The distribution and state of mangroves along the coast of Transkei,Eastern Cape Province,South Africa

The mangrove communities along the coastline of the former Transkei, now part of the Eastern Cape Province, have not been looked at in detail since Ward and Steinkes survey in 1982. Mangroves previously occurred in 17 estuaries but were now found in only 14 of the 76 estuaries visited, with a complete loss of mangroves evident in the Mnyameni, Mzimvubu and Bulungula estuaries amounting to 7.5 ha. Total mangrove loss amounted to 17.6 ha which represents a 6.5% loss over 17 years or 1.04 ha per annum. Tree cover had increased by 16.15 ha in eight other estuaries. This increase could be attributed to the inaccessibility of mangrove stands or to protection afforded by provincial nature reserves and hotel resorts. No new mangrove stands were recorded, although Steinke (pers. comm.) has recently recorded mangroves along the north bank of the Kei River. There has been little change in mangrove species composition in the different estuaries over the past 17 years. Total mangrove loss amounted to 17.6 ha which represents a 6.5% loss over 17 years or 1.04 ha per annum and the species recorded included Avicennia marina, Bruguiera gymnorhiza and Rhizophora mucronata. The mangrove fern Acrostichum aureum L. was recorded for the first time in the Mkozi estuary. Tree density for all estuaries was between 10 and 2594 trees ha^–1. The Mngazana and Mntafufu estuaries had the highest tree densities of 2594 and 1402 trees ha^–1 respectively, typical of riverine mangrove forests. Fringe mangroves were evident in most other systems. Removal of trees for wood has the greatest impact on mangrove cover. Fringe mangrove stands are particularly accessible to harvesters. Only 6% of the current area of trees is afforded some protection in conservation areas. The Mdumbi, Mzamba and Kobonqaba estuaries receive no conservation protection and harvesting has resulted in more than 50% of the trees being removed. The density of dead tree stumps was greater than the number of living trees and no seedlings or juvenile trees were found. Further removal of mangroves within the estuaries south of the Mzimvubu River is expected in the fringe mangroves, as most are unprotected and easily accessible.     

Mangrove growth in New Zealand estuaries: the role of nutrient enrichment at sites with contrasting rates of sedimentation

Mangrove forest coverage is increasing in the estuaries of the North Island of New Zealand, causing changes in estuarine ecosystem structure and function. Sedimentation and associated nutrient enrichment have been proposed to be factors leading to increases in mangrove cover, but the relative importance of each of these factors is unknown. We conducted a fertilization study in estuaries with different sedimentation histories in order to determine the role of nutrient enrichment in stimulating mangrove growth and forest development. We expected that if mangroves were nutrient-limited, nutrient enrichment would lead to increases in mangrove growth and forest structure and that nutrient enrichment of trees in our site with low sedimentation would give rise to trees and sediments that converged in terms of functional characteristics on control sites in our high sedimentation site. The effects of fertilizing with nitrogen (N) varied among sites and across the intertidal zone, with enhancements in growth, photosynthetic carbon gain, N resorption prior to leaf senescence and the leaf area index of canopies being significantly greater at the high sedimentation sites than at the low sedimentation sites, and in landward dwarf trees compared to seaward fringing trees. Sediment respiration (CO₂ efflux) was higher at the high sedimentation site than at the low one sedimentation site, but it was not significantly affected by fertilization, suggesting that the high sedimentation site supported greater bacterial mineralization of sediment carbon. Nutrient enrichment of the coastal zone has a role in facilitating the expansion of mangroves in estuaries of the North Island of New Zealand, but this effect is secondary to that of sedimentation, which increases habitat area and stimulates growth. In estuaries with high sediment loads, enrichment with N will cause greater mangrove growth and further changes in ecosystem function.     

Transport of sediment in mangrove swamps

The transport of suspended sediment in mangrove swamps is controlled by three dominant processes. First, the transport processes in the estuaries and coastal waters draining the swamp, including flocculation, tidal pumping, baroclinic circulation, trapping of the smallest particles in the turbidity maximum zone, and the effect of the mangrove tidal prism. Second, the mechanical and chemical reactions in mangrove waters destroying flocs of cohesive sediment in suspension. Third, biological processes have a dominant influence on the ultimate fate of clay particles in mangroves.     

能量标签技术及其在红树林生态系统能流研究中的应用

传统上认为红树林输出的有机质产生巨大的能流,支持了巨大的河口和近岸水域生态系统的次级生产。但能量标签技术的研究结果却显示红树林输出的有机质的作用并没有如此巨大。用红树碎屑难消化特性来解释此现象,此外数学模型模拟分析发现潮汐的稀释作用也可以解释这种现象。但这两者都不能解释,在其他初级生产者稀少时,红树材输出的有机质可以被大量利用的现象。在有红树林的河口和近海岸水域生态系统中,藻类等非红树初级生产者具有比红树植物更高的初级生产力,而且更容易被动物获得和消化。可以认为是藻类等巨大初级生产力的竞争作用导致红树初级生产在消费者组织中很难被发现,如此上面提到的难题就能得到很好的解决。此外能量标签技术检测出的是红树的初级生产在消费者组织中的相对比率,不是绝对数量值,从此角度看,能量标签技术的结果与传统观点不是矛盾而是互相补充的关系。由此推测红树的初级生产应该还是被消费者所利用,只是它们在消费者初级营养来源组成中占的比例并不大,但其绝对数量并不少。这与传统观点认为的红树的初级生产被大量利用,支撑了具有巨大的次级生产稍有不同。此外,能量标签技术在红树林生态系统中的适用性尚未检验;计算食物组成的数学工具不是很完善;实验设计上考虑的不够全面;对定量研究有一定的影响。     

Hydrodynamics of mangrove swamps and their coastal waters

Mangrove swamps help control the tidal hydrodynamics of many tropical estuaries. They generate an asymmetry of the tidal currents in both the tidal creeks and the mangrove swamps. This results in self-scouring of the tidal channels. Mangrove land reclamation results in siltation of the channel. Mangrove swamps control the flushing rates of the estuaries through the lateral trapping effect. Lateral trapping leads to the aggregation of mangrove litter along slick lines. Evapotranspiration plays a role in the hot dry season by forming a salinity maximum zone which isolates the estuary from the coastal waters for several months of the year. In the absence of runoff, evapotranspiration in the hot dry season generates an inverse estuarine circulation which can trap high salinity mangrove water, and mangrove detritus, along the bottom of a mangrove creek. This bottom layer can become anaerobic. Groundwater flow appears to play a key role in the nutrient budget of mangrove creeks, exporting salt left behind by evapotranspiration, and inhibiting runoff after rainfall. Particulates and dissolved nutrients outwelled from mangrove swamps to coastal waters are retained in a coastal boundary layer. This coastal boundary layer water can be trapped along the shore for long periods if the coast is straight and mangrove-fringed and the coastal waters are shallow. Headlands inhibit coastal trapping because they enhance mixing. Nutrient-rich coastal boundary layer waters may be ejected offshore as tidal jets peeling off headlands and locally enriching offshore waters.     

Regional philopatry of scalloped hammerhead sharks (Sphyrna lewini) to nursery areas in the Mexican Pacific

Hydrobiologia - The population genetic structure and female philopatry to nursery grounds of the scalloped hammerhead shark (Sphyrna lewini) were studied in different mangrove estuaries along the...     

Ecuador’s Mangrove Forest Carbon Stocks: A Spatiotemporal Analysis of Living Carbon Holdings and Their Depletion since the Advent of Commercial Aquaculture

In this paper we estimate the living carbon lost from Ecuador’s mangrove forests since the advent of export-focused shrimp aquaculture. We use remote sensing techniques to delineate the extent of mangroves and aquaculture at approximately decadal periods since the arrival of aquaculture in each Ecuadorian estuary. We then spatiotemporally calculate the carbon values of the mangrove forests and estimate the amount of carbon lost due to direct displacement by aquaculture. Additionally, we calculate the new carbon stocks generated due to mangrove reforestation or afforestation. This research introduces time and LUCC (land use / land cover change) into the tropical forest carbon literature and examines forest carbon loss at a higher spatiotemporal resolution than in many earlier analyses. We find that 80 percent, or 7,014,517 t of the living carbon lost in Ecuadorian mangrove forests can be attributed to direct displacement of mangrove forests by shrimp aquaculture. We also find that IPCC (Intergovernmental Panel on Climate Change) compliant carbon grids within Ecuador’s estuaries overestimate living carbon levels in estuaries where substantial LUCC has occurred. By approaching the mangrove forest carbon loss question from a LUCC perspective, these findings allow for tropical nations and other intervention agents to prioritize and target a limited set of land transitions that likely drive the majority of carbon losses. This singular cause of transition has implications for programs that attempt to offset or limit future forest carbon losses and place value on forest carbon or other forest good and services.     

A comparative ecology of estuaries in Nigeria

A bar-built estuary, a drowned river valley and two river delta estuaries were compared and contrasted to elucidate the impact of some abiotic factors, notably climate, salinity, and oxygen on the distribution of the aquatic fauna and flora.Salinity was recognizable as the key factor responsible for the population dynamics in these habitats. The mangrove community is characteristically zoned, but development and distribution of the trees is restricted in view of coastal geomorphic and hydrological processes to the western banks of the estuaries.The preponderance of marine fish species in estuaries was confirmed, although the wide salinity tolerance of some prolific-breeding Cichlidae from freshwater was noted. Forty-eight species of bony fishes were recorded in Lagos Lagoon, the Escravos, and Qua Ibo estuaries and classified by salinity tolerance into stenohaline and euryhaline marine species (66%), and freshwater species (34%). The same estuarine ichthyofauna were split into trophic groups: piscivores (46%), zooplankton feeders (10%), meiobenthos feeders (27%), and macrobenthos feeders (17%).     

Temporal Variations in Fish Catch Using Pop Nets in Mangrove and Saltmarsh Flats at Towra Point,NSW, Australia

Fish assemblages utilising saltmarsh and mangrove during spring tides were surveyed over a 12-month period using buoyant pop nets. A total of 48 net releases in the saltmarsh identified 16 species, at a density of 0.56 fish m⁻², with six species being of commercial importance. The same number of releases within the mangrove collected a total of 23 species at a density of 0.76 fish m⁻². However, fish density was higher within the saltmarsh than the adjacent mangrove when corrected for water volume. Multidimensional scaling revealed different assemblages of fish in the two habitats, with higher numbers of Ambassis jacksoniensis and Pseudomugil sp. in the saltmarsh and higher numbers of Mugilogobius sp. and Acanthopagrus australis in the mangrove. The result suggests a potentially significant role for saltmarsh as a fish habitat in the estuaries of southeast Australia.     

Mangrove Zonation: An Examination of Data from Seventeen Riverine Estuaries in Tropical Australia

To characterize mangrove zonation better under conditions innorthern Australia, surveys were undertaken within a numberof riverine estuaries along the eastern coast of Cape York Peninsula,in the Gulf of Carpentaria and at one location in Van DiemenGulf in the Northern Territory. Detailed records were takenof species occurrences along shore normal transects upstreamto tidal limits in each system. A simple numeric procedure developedby Williams, Bunt and Clay (Marine Ecology Progress Series 72 : 283–287, 1991) was applied to the data to definespecies sequential ordering across the intertidal surfaces.Considerable diversity in zonal pattern was found, partly theresult of floristic differences between and along rivers withinthe study area but also arising from variability in the centresof distribution of species across the intertidal surface. Closeexamination of the data suggests the variability to be attributableto the differing responses of individual species to the characterand pattern of environmental controls operating from point topoint locally and at larger scales throughout the mangrove environment.It is concluded that constancy of centres of distribution andof sequencing order under conditions in tropical Australia arenot to be expected. At the same time, within the range of alongtransect species distributions, conditions are likely to beencountered which permit individual species, whether rarelyor not, to occupy all or most of the intertidal surface. Anyother pattern of distribution is interpreted to represent conditionswhich, at least to a degree, are limiting for the species inquestion. In mangrove environments of considerable diversity,only extensive survey can be expected to reveal the full extentof zonal pattern in the associated vegetation. Mangroves; zonation; variability; riverine estuaries; Australian tropics     

The influence of mangrove-derived tannins on intertidal meiobenthos in tropical estuaries

Summary Mangrove-derived tannins negatively effected laboratory-reared nematode populations and natural communities of meiobenthos in tropical mangrove forests along the northeastern coast of Australia. In the low and mid intertidal zones of five mangrove estuaries, nearly all of the dominant meiofaunal taxa correlated negatively with concentrations of sediment tannins. Only nematodes correlated with low tannin concentrations in the high intertidal zones. The negative exponential equation y=be ^-mx represented the best-fit for most of the meiofauna-tannin relationships. The mangrove-dwelling nematode, Terschellingia longicaudata did not grow (r=0.001) in the laboratory on fresh, tannin-rich leaves of the red mangrove, Rhizophora stylosa. Population growth of the nematode was significantly greater on fresh, tannin-poor leaves of the grey mangrove, Avicennia marina (r=0.081) with best growth (r=0.112) attained on a diet of tannin-free, mixed cereal. These preliminary field and laboratory results suggest that hydrolyzable tannins leached from mangrove roots and leaf litter are an important factor regulating intertidal meiobenthic communities in tropical mangrove forests along the northeastern Australian coast.     

Shrimp ponds lead to massive loss of soil carbon and greenhouse gas emissions in northeastern Brazilian mangroves

Mangroves of the semiarid Caatinga region of northeastern Brazil are being rapidly converted to shrimp pond aquaculture. To determine ecosystem carbon stocks and potential greenhouse gas emissions from this widespread land use, we measured carbon stocks of eight mangrove forests and three shrimp ponds in the Acaraú and Jaguaribe watersheds in Ceará state, Brazil. The shrimp ponds were paired with adjacent intact mangroves to ascertain carbon losses and potential emissions from land conversion. The mean total ecosystem carbon stock of mangroves in this semiarid tropical landscape was 413 ± 94 Mg C/ha. There were highly significant differences in the ecosystem carbon stocks between the two sampled estuaries suggesting caution when extrapolating carbon stock across different estuaries even in the same landscape. Conversion of mangroves to shrimp ponds resulted in losses of 58%–82% of the ecosystem carbon stocks. The mean potential emissions arising from mangrove conversion to shrimp ponds was 1,390 Mg CO₂e/ha. Carbon losses were largely from soils which accounted for 81% of the total emission. Losses from soils >100 cm in depth accounted for 33% of the total ecosystem carbon loss. Soil carbon losses from shrimp pond conversion are equivalent to about 182 years of soil carbon accumulation. Losses from mangrove conversion are about 10‐fold greater than emissions from conversion of upland tropical dry forest in the Brazilian Caatinga underscoring the potential value for their inclusion in climate change mitigation activities.     

Watershed Deforestation and Down-Estuary Transformations Alter Sources,Transport, and Export of Suspended Particles in Panamanian Mangrove Estuaries

We identified eight Panamanian watersheds in which conversion from wet tropical forest to pastures differed and assessed the effects of degree of deforestation, and down-estuary transformations, on the suspended particulate matter discharged from the watersheds, entering, traversing through mangrove estuaries, and emerging into coastal waters. Deforested watersheds discharged larger concentrations of suspended particulate matter, with lower % C and N, higher mineral content, and heavier isotopic signatures into fresh reaches of estuaries. Down-estuary, sediment entrainment increased non-organic content of particulates, and watershed-derived imprints of deforestation on composition of particulate matter were mostly erased by within-estuary transformations. Isotopic signatures of C, N, and S in particulate matter demonstrated strong land-sea couplings, and indicated that the direction of the coupling was asymmetrical, with terrestrial and estuarine sources delivering particulate materials to coastal waters and sediments. Mangrove estuaries therefore both act as powerful modulators of human activities on land, while also exporting particulate materials to sea.     

Has the Herbicide Diuron Caused Mangrove Dieback? A Re-Examination of the Evidence

The claim that the herbicide Diuron in agricultural runoff caused dieback of the grey mangrove (Avicennia marina) in Central Queensland, Australia, has influenced government policies including programs to save the Great Barrier Reef. Several investigations on mangrove dieback in Central Queensland river estuaries have been published during the past decade. However, proof of a causal link between mangrove dieback and Diuron remains inconclusive. This study presents a systematic review of the evidence using Hill's Criteria of Causation. Our review shows that using concentrations of the chemical bound to sediment as a measure for biological availability in either glasshouse or field studies is inappropriate. The appropriate measure is Diuron concentration in solution and this parameter bears no simple relationship to concentration bound to sediment, and is not strongly correlated with mangrove health. Only when the herbicide is applied in experimental investigations at many orders of magnitude higher than measured in rivers has an impact on A. marina been demonstrated. Evidence from field studies suggests burial of pneumatophores, the plant's breathing roots, following flood events is a more likely causal factor in mangrove dieback, whereas any contribution from Diuron remains unproven.     

The primary carbon sources utilised by fishes in the Mngazi and Mngazana estuaries,South Africa: a preliminary assessment

A number of studies have used stable carbon isotopes to analyse aspects of the food web structure in South African estuaries but none has compared mangrove and non-mangrove estuarine food webs. This study compares the primary carbon sources utilised by the ichthyofauna in the Mngazana (with extensive mangroves) and Mngazi (without mangroves) estuaries. In contrast to the Kariega Estuary, where past research identified two basic carbon pathways, there were no clear carbon pathways within the Mngazi and Mngazana fish assemblages. Instead, the carbon isotopic values of fishes in both estuaries displayed a continuum rather than a tight clustering around particular energy sources. Most detritus feeders of the family Mugilidae from both estuaries were relatively more enriched (with carbon isotopic values ranging from ?16.9% to ?12.3%) than other fish taxa. The isotopic values of the mullet species suggest a diet derived from relatively enriched carbon sources such as benthic microalgae, the eelgrass Zostera and associated epiphytes. Based on the isotopic values, piscivorous fishes from both estuaries could not be linked to specific prey fish taxa, but clearly the mullet species were not their main food source. The invertebrate feeders that were common to both estuaries showed greater isotopic variations in the Mngazana Estuary (?24.7% to ?19.3%) than in the Mngazi Estuary (?21.9% to ?18.4%), probably reflecting the higher diversity of habitats and invertebrate prey items in the Mngazana system. Generally, the isotopic signatures of fishes from the Mngazi Estuary were more enriched than those from the Mngazana Estuary, thus indicating the possible effect of δ¹³C-depleted mangrove-derived carbon in the latter system.     

The ecology of Belizean mangrove-root fouling communities: patterns of epibiont distribution and abundance,and effects on root growth

The aerial prop roots of the neotropical red mangrove,Rhizophora mangle L., begin growing well above highest high water (HHW) and often extend well below lowest low water (LLW) before rooting in the benthic substratum. In Belize, Central America, prop roots growing below LLW are colonized by diverse assemblages of organisms, including macroalgae, hydrozoans, ascidians, sponges, anemones, hard corals, and isopod crustaceans. Mangroves, root-fouling epibionts, root herbivores, and benthic predators engage in complex interactions that are major determinants of mangrove growth and production. Species richness of root epibionts increases with distance from the mainland and with proximity to the barrier reef. Species richness decreases with variability in water temperature and salinity. Ascidians and sponges transplanted from Lark Cay into the coastal Placencia Lagoon failed to survive, but anemones from Lark Cay survived in Placencia Lagoon. Reciprocal transplants survived off-shore. The gastropod predator,Melongena melongena L., present only in mainland estuaries, reduced local barnacle abundance and epibiont species richness in Placencia Lagoon. Isopod species richness also increases with distance from shore, but the number of roots bored by these species decreases. These isopods can reduce root relative growth rate (RGR_root) by 55%. On off-shore cays, sponges and ascidians ameliorate negative effects of isopods. In mainland estuaries where epibionts are less common, isopod damage to roots is more severe. Experimental studies in mangrove swamps throughout the world would clarify the importance of plant-animal interactions in these widespread tropical ecosystems.     

红树林水生动物栖息地功能及其渔业价值

红树林生长于热带、亚热带海陆交界的生态敏感带,其根系为生活于潮间带高度异质环境下的生物提供了适宜生境:藻类、双壳类、甲壳类等大量附生于红树根部;红树的呼吸根、支柱根、树干、倒落的枝条和残骸等,与沉积物形成的松软基质为大量底栖动物提供栖息地,根部结构间的空隙成为虾类、蟹类和鱼类等游泳动物的优良避难所和索饵场。红树林凋落物以碎屑形式进入食物网,连同浮游植物、附生藻类和底栖微藻等,是红树林生态系统的碳循环重要组成部分,为红树林区水生动物提供了丰富的食物来源。可见,食物来源丰富、隐蔽性强、捕食压力低等特点使得红树林成为水生动物的理想栖息地,许多水生动物选择在其中度过部分或完整生活史。另外,红树林也是重要经济动物(鱼、虾、蟹类)的育苗场,为近岸鱼类种群的补充和渔业活动提供支持。为合理开发红树林区渔业和有效保护红树林湿地生态系统,从生境价值、凋落物在红树林生态系统食物网中的贡献等方面总结了红树林栖息地功能及其渔业价值,提出今后的研究方向应将红树林的栖息地功能从其它河口、近岸栖息地中分离出来,甄别不同栖息地间的动态关系及其对渔业的影响。     

Spatial patterns of fish communities along two estuarine gradients in southern Florida

In tropical and subtropical estuaries, gradients of primary productivity and salinity are generally invoked to explain patterns in community structure and standing crops of fishes. We documented spatial and temporal patterns in fish community structure and standing crops along salinity and nutrient gradients in two subtropical drainages of Everglades National Park, USA. The Shark River drains into the Gulf of Mexico and experiences diurnal tides carrying relatively nutrient enriched waters, while Taylor River is more hydrologically isolated by the oligohaline Florida Bay and experiences no discernable lunar tides. We hypothesized that the more nutrient enriched system would support higher standing crops of fishes in its mangrove zone. We collected 50 species of fish from January 2000 to April 2004 at six sampling sites spanning fresh to brackish salinities in both the Shark and Taylor River drainages. Contrary to expectations, we observed lower standing crops and density of fishes in the more nutrient rich tidal mangrove forest of the Shark River than in the less nutrient rich mangrove habitats bordering the Taylor River. Tidal mangrove habitats in the Shark River were dominated by salt-tolerant fish and displayed lower species richness than mangrove communities in the Taylor River, which included more freshwater taxa and yielded relatively higher richness. These differences were maintained even after controlling for salinity at the time of sampling. Small-scale topographic relief differs between these two systems, possibly created by tidal action in the Shark River. We propose that this difference in topography limits movement of fishes from upstream marshes into the fringing mangrove forest in the Shark River system, but not the Taylor River system. Understanding the influence of habitat structure, including connectivity, on aquatic communities is important to anticipate effects of construction and operational alternatives associated with restoration of the Everglades ecosystem.     

Tidal-Scale Hydrodynamics within Mangrove Swamps

Both the drag force and the horizontal eddy viscosity play a dominant role in the tidal-scale hydrodynamics in mangrove wetlands. Using field observations and basic fluid mechanics laws, the drag coefficient and the coefficient of dynamic eddy viscosity are found to be predictable as a function of the Reynolds Number based on the characteristic length scale of the vegetation. The characteristic length scale of the vegetation varies greatly with vegetation species, vegetation density and tidal elevation. Both these coefficients decrease with increasing values of the Reynolds Number. At the low range of the Reynolds Number both these coefficients reach much higher values than those typical of vegetation-poor estuaries and rivers. Consequently, the tidal flow within mangrove areas depends to a large degree upon the submerged vegetation density that varies with the tidal stage. These findings may be applied also in other vegetated tidal wetlands, including salt marshes.     

Holocene palynology and sea-level change in two estuaries in Southern Irian Jaya

Southern New Guinea leads the continental Australian plate into convergence to form the New Guinea Highlands. Extensive inter-tidal deltas are typical of much of this low gradient equatorial coastline. In two adjacent estuaries in Irian Jaya (West Papua), this study uses present elevations of mangrove species zones as a finite indicator of Holocene sea levels from pollen diagrams of estuarine cores. All cores showed mangroves at levels well below the present tidal range, with landward species zones being replaced over time by seaward zones. Results show tectonic subsidence in the recent period, with Late Holocene relative sea-level rise of 0.67 mm year⁻¹.