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1.
The lack of top-down control on Tripneustes gratilla, a sea urchin commonly known to graze on seagrass, and the bottom-up control of its feeding preference, led to the overgrazing of seagrass meadows of the species Thalassodendron ciliatum in Changuu Island (Zanzibar Archipelago). The impact of overgrazing on seagrasses was assessed by mapping the presence of grazed versus non-grazed seagrass patches in the study site, while the top-down control on T. gratilla was assessed by measuring the abundance of its fish predators. The feeding preference and distribution of T. gratilla were characterized by calculating the electivity indexes for each seagrass species and measuring sea urchin density, respectively. Approximately half of the patches of T. ciliatum were overgrazed, while predatory fishes of T. gratilla were absent from the site. The Vanderploeg and Scavia's Relativized Electivity Index indicated that T. gratilla had a feeding preference for T. ciliatum, which was also supported by higher urchin densities within T. ciliatum dominated patches. Bottom-up control of grazing activity was observed by quantifying and analyzing morphological, nutritional, and the chemical defense traits of the seagrass in relation to feeding preference and urchin density. Feeding was positively correlated to the seagrass tissue C:P ratio (ρ = 0.9), whereas urchin density showed no correlations. The bottom-up control of the feeding preference and agglomeration of T. gratilla in T. ciliatum meadows, together with the lack of evidence of substantial top-down control and the long recovery time of T. ciliatum led to the overgrazing of this species at this site. Overgrazing, therefore, was shown to be the result of multiple factors ranging from the traits of the seagrass and feeding preference of T. gratilla, to the abundance of predators in this area.  相似文献   

2.
Numerical simulations using a physiologically-based model of marine ecosystem size spectrum are conducted to study the influence of primary production and temperature on energy flux through marine ecosystems. In stable environmental conditions, the model converges toward a stationary linear log–log size-spectrum. In very productive ecosystems, the model predicts that small size classes are depleted by predation, leading to a curved size-spectrum.It is shown that the absolute level of primary production does not affect the slope of the stationary size-spectrum but has a nonlinear effect on its intercept and hence on the total biomass of consumer organisms (the carrying capacity). Three domains are distinguished: at low primary production, total biomass is independent from production changes because loss processes dominate dissipative processes (biological work); at high production, ecosystem biomass is proportional to primary production because dissipation dominates losses; an intermediate transition domain characterizes mid-production ecosystems. Our results enlighten the paradox of the very high ecosystem biomass/primary production ratios which are observed in poor oceanic regions. Thus, maximal dissipation (least action and low ecosystem biomass/primary production ratios) is reached at high primary production levels when the ecosystem is efficient in transferring energy from small sizes to large sizes. Conversely, least dissipation (most action and high ecosystem biomass/primary production ratios) characterizes the simulated ecosystem at low primary production levels when it is not efficient in dissipating energy.Increasing temperature causes enhanced predation mortality and decreases the intercept of the stationary size spectrum, i.e., the total ecosystem biomass. Total biomass varies as the inverse of the Arrhenius coefficient in the loss domain. This approximation is no longer true in the dissipation domain where nonlinear dissipation processes dominate over linear loss processes. Our results suggest that in a global warming context, at constant primary production, a 2–4 °C warming would lead to a 20–43% decrease of ecosystem biomass in oligotrophic regions and to a 15–32% decrease of biomass in eutrophic regions.Oscillations of primary production or temperature induce waves which propagate along the size-spectrum and which amplify until a “resonant range” which depends on the period of the environmental oscillations. Small organisms oscillate in phase with producers and are bottom-up controlled by primary production oscillations. In the “resonant range”, prey and predators oscillate out of phase with alternating periods of top-down and bottom-up controls. Large organisms are not influenced by bottom-up effects of high frequency phytoplankton variability or by oscillations of temperature.  相似文献   

3.
Processes acting on the early-life histories of marine organisms can have important consequences for the structuring of benthic communities. In particular, the degree of coupling between larval supply and adult abundances can wield considerable influence on the strength of trophic interactions in the ecosystem. These processes have been relatively well described in rocky systems and soft-sediment communities, and it is clear that they are governed by very different bottlenecks. Seagrass meadows make interesting study systems because they bear structural affinities to both soft sediments as well as rocky substrates. We examined the early-life history of Paracentrotus lividus, one of the dominant herbivores in Mediterranean seagrass meadows, to identify the drivers of population dynamics in this species. We measured spatial and temporal variability in sea urchin post-settlement in 10 Posidonia oceanica meadows in the North-Western Mediterranean over a period of two years, and compared the numbers with the one-year old cohort a year later (i.e. the new population recruitment) as well as between successive size–age groups. Urchin post-settlers differed substantially between meadows but were present in both years in all meadows surveyed, suggesting that larval supply was not limiting for any of the studied sites. However, in six of the studied meadows, the one-year cohort of urchins was absent in both years, indicating that post-settlement processes strongly affected urchins in these meadows. In contrast, in four of the studied meadows, there was a strong coupling between post-settlers and one-year cohort individuals. These meadows were structurally different from the others in that they were characterised by an exposed matrix of rhizomes forming a dense seagrass mat. This mat apparently strongly mediates post-settlement mortality, and its presence or absence dictates the successful establishment of urchin populations in seagrass meadows. As the population aged, the relationship between size–age groups decreased evidencing the action of other processes. Yet, these results indicate that differences in physical structure are a vital bottleneck for sea urchin populations in seagrass meadows. Exploring the interaction between ecosystem structure and early-life history may provide a broader and more unified framework to understand the dynamics of a range of benthic habitats, including rocky substrates, soft sediments and seagrass meadows.  相似文献   

4.
Control of walleye pollock (Theragra chalcogramma) recruitment in the Eastern Bering Sea involves complex interactions between bottom-up and top-down processes, although the mechanisms are poorly understood. We used statistical models to test the leading hypotheses linking recruitment variability to biotic and abiotic factors. Consistent with a “cold-pool hypothesis”, recruitment of pollock was significantly stronger if winters preceding the larval (age-0) and juvenile stages (age-1) were mild. However, our results did not support the proposed top-down mechanism (cannibalism) underlying this hypothesis. Several empirical relationships support an “oscillating control hypothesis”. As predicted by it, the effect of ice conditions on survival during the larval and early juvenile stages was modified by the abundance of adult pollock, implying stronger bottom-up control when adult abundance (hence cannibalism) was low. The proposed bottom-up mechanism predicts that the survival of pelagic-feeding walleye pollock (benthic-feeding yellowfin sole), should be higher during years with an early (late) ice retreat, which was confirmed by our analysis. Our results also provide additional evidence for a “larval transport hypothesis”, which states that cannibalism of larval and juvenile pollock is reduced in years when strong northward advection separates juveniles from cannibalistic adults.In addition to testing existing hypotheses, we identified new relationships between spawner-to-recruit survival rates of walleye pollock and several indicators of mixed layer dynamics during the spring and summer. Survival rates and recruitment were significantly reduced when larval or early juvenile stages experienced a delay in the (non-ice-associated) spring bloom as a result of stormy spring conditions, suggesting that the timing of the spring bloom is critical to both first-feeding larvae and age-1 juveniles. Furthermore, a dome-shaped relationship between pollock survival and summer wind mixing at the early juvenile stage is consistent with modeling and laboratory studies showing an increase in survival at low to moderate levels of wind mixing, but a decrease in feeding success at high levels of wind mixing.Top-down controls also regulate recruitment of walleye pollock. At least one-third of the variability in spawner-to-recruit survival could be accounted for by predation mortality at the early juvenile stage (age-1). Predation of juvenile pollock can be attributed largely to cannibalism, which varies with the abundance of adult pollock and with the availability of juveniles to adult predators. A simple index reflecting the spatial overlap between juvenile and adult pollock explained 30–50% of the overall variability in recruitment, similar to the variability explained by the best environmental predictors. Although environmental effects are difficult to separate from the effects of predation, we conclude that bottom-up and top-down processes are equally important in controlling the survival of pollock from spawning to recruitment at age 2. However, the magnitude of top-down control is itself modified by environmental factors that control the availability of juvenile pollock to adults (through impacts on spatial distribution) and the abundance of adult predators (through effects on productivity and carrying capacity).  相似文献   

5.
Acetylene reduction activity at a tropical seagrass bed in Papua New Guinea was studied. Blades of every species of seagrass studied (Enhalus acoroides, Thalassia hemprichii, Halodule uninervis, Syringodium isoetifolium) at the bed showed active acetylene reduction in the rage of 0.025–0.050 mol cm–2d–1. It is suggested that epiphytic blue-green algae would be responsible for active acetylene reduction. Acetylene reducing activity was observed also at rhizosphere of seagrasses, microbial communities on the detritus and periphyton on snail shells. It is estimated that total nitrogen fixed at the seagrass bed will be equivalent to 6.7% of that required for the growth of seagrasses.  相似文献   

6.
Previous studies conducted on a local scale emphasised the potential of trophic cascades in Mediterranean rocky reefs (involving predatory fish, sea urchins and macroalgae) in affecting the transition between benthic communities dominated by erected macroalgae and barrens (i.e., bare rock with partial cover of encrusting algae). Distribution patterns of fish predators of sea urchins (Diplodus sargus sargus, Diplodus vulgaris, Coris julis and Thalassoma pavo), sea urchins (Paracentrotus lividus and Arbacia lixula) and barrens, and fish predation rates upon sea urchins, were assessed in shallow (3-6m depth) sublittoral rocky reefs in the northern, central and southern sectors of the eastern Adriatic Sea, i.e., on a large spatial scale of hundreds of kilometres. No dramatic differences were observed in predatory fish density across latitude, except for a lower density of small D. sargus sargus in the northern Adriatic and an increasing density of T. pavo from north to south. P. lividus did not show any significant difference across latitude, whereas A. lixula was more abundant in the southern than in the central Adriatic. Barrens were more extended in the southern than in the central and northern sectors, and were related with sea urchin density. Fish predation upon adult sea urchins did not change on a large scale, whereas it was slightly higher in the southern sector for juveniles when predation rates of both urchins were pooled. Results show that: (1) assemblages of predatory fish and sea urchins, and barren extent change across latitude in the eastern Adriatic Sea, (2) the weak relations between predatory fish density and predation rates on urchins reveal that factors other than top-down control can be important over large scale (with the caveat that the study was conducted in fished areas) and (3) patterns of interaction among strongly interacting taxa could change on large spatial scales and the number of species involved.  相似文献   

7.
In October 2004, the North Pacific Marine Science Organization (PICES) sponsored a symposium to consider “Mechanisms that regulate North Pacific ecosystems: Bottom up, top down, or something else?” It sought to examine how marine populations, particularly the upper-trophic-level species, are regulated and to understand how energy flows through marine ecosystems. This introductory essay examines aspects of control mechanisms in pelagic marine ecosystems and some of the issues discussed during the symposium and in the 11 papers that were selected for this special issue. At global scales, the greatest biomass of fishes, seabirds and marine mammals tends to occur in regions of the world ocean with high primary production, e.g., the sub-arctic seas and up-welling regions of continental shelves. These large-scale animal distribution patterns are driven by food availability, not the absence of predators. At regional scales however, it is likely that current predation or past predation events have shaped local distributions, at least in marine birds and pinnipeds. Wasp-waist control occurs when one of the intermediate trophic levels is dominated by a single species, as occurs with small pelagic fishes of the world’s up-welling zones. Processes in these ecosystems may have features that result in a switch from bottom-up to top-down control.  相似文献   

8.
In this paper we present results from dynamic simulations of the Northern California Current ecosystem, based on historical estimates of fishing mortality, relative fishing effort, and climate forcing. Climate can affect ecosystem productivity and dynamics both from the bottom-up (through short- and long-term variability in primary and secondary production) as well as from the top-down (through variability in the abundance and spatial distribution of key predators). We have explored how the simplistic application of climate forcing through both bottom-up and top-down mechanisms improves the fit of the model dynamics to observed population trends and reported catches for exploited components of the ecosystem. We find that using climate as either a bottom-up or a top-down forcing mechanism results in substantial improvements in model performance, such that much of the variability observed in single species models and dynamics can be replicated in a multi-species approach. Using multiple climate variables (both bottom-up and top-down) simultaneously did not provide significant improvement over a model with only one forcing. In general, results suggest that there do not appear to be strong trophic interactions among many of the longer-lived, slower-growing rockfish, roundfish and flatfish in this ecosystem, although strong interactions were observed in shrimp, salmon and small flatfish populations where high turnover and predation rates have been coupled with substantial changes in many predator populations over the last 40 years.  相似文献   

9.
Seagrasses are habitats with significant ecological and economic functions but we have limited knowledge of seagrasses in Southeast Asia, the hypothesized centre-of-origin for tropical seagrasses. There have been only 62 ISI-cited publications on the seagrasses of Southeast Asia in the last three decades and most work has been in few sites such as Northwest Luzon in the Philippines and South Sulawesi in Indonesia. Our understanding of the processes driving spatial and temporal distributions of seagrass species here has focussed primarily on backreef and estuarine seagrass meadows, with little work on forereef systems. We used Pulau Tinggi, an island off the southeast coast of Peninsular Malaysia, as an example of a subtidal forereef system. It is characterized by a community of small and fast growing species such as Halophila ovalis (mean shoot density 1454.6 ± 145.1 m−2) and Halodule uninervis (mean shoot density 861.7 ± 372.0 m−2) growing in relatively low light conditions (mean PAR 162.1 ± 35.0 μmol m−2 s−1 at 10 m depth to 405.8 ± 99.0 μmol m−2 s−1 at 3 m water depth) on sediment with low carbonate (mean 9.24 ± 1.74 percentage dry weight), organic matter (mean 2.56 ± 0.35 percentage dry weight) and silt-clay content (mean 2.28 ± 2.43 percentage dry weight). The literature reveals that there is a range of drivers operating in Southeast Asian seagrass systems and we suggest that this is because there are various types of seagrass habitats in this region, i.e. backreef, forereef and estuary, each of which has site characteristics and ecological drivers unique to it. Based on our case study of Pulau Tinggi, we suggest that seagrasses in forereef systems are more widespread in Southeast Asia than is reflected in the literature and that they are likely to be driven by recurring disturbance events such as monsoons, sediment burial and herbivory.  相似文献   

10.
Surveys were undertaken on the shallow Bermuda marine platform between 2006 and 2008 to provide a baseline of the distribution, condition and environmental characteristics of benthic communities. Bermuda is located in temperate latitudes but coral reefs, tropical seagrasses and calcareous green algae are common in the shallow waters of the platform. The dominant organisms of these communities are all living at or near their northern latitudinal range limits in the Atlantic Ocean. Among the major benthic autotrophs surveyed, seagrasses were most restricted by light availability. We found that the relatively slow-growing and long-lived seagrass Thalassia testudinum is restricted to habitats with much higher light availability than in the tropical locations where this species is commonly found. In contrast, the faster growing tropical seagrasses in Bermuda, Syringodium filiforme, Halodule sp. and Halophila decipiens, had similar ecological compensation depths (ECD) as in tropical locations. Increasing sea surface temperatures, concomitant with global climate change, may either drive or allow the poleward extensions of the ranges of such tropical species. However, due to latitudinal light limitations at least one abundant and common tropical autotroph, T. testudinum, is able to occupy only shallower depths at the more temperate latitudes of Bermuda. We hypothesize that the poleward shift of seagrass species ranges would be accompanied by restrictions to even shallower depths of T. testudinum and by very different seagrass community structures than in tropical locations.  相似文献   

11.
Heavy nutrient loads in coastal waters often lead to excessive growth of microalgal and macroalgal epiphytes on seagrass leaves, with varying effects on the underlying seagrasses. This study evaluates the photosynthetic performance, epiphytic biomass and tissue nutrient content of two tropical seagrasses, Cymodocea serrulata and Thalassia hemprichii, in two intertidal areas along the Dar es Salaam coast in the Indian Ocean, a nutrient-rich region at Ocean Road (near the city centre), and a nutrient-poor region at Mjimwema (south of the city centre). Epiphyte biomass was significantly higher at the nutrient-rich site, and epiphytes were associated with reduced photosynthetic performance in both seagrass species at both sites. Likewise, nitrogen and phosphorus tissue content was higher in both species at the nutrient-rich site than at the nutrient-poor site. Epiphytic species composition on the seagrass leaves varied between seagrass species and between sites. Cymodocea serrulata had a higher number of epiphytic species at Mjimwema than at Ocean Road, whereas Thalassia hemprichii had more epiphytic species at Ocean Road than at Mjimwema. Seagrass photosynthetic performance, epiphytic biomass and nutrient content of the seagrasses were shown to be affected by nutrient concentration in the water column. Thus, for the future monitoring of the seagrass meadow, we recommend the use of combined measures such as seagrass performance, epiphytic biomass, nutrient contents and nutrient concentration levels in the water column.  相似文献   

12.
Fauna species living in seagrass meadows depend on different food sources, with seagrasses often being marginally important for higher trophic levels. To determine the food web of a mixed-species tropical seagrass meadow in Sulawesi, Indonesia, we analyzed the stable isotope (δ13C and δ15N) signatures of primary producers, particulate organic matter (POM) and fauna species. In addition invertebrates, both infauna and macrobenthic, and fish densities were examined to identify the important species in the meadow. The aims of this study were to identify the main food sources of fauna species by comparing isotopic signatures of different primary producers and fauna, and to estimate qualitatively the importance of seagrass material in the food web. Phytoplankton and water column POM were the most depleted primary food sources for δ13C (range −23.1 to −19.6‰), but no fauna species depended only on these sources for carbon. Epiphytes and Sargassum sp. had intermediate δ13C values (−14.2 to −11.9‰). Sea urchins, gastropods and certain fish species were the main species assimilating this material. Seagrasses and sedimentary POM had the least depleted values (−11.5 to −5.7‰). Between the five seagrass species significant differences in δ13C were measured. The small species Halophila ovalis and Halodule uninervis were most depleted, the largest species Enhalus acoroides was least depleted, while Thalassia hemprichii and Cymodocea rotundata had intermediate values. Fourteen fauna species, accounting for 10% of the total fauna density, were shown to assimilate predominantly (>50%) seagrass material, either directly or indirectly by feeding on seagrass consumers. These species ranged from amphipods up to the benthic top predator Taeniura lymma. Besides these species, about half of the 55 fauna species analyzed had δ13C values higher than the least depleted non-seagrass source, indicating they depended at least partly for their food on seagrass material. This study shows that seagrass material is consumed by a large number of fauna species and is important for a large portion of the food web in tropical seagrass meadows.  相似文献   

13.
Catherine  Dance 《Marine Ecology》1987,8(2):131-142
Abstract. Tagged individuals of the sea urchin Paracentrotus lividus followed in Port Cros Island showed mostly nocturnal movements, with individual peaks of activity which were not coordinated for the population during the night period. The straight line distance travelled in 24 h has a mean value of 49 cm, with a range of 0 to 240 cm. Neither sea urchin size nor depth affect activity. The urchins living on rocky substrate moved significantly more than those on seagrass beds. From one day to another, the distance travelled by the population of urchins varies significantly; this is not related to season. The activity is negatively correlated with the mean wind speed; during a period of turbulence lasting several hours, movement was significantly oriented to the deep. Periods of inactivity of 1 or 2 days were observed; they are not coordinated for the individuals followed, and the relation between feeding and movement is not clear. Urchins seem to stay in a quite small area, at least for some months.  相似文献   

14.
海草场的生态功能   总被引：5，自引：0，他引：5  
海草场作为特殊的海洋生态系统之一,尽管其面积占海洋总面积的比例很小,但其具有极高的初级生产力,在地球系统碳循环中起着不可忽视的作用。海草本身还可以作为其他附着生物的附着基,进一步提高了海草场的初级生产力,并为许多海洋动物提供了适宜的食物。海草场的存在,改变了海草场内的流体动力过程,加速了悬浮颗粒的沉降,对稳定海底底质和净化水质有着积极的作用,因此被称为"生态系统工程师"。海草本身可以作为一些海洋动物的食物,同时其复杂的地下和地上结构还为一些海洋动物提供了庇护和栖息场所。而海草植物体的大部分,将最终被微生物分解利用,所产生的营养盐可以满足其它初级生产者生长所需。本文通过总结世界范围内海草场生态学研究成果,旨在阐述海草场生态系统的特征及功能,以提高人们对海草场生态功能的认识,唤起人们对海草场的重视与保护意识。  相似文献   

15.
Seagrass meadows across north-eastern Australia, survive a range of environmental conditions in coastal bays, reefs, estuarine and deepwater habitats through adaptation of a range of structural, morphological and physiological features. The aim of this study was to investigate the influence of spatial features (habitat type, site and depth) and photon flux on the photosynthetic performance of 11 tropical seagrass species. Pulse amplitude modulated (PAM) fluorometry was used to generate rapid light curves from which measures of maximal electron transport rate (ETRmax), photosynthetic efficiency (α), saturating irradiance (Ek) and effective quantum yield (ΔF/Fm′) were derived. The amount of light absorbed by leaves (absorption factor) was also determined for each population. In intertidal habitats many seagrass species exhibited typical sun-type responses with a close coupling of both ETRmax and Ek with photon flux. Photosynthetic performance ranged from minima in Thalassodendron ciliatum to maxima in Syringodium isoetifolium. The absence of a coupling between photosynthetic performance and photon flux in subtidal populations was most likely due to highly variable light climates and possible light attenuation, and hence the photo-biology of estuarine and deepwater seagrasses exhibited photosynthetic responses indicative of light limitation. In contrast seagrass species from shallow reef and coastal habitats for the most part exhibited light saturation characteristics. Of all the variables examined ETRmax, Ek and ΔF/Fm′ were most responsive to changing light climates and provide reliable physiological indicators of real-time photosynthetic performance of tropical seagrasses under different light conditions.  相似文献   

16.
Abstract. We examined the responses of two tropical sea urchins, Lytechinus variegatus and Tripneustes ventricosus, to cues from predators, simulated predation events and food. Cues released from damaged conspecifics, heterospecifics and heteroclassics (holothurian) were used to simulate predation events in field experiments. Responses to the presence of seagrass, spiny lobster or both were tested in tanks. Findings were supplemented by natural history observations of dispersion patterns off Bermuda. In field experiments, neither species formed groupings in the presence of conspecific, heterospecific or heteroclassic cues. Flight responses were greatest in conspecific treatments; responses to heterospecific cues were intermediate to control and conspecific cues. Urchins in pre‐assembled associations remained in groups in control trials but dispersed when exposed to predation cues. Lytechinus exhibited greater sensitivity to predation cues than Tripneustes. Cues from a damaged sea cucumber invoked a response from Lytechinus but not Tripneustes. Both species employed a two phased response to cues from damaged conspecifics: initially a rapid, but ephemeral (2 min), alarm response followed by a slower (≈ 35 % lower) sustained flight phase for 6+ min, which in nature would disperse urchins downstream and away from a predator. In tank experiments, Lytechinus formed groupings only around food or food + predators. The presence of a predator reduced the aggregation response to food, suggesting that Lytechinus employed a risk aversion strategy. Tripneustes exhibited escape or refractory behavior in both control and experimental treatments in laboratory tanks.  相似文献   

17.
海草床是海岸带最富生产力的生态系统之一，支撑着各种各样的伴生生物。热带的印度和太平洋地区被认为拥有海草植物种类多样性最高，且分布面积最广，然而，这个区域的海草床大型底栖生物我们知之甚少。为了填补认知的空白，我们在该区域开展了一项生态调查，旨在描述该区热带海草床大型底栖生物的丰度和多样性，以及确定大型底栖生物丰度、物种丰富度和群落结构是否明显存在断面内的站间变化和样地间变化。2014年5月和2015年10月我们分别在北苏拉威西省东海岸和西海岸开展野外工作，使用柱状取样器采集海草床大型底栖生物样品。所得样品共计鉴定14大类149种底栖生物，种类最为丰富的类别为多毛类（56种，占26%的总个体数），十足类（20种，占9%的总个体数）和端足类（18种，占35%的总个体数）。东、西海岸海草床大型底栖生物表现出不同的空间分布模式。在东海岸，同一断面的大型底栖生物和端足类的丰度存在显著的站间差异；而在西海岸，大型底栖生物和多毛类的种类丰富度和丰度都表现出明显的站间变化，这可能归结于同一断面底质不均所造成。单因素ANOSIM以及MDS排序表明了北苏拉威西省东海岸和西海岸海草床大型底栖生物群落结构存在显著不同，正好对应于将海草床分成两大类型的栖息地，即西海岸的红树林-海草床-珊瑚连续体和东海岸的海草床-珊瑚连续体。与在热带海区开展的其他研究相比，本研究的大型底栖生物丰度和多样性处于中等水平。东、西海岸海草床大型底栖生物群落存在显著区别，其原因可能源于多方面，包括了沉积物模式，海草床结构和时间变化。  相似文献   

18.
The development of suitable reference states for ecosystem-based management requires documentation of changes in structure and functioning of marine ecosystems, including assessment of the relative importance of bottom-up and top-down processes as drivers of change. We used monitoring data available from St Helena Bay, the most productive bay and an important nursery area situated on the west coast of South Africa, during 1950–2010 to reveal changes in the abiotic and biotic components. St Helena Bay in the 1950s showed similarities to 2000–2010 in terms of wind patterns, hydrology and phytoplankton. Upwelling, oxygen and nutrient concentrations in subthermocline water displayed pronounced decadal-scale variability. Primary production in St Helena Bay is variable, but consistently higher than that on the adjacent Namaqua shelf. Zooplankton size composition and biomass in August have changed markedly since the 1950s. During 2001–2010, mesozooplankton biomass in autumn was considerably lower than in summer, probably due to predation by small pelagic fish. Pelagic fish catch patterns and distributions have altered dramatically. Conservation measures, implemented to reverse past negative human impact, have benefitted marine mammals, the abundance of which has increased in the area, but additional conservation measures are necessary to reverse the decline in African penguins Spheniscus demersus. St Helena Bay shows a muted response to long-term change in the southern Benguela, with marked decadal variability but no clear long-term trend in oceanography and biogeochemistry. Changes in ecosystem boundary conditions and fishing pressure cannot be ignored as important drivers of change in the southern Benguela since the 1950s.  相似文献   

19.
The contribution of seagrasses to coastal protection is examined through the review of the most relevant existing knowledge. Seagrasses are the largest submerged aquatic vegetation ecosystem protected in Europe and it is worth examining their contribution to coastal protection. The review performed highlights incident energy flux, density, standing biomass and plant stiffness as the main physical and biological factors influencing the efficiency of the protection provided by seagrasses. The main conclusion achieved is that seagrass meadows cannot protect shorelines in every location and/or scenario. The optimal conditions for enhancing the protection supplied might be achieved in shallow waters and low wave energy environments, with high interaction surface, at the vertical and horizontal dimension, between water flow and seagrasses. Likewise, the most favorable protection might be provided by large, long living and slow growing seagrass species, with biomass being largely independent of seasonal fluctuations and with the maximum standing biomass reached under the highest hydrodynamic forcings. It is shown that seawater warming, increasing storms and sea level rise, together with the increasing population and anthropogenic threats in the coastal area may lead to rates of change too fast to allow seagrasses to adapt and keep their coastal defense service. Finally, to amend the decline of seagrasses and consequent coastal protection loss, different artificial and natural adaptation measures are provided.  相似文献   

20.
The tropical coastal “seascape” often includes a patchwork of mangroves, seagrass beds, and coral reefs that produces a variety of natural resources and ecosystem services. By looking into a limited number of attempts at substitution and restoration of ecosystem services (e.g. artificial reefs, aquaculture in mangroves, artificial seawalls), we address the questions: (1) To what degree can technologies substitute for ecosystem services in the seascape? (2) How can ecosystem restoration reestablish not only the functions of direct value to humans, but also the ability of the systems to cope with future disturbance? Substitutions often imply the replacement of a function provided free by a solar powered, self-repairing resilient ecosystem, with a fossil-fuel-powered, expensive, artificial substitute that needs maintenance. Further, restoration usually does not focus on large-scale processes such as the physical, biological and biogeochemical interactions between mangroves, seagrass beds and coral reefs. Nonetheless, restoration might be the only viable management alternative when the system is essentially locked into an undesired community state (stability domain) after a phase-shift. We conclude that ecosystem services cannot be readily replaced, restored or sustained without extensive knowledge of the dynamics, multifunctionality and interconnectedness of ecosystems.  相似文献   

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