1km, forced by the assimilative JCOPE2 and JMA-GSM/MSM. The model successfully reproduces anticyclonic eddies that are significantly retained on the western side of the islands to promote lateral mixing in the area