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1.
Abstract. The influence of corallivores on coral community structure of eastern Pacific reefs has been considered less important then that of abiotic oceanographic factors. The data that support this assumption, however, are only available for Central American reefs. To assess the role of predation on hermatypic corals in a different regional reef environment, the abundance, spatial distribution and consumption rate of three corallivores: the echinoid Eucidaris thouarsii (Valenciennes), the asteroid Acanthaster planci (Linnaeus) and the teleostean fish Arothron meleagris (Bloch & Schneider), were estimated at Cabo Pulmo reef, Gulf of California, México (23°25' N, 109°25' W). Statistically, the abundances of the species did not change in any sections of the reef (mean values: E. thouarsii , 0.17 indiv. · m−2; A. planci , 1.9 indiv. · m−2; A. meleagris , 39 indiv. · ha−1). The average daily individual consumption rates of coral were calculated at 1.83 g CaCO3· m−2 for E. thouarsii , 118.4 cm2 for A. planci , and 16.38 g CaCO3· m−2 for A. meleagris , and were lower than those reported for Central American reefs. Considering the mean estimated carbonate production (7.9 kg CaCO3· m−2· a−1), corallivores eliminate less than 4% of the coral standing stock of Cabo Pulmo reef. The low corallivore population density and consumption rates, together with high local coral cover, indicate that corallivores are not key factors determining scleractinian abundance in this marginal reef.  相似文献   

2.
Human activities in the watersheds surrounding Maunalua Bay, Oahu, Hawaii, have lead to the degradation of coastal coral reefs affecting populations of marine organisms of ecological, economic and cultural value. Urbanization, stream channelization, breaching of a peninsula, seawalls, and dredging on the east side of the bay have resulted in increased volumes and residence time of polluted runoff waters, eutrophication, trapping of terrigenous sediments, and the formation of a permanent nepheloid layer. The ecosystem collapse on the east side of the bay and the prevailing westward longshore current have resulted in the collapse of the coral and coralline algae population on the west side of the bay. In turn this has lead to a decrease in carbonate sediment production through bio-erosion as well as a disintegration of the dead coral and coralline algae, leading to sediment starvation and increased wave breaking on the coast and thus increased coastal erosion. The field data and resulting coral reef ecohydrology model presented in this paper demonstrate and quantify the importance of biophysical processes leading to coral reef degradation as the result of urbanization. Coral restoration in Maunalua Bay will require an integrated ecosystem approach.  相似文献   

3.
Data from a three-year long field study of fine sediment dynamics in Cleveland Bay show that wave-induced liquefaction of the fine sediment bed on the seafloor in shallow water was the main process causing bed erosion under small waves during tradewinds, and that shear-induced erosion prevailed during cyclonic conditions. These data were used to verify a model of fine sediment dynamics that calculates sediment resuspension by both excess shear stress and wave-induced liquefaction of the bed. For present land-use conditions, the amount of riverine sediments settling on the bay may exceed by 50–75% the amount of sediment exported from the bay. Sediment is thus accumulating in the bay on an annual basis, which in turn may degrade the fringing coral reefs. For those years when a tropical cyclone impacted the bay there may be a net sediment outflow from the bay. During the dry, tradewind season, fine sediment was progressively winnowed out of the shallow, reefal waters.  相似文献   

4.
珊瑚礁地质稳定性研究现状   总被引：1，自引：1，他引：0  
为探究珊瑚礁稳定性的研究现状,对影响礁体稳定性的诸多因素如礁体结构、地形地貌、波浪、潮汐、潮流、风诸如热带气旋和风暴、海流等进行总结,结果表明以上诸多因素可在不同程度上决定或影响礁体稳定性。同时指出现阶段礁体稳定性研究所需注意的问题,其一是涉及珊瑚礁地质稳定性的礁体结构、地貌形态的研究尚少,需合理利用多波束测深仪、高分辨率多道地震探测系统、钻探及AUV、三维实时声学成像声呐技术获取礁体地形、地层及剖面形态,并借助GIS技术对滑坡等失稳地带进行圈定,其二是潟湖区、礁体边坡等区域的稳定性研究缺乏,需引入极限平衡法评估礁体边坡、泻湖等区域的稳定性,且需考虑波浪、潮汐引入的孔隙水压力、渗流力的影响。通过综述礁体稳定性研究现状及未来礁体稳定性研究的方向的展望,以期为珊瑚礁体上的工程建设及未来的珊瑚礁地质稳定性研究提供些许启示。  相似文献   

5.
Scleractinian corals create three-dimensional reefs that provide sheltered refuges, facilitate sediment accumulation, and enhance colonization of encrusting fauna. While heterogeneous coral habitats can harbor high levels of biodiversity, their effect on the community composition within nearby sediments remains unclear, particularly in the deep sea. Sediment macrofauna from deep-sea coral habitats (Lophelia pertusa) and non-coral, background sediments were examined at three sites in the northern Gulf of Mexico (VK826, VK906, MC751, 350–500 m depth) to determine whether macrofaunal abundance, diversity, and community composition near corals differed from background soft-sediments. Macrofaunal densities ranged from 26 to 125 individuals 32 cm−2 and were significantly greater near coral versus background sediments only at VK826. Of the 86 benthic invertebrate taxa identified, 16 were exclusive to near-coral habitats, while 14 were found only in background sediments. Diversity (Fisher’s α) and evenness were significantly higher within near-coral sediments only at MC751 while taxon richness was similar among all habitats. Community composition was significantly different both between near-coral and background sediments and among the three primary sites. Polychaetes numerically dominated all samples, accounting for up to 70% of the total individuals near coral, whereas peracarid crustaceans were proportionally more abundant in background sediments (18%) than in those near coral (10%). The reef effect differed among sites, with community patterns potentially influenced by the size of reef habitat. Taxon turnover occurred with distance from the reef, suggesting that reef extent may represent an important factor in structuring sediment communities near L. pertusa. Polychaete communities in both habitats differed from other Gulf of Mexico (GOM) soft sediments based on data from previous studies, and we hypothesize that local environmental conditions found near L. pertusa may influence the macrofaunal community structure beyond the edges of the reef. This study represents the first assessment of L. pertusa-associated sediment communities in the GOM and provides baseline data that can help define the role of transition zones, from deep reefs to soft sediments, in shaping macrofaunal community structure and maintaining biodiversity; this information can help guide future conservation and management activities.  相似文献   

6.
Total mass flux, size distribution of sediment particles and some chemical components such as total carbon (TC), total nitrogen (TN) and calcium carbonate (CaCO3) were monitored monthly using a multi-cup sediment traps at seven coral reef sites (6 reef flat and 1 reef slope) of the Marine Protected Areas around Ishigaki, Kohama, Kuroshima and Iriomote Islands in the southern Ryukyus, Japan from September 2000 to September 2001. The size distribution of trapped sediments revealed mostly uni-modal fine sand to mud in the reef flat and gravelly to coarse sand in the reef slope. The total mass flux ranged between 0.54 to 872 gm−2d−1, and showed a pronounced seasonality (high in summer-autumn and low in spring) at each site, which was consistent with the rainfall and typhoon regime. Exceptionally high values were observed on the reef slope (Iriomote) in February–March 2001 (1533 gm−2d−1) owing to a large amount of bottom sediment re-suspension. On the reef flat (Todoroki South and North; Ishigaki), values obtained in July–August 2001 (872 gm−2d−1) and August–September 2001 (800 gm− 2d−1) indicate the high terrestrial discharge from Todoroki River. Trapped sediment particles consist of CaCO3 (1.2–27.1%) and a non-carbonate fraction (98.8–72.9%), which contains total carbon (4.9–26%), carbonate carbon (CO2-C) (0.2–3.1%) and non-carbonate carbon (NC-C) (7.9–25.6%). Total nitrogen content was in the range 0.02–0.48%. TN is contained mainly in the carbonate fraction and NC-C may be contained in the non-carbonate fraction. The low TN/OC ratio of the trapped sediments suggests that they were mostly of terrestrial origin and that both fractions migrated. The high total mass flux derived from Todoroki River exceeded the threshold at which a lethal effect on coral community is caused. The results stress the importance of conducting seasonal studies of sedimentation over more than one year and at more than one location in south Japan coral reef ecosystems to gain an understanding of the processes controlling the total mass fluxes and their nutrients content, also to develop an awareness of how to prevent the damage of coral reef ecosystems and, if it does occur, to allow mitigation measures to be undertaken.  相似文献   

7.
Sponges are one of the principal agents of bioerosion and sediment production in coral reefs. They generate small carbonate chips that can be found in the sediments, and we investigated whether these could provide a means for assessment of bioerosion applicable to reef monitoring. We tested this hypothesis on samples from 12 Mexican coral reefs distributed along the Pacific coast, where boring sponges were particularly abundant, and quantified the amount of chips in samples of superficial sediment in three grain‐size fractions: fine (<44 μm), medium (44–210 μm) and coarse (>210 μm). The grain‐size distribution varied among reefs, with the majority of the sediment of most reefs being composed of coarse sands, and the medium and fine fractions dominating only at La Entrega and Playa Blanca. All the reefs presented clear evidence of bioerosion by sponges, with the characteristic chips present in the sediment, although at most sites the percentage of chips was very low (from 1% to 3% of the total sediment). Only at La Entrega and Playa Blanca did they constitute a significant fraction of the total sediment (18% and 16%, respectively). While not statistically significant, there was an interesting trend between sponge chips versus sponge abundance that suggests that quantification of the chips in the sediment could be used as a proxy for sponge erosion of the entire community, which cannot be estimated in by laboratory experiments. However, while this methodology could provide an integrated approach to monitor sponge bioerosion, more studies are necessary due to the influence of environmental factors on the transport and deposition of these chips.  相似文献   

8.
Despite the severity of tropical cyclone ‘Winifred’, which crossed the Great Barrier Reef on 1 February 1986, there were little long-term effects on lagoon surface sediments from reefs in its path. Short-term effects were apparent only at one particularly exposed area. These were: an increase in proportion of the coarse fraction, the establishment of sand ripples, and the destruction of the mounds produced by callianassid shrimps (normally the dominant topographic feature). Within six weeks this area was indistinguishable from a typical reef lagoon. This is probably the result of sediment reworking by callianassid shrimp, involving selective burial of the coarse fragments and transport to the surface of finer particles. Sediment turnover rates by callianassids are commensurate with change to the sediment within the relatively short period observed. The sediment fauna responded quickly to the changes in sediment type. Immediately after the cyclone the disturbed area supported a fauna typical of the coarse sediments on the shallow reef flat, as the sediment reverted to a more normal type so the fauna changed back to that typical of a reef lagoon.  相似文献   

9.
Sea cucumbers play an important role in sediment bioturbation in coral reef and rocky intertidal ecosystems, and bacteria were a commonly-reported component of ...  相似文献   

10.
Authigenic carbonates from methane seeps of the Congo deep-sea fan   总被引：1，自引：2，他引：1  
Submersible investigations with the ROV Victor 6000 of some pockmark structures on the seafloor of the Congo deep-sea fan have shown that they are active venting sites of methane-rich fluids, associated with abundant fauna and carbonate crusts. Moreover, methane hydrates have been observed both outcropping and deep in the sediments in the centre of the “Regab” giant pockmark. Authigenic carbonates, mostly calcite sometimes mixed with aragonite, are cementing the sedimentary matrix components and fauna; diatoms are abundant but only as moulds, indicating that biogenic silica dissolution occurred in situ synchronous with carbonate precipitation. The occurrence of diagenetic barite and pyrite in some carbonate crusts demonstrates that they can be formed either within the sulphate/methane transition zone or deeper in sulphate-depleted sediments. The oxygen isotopic compositions of the diagenetic carbonates (3.17–6.01‰ V-PDB) indicate that precipitation occurred with bottom seawater mixed with a variable contribution of water from gas hydrate decomposition. The very low carbon isotopic compositions of the diagenetic carbonates (−57.1 to −27.75‰ V-PDB) demonstrate that carbon derives mostly from the microbial oxidation of methane.  相似文献   

11.
We studied diatom assemblages and CaCO3 contents of methane-derived authigenic carbonates from the eastern margin of the Sea of Japan and assessed the formation time of these samples. Radioactive 14C date was determined in selected samples to obtain the maximum age of the time. The results of our study suggest mass formation of carbonate nodules in a glacial period within ∼40 ky, consistent with a published U/Th dating result of carbonate nodules in the study area. Diatom assemblages and contents in the carbonate nodules (abundance of ∼106/g, dominance of neritic-littoral species, warm/cold water species ratio lower than ∼25) differ from the near-seafloor sediments in the study area, which have characteristics of Holocene sediments in the Sea of Japan, and suggest cementation of glacial sediments. Laminated sediments in some nodule samples are glacial sediments because laminations are records of a low sea level period in the semi-enclosed ocean. Similarity of diatom assemblages and contents in all carbonate samples is another evidence of glacial sediments in nodules. Glacial sediments with oceanic cold water species as low as Holocene sediments restricts the sediment age to before 20 cal. ky BP. Carbonate contents higher than 78 wt% suggest the cementation of poorly compacted sediments near the seafloor, and the date of carbonate cementation is, therefore, close to that of the cemented sediments. Most carbonate nodule samples in this study were formed in a glacial period and detection of 14C restricts this period to within ∼40 ky.  相似文献   

12.
Rapa (27°36′ S, 144°20′ W) is a small (~40 km2) volcanic island isolated in the Southern Austral Archipelago, where direct anthropogenic stressors are extremely limited. Here, we present the results of the first quantitative survey of coral community structure across habitats and depths around the island. Despite its geographical isolation in the depauperate South Central Pacific, its small size and unfavourable environmental conditions (competition with macroalgae, low sea surface temperatures, reduced reef accretion), the diversity of scleractinian corals at Rapa is particularly high (112 species from 32 genera, including 37 species of Acropora) in comparison to other French Polynesian islands and subtropical Pacific locations. Our results indicate that the abundance (>100 colonies per 10 m2 recorded at nine of the 17 sampling stations) and cover (>40% at four stations) of corals are relatively high for a marginal reef location. Strong spatial heterogeneity was found, with high variation in diversity, abundance, cover and community composition among stations. Variation in community composition was related to habitat types, with distinct assemblages among fringing reefs within bays, reef formations at bay entrances, and those on the submerged platform surrounding the island. On the platform, a depth gradient was detected, with generic richness, abundance and cover generally greater at deeper stations (18–20 m depth) compared with medium‐depth (10–12 m) and shallow (1–3 m) stations. A gradient was also recorded along bays, with increasing coral diversity and abundance from the bay heads to the bay entrances. The coral community at Rapa was characterized by the presence of several taxa not found in other French Polynesian archipelagos and the rarity of others that are common and abundant in the Society and the Tuamotu islands. Another distinctive feature of reef communities at Rapa is the high cover and dominance of macroalgae, particularly in the shallower parts of the surrounding platform, which probably explains the lower densities of coral colonies recorded there. These characteristics of the diversity and biogeographical composition of coral assemblages at Rapa provide considerable ecological grounds for its conservation.  相似文献   

13.
珊瑚礁地层中的暴露面是海平面变化的忠实记录, 对地层层序的划分和研究珊瑚礁体的生长发育过程具有重要意义。本文基于手标本观察与薄片鉴定、碳氧同位素和矿物组成分析, 识别了南沙美济岛南科1井第四纪生物礁碳酸盐岩地层中的典型暴露面, 剖析了主要暴露面与海平面变化及珊瑚礁发育演化的关系。在南科1井(NK-1)第四纪地层中典型暴露面附近以出现大量溶蚀孔穴和红褐色或锈黄色钙质结核为特征, 其碳氧同位素偏负, 并富集Al、Th、Fe和稀土元素, 具有典型碳酸盐岩风化壳的特征。南科1井全新世礁体发育在晚更新世暴露面之上, AMS14C和U-Th年龄数据证实美济岛全新世珊瑚礁生长时段与南海其他珊瑚岛礁一致, 主要发育在8.2~4.7ka时期, 该时段海平面的缓慢上升为珊瑚礁连续垂向生长提供了有利环境。Sr同位素和古地磁年龄标定的南科1井更新世地层中的主要暴露面时代与南沙永暑礁和西沙群岛珊瑚礁地层中的暴露面时代基本一致, 主要暴露面对应于全球第四纪低海平面时期。  相似文献   

14.
The head of a canyon system extending along the western Porcupine Bank (west of Ireland) and which accommodates a large field of giant carbonate mounds was investigated during two cruises (INSS 2000 and TTR-13). Multibeam and sidescan sonar data (600?C1,150?m water depth) suggest that the pre-existing seabed topography acts as a significant factor controlling mound distribution and shape. The mounds are concentrated along the edges of the canyon or are associated with a complex fault system traced around the canyon head, comprising escarpments up to 60?m high and several km long. The sampling for geochemical and petrographic analysis of numerous types of authigenic deposits was guided by sidescan sonar and video recordings. Calcite-cemented biogenic rubble was observed at the top and on the flanks of the carbonate mounds, being associated with both living and dead corals (Lophelia pertusa, Madrepora oculata and occasional Desmophyllum cristagalli). This can plausibly be explained by dissolution of coral debris facilitated by strong currents along the mound tops and flanks. In turn, the dissolved carbon is recycled and precipitated as interstitial micrite. Calcite, dolomite and phosphatic hardgrounds were identified in samples from the escarpment framing the eastern part of the survey area. The laterally extensive phosphatic hardgrounds represent a novel discovery in the region, supplying hard substrata for the establishment of new coral colonies. Based on existing knowledge of regional oceanographic conditions, complemented with new CTD measurements, it is suggested that water column stratification, enhanced bottom currents, and upwelling facilitate the deposition of organic matter, followed by phosphatisation leading to the formation of phosphate-glauconite deposits. The occurrence of strong bottom currents was confirmed by means of video observations combined with acoustic and sampling data, providing circumstantial evidence of fine- to medium-grained sand. Evidently, slope breaks such as escarpments and deep-water canyon headwalls are important structural elements in the development of mature carbonate mounds induced by deep-water coral growth. Stable isotope data show no evidence of methane-derived carbon in the carbonates and lithified sediments of the Porcupine Bank Canyon mounds.  相似文献   

15.
Bioerosion is a natural process in coral reefs. It is fundamental to the health of these ecosystems. In the Eastern Tropical Pacific (ETP) coral reefs, the most important bioeroders are sponges, bivalves, sea urchins and the fish Arothron meleagris. In the 1980s, El Niño caused high coral mortality and an increase in macroalgal growth. As a result, greater sea urchin bioerosion occurred. This weakened the reef framework. Considering the high vulnerability of the ETP coral reefs, the goal of this study was to determine the current bioerosion impact of the sea urchin Diadema mexicanum along the western coasts of Mexico, El Salvador, Costa Rica and Panamá. The balance between coral bioaccretion and sea urchin bioerosion was also calculated. Between 2009 and 2010, in 12 coral reefs localities, D. mexicanum density, bottom cover and rugosity were quantified along band transects. The daily bioerosion rate was obtained from the amount of carbonates evacuated by sea urchins per unit time. The rate of coral accretion was calculated by multiplying the coral growth rate of the dominant genus by the density of their skeleton and by their specific coral cover. The localities were dissimilar (R = 0.765, P < 0.001) in terms of live coral cover, crustose calcareous algae, turf cover, rugosity index, and density and size of D. mexicanum. At all sites, with the exception of Bahía Culebra (Costa Rica), coral bioerosion was less than coral bioaccretion. Diadema mexicanum plays a dominant role in the balance of carbonates in the ETP, but this depends on reef condition (protection, overfishing, eutrophication) and so the impacts can be either positive or negative.  相似文献   

16.
The vertical distribution of macrobenthic fauna, heavy metals, and other physico‐chemical and biological characteristics of the sediments were studied in three sediment layers (0–7, 8–14, 15–21 cm) at seven stations in the Ubatuba region, north coast of São Paulo State, Brazil at several temporal and spatial scales. Six stations were located in the inner bay near the riverine run‐off, and one was outside the bay, distant from the riverine influence. The samples were collected four times in 1 year, on a seasonal basis. Sediments were basis comprised predominantly of very fine sand and the vertical distribution of grain size was uniform to a depth of 21 cm in all stations. Higher values of total organic matter, organic carbon, sulphur, heavy metals and phaeopigments were recorded at the inner Ubatuba Bay stations, probably due to the riverine influence. C/N ratios indicated a mixed origin of organic matter with a major contribution of terrestrial material in the inner stations. The vertical distribution of heavy metals showed a slight decline with sediment depth in the inner stations, indicating the present contribution. Most of the macrofauna was found at the surface sediment layer. Biological data showed that in the inner stations of Ubatuba Bay, which are under the influence of urban sewage and are moderately polluted, the fauna was distributed more superficially within the substrate than in St. 7, which is located in the external portion of the bay distant from sewage inputs. The environmental quality of the sites studied varied little throughout the year, at least in relation to the variables considered here. Temporal variation in the vertical distribution of benthic fauna was not evident in the four sampling surveys analysed. Only minor changes in the vertical distribution of the total fauna were detected in the seasonal scale, with the organisms located less deep within the sediment column in summer, indicating some influence of the tourism impact and/or rainy season.  相似文献   

17.
The widespread occurrence of microbialites in the last deglacial reef frameworks (16–6 Ka BP) implies that the accurate study of their development patterns is of prime importance to unravel the evolution of reef architecture through time and to reconstruct the reef response to sea-level variations and environmental changes.The present study is based on the sedimentological and chronological analysis (14C AMS dating) of drill cores obtained during the IODP Expedition #310 “Tahiti Sea Level” on the successive terraces which typify the modern reef slopes from Tahiti. It provides a comprehensive data base to investigate the microbialite growth patterns (i.e. growth rates and habitats), to analyze their roles in reef frameworks and to reconstruct the evolution of the reef framework architecture during sea-level rise.The last deglacial reefs from Tahiti are composed of two distinctive biological communities: (1) the coralgal communities including seven assemblages characterized by various growth forms (branching, robust branching, massive, tabular and encrusting) that form the initial frameworks and (2) the microbial communities developed in the primary cavities of those frameworks, a few meters (1.5 to 6 m) below the living coral reef surface, where they heavily encrusted the coralgal assemblages to form microbialite crusts. The dating results demonstrate the occurrence of two distinctive generations of microbialites: the “reefal microbialites” which developed a few hundred years after coralgal communities in shallow-water environments, whereas the “slope microbialites” grew a few thousands of years later in significantly deeper water conditions after the demise of coralgal communities.The development of microbialites was controlled by the volume and the shape of the primary cavities of the initial reef frameworks determined by the morphology and the packing of coral colonies. The most widespread microbialite development occurred in frameworks dominated by branching, thin encrusting, tabular and robust branching coral colonies which built loose and open frameworks typified by a high porosity (> 50%). In contrast, their growth was minimal in compact coral frameworks formed by massive and thick encrusting corals where primary cavities yielded a low porosity (~ 30%) and could not host a significant microbialite expansion.  相似文献   

18.
Mucus films, flocs or foams consisting of fine sand, algae and detritus frequently occur in the surface waters of rocky intertidal reef flats during incoming tide. These masses are referred to as mucus aggregates. We examined the developmental process of mucus aggregates and their abundance, distribution, migration and trophic composition. The trophic composition of mucus aggregates was then compared to those of sediments to evaluate their potential nutritional value for benthic animals. The organic matter content, chlorophyll a concentration, microalgal density and bacteria-derived fatty acid contents of mucus aggregates were higher than those observed in sediment, suggesting that mucus aggregates contain not only high levels of organic matter but also dense concentrations of microalgae and bacteria; therefore, mucus aggregates may serve as a qualitatively more energetic food source for benthic fauna compared to sediments. Benthic diatoms were the most abundant organisms in mucus aggregates. Large numbers of diatoms were trapped in fine mineral particles and mucilage-like strings, suggesting that a portion of the mucus is secreted by these benthic microalgae.Mucus aggregate accounted for only 0.01-3.9% of the daily feeding requirements of the dominant detritivore, Ophiocoma scolopendrina (Echinodermata: Ophiuroidea) over the entire sampling area. In contrast, for the species population on the back reef, where mucus aggregates ultimately accumulate, mucus aggregates provided from 0.4 to 113.3% of food for this species. These results suggest that mucus aggregate availability varies spatiotemporally and that they do not always provide adequate food sources for O. scolopendrina populations.  相似文献   

19.
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20.
We investigated microphytobenthic photosynthesis at four stations in the coral reef sediments at Heron Reef, Australia. The microphytobenthos was dominated by diatoms, dinoflagellates and cyanobacteria, as indicated by biomarker pigment analysis. Conspicuous algae firmly attached to the sand grains (ca. 100 μm in diameter, surrounded by a hard transparent wall) were rich in peridinin, a marker pigment for dinoflagellates, but also showed a high diversity based on cyanobacterial 16S rDNA gene sequence analysis. Specimens of these algae that were buried below the photic zone exhibited an unexpected stimulation of respiration by light, resulting in an increase of local oxygen concentrations upon darkening. Net photosynthesis of the sediments varied between 1.9 and 8.5 mmol O2 m−2 h−1 and was strongly correlated with Chl a content, which lay between 31 and 84 mg m−2. An estimate based on our spatially limited dataset indicates that the microphytobenthic production for the entire reef is in the order of magnitude of the production estimated for corals. Photosynthesis stimulated calcification at all investigated sites (0.2–1.0 mmol Ca2+ m−2 h−1). The sediments of at least three stations were net calcifying. Sedimentary N2-fixation rates (measured by acetylene reduction assays at two sites) ranged between 0.9 to 3.9 mmol N2 m−2 h−1 and were highest in the light, indicating the importance of heterocystous cyanobacteria. In coral fingers no N2-fixation was measurable, which stresses the importance of the sediment compartment for reef nitrogen cycling.  相似文献   

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