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1.
Ichthyoplankton assemblages were compared between regions dominated by the oligotrophic East Australian Current (EAC) and the inner-shelf waters off southeastern Australia, to determine if the early life history of fish was related to the separation of the EAC from the coast, producing different water masses as well as characteristic taxa. Samples were collected at the surface and in sub-surface waters, at 50 and 100 m isobath stations, during two summer research voyages in November 1998 and January 1999. On both voyages the study region was characterized by coastal and EAC waters in the north (∼31°S), and in the south by topographically induced upwelling (∼31°S), associated with narrowing of the continental shelf and separation of the EAC from the coast. Among the 111 families of larval fish, we observed distinctive assemblages of ichthyoplankton associated with the two different water masses. A greater abundance of the Carangidae, Labridae, Lutjanidae, Microcanthidae, Myctophidae and Scombridae was associated with the nutrient poor EAC water mass, while the Callionymidae, Clupeidae, Platycephalidae and Sillaginidae were mostly found in the cooler and/or fresher inner-shelf water mass. We assessed these patterns with opportunistic samples from an unusual, wind-driven upwelling event in the north (∼31°S) earlier in the November voyage. The relative abundance of these 10 characteristic families distinguished this wind-driven upwelling event from the subsequent relaxation and predominance of the EAC assemblage at this location just 6 d later. Distinctive and abundant families such as larval clupeids, relative to larval carangids, could be a useful marker of inner-shelf, EAC and mixed water masses in the absence of robust hydrographic data. This and related studies indicate contrast in early life histories of Sardinops sagax and Trachurus spp., which appear to spawn respectively in the inner-shelf and outer-shelf waters. The post-flexion stages of S. sagax predominate in the outer-shelf and Tasman Front, while post-flexion Trachurus spp. predominate in inner-shelf water masses.  相似文献   

2.
Seasonal variations of macrobenthos communities were analyzed over six years at two fixed stations at the depths of 500 m and 1,000 m in the bathyal zone off Sanriku, northeastern Japan, and were compared with those of the 80 m station established at the mouth of Otsuchi Bay, northeastern Japan. Significant seasonal variations of macrobenthos density were detected at the 80 m and 500 m stations. While the density increased in late spring and decreased until July at the 80 m station, it increased between May and August, and decreased in September at the 500 m station. At the 1,000 m station, no seasonal variation of macrobenthos abundance could be detected. The seasonal variations of the density observed at the 80 m and 500 m stations are probably related to that of the supply of organic materials derived from surface phytoplankton. Differences in the patterns of seasonal variations among the three stations may reflect the different periods of food supply to the sea floor. Significant seasonal variation in the feeding structure of the polychaete communities could be detected at the 80 m and 1,000 m stations.  相似文献   

3.
Due to the presence of few dominant predators on South African rocky shores, this coastline could be vulnerable to invasion by predatory crabs. This study applied horizon scanning to create an ordered watch list of alien crab species that could establish along this coastline under present-day and future temperature scenarios. This was done by: (i) identifying the species with both an invasion history and a possible pathway to South Africa; (ii) comparing the temperature ranges of the species’ native and introduced distributions to those of each of the four South African ecoregions; and (iii) ranking the species based on their potential ecological impacts. Of the 56 alien predatory crab species known worldwide, 28 species have pathways to South Africa. Incompatible temperature ranges excluded only two species from each ecoregion. Negative ecological impacts in their invaded ranges placed Japanese shore crab Hemigrapsus sanguineus, brush-clawed shore crab H. takanoi and Chinese mitten crab Eriocheir sinensis at the top of the watch list. This study highlights that many alien crab species have the potential to reach South Africa, with most likely to survive. This watch list should be used to support targeted monitoring and so facilitate early detection of these species, should they reach South Africa.  相似文献   

4.
This study focuses on the comparison of oceanic and coastal cold-core eddies with inner-shelf and East Australian Current (EAC) waters at the time of the spring bloom (October 2008). The surface water was biologically characterised by the phytoplankton biomass, composition, photo-physiology, carbon fixation and by nutrient-enrichment experiments. Marked differences in phytoplankton biomass and composition were observed. Contrasted biomarker composition suggests that biomarkers could be used to track water masses in this area. Divinyl chlorophyll a, a biomarker for tropical Prochlorophytes, was found only in the EAC. Zeaxanthin a biomarker for Cyanophytes, was found only within the oceanic eddy and in the EAC, whereas chlorophyll b (Chlorophytes) was only present in the coastal eddy and at the front between the inner-shelf and EAC waters.This study showed that cold-core eddies can affect phytoplankton, biomass, biodiversity and productivity. Inside the oceanic eddy, greater phytoplankton biomass and a more complex phytoplankton community were observed relative to adjacent water masses (including the EAC). In fact, phytoplankton communities inside the oceanic eddy more closely resembled the community observed in the inner-shelf waters. At a light level close to half-saturation, phytoplankton carbon fixation (gC d−1) in the oceanic eddy was 13-times greater than at the frontal zone between the eddy and the EAC and 3-times greater than in the inner-shelf water. Nutrient-enrichment experiments demonstrated that nitrogen was the major macronutrient limiting phytoplankton growth in water masses associated with the oceanic eddy. Although the effective quantum yield values demonstrate healthy phytoplankton communities, the phytoplankton community bloomed and shifted in response to nitrogen enrichments inside the oceanic eddy and in the frontal zone between this eddy and the EAC. An effect of Si enrichment was only observed at the frontal zone between the eddy and the EAC. No response to nutrient enrichment was observed in the inner-shelf water where ambient NOx, Si and PO4 concentrations were up to 14, 4 and 3-times greater than in the EAC and oceanic eddy. Although results from the nutrient-enrichment experiments suggest that nutrients can affect biomass and the composition of the phytoplankton community, the comparison of all sites sampled showed no direct relationship between phytoplankton biomass, nutrients and the depth of the mixed layer. This is probably due to the different timeframe between the rapidly changing physical and chemical oceanography in the separation zone of the EAC.  相似文献   

5.
Understanding species distributions and their community structure is increasingly important when taking an ecosystem‐based approach to conservation and management. However, knowledge of the distribution and community structure of species in mid‐range trophic levels (e.g. macroinvertebrates) is lacking in most marine ecosystems. Our study aimed to examine the spatial distribution and community‐level biogeographic patterns of common kelp forest–rocky reef macroinvertebrates in Southern California and to evaluate the effects of environmental gradients on these communities. Quantitative SCUBA surveys were used to estimate macroinvertebrate densities at 92 sites from 2008–2012. Non‐metric multidimensional scaling was used to evaluate spatial patterns of macroinvertebrate communities among Regions. We found that kelp forest–rocky reef macroinvertebrate communities are distinct among different island and mainland regions, and their community patterns exhibited a strong relationship with an environmental gradient (i.e. sea surface temperature) even after controlling for geographic distance between sites. High abundances of urchin species (Strongylocentrotus purpuratus and Strongylocentrotus franciscanus) were strong drivers of regional differences. Macroinvertebrate community patterns were driven by characteristic species that were typically more prevalent at warmer or colder sites. Our results provide the first quantitative analysis of macroinvertebrate community structure within the California kelp forest ecosystem. We also describe the distribution and abundance of 92 conspicuous kelp forest‐rocky reef macroinvertebrates among nine pre‐defined Regions. This study provides important preliminary information on these macroinvertebrate species that will be directly useful to inform management of invertebrate fisheries and spatial protection of marine resources.  相似文献   

6.
Understanding recreational aspects of the tourism industry developing around the KwaZulu-Natal sardine run is important for the protection and sustainability of the Pondoland Marine Protected Area (MPA), on the south-east coast of South Africa. Between June and July 2007, a total of 128 people visited this area to experience the sardine run using boat-based access. An onsite questionnaire survey of 108 (84.4%) participants at Port St Johns and Mbotyi indicated that the direct value of their visit was around R5.47 million (95% CI = R5.14–5.82 million). Although the benefits of the sardine run tourism industry extend throughout the South African economy, local indigenous communities receive little direct benefit. Almost half of all sardine run participants, however, showed a willingness to contribute R500 or more towards a community development programme. On a ranking from one (poor) to five (excellent), the average participant's response to overall quality of the experience and quality of the dive charter was 3.9 and 4.4, respectively. Over a quarter (27%) of participants never saw a sardine during their visit, despite the fact that their trip was marketed as ‘the sardine run’, so participant experiences often did not meet with expectations. The sardine run within the Pondoland MPA is currently an undermarketed and underexploited resource.  相似文献   

7.
The seismic stratigraphy and sedimentary architecture of the Amundsen Gulf Trough and adjacent slope, Canadian Beaufort Sea margin, are investigated using a grid of 2-D seismic reflection data. The inner-shelf of the Amundsen Gulf Trough is interpreted to be composed predominantly of exposed or near-surface bedrock, overlain by a spatially-discontinuous veneer of glacimarine to open-marine sediment. There is a seaward transition from exposed bedrock on the inner-shelf to a thick (up to 500 m) outer-shelf prograding wedge of acoustically semi-transparent sediment. Eight seismic sequences, divided into four megasequences, are described from the outer-shelf stratigraphy. Eight till sheets are identified from Megasequences A to C, providing evidence for at least eight Quaternary ice-stream advances through the Amundsen Gulf Trough to the shelf break. A trough-mouth fan with a minimum volume of about 10,000 km3 is present on the adjacent slope. The Amundsen Gulf ice stream probably represented the most northwesterly marine-terminating ice stream of the Laurentide Ice Sheet through much of the Quaternary, providing a major route for ice and sediment transfer to the Arctic Ocean. The youngest till sheet within the Amundsen Gulf Trough, Megasequence D, was probably deposited by a subsidiary ice stream, the Anderson ice stream, subsequent to retreat of the last, Late Wisconsinan Amundsen Gulf ice stream. This provides evidence of dynamic ice-sheet behaviour and the reorganisation of the northwest Laurentide Ice Sheet margin during the last deglaciation. A number of buried glacigenic landforms, including palaeo-shelf break gullies and a grounding-zone wedge with a volume of 90 km3, are described from the Amundsen Gulf Trough stratigraphy. Lateral grounding-zone wedges are identified at the northern and southern lateral margins of the Amundsen Gulf and M'Clure Strait troughs, respectively, and are interpreted to have been formed roughly contemporaneously by ice streams in Amundsen Gulf and M'Clure Strait.  相似文献   

8.
Although the organization patterns of fauna in the deep sea have been broadly documented, most studies have focused on the megafauna. Bivalves represent about 10% of the deep-sea macrobenthic fauna, being the third taxon in abundance after polychaetes and peracarid crustaceans. This study, based on a large data set, examined the bathymetric distribution, patterns of zonation and diversity–depth trends of bivalves from the Porcupine Seabight and adjacent Abyssal Plain (NE Atlantic). A total of 131,334 individuals belonging to 76 species were collected between 500 and 4866 m. Most of the species showed broad depth ranges with some ranges extending over more than 3000 m. Furthermore, many species overlapped in their depth distributions. Patterns of zonation were not very strong and faunal change was gradual. Nevertheless, four bathymetric discontinuities, more or less clearly delimited, occurred at about 750, 1900, 2900 and 4100 m. These boundaries indicated five faunistic zones: (1) a zone above ∼750 m marking the change from shelf species to bathyal species; (2) a zone from ∼750 to 1900 m that corresponds to the upper and mid-bathyal zones taken together; (3) a lower bathyal zone from ∼1900 to 2900 m; (4) a transition zone from ∼2900 to 4100 m where the bathyal fauna meets and overlaps with the abyssal fauna and (5) a truly abyssal zone from approximately 4100–4900 m (the lower depth limit of this study), characterized by the presence of abyssal species with restricted depth ranges and a few specimens of some bathyal species with very broad distributions. The ∼4100 m boundary marked the lower limit of distribution of many bathyal species. There was a pattern of increasing diversity downslope from ∼500 to 1600 m, followed by a decrease to minimum values at about 2700 m. This drop in diversity was followed by an increase up to maximum values at ∼4100 m and then again, a fall to ∼4900 m (the lower depth limit in this study).  相似文献   

9.
The composition and distribution of macrobenthic communities was investigated in three areas in the KwaZulu- Natal Bight, a section of shelf off the east coast of South Africa. Areas were pre-selected on the basis of three known oceanographic features, posited to deliver land- or Agulhas Current-derived nutrients onto the shelf and to drive ecosystem functioning in this region. Replicate sediment samples were collected with a 0.2 m2 van Veen grab, during two surveys (A, B) corresponding with normal periods of high and low rainfall, respectively. A subset of the full station array was selected across the shelf in an arrangement of increasing depths (inner-, mid- and outer shelf) through each feature area to investigate the spatial distribution and feeding modes of macrobenthic taxa. The two periods showed some differences in abundance and numbers of macrobenthic taxa, but were not statistically different. Total macrobenthic abundance from Survey A was 20 215 individuals from 642 taxa, decreasing to 18 000 individuals from 503 taxa during Survey B. Polychaeta and Crustacea were the dominant taxa sampled; abundance of the latter was attributed largely to a proliferation of Paguristes sp.1 at inner-shelf samples in the midbight (Thukela) region during Survey B. Similarity classification distinguished seven sample groups reflecting differences in feature areas and shelf positions under investigation. The Thukela River midshelf community supported the highest macrobenthic abundance, while the midshelf off the southern bight (Durban region) was most species rich. Findings were attributed to the habitat complexity of the midshelf which includes a palaeo-dune cordon at the 60 m isobath. Functionally, the community was dominated by interface- and deposit-feeding fauna, emphasising the importance of trophic plasticity in an environmentally variable and heterogeneous shelf environment.  相似文献   

10.
The presence and movement of Indo-Pacific bottlenose dolphins Tursiops aduncus were investigated using shore-based observations made during a humpback whale Megaptera novaeangliae migration survey at Cape Vidal, South Africa, undertaken between June and October 1988–1991. Occurrence was analysed as counts of dolphin sightings per hour within a generalised estimating equation framework. Bottlenose dolphin sightings decreased throughout the daily survey period, and there was a small seasonal peak in sightings during August. Data on movement trajectory were collected using a theodolite for 60% of the groups of dolphins sighted. Most groups were observed travelling in a northerly direction, with few groups recorded moving south. A generalised linear model indicated that northward-travelling dolphins were located closer to shore, travelled at higher speeds, and occurred in bigger groups under low-tide conditions, than those seen moving in a southerly direction. The southward-flowing Agulhas Current is close to shore at Cape Vidal, where the continental shelf is particularly narrow. We suggest that bottlenose dolphins in this area have adopted a specific movement regime to cope with the consistent fast-flowing currents that dominate their environment at Cape Vidal.  相似文献   

11.
Marine cave communities have been a continued source of ecological surprises, among other things because of their close ecological and evolutionary ties with the deep sea. The discovery of cladorhizid sponges, the deepest occurring poriferan family, in shallow Mediterranean caves in the 1990s was one such surprise, leading to the generally accepted hypothesis that the whole family was carnivorous, an unprecedented feeding mode for sponges. The recent observation of the cave species Asbestopluma hypogea in the Mediterranean bathyal, confirmed the view that some shallow caves can occasionally shelter otherwise deep‐dwelling species. Here we present new distribution data of A. hypogea, from deep Mediterranean locations, and for the first time from Atlantic locations. Among the new Atlantic records, the most surprising ones are located in three different geographic areas (Ria de Arousa, Groix Island and Cherbourg) of the NW European coasts, from the Iberian Peninsula to the English Channel, where A. hypogea reaches SCUBA depths (5–50 m), while not sheltered in marine caves. The carnivorous sponge however reaches its shallowest occurrence (5 m), in a small cave at Groix Island. The ecological significance of these discoveries, particularly the very patchy distribution and peculiar dynamics, are noteworthy, and the shallow occurrence of A. hypogea, together with other deep‐water or uncommon species, constitute unique assemblages that must be considered in conservation plans.  相似文献   

12.
O. N. Zezina 《Oceanology》2008,48(6):832-836
Of particular interest in the vertical biological zoning of the continental slope in seas and oceans are the substantial differences in the composition, diversity, and abundance (biomass and production) of benthic faunal communities populating the upper and lower subzones of the bathyal zone. The upper bathyal subzone (down to depths of 1500 ± 500 m) resembles the neritic (shelf) one, while its lower subzone is characterized by both reduced diversity and biomass, being similar in its biological parameters to the near-slope abyssal zone (>3000 m). The study of the bottom contour currents and related sediments (contourites) made the geological prerequisites for such differences understandable. Based on a comparative analysis of the bionomical boundaries and core depths of the contour currents in the North Atlantic and Gulf of Alaska, a changing of trophic zones, as well as quantitative and production characteristics of benthic communities, are established. It is shown that the differences between the biological parameters of the upper and lower bathyal subzones (the benthic biomass, the feeding mode of invertebrates, the growth rate, and the maximal sizes of adult specimens) are related to geological agents such as roiling and redeposition of the sediments.  相似文献   

13.
Although continental shelf and slope environments typically exhibit high epifaunal biomass and have been subjected to increasing fishing pressure, ecological information on assemblages of non‐commercial invertebrate species from subtidal and bathyal areas remains limited. Sea stars (Echinodermata: Asteroidea), which are known to influence communities through their feeding habits, have received less attention than structural taxa such as corals and sponges. To better understand the ecological roles of asteroids on continental shelves, we investigated ~30 species and assessed their distributions and co‐occurrence with other benthic invertebrates on the shelf and slope of Eastern Canada. Using fisheries data and in situ video footage, we compiled a large dataset covering ~600,000 km2 that included over 350,000 individual asteroid records (37–2243 m depth). Multivariate analyses revealed geographically distinct asteroid assemblages, with a maximal overall density at 400–500 m and the highest diversity at 500–700 m. The most abundant and densely occurring species was Ctenodiscus crispatus. We found that asteroids associate with corals, sponges, bivalves, and other echinoderms, and that depth and substrate influence these assemblages. We identified species likely to affect coexisting organisms by their burrowing behavior that can disrupt epi‐ and infauna (C. crispatus) and through predation on ecologically important corals (Hippasteria phrygiana). In addition to providing baseline distribution and ecological information for many bathyal asteroid species in the Northwest Atlantic, this work highlights the abundance and diversified roles of asteroids within continental shelf and slope ecosystems.  相似文献   

14.
在南沙岛礁区中西部实施海底岩石拖网,有3个站获得海底岩石样品。其中在巽它陆架东北缘水深200多米的NS99－17站见深灰色淤泥中有大量铁褐色含泥钙质结核,说明该区（可能在末次冰期时）处于滨海或海滩氧化环境形成结核,之后海平面上升了至少150m。在南威岛西北方康泰滩的NS99－57站1360m水深的海底获强溶蚀的白垩多孔灰岩,含底栖有孔虫化石,指示康泰滩是从早－中中新世或更早开始生长的生物礁。在永暑礁东北约55km处的NS99－52站新命名了“东生海山”,其西北坡获含第四纪钙质超微化石的礁灰岩,以及由抱球虫类等生物碎屑和胶磷矿组成的钙质磷块岩和含磷生物屑灰岩滚石,指示半深海环境,其中的钙质超微化石大部分时代为中始新世－渐新世,以始新世可能较大,小部分属第四纪。这次海底岩石拖网为研究南沙岛礁区中部写信为和油气资源评价提供了重要的“硬证据”,尤其是发现早第三纪半深海相钙质磷块岩离南海西南次海盆很近,不但证实了南沙海域广布下第三系海相沉积的推断,而且为判断南海的张开时间提供了补充依据。  相似文献   

15.
The prevalence of microplastics in the marine environment has resulted in a need to understand their association with various fauna. The aim of this study was to assess whether microplastic particles are present in the tube structure of the indigenous reef-building polychaete Gunnarea gaimardi, which occurs along the coast of South Africa. Sampling was conducted at nine stations along the west and southeast coastlines of South Africa, in July 2016 and April/May 2017. Microplastic particles were recorded in all tube structures of G. gaimardi, with the density ranging between 0.056 and 1.113 microplastic particles g–1 dry weight (dwt). There were no significant differences between sites (p > 0.05), resulting in an overall average of 0.275 (SD 0.215) microplastic particles g–1 (dwt). To the authors’ knowledge, this is the first study to show that synthetic polymers are found in the tubes of reef-building polychaetes along the coast of South Africa.  相似文献   

16.
The South African coastline has been invaded by numerous alien species. Rare pre-invasion (1980) and post-invasion datasets (2001 and 2012) exist for Marcus Island, a small land-tied island in Saldanha Bay, South Africa. These snapshot datasets of the island’s intertidal invertebrate community were complemented with monitoring across seasons, from 2014 to 2016. Invertebrate communities were compared among the summers of 1980, 2001, 2012, 2014, 2015 and 2016 to assess interannual differences, while invertebrates and algae were monitored quarter-annually to assess seasonal changes. In addition, the population dynamics of the alien mussel Mytilus galloprovincialis were monitored. Differences in invertebrate communities between consecutive summers were significant but much smaller than changes induced by the arrival of alien species. Invertebrate and seaweed communities differed among years and shore zones but not among seasons, whereas species diversity differed among years, seasons and shore zones, with zones having the strongest influence. The invasion by M. galloprovincialis, and ensuing spatial and temporal variability in its recruitment, emerged as the most important factor influencing community composition, overshadowing interannual and seasonal changes. This work highlights that the impacts of alien species can be distinguished from natural variability by combining long-term monitoring with surveys at finer temporal scales. This is an important step in extending our understanding of the impacts of marine alien species.  相似文献   

17.
The larval fish assemblage was investigated in the shallow, nearshore region of a proposed marine protected area in eastern Algoa Bay, temperate South Africa, prior to proclamation. Sampling was conducted at six sites along two different depth contours at ∼5 m and ∼15 m to assess shore association. Larvae were collected by means of stepped oblique bongo net tows deployed off a ski-boat, twice per season for 2 years between 2005 and 2007. In total, 6045 larval fishes were collected representing 32 families and 78 species. The Gobiidae, Cynoglossidae, Clupeidae, Engraulidae and Sparidae were the dominant fish families. Catches varied significantly among seasons peaking in spring with a mean of ∼200 larvae/100 m3. Mean overall larval density was higher along the deeper contour, at ∼15 m (40 larvae/100 m3). The preflexion stage of development dominated catches at the ∼5 m (80%) and ∼15 m (73%) depth contours. Body lengths of Argyrosomus thorpei, Caffrogobius gilchristi, Diplodus capensis, Heteromycteris capensis and Solea turbynei, all estuary associated species, were larger at the shallow sites nearer to shore. Larvae of coastal species that produce benthic eggs dominated catches (75%) in the shallow sites (∼5 m) but were less abundant (32%) farther from shore at the deeper (∼15 m) sites. All developmental stages of D. capensis, Engraulis capensis, H. capensis, Sardinops sagax and two Pomadasys species were found in the study area. It appears that some species use the shallow nearshore as a nursery area.  相似文献   

18.
Abstract.  Small-scale patterns of low-, mid- and high-shore intertidal species density, richness and abundance were systematically examined to assess the potential influence of 'mobility' on spatial community dynamics and diversity. Mobile taxa outnumbered sessile species by approximately 2:1, whereas sessile individuals were over 12 times as numerous as mobile ones over the entire study site. Patterns of density, richness and abundance differed with shore level and substratum. The results also highlighted the importance of clearly distinguishing between species density and species richness when examining spatially quantitative data. The rank order of habitats from highest to lowest species density was not the same when analysed in order of species richness. The potential for the mobile proportion of assemblages to influence small-scale diversity assessment through movement was greater at mid- and high-shore zones because the ratio of mobile species was higher and abundance (relative to the lower shore) was low. These observations might reflect the relative influence of the land–sea gradient of immersion on diversity and mobility in intertidal communities. The influence of mobile organisms on marine ecosystem function may be significant because small-scale patterns of movement are known to positively influence biodiversity. High richness paired with low abundance, however, may result in underestimating the functional role of mobile intertidal species.  相似文献   

19.
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   

20.
Transport between shelf and offshore environments supports a significant proportion of ocean primary productivity and is critical to the life cycle of many marine species. While fundamental differences in the underlying dynamics of eastern and western boundary currents have been recognized and studied for more than half a century, the implications for physical dispersal rates have received much less attention. In this study we explore how Australia’s two major boundary current systems, the East Australian Current and the Leeuwin Current, differ in their local retention and cross-shore transports in the upper water column and how these differences favor contrasting life histories of small pelagic fishes. The results suggest that the East Australian Current forms a partial barrier to onshore transport, but is effective in entraining shelf waters and transporting them offshore, particularly in the region where the current separates from the coast. Blue mackerel (Scomber australasicus) spawn on the outer-shelf in this separation region and may thereby maximize the dispersion of eggs and larvae in the mainly oligotrophic waters of the southern Coral Sea. In contrast, the Leeuwin Current system promotes onshore transport through the combined effects of mean onshore flow and eddy-induced mixing. In the Great Australian Bight, sardine (Sardinops sagax) and anchovy (Engraulis australis) may exploit the high coastal retention of the Leeuwin Current system by spawning on the inner-shelf during summer when the current is weakest and winds assist retention and enhance production through local upwelling.  相似文献   

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