Current Patterns and the Distribution of Benthic Habitats in a Coastal Lagoon of Mauritius

The oceanography around Mauritius (in the Western Indian Ocean) remains largely unstudied, hence there is an acute scarcity of marine environmental data for management purposes. Rigorous water depth and current measurements were made on a system of grids inside Le Morne lagoon (in the south western part of Mauritius) in March–April 2000 to generate semi-quantitative models of general flow pattern in the form of contour maps using SURFER 6 computer programme. A simultaneous survey on composition of bottom cover was conducted to examine possible relationships with current speed. A separate investigation recorded surface and bottom currents prevailing amongst various habitat types to demonstrate the nature of the resulting damping effect on surface current speed. Significant correlations generated from data analysis were discussed as a basis for real biophysical relationships. Some of the limitations in the current analysis and some of the seemingly contradictory results are acknowledged and addressed in the light of the general assumption that the structure of the lagoon is conditioned by current speeds. Much stronger current speeds just outside the reef (e.g. >0.5 m s⁻¹) than inside the lagoon (e.g. <0.32 m s⁻¹) indicated a substantial slow-down of water current by the reef barrier. Inshore bottom currents were weaker than at the surface and current speed correlated well with water depth. Bottom and surface current directions were generally similar, i.e. going northward during flood tide and southward during ebb. The lagoon would be classified as ‘restricted’, exhibiting well-defined tidal circulation, which is modified by wind forcing. Dense fields of branching Acropora corals slowed down surface current speed by as much as 87%, but the relationship between current speed and bottom cover appears to be variable, depending on the specific location within the lagoon under consideration. The contour plots of the flow pattern model generated reasonably high qualitative modelling of spatial current speed pattern in the lagoon, with stronger currents generally along the reef areas, at the reef passes and in the deeper zones. However, these plots did not match closely those displaying distribution of bottom cover, thus confirming results obtained from pair-wise correlation tests, namely the lack of a significant relationship between current speed and bottom cover. Most of the correlations would appear to represent biological relationships, with different types of communities enabling or excluding other types. Thus, the biophysical structure of the lagoon would be driven a priori by the distribution and abundance of corals rather than current speeds, which contradicts the above hypothesis. Recurrent natural hazards subject the benthic communities to a state of ‘perpetual knock-down and recovery’. Recovery, however, can be seriously impaired by the chronic ongoing degradation of the coastal marine environment of Mauritius. An urgent review of its coastal zone management and protection strategy would be desirable for the island.     

Wave-current interactions on a shallow reef (Nicaragua,Central America)

Measurements of wave height and currents associated with normal trade-wind conditions have been made on a linear reef that parallels the northern and northeastern coast of Great Corn Island, eastern shelf of Nicaragua, Central America. Analyses indicate that waves breaking over the reef crest generate lagoonward flow normal to the reef. Average reef-normal flow was in the range of 10 to 20 cm/s; however, individual wave surges reached values of up to 180 cm/s. The strength of the over-the-reef flow is modulated by the tide. Lagoon currents are weak (2–5 cm/s) and change direction with the tide as the lagoon fills and drains. Long-period oscillations in water level (30 s to 20 min) and in the current were observed, and may be important in transporting fine-grained sediments out of the reef-lagoon system. Strong, short-duration surge currents ( <5 s) transport coarse sediment from the breaker zone to the seaward margin of the backreef lagoon.     

Evaluation of indicators of nitrogen limitation in deep prairie lakes with laboratory bioassays and limnocorrals

A comparison of Kenyan reefs of different historical and observed levels of fishing exploitation showed that more exploited reef lagoons had greater sea urchin densities and sizes, fewer and smaller fish and less coral cover. In the most exploited lagoon the biomass of the burrowing sea urchin Echinometra mathaei increased five fold during the previous 15 years. An ecological study of the three most common omnivorous sea urchin species inhabiting hard substrate within these reef lagoons (E. mathaei, Diadema savignyi and D. setosum) suggests that they are ecologically separated by predation and avoid predators and competitors by occupying different size burrows or crevices within the lagoon. Predator removal through fishing activities may result in ecological release of the sea urchins and result in competitive exclusion of weaker competitors. The most exploited reef had a nearly monospecific barren of E. mathaei living outside burrows suggesting that E. mathaei may be the top competitor. Its ecological release appears to lead to a decrease in live coral cover, increased substrate bioerosion and eventually a loss of topographic complexity, species diversity, fish biomass and utilizable fisheries productivity. Data from the outer reef edge were more difficult to interpret but may indicate similar patterns. Within this area, physical stresses such as waves and currents may be a greater controlling force in regulating fishing activities and coral reef community structure.     

Physical processes in an Indian Ocean atoll

 Detailed measurements of water levels, and tide and wave-induced currents were undertaken to examine physical processes and their relationship with morphology in the Cocos (Keeling) Islands, a medium sized atoll in the Indian Ocean. Results indicate that the atoll structure controls both lagoon circulation and the spatial pattern of energy distribution. Lagoon circulation is tide dominated (currents 16–31 cms^-1) with flushing (2–5 days) of the lagoon occurring through the deep leeward passages. Wave- and tide-driven unidirectional flows through shallow passages (26–65 cms^-1) are important mechanisms of ocean to lagoon water exchange and contribute up to 24% of the lagoon neap tide prism. Reef flats are dominated by wave energy (maximum velocity 140 cms^-1, east) with measurements of the attenuation of wave energy between reef flats and shallow lagoon (80–90%) conforming to measurements from fringing and barrier reefs. Spectral analysis shows that the characteristics of wave energy vary on different sectors of the atoll, with gravity wave energy dominating the east, and infragravity wave energy dominating the southern reef flat and passages. Wave setup at the reef crest is considered to be responsible for an identified 0.1 m higher water level in the southern as opposed to eastern and northern atoll, which promotes higher reef flat growth. Transmission of gravity waves across reef flats requires threshold water depths of 0.65 (east) and 0.70 m (south). The higher southern reef is an effective filter of gravity wave energy for most tidal elevations. Differences in the type and magnitude of physical processes within the atoll are discussed with relation to geomorphic development on Cocos. Accepted: 28 February 1998     

Wave and tidal flushing in a near-equatorial mesotidal atoll

Most of the atolls found worldwide are under microtidal regimes, and their circulation mechanisms are widely documented and well known. Here, we describe the flushing mechanisms of a small-sized mesotidal atoll, based on water-level, wave and current data obtained during two different periods (total of 60 d). Rocas is the only atoll in the South Atlantic Ocean and is built primarily of coralline algae. Two reef passages connect the atoll lagoon to the ocean. Synchronous current profilers were deployed at the two reef passages, one inside and one outside the atoll, to characterize the influence of tides and waves on the circulation. Results showed that wind waves drove a setup on the exposed side of the atoll and that currents were predominately downwind, causing outflow at both reef passages. Waves breaking on the windward side supplied water to the atoll causing the lagoon water level to rise above ocean water level, driving the outflow. However, unlike microtidal atolls, at Rocas Atoll the water level drops significantly below the reef rim during low tides. This causes the reef rim to act as a barrier to water pumping into the lagoon by waves, resulting in periodic activation of the wave pumping mechanism throughout a tidal cycle. As result, inflow occurs in the wider passage during 27% of each tidal cycle, starting at low tides and reversing direction during mid-flood tide when the water level exceeded approximately 1.6 m (while overtopping the atoll’s rim). Our findings show that tides play a direct role in driving circulation on a mesotidal atoll, not only by modulating wave setup but also by determining the duration of wave pumping into the lagoon.

     

Reef demise and back-stepping during the last interglacial, northeast Yucatan

The elevation of reefs and coastal deposits during the last Interglaciation (MIS-5e) indicates that sea level reached a highstand of as much as 6 m above the present, but it is uncertain how rapidly this level was attained and how it impacted reef development. To investigate this problem, I made a detailed sedimentological analysis of a well-dated reef from the northeast coast of the stable Yucatan Peninsula. Two linear reef tracts were delineated which are offset and at different elevations. The lower reef tract crops out along northern shore for 575 m and extends from below present mean sea level to +3 m. The reef crest facies consists of large Acropora palmata colonies dispersed within a coral boulder-gravel and is flanked by an A. cervicornis-dominated reef-front and a large area of lagoonal framework formed by coalesced patches of A. cervicornis and Montastraea spp. Constituents in the upper centimetre of the lower tract are heavily encrusted by a cap of crustose corallines and, in places, are levelled by a discontinuous marine-erosion surface. The upper reef tract crops out ~150 m inland up to an elevation of +5.8 m and parallels the southern section of shore for ~400 m. It also consist of an A. palmata-dominated crest facies flanked by reef-front, back-reef and lagoonal frameworks. In this case, however, lagoonal frameworks are dominated by a sediment-tolerant assemblage of branching coralline algae. Also different is the lack of encrustation by corallines, and the infiltration of upper tract facies by beach-derived shell-gravels from regressive shoreface deposits above. These results indicate that the lower reef tract and lagoonal patch-reefs formed at a sea level of +3 m. Final capping by crustose corallines and discontinuous marine erosion indicates that the lower tract was terminated by the complete demise of corals on the crest but only patchy demise in the lagoon. Areas of continuous framework accretion between the lagoonal patch reefs and the upper reef-tract, however, require that the demise of this reef was ecologically synchronous with initiation of the upper reef-tract, which had back-stepped 100 m into the lagoon. In this new position, the upper tract developed a reef crest that corresponded to a final sea-level position of +6 m. Reef flat development at +5 m and large in-place colonies of A. palmata at the base of the crest unit indicate, however, that sea level must have risen rapidly from +3 to more than +5 m to accommodate back-stepping. This sea-level jump created a higher energy wave field that mobilized back-reef and lagoonal sediments, and the resulting high sediment flux eroded lagoonal framework and prevented the recovery of the submerged lower reef crest. So this single jump in sea level was responsible not only for reef demise and back-stepping but also for marine erosion and suppression of subsequent reef development—features that elsewhere have been used to support multiple sea-level excursions during the last interglacial.     

南沙群岛美济礁海域夏季浮游动物群落特征

浮游动物是珊瑚礁生态系统中的重要组成部分,但国内相关研究相对较少。利用2012年7月在南沙群岛美济礁海域开展的13个站次的海洋生物调查数据,对美济礁浮游动物的群落特征进行研究,表明:(1)美济礁浮游动物种类组成丰富,共出现15个类群138种(类)浮游动物;(2)优势种组成复杂、区域变化明显,单一种的优势度不高;(3)浮游动物平均密度和湿重生物量分别为117.70个/m3、69.01 mg/m3,浮游幼虫在总密度中所占比例最大;(4)浮游动物非常丰富,多样性程度较高。平均丰富度、多样性指数和均匀度分别为4.93、3.33和0.67;(5)浮游动物可划分为2个群落,分别为潟湖-礁坪区群落和向海坡群落;(6)受珊瑚礁不同生物地貌带的空间异质性和水动力条件的影响,美济礁浮游动物群落区域差异明显。潟湖区浮游动物多样性较高,生物量最低;礁坪区浮游动物数量大,但分布不均匀,且多样性水平最低;向海坡区浮游动物种类最多、多样性非常丰富,但栖息密度最低。     

Altered sediment biota and lagoon habitat carbonate dynamics due to sea cucumber bioturbation in a high‐pCO2 environment

The effects of global change on biological systems and functioning are already measurable, but how ecological interactions are being altered is poorly understood. Ecosystem resilience is strengthened by ecological functionality, which depends on trophic interactions between key species and resilience generated through biogenic buffering. Climate‐driven alterations to coral reef metabolism, structural complexity and biodiversity are well documented, but the feedbacks between ocean change and trophic interactions of non‐coral invertebrates are understudied. Sea cucumbers, some of the largest benthic inhabitants of tropical lagoon systems, can influence diel changes in reef carbonate dynamics. Whether they have the potential to exacerbate or buffer ocean acidification over diel cycles depends on their relative production of total alkalinity (A_T) through the dissolution of ingested calcium carbonate (CaCO₃) sediments and release of dissolved inorganic carbon (C_T) through respiration and trophic interactions. In this study, the potential for the sea cucumber, Stichopus herrmanni, a bêche‐de‐mer (fished) species listed as vulnerable to extinction, to buffer the impacts of ocean acidification on reef carbonate chemistry was investigated in lagoon sediment mesocosms across diel cycles. Stichopus herrmanni directly reduced the abundance of meiofauna and benthic primary producers through its deposit‐feeding activity under present‐day and near‐future pCO₂. These changes in benthic community structure, as well as A_T (sediment dissolution) and C_T (respiration) production by S. herrmanni, played a significant role in modifying seawater carbonate dynamics night and day. This previously unappreciated role of tropical sea cucumbers, in support of ecosystem resilience in the face of global change, is an important consideration with respect to the bêche‐de‐mer trade to ensure sea cucumber populations are sustained in a future ocean.     

一株高效拮抗向日葵菌核菌的细菌菌株YC16的筛选及其作用效果研究

[目的]筛选高效拮抗向日葵菌核菌的细菌菌株,为开发防治菌核菌病害、提高向日葵产量的生物菌剂提供菌种资源。[方法]以羧甲基纤维素钠(CMC)、小麦秸秆纤维素为唯一碳源的无机盐培养基,分离高效降解纤维素的细菌菌株;采用纤维素降解菌与菌核菌的平板对峙方法,进一步筛选拮抗菌核菌的菌株;利用16S rDNA序列鉴定菌株、PDYA平板对峙实验检验上述所选拮抗菌株的抑菌谱;采用离体向日葵新鲜叶片、草炭土基质盆栽实验,观察拮抗菌菌株抑制菌核菌生长的能力;温室盆栽和田间试验条件下,研究其防治向日葵菌核菌病害、促进生长和提高产量的效果。[结果]筛选了一株高效抑制菌核菌的细菌YC16,经过16S rDNA序列分析,鉴定为解淀粉芽孢杆菌。YC16菌株能够抑制8种病原真菌生长,包括齐整小核菌、腐皮镰孢菌、尖孢镰刀菌、稻梨孢、辣椒疫霉、镰刀菌、尖镰孢黄瓜专化型和向日葵菌核菌;抑制菌核菌感染叶片,抑制率达到了80.42%;抑制盆栽基质中菌核菌的菌丝生长,基质表面菌丝密度比对照减少了50%以上。盆栽接种YC16的向日葵生物量比对照提高54.9%,田间向日葵接种YC16菌剂对菌核菌引发的盘腐病防治效果达39%-100%,产量提高24.4%-30.2%。[结论]YC16生物菌剂施用于土壤,能够有效防治向日葵的茎腐病和盘腐病,展现了防治向日葵菌核病和提高产量的双重效果,是一株具有良好应用前景的高效菌种资源。     

Evolution of fringing reefs: space and time constraints from the Gulf of Aqaba

This study documents the pattern and rate of reef growth during the late Holocene as revealed by unique geological conditions at the subsiding NW Gulf of Aqaba. We discovered that the modern fringing reef near the city Elat grows on top of a fossil submerged mid-Holocene reef platform. Four coral cores from the fossil platform were dated using the radiocarbon and U-Th methods. The fossil corals range from 5.6±0.1 to 2.4±0.03 ka, constraining the initiation of the modern reef to 2,400 years ago at most. We documented the detailed morphology of the reef using aerial photographs and scuba diving. The survey shows that at its northern end, growth of the 2-km-long reef is inhibited by an active alluvial fan, and it is composed of isolated knolls that are just approaching the sea surface. Towards the south, the knolls are progressively larger and closer together, until they form a continuous reef platform. Along this north-to-south trend we follow the evolution of reef morphology, changes in coral distribution, and the development of a lagoon separated from the open sea. Based on these observations, we suggest a four-stage reef growth model: (a) the reef initiates as coral colonies, forms knolls, and begins to grow upward, limited by the sea surface. (b) Upon reaching the surface, the knolls spread laterally, preferentially parallel to the dominant wave direction assuming an elongated morphology. (c) Continued growth results in adjacent knolls eventually coalescing to form a continuous jagged reef. We interpret the spurs-and-grooves morphology that can be traced across the reef at Elat as remnants of the original trends of knolls. (d) While reef expansion continues, the original knoll trends may be obscured as a massive reef front takes shape. Considering reef growth rates and observations from the modern reef at Elat, this evolution scheme predicts an age range of 10³ years for corals on the reef platform. The range and distribution of radiometric ages we obtained from the fossil reef platform underlying the living Elat reef confirm this hypothesis.     

Extreme climatic events reduce ocean productivity and larval supply in a tropical reef ecosystem

Increasing ocean temperatures due to global warming are predicted to have negative effects on coral reef fishes. El Niño events are associated with elevated water temperatures at large spatial (1000s of km) and temporal (annual) scales, providing environmental conditions that enable temperature effects on reef fishes to be tested directly. We compared remote sensing data of sea surface temperature (SST) anomalies, surface current flow and chlorophyll‐a (Chl‐a) concentration with monthly patterns in larval supply of coral reef fishes in nearshore waters around Rangiroa Atoll (French Polynesia) from January 1996 to March 2000. This time included an intense El Niño (April 1997–May 1998) event between two periods of La Niña (January–March 1996 and August 1998–March 2000) conditions. There was a strong relationship between the timing of the El Niño event, current flow, ocean productivity (as measured by Chl‐a) and larval supply. In the warm conditions of the event, there was an increase in the SST anomaly index up to 3.5 °C above mean values and a decrease in the strength of the westward surface current toward the reef. These conditions coincided with low concentrations of Chl‐a (mean: 0.06 mg m^?3, SE ± 0.004) and a 51% decline in larval supply from mean values. Conversely, during strong La Niña conditions when SST anomalies were almost 2 °C below mean values and there was a strong westward surface current, Chl‐a concentration was 150% greater than mean values and larval supply was 249% greater. A lag in larval supply suggested that productivity maybe affecting both the production of larvae by adults and larval survival. Our results suggest that warming temperatures in the world's oceans will have negative effects on the reproduction of reef fishes and survival of their larvae within the plankton, ultimately impacting on the replenishment of benthic populations.     

Sensitive determination of bromhexine hydrochloride based on its quenching effect on luminol/H2O2 electrochemiluminescence system

In this paper, the electrochemiluminescence (ECL) behavior of luminol/H₂O₂ system in the presence of bromhexine hydrochloride (BrH) was investigated. It was found that the ECL intensity of luminol/H₂O₂ system on a platinum electrode could be intensely quenched by BrH owing to the scavenging superoxide radical ability of BrH, and therefore the sensitive determination of BrH was possible. Under optimal conditions, the quenched ECL intensity was linear to the concentration of BrH in a wide range of 0.08 to 500 μM, with a detection limit of 0.02 μM (signal‐to‐noise ratio (S/N) = 3). This ECL method possessed the merits of rapid, simple and sensitive, and was successfully applied to the BrH quantification in pharmaceutical preparations with satisfactory recoveries of 91.0 ± 4.0 to 106.5 ± 3.4%. The possible route of the quenched ECL of luminol/H₂O₂ in the presence of BrH was also discussed.     

Exposure to elevated carbon dioxide does not impair short‐term swimming behaviour or shelter‐seeking in a predatory coral‐reef fish

Adult bluespotted rockcod Cephalopholis cyanostigma, a coral‐reef grouper, were acclimated to either ambient (mean ± s.d. 406 ± 21 μatm; 1 atmos = 101325 Pa) or high pCO₂ (945 ± 116 μatm) conditions in a laboratory for 8–9 days, then released at the water surface directly above a reef (depth c. 5 m) and followed on video camera (for 191 ± 21 s) by scuba divers until they sought cover in the reef. No differences were detected between groups in any of the six measured variables, which included the time fish spent immobile after release, tail beat frequency during swimming and the time required to locate and enter the protective shelter of the reef.     

Dinoflagellate-targeted PCR reveals highly abundant and diverse communities of parasitic dinoflagellates in and near Zhubi Reef,South China Sea

While diversity of symbiodiniacean dinoflagellates has been a focus of coral reef ecological research, information on the diversity of planktonic dinoflagellates in reef ecosystems remains limited. We used dinoflagellate-targeted PCR to investigate dinoflagellate diversity for a coral reef ecosystem. In the summer of 2007, plankton samples were collected from a lagoon, atoll, and open sea area of Zhubi Reef in the Nansha Islands, South China Sea. Sequencing of dinoflagellate-specific SSU rDNA clone libraries from samples in each of these habitats revealed high diversity and numerous novel dinoflagellate lineages. Gymnodiniales were most abundantly represented in all three water areas. Lagoon assemblages were co-dominated by Syndiniales and Gonyaulacales, the atoll by Gonyaulacales and Peridiniales, and the open sea by Syndiniales and Prorocentrales taxa. Species in the Syndiniales (group II) genus Amoebophrya were represented by eight new sequences and 13 previously described clades and were dominated by species reported to infect Gymnodiniales, Gonyaulacales, Peridiniales, and Prorocentrales taxa. And Amoebophrya were particularly abundant and diverse in the lagoon. Our results suggest that Amoebophrya probably play an important role in regulating dynamics of dinoflagellate assemblages in the Zhubi Reef coral ecosystem. In contrast, the few symbiodiniacean taxa detected occurred only in the open sea, suggesting planktonic aposymbiotic Symbiodiniaceae rarely occur in the reef ecosystem. We demonstrate the usefulness of a dinoflagellate-specific molecular technique for profiling dinoflagellate communities, and uncover diversity and the potential importance of parasitic lineages in a coral reef ecosystem.

     

Modelling wind driven circulation One Tree Reef,Southern Great Barrier Reef

A vertically integrated numerical model of wind-driven circulation at One Tree Reef is presented. The model is the numerical model SURGE developed originally to study tropical cyclone surge. Current data collected in the reef lagoon and over the reef flats is used to test the model's applicability. The reef topography has been modelled explicitly, rather than using an assumed reef shape, with a grid spacing of 150 m. The model corresponds well to the measured current behaviour. The greatest drawback to use of the model is that, at low tide, currents reverse with depth due to lagoon enclosure and a depth integrated model cannot produce water velocity depth profiles. The model can be used to predict current behaviour in strong wind conditions, such as during a cyclone, and to estimate net flows into and out of the lagoon.     

Observations of the thermal environment on Red Sea platform reefs: a heat budget analysis

Hydrographic measurements were collected on nine offshore reef platforms in the eastern Red Sea shelf region, north of Jeddah, Saudi Arabia. The data were analyzed for spatial and temporal patterns of temperature variation, and a simple heat budget analysis was performed with the goal of advancing our understanding of the physical processes that control temperature variability on the reef. In 2009 and 2010, temperature variability on Red Sea reef platforms was dominated by diurnal variability. The daily temperature range on the reefs, at times, exceeded 5°C—as large as the annual range of water temperature on the shelf. Additionally, our observations reveal the proximity of distinct thermal microclimates within the bounds of one reef platform. Circulation on the reef flat is largely wave driven. The greatest diurnal variation in water temperature occurs in the center of larger reef flats and on reefs protected from direct wave forcing, while smaller knolls or sites on the edges of the reef flat tend to experience less diurnal temperature variability. We found that both the temporal and spatial variability in water temperature on the reef platforms is well predicted by a heat budget model that includes the transfer of heat at the air–water interface and the advection of heat by currents flowing over the reef. Using this simple model, we predicted the temperature across three different reefs to within 0.4°C on the outer shelf using only information about bathymetry, surface heat flux, and offshore wave conditions.     

Benthic Primary Production Budget of a Caribbean Reef Lagoon (Puerto Morelos,Mexico)

High photosynthetic benthic primary production (P) represents a key ecosystem service provided by tropical coral reef systems. However, benthic P budgets of specific ecosystem compartments such as macrophyte-dominated reef lagoons are still scarce. To address this, we quantified individual and lagoon-wide net (P_n) and gross (P_g) primary production by all dominant functional groups of benthic primary producers in a typical macrophyte-dominated Caribbean reef lagoon near Puerto Morelos (Mexico) via measurement of O₂ fluxes in incubation experiments. The photosynthetically active 3D lagoon surface area was quantified using conversion factors to allow extrapolation to lagoon-wide P budgets. Findings revealed that lagoon 2D benthic cover was primarily composed of sand-associated microphytobenthos (40%), seagrasses (29%) and macroalgae (27%), while seagrasses dominated the lagoon 3D surface area (84%). Individual P_g was highest for macroalgae and scleractinian corals (87 and 86 mmol O₂ m⁻² specimen area d⁻¹, respectively), however seagrasses contributed highest (59%) to the lagoon-wide P_g. Macroalgae exhibited highest individual P_n rates, but seagrasses generated the largest fraction (51%) of lagoon-wide P_n. Individual R was highest for scleractinian corals and macroalgae, whereas seagrasses again provided the major lagoon-wide share (68%). These findings characterise the investigated lagoon as a net autotrophic coral reef ecosystem compartment revealing similar P compared to other macrophyte-dominated coastal environments such as seagrass meadows and macroalgae beds. Further, high lagoon-wide P (P_g: 488 and P_n: 181 mmol O₂ m⁻² lagoon area d⁻¹) and overall P_g:R (1.6) indicate substantial benthic excess production within the Puerto Morelos reef lagoon and suggest the export of newly synthesised organic matter to surrounding ecosystems.     

Ion currents and the nitrogen status of roots of Hordeum vulgare and non-nodulated Trifolium repens

Abstract. Profiles of self-generated ion currents associated with the growing primary root tips of intact Hordeum vulgare L. and Trifolium repens L. (nonnodulated) seedlings were measured using a highly sensitive vibrating electrode in media containing NH⁺₄ or NO^-₃, and compared to control roots growing in nitrogen free media. Under these three nutrient regimes, positive current entered the root at regions corresponding to the meristematic tissues and main elongation zones of root tips and left from the mature root tissues. Mapping the surface of the roots with a pH-sensitive microelectrode revealed regions of external alkalinity where positive electrical current entered the root, and external acidity where positive current exited. The correlation between pH-profile and the pattern of ion current generation in these experiments suggests that H⁺ ions were responsible for carrying the bulk of the root-generated current. Assimilation of NHJ results in net H⁺ extrusion while assimilation of NO^-₃, results in net OH^-₃ efflux. Growth on NH⁺₄, as compared to growth on NO^-₃, stimulated the magnitude of the electrical current but did not affect significantly the growth rate of the roots. However, despite the differing stresses on internal pH regulation that arise due to growth on the two exogenous forms of combined nitrogen, the current profiles were qualitatively similar under the different conditions that were examined. The role of the circulating proton current is not yet known; however, the constancy of the current profile under different nutrient regimes sustains the hypothesis that the current may have a role in the regulation of root polarity.     

Currents and flusing of Britomart reef lagoon,Great Barrier Reef

Currents, sea levels and weather were measured for a 60 d period at a number of sites in and near Britomart reef, Great Barrier Reef. The tidal currents were primarily semi-diurnal in character. The lowfrequency currents were due to a simple balance, in the longshore direction, between the wind stress and the bottom frictional stress, and, in the cross-shelf direction, between the bottom frictional stress and the pressure gradient generated by the passage of wind-driven continental shelf waves. The bottom friction coefficient, for the nonlinear drag law, is estimated to be of the order of 0.02. By contrast, the currents over the surrounding continental shelf are primarily controlled by continental shelf wave dynamics and are not dominated by friction. It thus appears that the water bodies in, respectively, the reef matrix and the Lagoon of the Great Barrier Reef are relatively uncoupled. Tidal and low-frequency currents contributed roughly equally to the flushing of Britomart reef lagoon with an estimated flushing time of the order of 4 d.     

A Kinetic Model with Ordered Cytoplasmic Dissociation for SUC1, an Arabidopsis H+/Sucrose Cotransporter Expressed in Xenopus Oocytes

To elucidate the kinetic properties of the Arabidopsis H⁺/sucrose cotransporter, SUC1, with respect to transmembrane voltage and ligand concentrations, the transport system was heterologously expressed in Xenopus laevis oocytes. Steady-state plasma membrane currents associated with transport of sucrose were measured with two-electrode voltage clamp over the voltage range −180 to +40 mV as a function of extracellular pH and sugar concentrations. At any given voltage, currents exhibited hyperbolic kinetics with respect to extracellular H⁺ and sugar concentrations, and this enabled determination of values for the maximum currents in the presence of each ligand (i ^H _max , i ^S _max for H⁺ and sucrose) and of the ligand concentrations eliciting half-maximal currents (K ^H _m , K ^S _m ). The i ^H _max and i ^S _max exhibited marked and statistically significant increases as a function of increasingly negative membrane potential. However, the K ^H _m and K ^S _m decreased with increasingly negative membrane potential. Furthermore, at any given voltage, i ^S _max increased and K ^S _m decreased as a function of the external H⁺ concentration. Eight six-state carrier models—which comprised the four possible permutations of intracellular and extracellular ligand binding order, each with charge translocation on the sugar-loaded or -unloaded forms of the carrier—were analyzed algebraically with respect to their competence to account for the ensemble of kinetic observations. Of these, two models (first-on, first-off and last-on, first-off with respect to sucrose binding as it passes from outside to inside the cell and with charge translocation on the loaded form of the carrier) exhibit sufficient kinetic flexibility to describe the observations. Combining these two, a single model emerges in which the binding on the external side can be random, but it can only be ordered on the inside, with the sugar dissociating before the proton. Received: 23 January 1996/Revised: 16 April 1997