Daniel M. Alongi

Present state and future of the world's mangrove forests

Mangroves, the only woody halophytes living at the confluence of land and sea, have been heavily used traditionally for food, timber, fuel and medicine, and presently occupy about 181 000 km 2 of tropical and subtropical coastline. Over the past 50 years, approximately one-third of the worlds mangrove forests have been lost, but most data show very variable loss rates and there is considerable margin of error in most estimates. Mangroves are a valuable ecological and economic resource, being important nursery grounds and breeding sites for birds, fish, crustaceans, shellfish, reptiles and mammals; a renewable source of wood; accumulation sites for sediment, contaminants, carbon and nutrients; and offer protection against coastal erosion. The destruction of mangroves is usually positively related to human population density. Major reasons for destruction are urban development, aquaculture, mining and overexploitation for timber, fish, crustaceans and shellfish. Over the next 25 years, unrestricted clear felling, aquaculture, and overexploitation of fisheries will be the greatest threats, with lesser problems being alteration of hydrology, pollution and global warming. Loss of biodiversity is, and will continue to be, a severe problem as even pristine mangroves are species-poor compared with other tropical ecosystems. The future is not entirely bleak. The number of rehabilitation and restoration projects is increasing worldwide with some countries showing increases in mangrove area. The intensity of coastal aquaculture appears to have levelled off in some parts of the world. Some commercial projects and economic models indicate that mangroves can be used as a sustainable resource, especially for wood. The brightest note is that the rate of population growth is projected to slow during the next 50 years, with a gradual decline thereafter to the end of the century. Mangrove forests will continue to be exploited at current rates to 2025, unless they are seen as a valuable resource to be managed on a sustainable basis. After 2025, the future of mangroves will depend on technological and ecological advances in multi-species silviculture, genetics, and forestry modelling, but the greatest hope for their future is for a reduction in human population growth.

Bacterial productivity and microbial biomass in tropical mangrove sediments

Bacterial productivity (3H-thymidine incorporation into DNA) and intertidal microbenthic communities were examined within five mangrove estuaries along the tropical northeastern coast of Australia. Bacteria in mangrove surface sediments (0–2 cm depth) were enumerated by epifluorescence microscopy and were more abundant (mean and range: 1.1(0.02–3.6)×1011 cells·g DW−1) and productive (mean: 1.6 gC·m−2· d−1) compared to bacterial populations in most other benthic environments. Specific growth rates (¯x=1.1) ranged from 0.2–5.5 d−1, with highest rates of growth in austral spring and summer. Highest bacterial numbers occurred in winter (June–August) in estuaries along the Cape York peninsula north of Hinchinbrook Island and were significantly different among intertidal zones and estuaries. Protozoa (105−106·m−2, pheopigments (0.0–24.1μg·gDW−1) and bacterial productivity (0.2–5.1 gC·m−2·d−1) exhibited significant seasonality with maximum densities and production in austral spring and summer. Algal biomass (chlorophylla) was low (mean: 1.6μg·gDW−1) compared to other intertidal sediments because of low light intensity under the dense forest canopy, especially in the mid-intertidal zone. Partial correlation analysis and a study of possible tidal effects suggest that microbial biomass and bacterial growth in tropical intertidal sediments are regulated primarily by physicochemical factors and by tidal flushing and exposure. High microbial biomass and very high rates of bacterial productivity coupled with low densities of meiofaunal and macroinfaunal consumers observed in earlier studies suggest that microbes may be a sink for carbon in intertidal sediments of tropical mangrove estuaries.

Carbon sequestration in mangrove forests

Mangrove forests are highly productive, with carbon production rates equivalent to tropical humid forests. Mangroves allocate proportionally more carbon belowground, and have higher below- to above-ground carbon mass ratios than terrestrial trees. Most mangrove carbon is stored as large pools in soil and dead roots. Mangroves are among the most carbon-rich biomes, containing an average of 937 tC ha-1, facilitating the accumulation of fine particles, and fostering rapid rates of sediment accretion (∼5 mm year -1) and carbon burial (174 gC m-2 year -1). Mangroves account for only approximately 1% (13.5 Gt year -1) of carbon sequestration by the world’s forests, but as coastal habitats they account for 14% of carbon sequestration by the global ocean. If mangrove carbon stocks are disturbed, resultant gas emissions may be very high. Irrespective of uncertainties and the unique nature of implementing REDD+ and Blue Carbon projects, mangroves are prime ecosystems for reforestation and restoration.

The influence of forest type on microbial-nutrient relationships in tropical mangrove sediments

Microbial productivity, nutrient chemistry and rates of nutrient regeneration were examined in muds of different mangrove forests within the Fly Delta, Papua New Guinea, to assess the effect of forest type on microbial and nutrient processes, and their interactions. Three major forest types were examined: Rhizophora-Bruguiera, Nypa and Avicennia-Sonneratia forests. For most variables, variations within a forest type were as great as, or greater than, differences between forest types. A high-intertidal Nypa site was most different in edaphic characteristics compared to five low-intertidal stations (two stations in each forest type) suggesting that differences among forest types in earlier studies were mainly a function of tidal elevation rather than species-specific ability of mangroves to influence redox and nutrient status. Dissolved inorganic nutrients were dominated by high (~ 200–500 μM) concentrations of silicates, but porewater phosphate levels were usually below detection limits (<0.02μM). Measured rates of nutrient regeneration were either slow into the sediment, or undetectable, despite a high concentration gradient for some solutes such as silicate. Rates of bacterial DNA and protein synthesis, and patterns of benthic primary production, indicate uptake of nutrients at the sediment-water interface by epibenthic microalgac and sequestering of porewater solutes by very active, subsurface bacterial communities. Rapid growth of these bacteria may be partially maintained by the decomposition and release of nutrients of mangrove roots and rhizomes, as suggested by the dominance of silicate in the porewater. Correlation analysis supports the notion of nutrient (mainly P) limitation of bacteria and microalgac in mangrove muds. It appears that a close microbe-nutricnt-planl connection serves as a mechanism to conserve scarce nutrients necessary for the existence of these tropical tidal forests.

Intertidal zonation and seasonality of meiobenthos in tropical mangrove estuaries

Intertidal zonation and seasonality of tropical meiobenthic communities were examined within five mangrove estuaries along the northeastern coast (Cape York peninsula) of Australia from May 1985 to January 1986. Partial correlation analysis revealed that environmental cues such as temperature and sediment granulometry were the most important factors regulating the zonation patterns of meiofauna. Seasonality was greatly influenced by monsoonal rains. During austral summer, prolonged monsoonal rains occurred along the coast north of 18°S latitude (Hinchinbrook Island), resulting in increased river discharge and scouring of surface silts and clays, organic matter and bacteria from most tidal sediments. Despite scouring, meiofaunal densities increased in the summer wet season, probably due to warmer temperatures and the high resilience of meiobenthos to sediment disturbance. In mangrove sediments not subjected to torrential rains (Hinchinbrook Island), meiofaunal densities were highest in austral autumn and winter (sediment temperature: 23 to 27°C) and lowest in austral spring and summer (28° to 40°C). Turbellarians were the dominant meiofaunal group, accounting for 58 to 67% of total faunal densities which generally decreased with elevation in all of the estuaries. Meiofauna in tropical Australian mangroves, as in other organic-rich muds and in coral reefs, appear to exert little impact on microbial standing stocks when intercorrelated variables are accounted for. The abundances of hard-bodied meiofauna were low compared with temperate communities, lending further support to Moores (1972) contention that tropical intertidal communities are subjected to greater physical stress than their temperate counterparts.

The impact of shrimp pond effluent on water quality and phytoplankton biomass in a tropical mangrove estuary

Abstract Water quality and phytoplankton biomass were examined over a three-year period in a mangrove estuary receiving periodic inputs of effluent from adjacent shrimp ponds, and in two adjacent, non-impacted estuaries, in north Queensland, Australia. Chlorophyll a, dissolved oxygen (DO), biological oxygen demand (BOD), pH, and salinity at the discharge site in the receiving estuary were significantly higher than in the two control estuaries. There were no significant differences between the impacted and control estuaries in total suspended solids (TSS) and dissolved nutrient concentrations. Water quality and phytoplankton biomass were within ambient levels within 1 km downstream of the discharge site, based on a comparison with long-term, pre-impact data for the estuary. Within 1–2 months after pond discharge ceased, water quality and phytoplankton biomass at the discharge site returned to levels equivalent to those in the control estuaries. The limited spatial and temporal impact suggests that the effluent was dissipated by tides and assimilated and/or mineralized by the estuarine food web. Our results imply that tidal mangrove estuaries have some capacity, at least over short spatial and temporal scales, to process intermittent inputs of pond-derived nutrients.

The influence of stand age on benthic decomposition and recycling of organic matter in managed mangrove forests of Malaysia

Decomposition of sediment organic matter was examined in relation to forest age in 2-, 15- and 60-year old, managed Rhizophora apiculata (Blume) stands in the Matang Mangrove Forest Reserve of peninsular Malaysia. Rates of O2 consumption (range: 11.5–21.4 mmol m−2 d−1) and CO2 production (range: 8.9–20.9 mmol m−2 d−1) were equivalent among the forests indicating that early diagenesis is not linked to stand age and age-related differences in rates of forest production. There were, however, site differences in the dominance of specific diagenetic pathways. Rates of sulfate reduction (to 40 cm depth) averaged 8.9±3.1 mmol S m−2 d−1 and 7.2±0.3 mmol S m−2 d−1 in the 15- and 60-year old forests, respectively, accounting for most (75–125%) of the total mineralization. In contrast, sulfate reduction (3.0±0.5 mmol S m−2 d−1) constituted a considerably smaller proportion (42%) of total organic matter oxidation at the 2-year old forest. Rates of solute efflux across the sediment-water interface and vertical profiles of dissolved Mn and NO2−+NO3− suggest that manganese reduction and denitrification–nitrification, coupled with aerobic respiration, account for most oxidation of organic matter at the 2-year old forest. The loss of particulate organic matter and the increased importance of aerobic and suboxic processes in the 2 year-old forest suggest some impact from disturbance of tree removal. A shift to proportionally less sulfate reduction in sediments of regenerating forests may result in greater availability of dissolved nutrients and some trace metals, and serve to reduce exposure of R. apiculata seedlings to anoxic, toxic solutes (e.g., sulfides). This diagenetic shift may facilitate rapid seedling growth and regeneration of forests.

The influence of fish cage aquaculture on pelagic carbon flow and water chemistry in tidally dominated mangrove estuaries of peninsular Malaysia

The impact of floating net cages culturing the seabass, Lates calcarifer, on planktonic processes and water chemistry in two heavily used mangrove estuaries in Malaysia was examined. Concentrations of dissolved inorganic and particulate nutrients were usually greater in cage vs. adjacent (approximately 100 m) non-cage waters, although most variability in water-column chemistry related to water depth and tides. There were few consistent differences in plankton abundance, production or respiration between cage and non-cage sites. Rates of primary production were low compared with rates of pelagic mineralization reflecting high suspended loads coupled with large inputs of organic matter from mangrove forests, fishing villages, fish cages, pig farms and other industries within the catchment. Our preliminary sampling did not reveal any large-scale eutrophication due to the cages. A crude estimate of the contribution of fish cage inputs to the estuaries shows that fish cages contribute only approximately 2% of C but greater percentages of N (32-36%) and P (83-99%) to these waters relative to phytoplankton and mangrove inputs. Isolating and detecting impacts of cage culture in such heavily used waterways--a situation typical of most mangrove estuaries in Southeast Asia--are constrained by a background of large, highly variable fluxes of organic material derived from extensive mangrove forests and other human activities.

Bathyal meiobenthos of the western Coral Sea: distribution and abundance in relation to microbial standing stocks and environmental factors

The distribution and abundance of meiobenthos in relation to microbial densities and environmental factors were examined at 24 stations in a bathyal (298–1610 m) region of the western Coral Sea. Densities of metazoan meiofauna were low (x = 57; range: 19–170 individuals 10 cm−2) compared with other bathyal communtiesm but when densities of living Foraminifera (x = 560; range: 0–3410 individuals 10 cm−2) were included, total faunal densities were high (x = 610 10 cm−2) and ranged from 73 to 3465 individuals 10 cm−2. Soft-bodied (non-chitinous) taxa (e.g. turbellarians) were not detected beyond the continental slope. Densities of all metazoan taxa, excluding nematodes, decreased significantly with bathymetric depth. When the effect of ocean depth was held constant, only a few significant correlations of meiobenthos with microbes and sediment characteristics were found. Our data, coupled with earlier findings of low bacterial densities and organic conditions, suggest that low densities of metazoan meiobenthos in the western Coral Sea are due to low rates of detrital input. However, densities of Foraminifera and other protozoans increased with bathymetric depth (Alongi, 1987, Deep-Sea Research, 34, 1245–1254), indicating their ability to exploit oligotrophic conditions and to gain numerical dominance in benthic food webs of the deep sea.

The Impact of Climate Change on Mangrove Forests

Mangrove forests have survived a number of catastrophic climate events since first appearing along the shores of the Tethys Sea during the late Cretaceous-Early Tertiary. The existence of mangrove peat deposits worldwide attests to past episodes of local and regional extinction, primarily in response to abrupt, rapid rises in sea level. Occupying a harsh margin between land and sea, most mangrove plants and associated organisms are predisposed to be either resilient or resistant to most environmental change. Based on the most recent Intergovernmental Panel on Climate Change (IPCC) forecasts, mangrove forests along arid coasts, in subsiding river deltas, and on many islands are predicted to decline in area, structural complexity, and/or in functionality, but mangroves will continue to expand polewards. It is highly likely that they will survive into the foreseeable future as sea level, global temperatures, and atmospheric CO2 concentrations continue to rise.