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Gitai Yahel, PhD | Marine ecology

Yahel@Ruppin.ac.il | W. +972 (9) 830 4110 | H. +972 (77) 7800 637 |  Skype: gitaiyahel

Research interests Publication list Published In preparation Reports and other Conference

Functional convergence of microbes associated with temperate marine sponges Ribes, M., Jiménez  E., Yahel G., López-Sendino P., Diez B., Massana, R., Sharp J.H., and Coma R. Environmental Microbiology, in press Most marine sponges establish a persistent association with wide arrays of phylogenetically and physiologically diverse microbes. To date, the role of these symbiotic microbial communities in the metabolism and nutrient cycles of the sponge-microbe consortium remains largely unknown. We identify and quantify the microbial communities associated with three common Mediterranean sponge species, Dysidea avara, Agelas oroides and Chondrosia reniformis (Desmospongiae) that cohabitate in the coralligenous community. For each sponge we also quantified its particulate diet and the uptake and release of dissolved organic carbon, phosphate, dissolved organic nitrogen (DON) and carbon (DOC), and inorganic nitrogen. Low microbial abundance (<6% of the 30 tissue occupied by microbes) and no evidence for DOC uptake or nitrification was found for D. avara. In contrast O. oroides and C. reniformis showed high microbial abundance (30% and 70% of their tissue occupied by microbes, respectively) and both species exhibited high nitrification and high DOC and NH4^+ uptake. Surprisingly, these unique metabolic pathways were mediated in each sponge species by a different, and host specific, microbial community. The results point to a functional convergence of microbial consortia in some sympatric sponge species suggesting that these metabolic processes may be of special relevance to the success of the holobiont The sponge pump: the role of current induced flow in the design of the sponge body plan Leys, S.P., G. Yahel, M. A. Reidenbach, V. Tunnicliffe, U. Shavit, and H. M. Reiswig PLOS ONE 6(12): e27787. doi:10.1371/journal.pone.0027787 Sponges are suspension feeders that use flagellated collar-cells (choanocytes) to actively filter a volume of water equivalent to many times their body volume each hour. Flow through sponges is thought to be enhanced by ambient current, which induces a pressure gradient across the sponge wall, but the underlying mechanism is still unknown. Studies of sponge filtration have estimated the energetic cost of pumping to be <1% of its total metabolism implying there is little adaptive value to reducing the cost of pumping by using “passive” flow induced by the ambient current. We quantified the pumping activity and respiration of the glass sponge Aphrocallistes vastus at a 150 m deep reef in situ and in a flow flume; we also modeled the glass sponge filtration system from measurements of the aquiferous system. Excurrent flow from the sponge osculum measured in situ and in the flume were positively correlated (r>0.75) with the ambient current velocity. During short bursts of high ambient current the sponges filtered two-thirds of the total volume of water they processed daily. Our model indicates that the head loss across the sponge collar filter is 10 times higher than previously estimated. The difference is due to the resistance created by a fine protein mesh that lines the collar, which demosponges also have, but was so far overlooked. Applying our model to the in situ measurements indicates that even modest pumping rates require an energetic expenditure of at least 28% of the total in situ respiration. We suggest that due to the high cost of pumping, current-induced flow is highly beneficial but may occur only in thin walled sponges living in high flow environments. Our results call for a new look at the mechanisms underlying current-induced flow and for reevaluation of the cost of biological pumping and its evolutionary role, especially in sponges. Resuspension by fish facilitates the transport and redistribution of coastal sediments Katz T., Yahel G., Reidenbach M., Tunnicliffe T., Herut B., Crusius J., Whitney F., and Lazar B. (Limnol. Oceanogr., in press) Oxygen availability restricts groundfish to the oxygenated, shallow margins of Saanich Inlet, an intermittently anoxic fjord in British Columbia, Canada. New and previously reported ^210Pb measurements in sediment cores compared with flux data from sediment traps indicate major focusing of sediments from the oxygenated margins to the anoxic basin seafloor. We present environmental and experimental evidence that groundfish activity in the margins is the major contributor to this focusing. Fine particles resuspended by groundfish are advected offshore by weak bottom currents, eventually settling in the anoxic basin. Transmittance and sediment trap data from the water column show that this transport process maintains an intermediate nepheloid layer (INL) in the center of the Inlet. This INL is located above the redox interface and unrelated to water density shifts in the water column. We propose that this INL is shaped by the distribution of groundfish (as resuspension sources) along the slope and hence by oxygen availability to these fish. We support this conclusion with a conceptual model of the resuspension and off-shore transport of sediment. This fish-induced transport mechanism for sediments is likely to enhance organic matter decomposition in oxygenated sediments and its sequestration in anoxic seafloors. Glass sponge reefs as a silicon sink Chu J.W.F., Maldonado M., Yahel G., Leys S.P. Mar. Ecol. Prog. Ser. (411, 1-14, 2011) Glass sponge reefs concentrate large amounts of biological silicon (Si) over relatively small areas of the seafloor. We examined the role of glass sponges in biological silicon (Si) cycling by calculating a Si budget for 3 glass sponge reefs (Howe, Fraser, and Galiano) in the Strait of Georgia (SOG), British Columbia, Canada. The main reef-forming glass sponge Aphrocallistes vastus is heavily silicified, with 80% of its dry weight composed of biogenic silica (bSi). We used a combination of field sampling and surveys with a remote-operated vehicle to estimate the volume, mass, and bSi content of the reefs. BSi content ranged from 7 to 11 kg m^−2 among reefs, amounting to a total of 915 t of bSi locked in the exposed portion of the 3 reefs. Water column measurements of dissolved Si (dSi) indicated that the SOG is a region of high dSi, with average dSi concentrations of 50 µmol l^−1 in waters over the reefs. The skeletons of glass sponges showed very little dissolution after 8 mo immersion in seawater, as determined by changes in dSi in samples and scanning electron microscopy of the spicules. In contrast, diatom frustules, the main source of bSi in surface waters of the SOG, were ~200 times more soluble. Our calculations of Si flux suggest that glass sponge reefs can equate to 65% of the dSi reservoir (3.6 × 10^9 mol Si) in the SOG and represent a substantial Si sink in the continental shelf waters of the northeastern Pacific Ocean. In situ size-independent retention of phytoplankton and bacteria by the tropical bivalve Lithophaga simplex Gitai Yahel, Dominique Marie, Peter G. Beninger, Shiri Eckstein, and Amatzia Genin Aquatic Biology (6, 235-246) Few feeding studies have been performed on tropical bivalves, and in situ feeding studies are lacking altogether. We investigated retention efficiencies for natural particles in the coral-boring tropical mytilid Lithophaga simplex. Using the in-situ InEx technique (Yahel et al. L&O Methods, 3, 46-58, 2005) SCUBA divers carefully collected samples from the water inhaled and exhaled by undisturbed bivalves at the coral reef of Eilat (Gulf of Aqaba). Particle retention efficiencies were determined using flow cytometry analysis of the paired water samples. The photosynthetic bacterium Synechococcus (0.9±0.1 µm) and larger eukaryotic algae (1-10 µm) were preferentially retained by the bivalve with removal efficiencies of up to 90% (1996-2000 average 69±14% and 60±17%, respectively, N =74 individual bivalves). The minute photosynthetic bacterium Prochlorococcus (size 0.4±0.1 µm) was also moderately retained (41±19%). Only a small proportion of the non-photosynthetic bacteria (0.3±0.1 μm) were retained (5±18%, median 9%), despite their numerical dominance in the plankton and considerable size overlap with Prochlorococcus. Size-independent preferential retention was also observed within particle types: (1) L. simplex more efficiently retained Prochlorococcus and picoeukaryotic algae cells with higher chlorophyll content, and (2) the small fraction of non-photosynthetic bacteria retained did not differ in size, but had higher nucleic acid content (compared to the inhaled population) an indication for viable and active bacteria. We conclude that particle retention is not strictly size - dependent in L. simplex, and probably involves other cell attributes such as cell surface properties and/or motility. Home Intense benthic grazing of phytoplankton by the coral reef community Amatzia Genin, Stephen G. Monismith, Matthew A. Reidenbach, Gitai Yahel , and Jeffrey R. Koseff Limnology and Oceanography (54, 938–951) This paper presents a study of grazing on phytoplankton by a section of the fringing coral reef located in the Gulf of Aqaba near Eilat, Israel. To make in-situ measurements of grazing rates, we used the control volume approach common in engineering. We sampled phytoplankton biomass concentrations using 4 arrays of that defined an imaginary box extending from the bed to the surface, the control volume. To compute fluxes of materials in and out of the box, we made velocity measurements made at the center of the control volume using an ADCP and a set of ADVs. While phytoplankton patchiness and other turbulent variations in concentration introduced significant uncertainty into our results, mass balances of fluxes into and out of the box indicate a flow-dependent grazing rate of ca. 10 to 20 m d^-1 for the Eilat reef. This value is somewhat larger (but not statistically different) from a value of ca. 3 m d^-1 derived from a priori estimates of grazing rates given the observed benthic community and a value of 4 m d^-1computed from the observed near-bed concentration gradient. Our measurements show the importance of time averaging of sampled concentrations to reduce uncertainty Home Groundfish overfishing, diatom decline and the marine silica cycle – Lessons from Saanich Inlet, Canada and the Baltic Sea cod crash Timor Katz, Gitai Yahel, Ruthy Yahel, Verena Tunnicliffe, Barak Herut, Paul Snelgrove and  Boaz Lazar Global Biogeochem. Cycles, 23, GB4032 In this study, we link groundfish activity to the marine silica cycle suggesting that the drastic mid 1980s crash of the Baltic Sea cod population triggered the observed decrease in dissolved silica (DSi) and diatom abundance in the water. We suggest that this seemingly unrelated sequence of events was caused by a marked decline in sediment resuspension due to reduced groundfish activity resulting from the cod crash. In a study in Saanich Inlet, BC, Canada, we discovered that, by resuspending bottom sediments, groundfish triple DSi fluxes from the sediments and reduce silica accumulation therein. Using these findings and the available oceanographic and environmental data from the Baltic Sea, we estimate that overfishing and recruitment failure of Baltic cod reduced by 20% the DSi supply from bottom sediments to the surface water leading to a decline in the diatom population in the Baltic Sea. The major importance of the marginal ocean in the marine silica cycle and the associated high population density of groundfish suggest that groundfish play a major role in the silica cycle. We postulate that the dwindling groundfish populations caused by anthropogenic perturbations, e.g. overfishing and bottom water anoxia, may cause shifts in marine phytoplankton communities. Home Fish activity, a major mechanism for nutrient and carbon recycling from coastal marine sediments Gitai Yahel, Ruthy Yahel, Timor Katz, Boaz Lazar, Barak Herut and Verena Tunnicliffe (2008) Marine Ecology Progress Series 372, 195-209 Sediments underlying deep ocean basins accumulate carbon, nutrients and many trace compounds and contaminants.  In a northeast Pacific basin, we find that organic mater remineralization and the redistribution of bottom sediments are substantially affected by the resuspension activity of benthic fish.  Fish rework >40% of the seabed daily with over 100 disturbances m^-2 day^-1.  This resuspension activity reduces organic carbon sequestration by 230 mg C m^-2 day^-1, equivalent to ~1/2 of its downward flux and enhances near-bottom turbidity, oxygen demand, and nutrients regeneration.  To date, these processes are missing from geochemical models.  Exploitation of ground fish stocks is likely to have a significant effect on global geochemical cycles. Home Size independent selective filtration of ultraplankton by hexactinellid glass sponges Gitai Yahel, Dafne I. Eerkes-Medrano, and Sally P. Leys (2006) Aquatic Microbial Ecology 52,428-440 Selective feeding is an important mechanism by which animals can maximize their energy gain, avoid harmful or toxic food items, and optimized their diet. Selective predation by flagellates and ciliated protists is a major mechanism shaping the structure of microbial communities in pelagic habitats. Microbial communities at the benthic boundary layer take advantage of enhanced flux of organic materials but at the same time are exposed to enhanced predation by benthic suspension feeders. Our understanding of the functioning of this "benthic-pelagic loop" is yet limited but sponges are considered to be key grazers in many benthic habitats. We studied the feeding preferences of two of the most common glass sponges of the North-East Pacific coast, Rhabdocalyptus dawsoni and Aphrocallistes vastus. Sponges were maintained in large darkened tanks supplied with running sea water from the nearby fjord. The water inhaled and exhaled by the sponges was simultaneously sampled and analyzed using a flow cytometer. Both sponges showed a similar (but not identical) feeding pattern, efficiently removing up to 99% of the most abundant bacteria cells whereas clays, silts and debris particles were expelled into the exhaled water. Surprisingly, filtration efficiencies were maximal for the relatively large and rare eukaryotic algae (3-5 µm, 86 ± 9%) and for small non photosynthetic bacteria (<0.4 µm, 89 ± 10%) while intermediate size non-photosynthetic bacteria characterized with higher nucleic acid content were efficiently removed in February (92±3%) when overall plankton concentration was low but not in July (28 ± 16%). The intermediate size photosynthetic prokaryote Synechococcus (1.1-1.5 µm) was also less preferred. Based on detailed analysis of the ultra-structure of the glass sponge filtration apparatus we argue that the selective filtration we observed involved individual processing, recognition, sorting, and transport of each particle through the sponge syncytial tissue. Selective grazing by glass sponges, like their protozoans pelagic counterparts can potentially be an important mechanism shaping the microbial communities in the deep water of North-East Pacific fjords. Home In situ feeding and metabolism of glass sponges (Hexactinellida, Porifera) studied in a deep temperate fjord with a remotely operated submersible. Yahel G., Whitney F., Reiswig H.M., Eerkes-Medrano D.I., and Leys S.P.  (2007) Limnol. Oceanogr. 52,428-440 Glass sponges are conspicuous inhabitants of the deep benthic community of temperate Northeast Pacific fjords. Using the Canadian remotely operated submersible ROPOS and a new device, the SIP, for clean sampling of the water inhaled (incurrent) and exhaled (excurrent) by the sponges, we measured in situ the removal of total organic carbon (TOC) and living cells, as well as the excretion of nitrogenous waste, by two species of glass sponge at 120-160 m. At the deep fjord habitat, ambient water resembled true deep-sea water with low oxygen concentrations (<2 mL L^-1), high dissolved silica (>55 µmol L^-1), and low organic content (TOC<53±6 µmol L^-1). The turbid water (<35% m^-1 transmissivity) contained high suspended sediment load (7.1±0.9 mg L^-1) and the organic content of the seston was low (<2%). Both Aphrocallistes vastus and Rhabdocalyptus dawsoni were effective bacteriovores, removing up to 95% of the bacteria (median removal was 79% for both species). Nano heterotrophic protists (4-10 µm) were also removed at similar efficiencies and contributed a significant proportion (~30%) of carbon to the sponges’ nutrition. Ultra plankton (<10 µm) removal (2.2±1.3 µmol L^-1) accounted for the entire TOC uptake and ammonia excretion (0.20±0.13 µmol L^-1) by the sponge with no evidence for dissolved organic uptake. Despite the massive siliceous skeleton of both sponges, silica uptake was below detection levels (0.28 µmol L^‑1). This low silica uptake supports previous suggestions of low growth rates in Hexactinellida. Our results suggest that glass sponges are efficient filter feeders, selectively removing the scarce microbial cells from a 'soup' of suspended clay and detritus particles. Reported sponge abundances of up to 250 individuals 10 m^-3 suggest that the filtering activity of these organisms could significantly impact benthic-pelagic mass exchange in NE Pacific fjords. Home Environmental Impacts of air-gun surveys on Glass Sponges PI: Tunnicliffe V. and Chapman R. | Draft prepared by Yahel G. and Wimut M.  (2007) Ministry of Energy and Mines and University of Victoria, Social Science and Science Partnerships Air-gun surveys associated with the oil and gas exploration in the Queen Charlotte Basin will insonify the seafloor with broadband, high intensity noise, thereby exposing the unique glass sponge reef systems of that area to acoustic impacts. We used the opportunity offered by an NSERC funded research cruise to the Fraser Ridge glass sponge reef to conduct a brief, and preliminary, study of the immediate biological response of one of the major reef building glass sponges (Aphrocallistes vastus) to acoustic vibrations. The measure of response in these animals was reduction in their feeding currents. These currents were measured in the exit orifice by deployed instruments that had no physical contact with the animals. The sound source was a small, surface air-gun (164 cm^3) that generated average sound exposure levels of 151 dB re μPa^2s^-1 at the sponge location. The air-gun signal was distinct from background noise below 300 Hz. The question of whether the sponge’s excurrent flow responded to the pressures from the series of air-gun shots was addressed statistically. The sponge response to ambient conditions was compared to the excurrents measured at the air-gun shot times. For this single sample of 16 air-gun shots, the statistical analysis indicated that there was little or no evidence that the acoustic pressure from the air-gun influenced the physiological functions. The experimental work carried out here was very challenging, and many factors were difficult to control. First, since the ambient flow speed accounted for ~ 70% of the observed variation in the excurrent flow, it was impossible to separate this factor from the analysis. Second, the complexity of the approach constrained the sample size and duration of the observations. Daylight hours, marine mammal intrusion, preparation time for the ROV (Remote Operated Vehicle) and instrument malfunction all became confounding factors. Nonetheless, the experimental study is novel and the statistical approach is a benchmark for future studies. Further laboratory work and field studies are necessary before meaningful conclusions can be drawn. New research should make use of more realistic sound levels, and study recovery times, habituation and longer term effects on tissue and skeletal integrity. Home Boundary layer turbulence and flow structure over a fringing coral reef Reidenbach M.A., Monismith S.G., Koseff J.R., Yahel G., and Genin A. (2006) Limnol. Oceanogr. 51, 1956-1968 Hydrodynamics control key processes in coral reef ecology, from advective transport of food and larvae, through turbulence-dependent mass fluxes across the boundary layer over single organisms. Here we report on the flow structure and its interaction with the bed roughness over a fringing coral reef in the Gulf of Aqaba, Red Sea. Measurements were made at two fore-reef sites and a nearby sandy slope. The shear velocity, u* and drag coefficient, CD were determined directly from turbulent Reynolds stresses measured using Acoustic Doppler Velocimeters. Values of CD for the coral substrates ranged from 0.009 to 0.015, three to five times greater than the sandy bottom. The turbulence dissipation rate, ε, was determined by fitting spectra of vertical velocity to the theoretical "5/3" law expected for the inertial subrange of turbulence. A local balance was found between production and dissipation of turbulent kinetic energy (TKE). This equilibrium signified that we could estimate u* from the either the mean velocity profile, turbulence, or dissipation rate of TKE. Estimates from all three measures agreed well, indicating that existing turbulent boundary layer flow theory can be applied to flows over the rough topography of coral reefs. This has important implications for both numerical modeling of the flow and estimating dispersion rates over reef systems. The results also suggest that the bottom topography, by enhancing both reef scale and local drag and mixing levels, allows reef biota to more effectively exchange dissolved and particulate matter with oceanic waters.   Home Phytoplankton grazing by epi- and in-fauna inhabiting exposed rocks in coral reefs Yahel G., Zalogin T., Yahel R., and Genin A. (2005) Coral Reefs 125, 153-163 Exposed rocks with no visible macro-fauna are abundant in all coral reefs. Depletion of phytoplankton cells and pigments by the minute crypto fauna inhabiting the outer few centimeters of such rocks was experimentally studied over an annual cycle in the Gulf of Aqaba, Red Sea. Different substrata were introduced into small (3.6 L), well mixed, tanks that were fed by running seawater pumped directly from the reef at a rate of 11±1 L hr^-1. A steady-state reduction in phytoplankton abundance and chlorophyll a concentration of 38±26% (mean ± 1 SD) was found for untreated rocks but not for sand, gravel, or killed controls. Average areal clearance rate by untreated rocks was 17.3±8.0 ml cm^-2 hr^-1. Conservative extrapolation of this rate to the whole reef community, suggests that the fauna inhabiting exposed rocks clears 2.1± 0.9 m^3 m^-2 d^-1 at Eilat. Phytoplankton removal by untreated rocks varied from 1.5 ng chlorophyll a cm^-2 hr^-1 during the oligotrophic summer conditions to 6 ng chlorophyll a cm^-2 hr^‑1 during the spring bloom. These values correspond to a potential nitrogen gain of 1.3 and 5.2 mmol N m^-2 day^-1, respectively. Cryptic reef-rock fauna can have a key role in the biogeochemical functioning of coral reef communities. Home Thermally driven exchanges between a coral reef and the adjoining ocean Monismith S.G., Genin A., Reidenbach, M.A., Yahel G., and Koseff J.R. (2006) Journal of Physical Oceanography 36, 1332-1347 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 slope through the inertial scaling, ΔV ~β^1/3uffirst proposed by Phillips in the 1960s as a model of buoyancy-driven flow in the Red Sea. However, it is found that turbulent stresses associated with the long shore tidal flows and unsteadiness due to the periodic nature of the buoyancy forcing can act to weaken the sheared exchange flow. Nonetheless, the measured exchange flow transport agrees well with previous field and laboratory work. The paper is concluded by noting that the “thermal siphon” observed on the Eilat reef may be a relatively generic feature of the nearshore physical oceanography of reefs and coastal oceans in general. Home Fouling reefal communities on artificial reefs: Does age matter? Perkol-Finkel S., Shashar N., Barneah O., Ben-David-Zaslow R., Oren U., Reichart T., Yacobovich T., Yahel G., Yahel R., and Benayahu Y. (2005) Biofouling 21, 127-140 Man-made submerged structures, including shipwrecks, offering substrata for fouling organisms and fish, have been classified secondarily as artificial reefs (ARs). The current approach in AR design is that of low-profile structures placed on the seabed and attempting to mimic natural reef (NR) communities with the aim of mitigating degraded marine ecosystems. To examine the validity of this concept, a long-term comparison of the developing AR fouling communities to those of nearby NRs is required. A survey of the fouling reefal organisms was conducted on seven shipwrecks (Red Sea, Egypt), comprising three young (ca 20 years old) and four old (4100 years old) unplanned ARs, in comparison to nearby NR communities. The hypothesis tested was that the age of the ARs shapes the structure of their fouling coral communities. The results demonstrated distinct differences between ARs and NRs and between young and old ARs. While the species composition on ARs may resemble that of NRs after approximately 20 years, obtaining a similar extent of coral cover may require a full century. Moreover, differences in structural features between ARs and NRs may lead to differences in species composition that persist even after 100 years. Home “InEx” – a direct in situ method to measure filtration rates, nutrition and metabolism of active suspension feeders Yahel G., Marie D., Hadas E., and Genin A.  (2005) Limnol. Oceanogr. Methods 3, 46-58 Sponges, bivalves and tunicates play an important role in the trophic dynamics of many benthic communities. However, accurate in situ measurements of their filtration and excretion rates are lacking. Our knowledge of these rates is based on indirect, artifact-prone, mostly in vitro measurements.  This paper presents and evaluates an in situ, non-intrusive technique to directly measure the rate and efficiency by which an active suspension feeder removes (or discharges) substances from (to) the water it filters. The technique, termed “InEx”, is based on the simultaneous, pair-wise collection of the water Inhaled and Exhaled by the animal. The difference in the concentrations of a substance among a pair of samples provides a measure of the retention (or excretion) of the substance by the animal. Calculations of feeding (or excretion) rates are obtained by multiplying the concentration difference by pumping rate. The latter is concurrently measured by recording the movement of a dye front in a transparent tube positioned within the ex-current jet. An important quality of the InEx technique is the lack of any manipulation of the studied organisms thus allowing realistic estimates of the organism’s performance under natural conditions.  Preliminary results showing the diet composition, feeding rates and removal efficiencies of some coral reef sponges, bivalves and tunicates are presented and discussed. Home Diel pattern with abrupt crepuscular changes of zooplankton over a coral reef Yahel R., Yahel G., and Genin (2005) Limnol. Oceanogr. 50, 930-944 Zooplankton abundance and emergence patterns were studied over coral reefs in the Gulf of Aqaba using high resolution acoustics (multi-beam imaging sonar, FTV, 1.6MHz and WH600 ADCP, 614 KHz), emergence traps, net tows, and underwater pump arrays. These measurements were supplemented with field observations on feeding behavior of zooplanktivorous fish. The zooplanktonic community over coral reefs is a complex and highly dynamic mixture of pelagic plankton advected into the reef, larvae and eggs released by benthic animals, and demersal plankton. At sunset (±4 min, 95% confidence interval) acoustic back-scattering intensity and zooplankton biomass started to increase rapidly. In contrast to this ascent, the pre-dawn decline began earlier during the dark (82±5 min before sunrise, mean±95% confidence interval) and terminated in complete darkness 43±8 min before sunrise. This light dependent diel behavior was highly consistent throughout the year, regardless of seasonal and environmental changes. Direct sampling indicated that smaller zooplankters (500-700 µm) ascended first and that demersal zooplankton accounted for most of the increase in zooplankton soon after sunset. Surprisingly, the emergence of zooplankton occurred while large schools of daytime zooplanktivorous fish were still foraging in the water column. However, at that time the fish’ predatory efficiency is greatly reduced and corals do not yet expand their tentacles. We suggest that the early emergence of reef zooplankton utilizes a “window of opportunity” when nocturnal predatory risk by sessile zooplanktivores (e.g., corals) is still low and the efficiency of visual predators is already reduced. Home Near-bottom depletion of zooplankton over coral reefs: I. Diurnal dynamics and size distribution Yahel R., Yahel G., and Genin A. (2005) Coral Reefs 24, 75-85 Changes in the near-bottom abundance of zooplankton on scales of centimeters to meters and hours to seasons are of great importance to corals and other benthic zooplanktivores. Our objective was to characterize such spatio-temporal changes over several coral reefs in the Gulf of Aqaba, Red Sea. Using arrays of underwater pumps, we found a substantial depletion of zooplankton near the bottom. Vertical gradients in zooplankton abundance were steeper during the night than day, mostly due to a greater nocturnal increase in zooplankton biomass higher in the water column. On average, the layer <1 m above bottom (mab) was depleted by 2.6±2.2 mg m^-3(46±35%) and 1.4±1.4 mg m^-3 (37±43%) during night and day, respectively. A long time series of bi-weekly samples at 0.5 mab, lasting 1.5 yrs, indicated a doubling of the biomass during the night with no apparent seasonality. The diel change was due to an increase in the abundance of only large (>200 µm) zooplankters around dusk and their disappearance in the morning. Diurnal predation by zooplanktivorous fish, sediment resuspension by benthivorous fish and zooplankton behavior appear to control the dynamics of suspended particles over the reef, creating sharp vertical gradients and a remarkable diel cycle in the ratio between nutritious plankton and inorganic particles. Home In situ feeding and element removal in the symbiont-bearing sponge Theonella swinhoei: Bulk DOC is the major source for carbon Yahel G., Sharp J.H., Marie D., Häse C., and Genin A. (2003) Limnol. Oceanogr. 48 141-149. The vast majority of organic matter in the world ocean is found in the dissolved pool.  However, no evidence has been demonstrated for direct uptake of bulk dissolved organic matter (DOM) by organisms other than bacteria and some invertebrate larvae. The total organic carbon (TOC) is 10-30% higher in coral reefs than in adjacent open waters. The dissolved organic carbon (DOC) accounts for > 90% of the TOC.  Using a new in situ technique for clean sampling of the seawater inhaled and exhaled by benthic suspension feeders we measured directly the removal of DOC in the symbiont-bearing reef sponge Theonella swinhoei. The sponge removed up to 26% (12±8%, mean±1SD) of the TOC (dissolved and particulate) from the water it filtered during a single passage through its filtration system.  Most of the carbon gained by the sponge was from the dissolved pool (10±7 µmol C liter^-1), an order of magnitude greater than the carbon gained from the total living cells (phytoplankton and bacteria) the sponge removed (2±1 µmol C liter^-1).  In T. swinhoei, over two thirds of the sponge biomass consists of symbiotic bacteria, likely having an important role in the DOC uptake. Our findings indicate that the role of DOC in metazoan nutrition as well as the role of metazoans in DOC cycling may have been grossly underestimated. Home Daily Cycles of Suspended Sand at Coral Reefs: A Biological Control Yahel R., Yahel G., and Genin A. (2002) Limnol. Oceanogr. 47: 1071-1083. Sediment resuspension has a major ecological role in many coastal habitats, lakes and marine environments. Waves and currents are generally thought to be the major control on sediment resuspension. In this study, the temporal and spatial distributions of Suspended Sand Concentrations (SSAC) were measured at five coral reefs in the northern Gulf of Aqaba, Red Sea, by directly filtering seawater through 100 µm mesh nets. Levels of near-bottom SSAC were twice as high during the day (4.8 – 9.7 mg m^-3) than at night (2.3 – 5.2 mg m^-3). Vertical profiles showed a sharp increase of SSAC (2-6 folds) toward the bottom during the daytime, but not at night. The contributions of wind, waves and currents to variations in SSAC were non-significant except during rare southern storms. Temporal and spatial changes in the level of SSAC during the day corresponded with the abundance of benthivorous fish and their activity pattern. Resuspension activities by fish numbered >1.5 resuspension events m^-2 h^-1 and were confined to daytime. An in situ experiment in which fish were excluded from a large section (250 m^2) of the reef resulted in elimination of the day - night differences and a significant decrease in daytime SSAC over the treated reef section in comparison to the open, control sites. This study is the first report of a daily pattern of suspended sediment at coral reefs with fish activity as it’s causative agent. Home Intense benthic grazing on phytoplankton in coral reefs revealed using the control volume approach Genin A., Yahel G., Reidenbach M.A., Monismith S.G., Koseff J.R. (2002) Oceanography 15, 90-96. A major objective of biogeochemical studies of coral reefs is to quantify fluxes of particulate and dissolved matter between the reef and overlying waters. However, direct measurements of these fluxes are hard to obtain due to the typically small concentration changes as the water flows over the bottom and due to shear, turbulent mixing and concentration gradients characterizing benthic boundary layers. Using state-of-the-art underwater technology, we were able to apply the “Control Volume” approach to measure in situ phytoplankton grazing on a scale of a whole coral-reef community. The results indicate that the import of carbon and nutrients via this grazing is a major , previously underestimated, trophic pathway in coral reefs. The amount of phytoplankton grazed by 1 m ^2 of reef is similar to the total phytoplankton produced in the entire water column of the surrounding sea under 1 m ^2 of sea-surface. The import of allochthonous nutrients into the reef via this grazing balances the downstream leak of dissolved nutrients. Physically, the flow over the rough topography of the reef produces enhanced turbulence, enabling high grazing rates to be sustained, while on larger scales, the exchange between the offshore ocean and the reef is supported by buoyancy- driven flows. With the advent of underwater technology, the control volume technique is no longer limited to unique situations (e.g. closed lagoons, shallow flats), but should be generally applicable for measurements of benthic-pelagic fluxes in oceans and lakes. Home Phytoplankton distribution and grazing near coral reefs Yahel G., Post A.F., Fabricius K.E., Marie D., Vaulot D., and Genin A. (1998) Limnol. Oceanogr. 43, 551-563. Depletion of phytoplankton cells and pigments over coral reefs were studied in the Gulf of Aqaba, Red Sea, during 1994-1996. Phytoplankton abundance and chlorophyll a concentration were 15-65% lower near the reefs than in the adjacent open waters. The decrease in chlorophyll near the reef was typically associated with an increase in the concentration of its degradation products, the phaeopigments.  The steepest slope of these cross-shore gradients occurred within 1-3 m above bottom. Over 50% of the variation in the extent of the chlorophyll gradients, but not of phaeopigments, could be explained by the advection of water during 2 hrs preceding the transect and by the concentration of chlorophyll a in the open water. No cross-shore gradients were observed at a sandy-bottom site without reef. Eukaryotic phytoplankton (<5 mm) contributed >70% of the total depleted carbon near the reef during winter, while the cyanobacterium Synechococcus (1 mm) contributed the largest share in summer. The proportions of different taxa in depleted fractions were similar to those in ambient waters, indicating no size selectivity. Direct measurements of phytoplankton removal rates were made in water passing through a unique 5 m-long perforated reef, dominated by herbivorous soft corals. The waters downstream of that reef were strongly depleted of phytoplankton (10 to >36%, or 32 to >100 ng chlorophyll a l^-1). When converted to carbon fluxes, these rates greatly exceeded reported values of carbon input to coral reefs via zooplankton predation. Phytoplankton grazing is an important component of benthic-pelagic coupling in coral reefs. Home In situ depletion of phytoplankton by an azooxanthellate soft coral Fabricius K.E., Yahel G., and Genin A. (1997) Limnol. Oceanogr. 43, 354-356. Abstract - The in situ removal of phytoplankton by the soft coral Dendronephthya hemprichi was investigated by taking small-scale measurements of chlorophyll concentrations around colonies on a reef of the Northern Red Sea. The chlorophyll concentration downstream of a 0.75 m deep colony thicket was depleted by 6.4% (± 1.4% SE) compared with the water upstream. Neighboring organisms are thus exposed to water, which is significantly depleted of phytoplankton. A 0.75 x 0.1 x 0.1 m passage of actively feeding colonies removed 34 mg carbon per day from the water, equivalent to ~ 1.3 times their respiratory carbon demand. Rates of algae intake were also estimated by determining the decrease in gut fluorescence in starved colonies. The in situ depletion technique showed a three to six-fold greater sensitivity compared with the gut fluorescence technique, and should be preferred as a technique for estimating feeding rates. Home Suspension feeding in coral reefs - what about dissolved matter? Yahel G. (1998) Isr. J. Zool. 44, 90-91. Benthic grazing on phytoplankton is a principal trophic pathway in shallow, temperate coastal habitats. In tropical reefs, on the other hand, studies of benthic-pelagic coupling have focused on zooplankton, rather than phytoplankton, as the principal source of allochthonous prey. Strong phytoplankton grazing is to be expected at the reef, as numerous members of the coral-reef community are known to feed on particles within the size range of phytoplankton. Conceptual considerations also support expectations for intense phytoplanktivory at the reef, as phytoplankton biomass in coral reefs commonly exceeds that of zooplankton by an order of magnitude. Furthermore, the bottom topography in coral reefs is typically rough, with numerous living and non-living objects having a high “slenderness ratio” (height over width), which is an optimal morphology for feeding on small suspended particles. Direct evidence showing that reef dwellers actually feed on phytoplankton are nevertheless scarce and the role of phytoplankton as a source for allochthonous carbon and nutrients in coral reefs is poorly understood. Moreover, the role of Dissolved Organic Carbon (DOC) as food for the guild of benthic suspension feeders is virtually unknown. Recently we have found that the boundary layer over coral reefs in the Red Sea, a few meters in thickness, is significantly depleted (up to 60%) of phytoplankton, compared with off-shore waters, and that DOC is substantially higher near the reef. In order to identify the major phytoplankton grazers at the reef, we have developed a technique to sample, in situ, uncontaminated exhaled waters and to measure pumping rates of active suspension feeders. Using this method we have measured the diet composition, pumping rates and clearance efficiencies for seven most conspicuous members of the active suspension feeders guild in the coral reefs of Eilat, Israel. A flow cytometer was to used  measure concentrations of phytoplankton and bacteria, a Flow Injection Analyzer was used to measure the concentrations of NH3 and NO2, and a high temperature TOC analyzer was used to measure concentrations of DOC. Diet composition, filtration efficiency and pumping rates were nearly uniform within taxa but vary considerably between taxa. Synechococcus was the most efficiently removed prey by all taxa (55-95% removal efficiency). Heterotrophic bacteria were efficiently removed by sponges (60-88%) but not by bivalve or ascidians (<15%). During the spring bloom eukaryotic algae (2-5mm) were the most abundant phytoplankton (biomass wise) at the study site, and although being less efficiently removed by suspension feeders, the carbon gained from these relatively large cells exceeded that of smaller prey. NH3, but not NO2, was significantly elevated in exhaled waters of sponges and bivalves, but not of ascidians. Preliminary results indicate a substantial removal of DOC by sponges and ascidians (but not by bivalves). If corroborated, these findings suggests that the carbon gained via DOC removal exceeds that from particulate matter (phytoplankton, bacteria and detritus) by an order of magnitude. The contribution of carbon and nutrients via consumption of DOC, phytoplankton and bacteria to the individual feeders and the reef community need to be re-evaluated. Home Herbivory in Soft Corals: Correction Fabricius K.E., Yahel G., and Genin A. (1997) Science 273, 293b-297 We have demonstrated the ability of several azooxanthellate soft corals to feed on phytoplankton (Reports, 7 Apr. 1995, p. 90) (1). The conversion of gut fluorescence values to chlorophyll concentrations yields nanograms of chlorophyll in the gastrovascular system of Dendronephthya hemprichi, not micrograms, as we erroneously stated. Hence the data presented in our "third line of evidence" for herbivory, and in figure 1B, should have read "Chlorophyll a content (nanograms per polyp)" instead of microgram. Gut chlorophyll contents of well-fed D. hemprichi are thus comparable to those of herbivorous copepods with similar body weight (2). As these corrections could change our conclusion with regard to herbivory in D. hemprichi, we have estimated the coral's feeding rate using a new approach, based on in situ measurements of chlorophyll removal from natural seawater. Chlorophyll a concentrations in seawater upstream of large D. hemprichi colonies were compared with those downstream of the corals with a time lag between pairs of samples equal to the water passage time through the coral thicket. On average, D. hemprichi removed 0.035 microgram per liter ± 0.064 (SD) or 6.4% ± 11.6 of chlorophyll at a mean flow speed through the colonies of 3.5 centimeters per second. This removal rate is equivalent to an uptake of 16.34 milligrams of phytoplankton carbon per gram of ash-free dry weight (AFDW) of the coral per day, which is 2.5 to 3.6 times the corals' daily respiratory carbon demand (3). The chlorophyll depletion of the water and recent electron microscopic documentation of digested algal cells within the endodermal tissue of the polyps suggest that our finding of herbivory in these soft corals is valid. However, our initial method of analyzing gut fluorescence underestimated the actual rates of intake and digestion of phytoplankton by D. hemprichi. Home Selective, size independent, filtration of pico-plankton by three tropical suspension feeders: a sponge, an ascidian, and a bivalve Yahel G., Marie D., Beninger P., and Genin A. Diet composition and prey preference patterns were studied in three benthic coral-reef suspension feeders: a sponge (Theonella swinhoei), a mollusk (the boring bivalve Lithophaga simplex), and a tunicate (the solitary ascidian Halocynthia gangelion). The water inhaled and exhaled by undisturbed specimens was cleanly collected in situ. Flow cytometric comparison of these water samples provided a direct measure of the grazer's filtration efficiencies over the course of four years (1996-2000) covering a wide range of environmental conditions. Ultraplankton (<8 µm), which dominated the planktonic biomass in the oligotrophic waters overlying the reef, was removed efficiently by each of the three grazers exhibiting a unique species-specific selectivity patterns. Synechococcus removal efficiencies were 70±14%, 76±16%, and 92±8% (mean±1SD), for the bivalve, ascidian and sponge, respectively. Coccoid photosynthetic bacteria were preferred by all three taxa over both the larger eukaryotic algae and the somewhat smaller non-photosynthetic bacteria. The ascidian and the bivalve efficiently removed the minute photosynthetic bacteria Prochlorococcus (~0.6 µm, 65±19% and 41±19%, respectively) but not non photosynthetic bacteria (8±7% and 5±19%, respectively). The negative selection against non photosynthetic bacteria was surprising considering their dominance in the planktonic community and the considerable size overlap on non photosynthetic bacteria with the Prochlorococcus population. The small fraction of non-photosynthetic bacteria retained by the bivalve and the ascidian did not differ in size but had higher apparent nucleic acid content in comparison with the ambient (inhaled) population. Considerable retention of non photosynthetic bacteria was exhibited only by the sponge (84±8%) which was the most efficient and least selective suspension feeder (T. swinhoei removal efficiencies ranged from 73±27% for eukaryotic algae to 95±7% for Prochlorococcus). Selectivity for cell attributes within prey taxon was evident for the bivalve which preferred Prochlorococcus and eukaryotic algae with higher chlorophyll content. The sponge preferred the smaller photosynthetic cells but showed no preferential size selection for non-photosynthetic bacteria. We suggest that for benthic tropical picoplankton grazers, selectivity is not size dependent and probably relies on other cell attributes such as cell surface properties and/or motility. The mechanisms underlying the observed selectivity patterns are still unresolved and - as each phylum relies on a unique filtration mechanism – should vary between the three phyla. As the selectivity did not appear to maximize carbon or energy gain, it is suggested that at the reef, where carbon is not a rare commodity, suspension feeders have evolved to optimize the gain of other nutrients or to avoid harmful prey taxa.