Norman C. Duke

Mangrove production and carbon sinks: A revision of global budget estimates.

results in a conservative estimate of 218 ± 72 Tg C a 1 . When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at 112 ± 85 Tg C a 1 , equivalent in magnitude to 30–40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO2 efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far.

Factors influencing biodiversity and distributional gradients in mangroves

Numerous factors affect the distribution of mangrove plants. Most mangrove species are typically dispersed by water-buoyant propagules, allowing them to lake advantage of estuarine, coastal and ocean currents both to replenish existing stands and to establish new ones. The direction they travel depends on sea currents and land barriers, but the dispersal distance depends on the time that propagules remain buoyant and viable. This is expected to differ for each species. Similarly, each species will also differ in establishment success and growth development rate, and each has tolerance limits and growth responses which are apparently unique. Such attributes are presumably responsible for the characteristic distributional ranges of each species, as each responds to the environmental, physical and biotic settings they might occupy. In practice, species are often ordered by the interplay of different factors along environmental gradients, and these may conveniently be considered at four geographic scales-global, regional, estuarine and intertidal. We believe these influencing factors act similarly around the world, and to demonstrate this point, we present examples of distributional gradients from the two global biogeographic regions, the Atlantic East Pacific and the Indo-West Pacific.

Mangroves as nursery sites: comparisons of the abundance and species composition of fish and crustaceans in mangroves and other nearshore habitats in tropical Australia

Daytime sampling of mangrove and seagrass (Halophila/Halodule community) habitats every 7 wk at Alligator Creek, Queensland, Australia, over a period of 13 mo (February 1985–February 1986) using two types of seine net, revealed distinct mangrove and seagrass fish and crustacean faunas. Total abundance of fish and relative abundance of small and large fish also varied between habitats and seasonally. Post-larval, juvenile and small adult fish captured with a small seine-net (3 mm mesh) were significantly more abundant (4 to 10 times) in the mangrove habitat throughout the 13 mo of sampling. Mangrove fish abundance showed significant seasonality, greatest catches being recorded in the warm, wet-season months of the year. Relative abundances of larger fish (captured in a seine net with 18 mm mesh) in the two habitats varied throughout the year, but did not show a seasonal pattern. At the same site, small crustaceans were significantly more abundant in the mangroves in all but one dryseason sample. Similar comparisons for three riverine sites, sampled less frequently, in the dry and wet seasons of 1985 and 1986, respectively, showed that in general mangrove habitats had significantly more fish per sample, although the relative abundance of fish in mangroves and other habitats changed with season. Crustacean catches showed a similar pattern, except that densities among sites changed with season. Fish and crustacean abundance in mangroves varied among sites, indicating that estuaries differ in their nursery-ground value. The juveniles of two commercially important penaeid prawn species (Penaeus merguiensis and Metapenaeus ensis) were amongst the top three species of crustaceans captured in the study, and both were significantly more abundant in the mangrove habitat. By contrast, mangroves could not be considered an important nursery for juveniles of commercially important fish species in northern Australia. However, based on comparisons of fish catches in other regions, the results of the present study indicate the importance of mangroves as nursery sites for commercially exploited fish stocks elsewhere in South-East Asia.

Mangrove fish-communities in tropical Queensland, Australia: spatial and temporal patterns in densities, biomass and community structure *

Regular daylight sampling over 13 mo (February 1985–February 1986) in and adjacent to intertidal forested areas, in small creeks and over accreting mudbanks in the mainstream of a small mangrove-lined estuary in tropical northeastern Queensland, Australia, yielded 112 481 fish from 128 species and 43 families. Species of the families Engraulidae, Ambassidae, Leiognathidae, Clupeidae and Atherinidae were numerically dominant in the community. The same species, with the addition ofLates calcarifer (Latidae).Acanthopagrus berda (Sparidae) andLutjanus agentimaculatus (Lutjanidae) dominated total community biomass. During high-tide periods, intertidal forested areas were important habitats for juvenile and adult fish, with grand mean (±1 SE) density and biomass of 3.5±2.4 fish m−3 and 10.9±4.5 g m−3, respectively. There was evidence of lower densities and less fish species using intertidal forests in the dry season (August, October), but high variances in catches masked any significant seasonality in mean fish biomass in this habitat. On ebb tides, most fish species (major families; Ambassidae, Leiognathidae, Atherinidae, Melanotaeniidae) moved to small shallow creeks, where mean (±1 SE) low-tide density and biomass were 31.3±12.4 fish m−2 and 29.0±12.1 g m−2, respectively. Large variances in catch data masked any seasonality in densities and biomasses, but the mean number of species captured per netting in small creeks was lowest in the dry season (July, August). Species of Engraulidae and Clupeidae, which dominated high-tide catches in the forested areas during the wet season, appeared to move into the mainstream of the estuary on ebbing tides and were captured over accreting banks at low tide. Accreting banks supported a mean (±1 SE) density and biomass of 0.4±0.1 fish m−2 and 1.7±0.3 g m−2, respectively, at low tide. There were marked seasonal shifts in fish community composition in the estuary, and catches in succeeding wet seasons were highly dissimilar. Comparison of fish species composition in this and three other mangrove estuaries in the region revealed significant geographic and temporal (seasonal) variation in fish-community structure. Modifications and removal of wetlands proposed for north Queensland may have a devastating effect on the valuable inshore fisheries of this region, because mangrove forests and creeks support high densities of fish, many of which are linked directly, or indirectly (via food chains) to existing commercial fisheries.

Recruitment, growth and residence time of fishes in a tropical Australian mangrove system

Twenty fish species accounted for > 96% of the catch by numbers in mangrove habitats in Alligator Creek, in tropical Queensland, Australia. The timing of recruitment, residency status, the period of residence and growth of fish during the time they spent in the mangrove habitat was assessed by examining gonad maturity and following changes in size-frequency plots for each species over 13 months. Five species were permanent residents, completing their life-cycles in mangrove swamps; eight were ‘long-term’ temporary residents, being present for not, vert, similar 1 year as juveniles before moving to other near-shore habitats; and seven were ‘short-term’ residents or sporadic users of the mangrove habitat. Amongst the latter group, four species lived in the mangrove habitat for between 1 and 4 consecutive months, while three engraulid species appeared to move rapidly, and often, between mangrove and other near-shore habitats. One of the resident species spawned and recruited throughout the year, but recruitment for most species was highly seasonal, being concentrated in the late dry season (October) to mid wet season (February) period. Temporary resident species dominated the fish community in the wet season (December–April), but resident species comprised more than 90% of total fish numbers in the mid dry season (August) after temporary residents left the mangroves in the early dry season. Several species had more than one peak of recruitment during the wet season. The cohort of 0 + agedLeiognathus equulus which recruited in December grew more rapidly and remained in the mangroves for a shorter period than the cohort which recruited later in the wet season (February). Only nine of the 20 species examined are strictly dependent on mangrove-lined estuaries, the remaining 11 are captured in significant numbers in other near-shore habitats. Only four of the 20 species are of direct commercial importance in Australia, but most are major prey for several valuable, commercial species harvested both within mangrove habitats and in adjacent shallow shelf habitats.

Global distribution and genetic discontinuities of mangroves – emerging patterns in the evolution of Rhizophora

Abstract. Mangroves are often described as a group of plants with common features and common origins based mostly on their broad distributional patterns, together with an erroneous view of comparable abilities in long-distance dispersal. However, whilst mangroves have common needs to adapt to rigorous environmental constraints associated with regular seawater inundation, individual taxa have developed different strategies and characteristics. Since mangroves are a genetically diverse group of mostly flowering plants, they may also have evolved at quite different geological periods, dispersed at different rates from different locations and developed different adaptive strategies. Current distributions of individual taxa show numerous instances of unusual extant distribution which demonstrate finite dispersal limitations, especially across open water. Our preliminary assessment of broad distribution and discontinuities reveals important patterns. Discontinuities, in the absence of current dispersal barriers, may be explained by persistent past barriers. As we learn more about discontinuities, we are beginning to appreciate their immense implications and what they might tell us about past geological conditions and how these might have influenced the distribution and evolution of mangroves. In this article, we describe emerging patterns in genetic relationships and distributions based on both current knowledge and preliminary results of our studies of molecular and morphometric characteristics of Rhizophora species in the Indo West Pacific region.

Effect of Bioremediation on the Microbial Community in Oiled Mangrove Sediments

Bioremediation was conducted in the field on a mature Rhizophora stylosa mangrove stand on land to be reclaimed near Fishermans Landing Wharf, Gladstone Australia. Gippsland crude oil was added to six large plots (>40 m(2)) and three plots were left untreated as controls. Bioremediation was used to treat three oiled plots and the remaining three were maintained as oiled only plots. The bioremediation strategy consisted of actively aerating the sediment and adding a slow-release fertilizer in order to promote oil biodegradation by indigenous micro-organisms. Oil addition stimulated the numbers of alkane-degrading bacteria slightly to levels of 10(4)-10(5)/g sediment. Bioremediation of the oiled sediment had a marked effect on the alkane-degrading population, increasing the population size by three orders of magnitude from 10(5) to 10(8) cells/g of sediment. An effect of bioremediation on the growth of aromatic-degraders was detected with numbers of aromatic-degraders increasing from 10(4) to 10(6) cells/g of sediment. Active aeration and nutrient addition significantly stimulated the growth of hydrocarbon-degraders in oiled mangrove sediment in the field

Genetic diversity, distributional barriers and rafting continents - more thoughts on the evolution of mangroves

Without continental drift, the diversity and distribution of many species, including mangrove plants, would be very different today. First, there would be fewer pantropic genera and many more endemics. Second, their characteristics would not be as common and widespread as some are today. Continental drift has brought about the massive mixing and dispersal of genes in geologically recent times, greatly enhancing the evolutionary process; particularly for flowering plants — the angiosperms, which evolved during the period.Mangrove plants are comprised of approximately 70 species from 20 quite different angiosperm families. Most taxa are characterized by special physiological abilities and structural forms, enabling them to live in both seasonally fluctuating saline conditions, and water-saturated soils. Their occurrence is mostly tropical, perhaps because of harsh physiological conditions of intertidal habitats; but distributions of specific taxa do not fully concur with the idea of a completely tropical evolution, at least for some important species.At least one genus of mangrove tree, Avicennia, occurs around the world, chiefly in tropical estuarine habitats, although they also range into temperate latitudes, especially in the south. Around the world, there are no more than ten species of Avicennia recognised today, but their diagnostic determinants were inadequate prior to recent studies using both numerical analyses of morphological parameters and isozymes. Such analyses significantly reduced the number of apparent species, notably around Australia, and provided a basis for the revision of distributional records throughout the Indo West Pacific region. One species, A. marina, was found to be widespread and morphologically variable with genes divided into characteristic groupings of at least three geographic areas in the region. Based on these findings, there are several novel inferences to be made regarding the evolution of this genus. A western Gondwanan origin is proposed, with subsequent radiation of several taxa facilitated via the tectonic dispersal of southern continental fragments.

Physical determinants of inter-estuary variation in mangrove species richness around the tropical coastline of Australia

The hypothesis that species richness in tropical, tidal (mangrove) forests in the eastern and western portions of the Australian continent is influenced by different suites of environmental parameters is investigated. Data for species presence/absence and quantitative measures of nine environmental variables from ninety-two estuaries around the tropical coast of Australia were used to test the hypothesis. Linear models were developed and compared for each region. For eastern Australia -53% of the variance in species richness was explained com- pared to -70.0% of the variance for western Australian mangrove forests. Maximum and minimum temperatures were positively correlated with species richness in both regions. Increasing temperatures led to increases in species richness. Tidal amplitude is inversely related to species richness in the east and west. Estuaries with larger tidal amplitudes have fewer species than estuaries with smaller tidal ranges. Estuary length, the size of the surrounding catchment, rainfall variation and the frequency of tropical cyclones have significant effects on species richness in eastern but not western mangrove forests. Estuaries which are long and have large catchments tend to have more species than those being shorter with smaller catchments. High interannual rainfall variability and frequent cyclones tend to decrease species richness in the east. The amount of freshwater runoff is inversely related to species richness in the western mangrove forests but not in eastern forests. Although similarities do exist, there are important differences between eastern and western mangrove forests in the response of species richness to

Halophytes – A resource for the future

One of the more urgent global problems, particularly in arid and semi-arid regions of the world, is finding enough water and land to support the world’s growing food needs. By some estimates, an additional 500 million acres of new croplands are required over the next thirty years to feed burgeoning populations of the tropics and subtropics. Yet only 230 million acres are available in these areas for farming to expand. And, much of this land is forested and should realistically be preserved to maintain the integrity of remaining natural stands and ecosystem functioning. Furthermore, saline soils are a major problem of cultivated lands in semi-arid and arid areas. About 23% of the world’s 1.5 × 10 9 ha of cultivated land is saline and 37% is sodic. Approximately one third of the world’s irrigated land is salt-affected due chiefly to unsustainable irrigation practices. Freshwater resources are also being rapidly depleted and current agricultural irrigation practices are steadily increasing salinity levels in many regions. Rapid increases in population pressure are leading to greater utilization of remaining meager freshwater supplies for drinking, leaving even less for agriculture. In this complex scenario, it has become imperative to consider development of non-conventional agricultural technologies which might more effectively utilize degraded, marginal and saline lands for agriculture by using, the ‘so-called’, poor quality water (brackish water, seawater, and wastewater). An attractive option is the use of seawater or brackish water agriculture, and this has worked well in both inland and coastal sandy soils of some desert environments. Saline water aquifers exist in many arid lands of the world but these have been greatly under-utilized. This brackish water could be a major resource in saline agriculture to produce food, feed, and fiber and oil seeds on currently non-productive, saline arid lands. In further support of saline agriculture, there would be no shortage of water since seawater in the oceans makes up 97% of water on earth. Desert land is also plentiful, with around 43% of the earth’s total land surface being arid or semi-arid. A small portion of this (about 15%) is close to the sea and this would be convenient for growing crops using saline agriculture. This amounts to 130 million hectares of ‘new’ land that could arguably be brought into human or animal food production – without cutting down more forests or diverting scarce fresh water resources from current agricultural and community needs. Salt tolerant plants (halophytes, including salt marsh and mangrove plants) are highly evolved and specialised organisms with well-adapted morphological and physiological characteristics allowing them to proliferate in the soils possessing high salt concentrations. It is tempting to think we might exploit these plants for the better utilization of saline water, and for the rehabilitation of highly saline soils. Saline agriculture, however, must fulfill two conditions to be cost-effective. First, it must produce useful crops at yields high enough to justify the expense of pumping salty water. Second, researchers must develop successful agronomic techniques for growing saline, water-irrigated crops in a sustainable manner. These methods must also not contribute to further damage of natural environments. If applied successfully, this approach would lead to the domestication of wild, salttolerant plants for use as food, forage, and oilseed crops.