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1.
Much of coral reef ecology has focused on how human impacts change coral reefs to macroalgal reefs. However, macroalgae may not always be a good indicator of reef decline, especially on reefs with significant sea urchin populations, as found in Kenya and Hawaii. This study tests the effects of trophic interactions (i.e. herbivory by fishes and sea urchins) and spatial competition (between algae and coral) on algal community structure of reefs surrounding two Hawaiian Islands that vary in their level of human impacts. Reef‐building organisms (corals and crustose coralline algae) were less abundant and turf algae were more abundant on Maui as compared to Lanai, where human impacts are lower. In contrast to previous studies, we found no evidence that macroalgae increased with human impacts. Instead, low turf and macroalgal abundance were best explained by the interactive effects of coral cover and sea urchin abundance. Fishing sea urchin predators appeared to have cascading effects on the benthic community. The absence of sea urchin predators and high sea urchin densities correspond to a disproportionately high abundance of turf and crustose coralline algae. We propose that high turf algal abundance is a better indicator of reef decline in Hawaii than high macroalgal abundance because turf abundance was highest on reefs with low coral cover and few fish. The results of this study emphasize that understanding changes in community composition are context‐dependent and that not all degraded reefs look the same.  相似文献   

2.
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   

3.
Sea urchins are important ecosystem engineers in subtidal ecosystems worldwide, providing biogenic structure and altering nutrient dynamics through intensive grazing and drift algal capture. The current work evaluates red urchin (Strongylocentrotus franciscanus) density on fixed transects through time, individual displacement, and urchin‐associated benthic community composition using a field‐based approach at multiple depths (in and outside of the macroalgal zone) and replicated across sites in the San Juan Archipelago, Washington. Urchins exhibited no large‐scale, temporal or directional changes in density among depths. Furthermore, 87% of individual urchins observed in repeated small‐scale surveys over 3 weeks exhibited no change in position. Individual displacement was negatively correlated to drift algal capture. Evidence of sedentary behavior from the displacement surveys was supported by the sessile and mobile community composition in areas directly under versus adjacent to (control) urchins. The benthos under urchins had a higher percentage of bare space, crustose coralline algae, and increased density of snails, crabs and shrimp relative to associated control plots. Abundance of mobile organisms associating with urchins increased relative to control plots at the deepest survey depth (30 m), indicating a greater strength of interaction with distance from macroalgal production. This work presents evidence of food availability‐related behavior in red urchins and indicates that even when sedentary, urchins have a strong influence on ecosystem structure through increasing availability of shelter and macroalgal detritus to the benthos.  相似文献   

4.
《Oceanologica Acta》1999,22(6):567-578
In New Caledonia, core data indicate that the fringing reefs grew during the last interglacial and the Holocene, while the barrier reefs developed during several high sea level stands of the Quaternary. These growth periods are archived in a 128-m-long core from Ilot Amédée, offshore of Nouméa. Directly upon the peridotitic substrate (reached at 126.50 m), a basal unit comprising abundant rhodoliths, molluscs and rare corals is characteristic of the pioneer stage of barrier reef development. Above it, a severely calcitized unit contains corals, molluscs and abundant micritic levels. Then, a sequence punctuated by several minor discontinuities and by a downward increase in diagenetic alteration is found. At 47 m core depth, a unit, which is thought to be related to isotopic stages 7 and 9, contains thin beds of coralgal frameworks and muddy detritus. The 125 ka-old reef, from 14 to 37 m core deep, is predominantly composed of biocalcarenites and rare coral colonies. Finally, the Holocene reef is composed of sands and scarce coral buildups. Drilling results indicate that the distributional patterns of the 125-ka-old reef bodies around New Caledonia express an increasing tendency of island subsidence northward, southwestward and more markedly seaward, mainly controlled by isostatic readjustments and margin collapse.  相似文献   

5.
Effective conservation requires knowledge of the effects of habitat on distribution and abundance of organisms. Although the structure of coral reef fish assemblages is strongly correlated with attributes of reef structure, data relating reef types to fish assemblages are scarce. In this study we describe the influence of gross habitat characteristics and seasonality on coral reef fish assemblages of fringing and patch reefs in Kenya. Results showed that total fish abundance was not significantly different between the reefs; however, the fringing reef had higher species diversity during both the northeast (42 spp.) and southeast (36 spp.) monsoon seasons when compared to the patch reef. The more fished species (e.g. Siganus sutor and Lethrinus mahsena) were more abundant on the patch reef in both seasons. Statistical analysis indicated common species between the reefs were more abundant on the fringing reef. Seasons affected abundance of the more vagile species (S. sutor), whereas the reef‐attached sky emperor, L. mahsena was affected more by reef type than by seasons. No significant interaction effects of habitat and seasons were found, indicating independence of habitat and environmental variability in affecting fish assemblages on the reefs. Smaller sized fish dominated the fringing reef more than the patch reef, whereas the skewness index (Sk) indicated a normal‐sized frequency distribution on the patch reef. Trophic structure of the fishes varied more within than between reefs, whereas fish assemblage structure was affected more by seasons on the fringing reef. These results suggest that conservation measures such as marine protected area (MPA) design and setting should consider effects of reef morphology and environmental variability on coral‐reef fish assemblage structure.  相似文献   

6.
As part of a broader study on benthic-pelagic coupling in the southern North Sea, specimens of the common heart urchin Echinocardium cordatum were sampled for analyses on phytopigments and fatty acids in their guts. Results were interpreted in the context of feeding and ecological functioning of the heart urchins in the benthic system. Ingestion selection factors for both component groups were relatively high, 5 to 9 for chlorophyll a and 9 to 130 for total fatty acids. The data point to at least partially different sources of the pigments and of the fatty acids. Next to algal detritus, small infauna relatively rich in fatty acids might be preferentially co-ingested with the detritus. Due to digestive breakdown and absorption, the concentrations of pigments and fatty acids were importantly decreased, indicating a rather high digestion efficiency for this subsurface deposit feeder, up to 80%. The results indicate that E. cordatum increases its energy acquisition by strong selectivity and a high digestive efficiency. Optimal foraging is likely to apply on deposit-feeding invertebrates in relatively food-rich coastal environments as much as it does in the food-poor deep-sea environment. Using chlorophyll a as a proxy for carbon, the contribution of the urchin population to the momentary benthic carbon budget was calculated at 7% to 42%.  相似文献   

7.
Numerous studies of interactions between urchins and algae in temperate areas have shown an important structuring effect of sea urchin populations. These studies focused almost wholly on the effect of high urchin densities on laminarian forests. In contrast, algal communities below 5–6 m depth in the northwestern Mediterranean are characterised by low sea urchin densities (<5 ind m−2) and the absence of laminarian forests. No previous research has addressed sea urchin/algal interactions in this type of community. To determine the effect of the most abundant echinoid species, Paracentrotus lividus, on well-established algal communities in this area, we performed a removal–reintroduction experiment in rocky patches located between 13 and 16 m depth in the northwestern Mediterranean, where sea urchin densities ranged between 0.9 and 3.4 ind m−2. After 6 months, the cover of non-crustose algae was significantly higher in the plots from which sea urchins had been removed than in control plots (84 vs 67% cover). These removal plots reverted to their original state upon reintroduction of sea urchins. The non-crustose algae consisted of turfing and frondose forms, with the former representing some 70% of the non-crustose algal cover. Change in the cover of turfing algae was responsible for the significant increase in algal development in the sea urchin removal plots. The response of frondose algae to the treatment varied between algal species. It is concluded that grazing by P. lividus exerts a significant effect on habitat structure, even in communities with low sea urchin densities, such as those found in vast areas of the Mediterranean sublittoral.  相似文献   

8.
南沙群岛永暑礁7个表层礁坪原生块状珊瑚的高精度TIMS U-Th和12个常规^14C测年结果表明,永暑礁表层礁坪形成于现代,与钻探研究结果吻合。全新世永暑礁系从约7300aBP开始,以现代礁坪面以下17m左右的晚更新世礁灰岩为基底连续发育至今,因此,与南海周边地区的珊瑚礁不同,南沙群岛珊瑚礁的表层块状珊瑚不能够提供全新世高海平面的证据。由于现代珊瑚礁的堆积速率大于地壳沉降速率,可能会形成更多的灰沙洲或灰沙岛。  相似文献   

9.
Previous field observations have suggested an association between the urchin Parechinus angulosus and juveniles of the abalone Haliotis midae. To test the generality and nature of this association, surveys were carried out at five sites between Cape Point and Danger Point in the kelp beds of the South-Western Cape, South Africa. These showed that both species occupy primarily hard substrata, showing preferences for encrusting coralline algae. They also confirmed a strong, positive relationship between urchins and juvenile abalone. Of the juvenile abalone sampled, more than 98% were found beneath sea urchins. All small (3–10 mm) and medium-sized(11–20 mm) juvenile abalone were under urchins, whether on flat or vertical reef, or in crevices. A small proportion (～10%) of larger juveniles(21–35 mm) was not found under urchins, and in these instances they occupied crevices instead. These findings are of particular importance in terms of their implications for the lucrative commercial abalone fishery in South Africa, indicating that urchins are of critical importance to the continued survival of viable abalone populations. There has been a dramatic decrease in natural populations of sea urchins over the past five years in the heart of the abalone fishing grounds, and the present findings suggest that this will lead to recruitment failure of abalone, because juvenile abalone seem dependent on the urchins. The long-term consequences for the industry may be crucial.  相似文献   

10.
珊瑚礁是全球生物多样性最高的海洋生态系统之一,底栖贝类是该生态系统的重要组成类群。为了解北部湾涠洲岛珊瑚礁底栖贝类的群落现状及特征,于2015年秋季(10月)与2018年春季(5月)采用水肺潜水截线样条定量调查法对涠洲岛珊瑚礁区6个断面的底栖贝类进行了调查,并分析了物种组成、丰度、生物多样性指数等群落特征。综合两次调查结果显示涠洲岛珊瑚礁区共有底栖贝类128种,分别属于多板纲1科1属3种,腹足纲25科46属68种,双壳纲22科31属57种。优势种为斑顶拟舌骨牡蛎、粗衣蛤、刺荔枝螺、马蹄螺、杂色牙螺、青蚶、旗江珧、甲虫螺、蕾丝蟹守螺、珠母爱尔螺。2018年春季定量断面采集到的样品为2纲14科43种,各断面的丰度、生物量、多样性指数、物种丰度指数和均匀度指数均值分别为3.39个/m2、86.94 g/m2、3.31、3.50、0.37。通过对两年的调查数据比较,发现2015?2018年涠洲岛珊瑚礁区的贝类生物群落呈现良好演替发育趋势。南海珊瑚礁区贝类群落结构可能受到了人为干扰强度和纬度的双重影响。本研究全面掌握了涠洲岛珊瑚礁底栖贝类的种类、分布区及群落的结构与变化,可为该地区海洋生物资源开发利用、珊瑚礁保护和生态修复等工作提供数据支持。  相似文献   

11.
Abstract Lytechinus variegatus and Arbacia punctulata have been studied primarily in inshore, shallow‐water areas. However, they are abundant in deeper waters on the Florida gulf‐coast shelf and seem important components of the benthic communities there. Lytechinus variegatus occurs alone on sand bottoms and A. punctulata occurs alone on rubble bottoms in these deeper waters. The species also co‐occur there on ­heterogeneous bottoms, each in a distinct microhabitat with A. punctulata on rubble and L. variegatus on surrounding sand. Characteristics of the sea urchins in these different deeper‐water habitat types and at one nearshore site with a heterogeneous rubble‐sand bottom were compared. Over the 2‐year study, offshore individuals of both species had low gut and gonad indices and the maximum size of individuals did not change. This suggests food limitation and low production. Offshore, A. punctulata had a higher Aristotle's lantern index and lower gut and gonad indices in populations where it ­co‐occurred with L. variegatus compared to populations where it occurred alone. The ­Aristotle's lantern index of L. variegatus did not differ among the offshore sites. Neither species seemed food limited at the nearshore site. Although productivity is lower at the offshore sites, both species extend their distribution and reproduction potential by existing there.  相似文献   

12.
Quantitative research on composition, biomass and production rates of zooplankton community is crucial to understand the trophic structure in coral reef pelagic ecosystems. In the present study, micro‐ (35–100 μm) and net‐ (>100 μm) metazooplankton were investigated in a fringing coral reef at Tioman Island of Malaysia. Sampling was done during the day and night in August and October 2004, and February and June 2005. The mean biomass of total metazooplankton (i.e. micro + net) was 3.42 ± 0.64 mg C·m?3, ranging from 2.32 ± 0.75 mg C·m?3 in October to 3.26 ± 1.77 mg C·m?3 in August. The net‐zooplankton biomass exhibited a nocturnal increase from daytime at 131–264% due to the addition of both pelagic and reef‐associated zooplankton into the water column. The estimated daily production rates of the total metazooplankton community were on average 1.80 ± 0.57 mg C·m?3·day?1, but this increased to 2.51 ± 1.06 mg C·m?3·day?1 if house production of larvaceans was taken into account. Of the total production rate, the secondary and tertiary production rates were 2.20 ± 1.03 and 0.30 ± 0.06 mg C·m?3·day?1, respectively. We estimated the food requirements of zooplankton in order to examine the trophic structure of the pelagic ecosystem. The secondary production may not be satisfied by phytoplankton alone in the study area and the shortfall may be supplied by other organic sources such as detritus.  相似文献   

13.
Abstract. The population densities, spatial distributions, size frequencies, growth rates, longevity and reproductive activities of sub‐populations of the sea urchin Lytechinus variegatus were investigated over a two‐year period. Sea urchins were examined in three habitats in Saint Joseph Bay, Florida, which is within the northern limits of their distribution. Densities of sea urchins, which ranged as high as 35 individuals ·?2, fluctuated seasonally at all sites and were higher in seagrass beds comprised of Thalassia testudinum than Syringodium filiforme or on a sand flat. A cold front caused large‐scale, catastrophic mortality among adult, and especially juvenile, sea urchins in nearshore habitats of the Bay in the spring of 1993, leading to a dramatic decline in sea urchin densities at the Thalassia seagrass site. The population recovered over 6 months at this site and was attributable to immigration of new adults. Juvenile recruitment displayed both interannual and site‐specific variability, with recruitment being highest in seagrass habitats in fall and spring. The most pronounced recruitment event occurred in fall 1993 at the Thalassia site. Spatial distributions of adult individuals ascertained monthly never varied from random in the seagrass beds (T. testudinum and S. filiforme) or during spring, summer or fall months on the sand flat. Nonetheless, aggregations of adult sea urchins were observed on the sand flat in the winter months and were associated with patchy distributions of plant food resources. Juvenile sea urchins (< 25 mm test diameter) exhibited aggregations at all sites and 67 % of all juveniles under 10 mm test diameter (91 of 165 individuals observed) were found under the spine canopies of adults. Measurements of the inducibility of spawning indicated peak gametic maturity in all three sub‐populations in spring and summer. Gonad indices varied between habitats and years, but distinct maxima were detected, particularly in spring 1993 and late summer 1994. The mean gonad index of individuals at the Syringodium seagrass site was 2‐ to 4‐fold higher than the other sites in spring 1993 and gonad indices were much higher at all sites in spring of 1993 than 1994. Estimates of growth based on changes in size frequency cohorts coupled with measurements of growth bands on lantern demipyramids indicated that L. variegatus in three habitats of Saint Joseph Bay have similar growth rates and attain a mean test diameter of approximately 35 mm in one year. In contrast to populations within the central biogeographical range of the species, which may attain test diameters up to 90 mm, the largest individuals recorded in Saint Joseph Bay were 60 mm in test diameter, and almost all individuals were no more than 45 mm in test diameter or two years of age. The demographics of L. variegatus in the northern limits of their distribution appear to be strongly influenced by latitudinally driven, low‐temperature events and secondarily by local abiotic factors, especially springtime low salinities, which may negatively impact larval development and recruitment.  相似文献   

14.
During the strong warm El Niño (EN) that occurred in 1997/98, Independence Bay (14°S, Peru) showed a ca. 10 °C increase in surface temperatures, higher oxygen concentrations, and clearer water due to decreased phytoplankton concentrations. Under these quasi-tropical conditions, many benthic species suffered (e.g. macroalgae, portunid crabs, and polychaetes) while others benefited (e.g. scallop, sea stars, and sea urchins). The most obvious change was the strong recruitment success and subsequent proliferation of the scallop Argopecten purpuratus, whose biomass increased fiftyfold. To understand these changes, steady-state models of the bay ecosystem trophic structure were constructed and compared for a normal upwelling year (1996) and during an EN (1998), and longer-term dynamics (1996–2003) were explored based on time series of catch and biomass using Ecopath with Ecosim (EwE) software. Model inputs were based on surveys and landings data collected by the Instituto del Mar del Perú (IMARPE). Results indicate that while ecosystem size (total throughput) is reduced by 18% during EN, mainly as a result of decreased total primary production, benthic biomass remains largely unchanged despite considerable shifts in the dominant benthic taxa (e.g. scallops replace polychaetes as secondary consumers). Under normal upwelling conditions, predation by snails and crabs utilize the production of their prey almost completely, resulting in more efficient energy flow to higher trophic levels than occurs during EN. However during EN, the proliferation of the scallop A. purpuratus combined with decreased phytoplankton increased the proportion of directly utilized primary production, while exports and flows to detritus are reduced. The simulations suggest that the main cause for the scallop outburst and for the reduction in crab and macroalgae biomass was a direct temperature effect, whereas other changes are partially explained by trophic interactions. The simulations suggest that bottom-up effects largely control the system.  相似文献   

15.
The effect of harvesting on the diversity and structure of benthic assemblages is nowadays a relevant component of fishery management. Invertebrate harvesting is increasing worldwide and sea urchins are one of the main invertebrate target species. However, current knowledge about the effects of sea urchin harvesting on benthic assemblages is still poor. The aim of this study is to evaluate the influence of Paracentrotus lividus (Lamark, 1816) harvesting on tide pool assemblages from intertidal rocky shores. To achieve this objective, a field removal experiment was done for 10 months. Results of univariate and multivariate analyses showed that P. lividus harvesting had a significant effect on the number of taxa and structure of the benthic assemblage. Assemblages from pools where P. lividus were removed showed significant differences against controls; namely, where P. lividus were eliminated, pools showed higher number of taxa than control pools. This result contrasts with previous studies assessing effects of invertebrate harvesting that generally have found a biodiversity reduction in the exploited habitat. Despite this apparently positive effect of P. lividus harvesting, we should be cautious because effects of P. lividus harvesting on higher trophic levels or during long periods of time remain unexplored.  相似文献   

16.
In kelp beds of the South-Western Cape, South Africa, a strong positive relationship exists between the sea urchin Parechinus angulosus and juveniles of the abalone Haliotis midae. Field surveys reported here revealed a positive, but weak, association between this urchin and H. midae recruits (i.e. individuals <3 mm shell length). Selectivity indices showed that recruits occurred exclusively on encrusting coralline algae and showed preferences for strongly textured corallines, rather than those with smooth texture. On smooth corallines, around 80% of recruits were under urchins, compared with about 25% on textured corallines, suggesting that any need for shelter beneath urchins is greatest on smooth surfaces.  相似文献   

17.
As coral reef ecosystems decline in health worldwide, reef‐associated fishes are being impacted by changes to their coral reef habitats. While previous studies have shown coral reef structure to affect the demography of reef fishes, changes in reef conditions may also impact the behavior of reef fishes as they cope with altered habitats. In this study, we examined spatial patterns of intraspecific behavioral variation in the bicolor damselfish (Stegastes partitus) across the fringing reefs of Curaçao (Caribbean Sea), and explored how this behavioral variation associated with physical and social conditions on the reef. Principal components analysis (PCA) condensed physical parameters of the reef into principal component 1 (PC1), comprising depth, coral cover (%), rugosity, and average hole size (cm2), and principal component 2 (PC2), which represented the number of holes. PC1, but not PC2, increased spatially across the reef as the habitat transitioned from coral rubble in the shallows to live coral on the reef slope. This transition in reef structure was paralleled by changes in social conditions including decreases in bicolor damselfish density in habitats with higher PC1 values. The behavior of bicolor damselfish also varied spatially with greater aggression and more frequent shelter use in habitats with lower PC1 values. Path analysis revealed robust associations between this behavioral variation and physical habitat conditions of the reef, indicating that physical – rather than social – habitat variation is the primary determinant of these spatial patterns of intraspecific behavioral variation. Taken as a whole, this coupling between physical reef structure and behavior suggests that reef fish may show altered behaviors on coral reefs degraded by anthropogenic impacts.  相似文献   

18.
Recent settlers of many marine benthic invertebrates are cryptic, which exposes them to a suite of animals that differs from those they may experience as adults, potentially resulting in interactions causing mortality and/or reducing growth. Previous field experiments have indicated that such is the case with small juvenile green sea urchins Strongylocentrotus droebachiensis but which taxa are responsible for the mortality and reduced growth was not determined. A laboratory study was conducted to examine the effects of small macro‐benthic invertebrates, specifically chitons, scaleworms and larger juvenile conspecifics, as well as the full suite of cobble‐dwelling organisms, on the mortality, growth and behaviour of small (<3 mm) juvenile sea urchins. The likelihood of survival of small juvenile sea urchins was lower in the presence of larger juvenile sea urchins or with the full suite of cobble‐dwelling organisms than in the absence of animals. The small juvenile sea urchins survived and grew the best when they were with chitons and scaleworms. The behaviour of small sea urchins with the full suite of cobble‐dwelling organisms was more cryptic than the behaviour of urchins with scaleworms. This study indicates that interactions with the suite of small organisms living amongst cobbles can affect survival, growth and behaviour of small juvenile sea urchins, and that larger juvenile sea urchins can be a source of mortality for smaller conspecifics.  相似文献   

19.
Population outbreaks of crown-of-thorns sea star (Acanthaster planci L.) remain one of the most significant biological disturbances on tropical coral reefs although the increasing attention given to other threats has greatly limited recent progress in understanding the cause and consequences of this phenomenon. In September 2005 dramatic increases in the abundance of A. planci were observed on reefs within Bootless Bay, Central Province, Papua New Guinea, where few crown-of-thorns have previously been reported. Densities of A. planci peaked at 162 sea stars per hectare and caused extensive coral mortality. This outbreak killed upwards of 55% of live corals, reducing overall coral cover from 42.4% in 2005 down to just 19.1% in March 2006. Declines in coral cover were largely driven by widespread mortality of Acropora spp. which dominated reef assemblages prior to the outbreak. The extensive depletion of Acropora spp. greatly altered the coral composition as well as the physical structure of reef habitats. In the absence of any other major disturbances these coral communities are likely to quickly recover, but this outbreak highlights the ongoing contribution of A. planci to degradation of coral reef environments.  相似文献   

20.
This paper examines fluctuations in water level over a Caribbean reef flat at Punta Galeta, Panamá. In an analysis of approximately ten years of records, the mean diurnal range of the tides was 24·5 cm and varied <2 cm from year to year. Daily mean water levels varied erratically over a range of approximately 30 cm. Monthly mean water levels fluctuated seasonally over a range of about 10 cm and were consistently higher than those at the regional tidal reference station at Cristóbal. On days with more wave action, water levels at Galeta increased relative to those at Cristóbal, suggesting that waves were ‘pumping’ water onto the reef flat. The monthly mean water levels at the two sites were not correlated, indicating that tidal data from conventional stations in deeper water cannot be extrapolated to reef flats, except as estimates of minimum potential water levels.Most of the reef flat was within 6 cm vertical span just below mean lower low water. The highest elevations within this range were exposed above water level for an average of 918 hours per year, as compared with 144 hours per year at the lowest elevation. Most exposures lasted less than 10 hours, with a modal duration of 3–5 hours; however, exposures longer than 12 hours occurred at nearly all elevations in all years. Exposures of the reef flat were most frequent between February and June and between August and November, a pattern apparently caused by a combination of seasonal oscillations of regional sea levels, the annual pattern of the solstitial tides, and waves generated by seasonal trade winds.The fluctuations in water levels apparently affected the abundances of some species of sea urchins on the reef flat. Populations of Lytechinus variegatus and Diadema antillarum declined or disappeared from the reef flat during seasons of repeated subaerial exposures, but recolonized the habitat in periods of higher water levels. Although they are reported to suffer heavy mortality during emersion, Echinometra lucunter and E. viridis did not exhibit reductions in abundance that were synchronous with the seasonal exposures of the reef flat.  相似文献   

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