Amatzia Genin

Flow enhances photosynthesis in marine benthic autotrophs by increasing the efflux of oxygen from the organism to the water

Worldwide, many marine coastal habitats are facing rapid deterioration due in part to human-driven changes in habitat characteristics, including changes in flow patterns, a factor known to greatly affect primary production in corals, algae, and seagrasses. The effect of flow traditionally is attributed to enhanced influx of nutrients and dissolved inorganic carbon (DIC) across the benthic boundary layer from the water to the organism however, here we report that the organism’s photosynthetic response to changes in the flow is nearly instantaneous, and that neither nutrients nor DIC limits this rapid response. Using microelectrodes, dual-pulse amplitude-modulated fluorometry, particle image velocimetry, and real time mass-spectrometry with the common scleractinian coral Favia veroni, the alga Gracilaria cornea, and the seagrass Halophila stipulacea, we show that this augmented photosynthesis is due to flow-driven enhancement of oxygen efflux from the organism to the water, which increases the affinity of the RuBisCO to CO2. No augmentation of photosynthesis was found in the absence of flow or when flow occurred, but the ambient concentration of oxygen was artificially elevated. We suggest that water motion should be considered a fundamental factor, equivalent to light and nutrients, in determining photosynthesis rates in marine benthic autotrophs.

Dynamics of temperature and chlorophyll structures above a seamount: an oceanic experiment

Three hydrographic surveys comprised of densely spaced XBT and CTD stations were conducted over Minami-kasuga Seamount, in the northwest Pacific (21°36’N, 143O38’E). A cold dome, similar to a Taylor column, was observed above the seamount top during the first survey. Uplifted isotherms penetrated to the lower euphotic zone and were associated with higher chlorophyll concentrations. Vertical displacement of uplifted isotherms decayed with elevation above the seamount, so that both temperature and chlorophyll anomalies were undetectable at depths less than 80 m. Relatively high chlorophyll concentrations in a layer from 80 m to 100 m depth formed a distinctive deep chlorophyll maximum (DCM) which was less well defined away from the seamount. Calculations based on the observed chlorophyll increase and on estimated phytoplankton growth rate suggested a minimal residence time of the cold dome on the order of several days. Zooplankton densities were also higher over the seamount top, both within and above the cold dome. No cold dome, chlorophyll increase, or high zooplankton biomass were detected above the seamount on the second and third surveys, carried out 2 and 17 days later, respectively. Mixing and deflections of isotherms occurred within a “boundary zone” around the seamount slope during the first and third surveys. Our observations suggest that seamounts are a source of both biological and physical patchiness in the surrounding Ocean as features developed above them are swept away. The importance of the seamount-generated “experiment” is discussed in relation to field studies of the DCM. Specifically, our observations suggest that a sharp chlorophyll maximum can be formed by enhanced in situ growth following a sub-surface upwelling event.

Herbivory in Asymbiotic Soft Corals

A zooxanthellae-free soft coral from the Red Sea feeds almost exclusively on phytoplankton, a mode of nutrition so far unknown for corals. Herbivory was also found in three other azooxanthellate soft corals. In tropical oligotrophic waters, phytoplankton biomass density may be an order of magnitude higher than that of zooplankton. Use of this resource allows these azooxanthellate cnidarians to be highly productive in flow-exposed oligotrophic reef waters.

The use of a Canadian (perspective) grid in deep-sea photography

The construction and characteristics of a Canadian (perspective) grid as applicable for oblique deep-sea photographs are described. The implementation of such a grid through an automated digitizing system allow fast accurate and precise calculations of sizes and densities of animals and other objects. Changes in the cameras inclination due to “nose up” or “nose down” motion over rough topography can cause erroneous calculations of size. The magnitude of this error increases quasi-exponentially with distance of objects from the lower edge of the image and is particularly large for camera systems with a small angle of inclination. The area in a photograph from which accurate counts can be taken, should be determined a posteriori for each type of object (e.g. species), based on a plot of density vs distance from the camera.

A coral reef refuge in the Red Sea.

The stability and persistence of coral reefs in the decades to come is uncertain due to global warming and repeated bleaching events that will lead to reduced resilience of these ecological and socio-economically important ecosystems. Identifying key refugia is potentially important for future conservation actions. We suggest that the Gulf of Aqaba (GoA) (Red Sea) may serve as a reef refugium due to a unique suite of environmental conditions. Our hypothesis is based on experimental detection of an exceptionally high bleaching threshold of northern Red Sea corals and on the potential dispersal of coral planulae larvae through a selective thermal barrier estimated using an ocean model. We propose that millennia of natural selection in the form of a thermal barrier at the southernmost end of the Red Sea have selected coral genotypes that are less susceptible to thermal stress in the northern Red Sea, delaying bleaching events in the GoA by at least a century.

Zooplankton patch dynamics: daily gap formation over abrupt topography

Abstract Net tow and acoustic surveys of zooplankton distributions were made over and around Sixtymile Bank (110 km southwest of San Diego, California). Gaps devoid of vertically migrating zooplankton were formed every evening above the summit of the bank. Interactions between the migrating animals, their predators, physical advection and the local topography appear to determine the gap formation and dynamics. Gaps were transported downstream during the night and appeared to disintegrate slowly through vertical swimming behavior, current shear and mixing processes. Patch dynamics following gap formation, mediated by both ocean currents and animal behavior, should augment the spatial heterogeneity of zooplankton and affect marine food webs in areas where abrupt topography features are common.

Thermally Driven Exchanges between a Coral Reef and the Adjoining Ocean

Abstract In this paper hydrographic observations made over a fringing coral reef at the northern end of the Gulf of Aqaba near Eilat, Israel, are discussed. These data show exchange flows driven by the onshore–offshore temperature gradients that develop because shallow regions near shore experience larger temperature changes than do deeper regions offshore when subjected to the same rate of heating or cooling. Under heating conditions, the resulting vertically sheared exchange flow is offshore at the surface and onshore at depth, whereas when cooling dominates, the pattern is reversed. For summer conditions, heating and cooling are both important and a diurnally reversing exchange flow is observed. During winter conditions, heating occupies a relatively small fraction of the day, and only the cooling flow is observed. When scaled by ΔV, the observed profiles of the cross-shore during cooling velocity collapse onto a single curve. The value of ΔV depends on the convective velocity scale uf and the bottom s...

Distribution of epibenthic megafauna and lebensspuren on two central North Pacific seamounts

Abstract The abundance, composition and spatial distribution of megafaunal communities and lebensspuren assemblages at three sites on two deep seamounts in the central North Pacific were surveyed photographically using still cameras mounted on the research submersible Alvin . Photographic transects were made on the summit cap (∼1500 m depth) and summit perimeter (∼ 1800 m depth) of Horizon Guyot and on the summit cap (∼3100 m depth) of Magellan Rise. The summit caps of both seamounts were covered with foraminiferal sand, while the summit perimeter of Horizon Guyot was characterized by numerous rock outcroppings (basalt and chert encrusted with ferromanganese oxides) on which was situated a speciose assemblage of suspension-feeding organisms. The most abundant megafauna at all three sites were large, sediment-agglutinating protists belonging to the class Xenophyophorea. Among the three sites, the Horizon Guyot summit cap supported the highest densities of fishes and lebensspuren and the fewest echinoderms, while the Magellan Rise summit cap was populated by a diverse community of deposit-feeding echinoderms. Megafaunal abundances on Horizon Guyot were lower than those at equivalent depths on the western North Atlantic continental slope, while those on Magellan Rise were higher. The faunal differences observed between the two seamounts were attributed primarily to differences in hydrodynamic conditions, substrate availability and nutrient availability. Most of the lebensspuren on these seamounts appeared to be patchily distributed on spatial scales of 10–1000 m, while xenophyophore distributions were predominantly random on the same spatial scales. Biogeographically the species identified exhibited predominantly widespread to cosmopolitan distributions with Indo-West Pacific faunal affinities, typical of other seamounts in the same depth range and biogeographic province.

Effects on growth and reproduction of the coral Stylophora pistillata by the mutualistic damselfish Dascyllus marginatus

Although coral dwelling fishes are common on coral reefs, the nature of their effect on the host corals is poorly understood. The present study, conducted in the Gulf of Eilat (Red Sea) between July 1989 and August 1990, demonstrated that the branching coral Stylophora pistillata (Esper) benefits, in two components of coral fitness, from the presence of the damselfish Dascyllus marginatus (Rüppell), an obligate coral dweller. The growth rate of damselfish-inhabited corals was significantly higher than that of corals without damselfish. This was observed, using two growth assessment methods, in long-term (>7 mo) comparisons between: (1) corals where the damselfish were experimentally removed versus corals with unaltered fish groups; and (2) naturally inhabited versus non-inhabited corals. The presence of damselfish did not affect the corals specific (per surface area) reproductive output, whether it was assessed by the number of female gonads per polyp or by the number of planulae released cm-2 surface area d-1. However, the more rapid increase in branch size in damselfish-inhabited corals resulted in an apparent increase in the total reproductive output, with age, in growing corals. These findings demonstrate that the association between the damselfish D. marginatus and its host coral, S. pistillata, is mutualistic.

Tidal currents and anticyclonic motions on two North Pacific seamounts

Near-bottom currents were measured for several days at three sites on the summits of Fieberling Guyot (32°26′N, 127°46′W) and Horizon Guyot (19°15′N, 160°00′W). Three moorings comprised of two current meters were deployed on each summit; two moorings were deployed on opposite sides of the rim of the summit and one mooring was deployed near the center of the summit. The observed currents were strong, with maximum speeds of 48 and 24 cm s−1 on Fieberling and Horizon, respectively. The currents at specific frequencies were enhanced relative to those in the surrounding ocean. Diurnal currents were the dominant component of the current field on Fieberling Guyot. They accounted for 39–68% of the energy and had amplitudes around 12 cm s−1. We suspect that these diurnal currents were waves trapped over the seamount. Semidiurnal internal tidal currents were the strongest currents over Horizon Guyot, with amplitudes around 4 cm s−1. The flow patterns determined in this study seemed to affect the biological and geological characteristics of the seamounts.