Mangroves and Salt Marshes

Many studies in the muddy intertidal zone of temperate regions haveindicated meiofaunal communities to be mainly affected by epibenthicpredation and disturbance rather than competition. Few studies, however,have dealt with mangrove sediments of tropical areas. In addition to aparallel study in a Ceriops tagal (Perr.) Rob. zone, a manipulativeexclusion technique was used to trace the dominant biological interactionsstructuring the meiobenthos of an East African Avicennia marina (Forsk.)Vierh. mangrove forest. The densities of the major meiobenthic taxa and nematode genera and abroad range of environmental factors were monitored over a depth profile forone year of caging. Cages (1 m2) excluded all epibenthos (> 2 mm)for one year and were procedurally controlled. Procedural and exclusioneffects were traced, using a factorial and mixed ANOVA design. Significant exclusion effects were indicated for oligochaetes and for oneof the dominant epistratal feeding nematode genera. They are discussed interms of epibenthic composition and density, feeding behaviour, foodresources, and the abiotic environment. The conclusion is that the observed meiobenthos (especially oligochaetesand nematodes) is influenced mainly by exploitative or resource competitionwith the epibenthos. The common food source was indicated to be muddydetritus and microalgae. Consequently, the role of the meiobenthos is mainlysituated in an isolated, detrital food web with only minor energy fluxes tothe epibenthos.

Sáu trạm đo hải dương học đã được duy trì trong 8 tuần tại miệng cửa sông Fly có rừng ngập mặn từ tháng 4 đến tháng 6 năm 1995, trong mùa gió mậu dịch đông nam. Bốn trạm đo bổ sung cũng đã được triển khai trong 8 tuần dọc theo kênh cửa sông vào năm 1992, cũng trong mùa gió mậu dịch đông nam. Dữ liệu này được sử dụng để ước lượng sự trao đổi ròng của trầm tích lơ lửng giữa cửa sông và Vịnh Papua. Phát hiện được một luồng trầm tích mịn đáng kể từ đại dương ven bờ vào cửa sông, khoảng 40 tấn/s, tức là khoảng 10 lần tốc độ dòng chảy từ sông, dẫn đến một tỷ lệ lắng đọng trung bình theo chiều dọc được tính toán là 2 mm/năm. Rừng ngập mặn có thể chiếm khoảng 6% lượng trầm tích từ sông vào hoặc khoảng 1/4 lượng trầm tích đất sét từ sông. Nếu trầm tích này chỉ phân bố trên các khu vực tích lũy quan sát được gần các hòn đảo, thì tỷ lệ tích lũy cục bộ trong các khu vực này có thể đạt tới 4 cm/năm. Các ước tính về tỷ lệ tích lũy khối lượng trầm tích mềm (1–10 kg/m2/năm) trong kênh từ việc đo Pb-210 và C-14 từ các mẫu bùn rừng ngập mặn delta không thể giải thích cho tỷ lệ tích lũy này theo quy mô thời gian từ 100–1000 năm. Số phận của lượng trầm tích còn lại vẫn chưa rõ, nó có thể được xuất khẩu từ cửa sông trong mùa mưa.

The black mangrove, Avicennia germinans, occurs sympatric withSpartina alterniflora in coastal Louisiana. Zonation exists alongan elevational gradient with A. germinans dominant at higherelevation creekbanks and S. alterniflora at interior, lowerelevation sites with greater depth and duration of flooding.Establishment of A. germinans seedlings was examined in cagesthat excluded predators and limited horizontal, but not vertical,movement of propagules by the tides and showed that blackmangrove could readily establish in the Spartina zone. Survivalof A. germinans seedlings after one year in cages was notsignificantly different between the two zones, and seedlings weresignificantly taller in the Spartina zone. Thus, neitherinundation per se nor other abiotic factors alone could accountfor the absence of A. germinans in the interior marsh. Althoughpropagules were dispersed into both zones, a net removal ofuncaged propagules from plots in the Spartina zone (–1.3 ± 0.6m-2 d-1) compared to a net addition to plots in the Avicennia zone (+0.5 ± 0.4 m-2d-1) indicated that retention of propagulesdiffered between zones. Causes of mortality were decay (Spartina> Avicennia zone), desiccation (Avicennia > Spartina zone), andpredation (Spartina > Avicennia zone). Although few propaguleswere completely consumed by predators (snails and crabs), damageto the cotyledons promoted decay. The results suggest that tidalaction limits retention and settlement of A. germinans propagulesin the Spartina zone, and a combination of predator damage andfrequent flooding leads to rapid decay of propagules that strandthere.

The tidal currents in mangrove forests are impeded by the friction caused by the high vegetation density. The tidal currents are also complex comprising eddies, jets and stagnation zones. The sediment particles carried in suspension into the forest during tidal inundation are cohesive, mainly clay and fine silt, and form large flocs. These flocs remain in suspension as a result of the turbulence created by the flow around the vegetation. The intensity of sedimentation is largest for trees forming a complex matrix of roots such as Rhizophora sp. and smallest for single trees such as Ceriops sp. The flocs settle in the forest around slack high tide. At ebb tides the water currents are too small to re-entrain this sediment. Hence the inundation of coastal mangrove forests at tidal frequency works as a pump preferentially transporting fine, cohesive sediment from coastal waters to the mangroves. Mangroves are thus not just opportunistic trees colonising mud banks but actively contribute to the creation of mud banks.

The water budget of a tropical tidal salt flat in dry tropical Australia has been studied with particular emphasis on estimating the groundwater fluxes. Salt was used as a passive tracer to determine some of these fluxes. Groundwater salt accumulation (or loss) was less than 1 kg/m3/month. Surface water leaving the flat at a higher salt concentration than flood water causes a net outwelling of salt of between 0.4 and 1 kg/m2/month. Evaporation from the salt flat was estimated to be 70 mm/month. Using these measurements and a simple model of the groundwater flows, it is concluded that the groundwater loss is less than 40 mm/month. The accuracy of the salt budget was insufficient to rule out the possibility that the net groundwater motion was upwards. Measurements of Si and PO_4 fluxes indicated that the net outwelling due to surface water flows were respectively 3 mmole/m2/month and 0.014 mmole/m2/month. A net groundwater loss of 10 mm would produce a similar magnitude outwelling due to groundwater discharge. The mangrove-fringed tropical tidal salt flats are the source of large quantities of nutrients that are released to the near-shore zone.

The processes of release and retention of phosphorus (P) in soils of a tidal floodplain were studied at Combu Island (Belém/Brazil). Three parcels of 220m2 were chosen: low floodplain (LF), intermediate floodplain (MF) and high floodplain (HF). Two cores to 0.4 m depth were collected in each parcel, one week after the highest and lowest tides of the year. Organic phosphorus (P—OM) concentrations ranged from 0.02 ± 0.01 to 0.07 ± 0.02 mg/g, the highest concentrations were found in the HF parcel. On the other hand the HF parcel had the lowest concentrations of total inorganic phosphorus (TIP): 0.10 ± 0.03 to 0.11 ± 0.33 mg/g. The speciation of inorganic P suggested that P—Fe/Al is the main chemically bound P in soil (0.05 ± 0.01 to 0.20 — 0.04 mg/g). The tidal inundation of the floodplain appears to influence these chemical reactions involving phosphate retention and release.

Chemical changes in anoxic tidal waters, emerging from the mangrove forest of Cananeia, (48° W, 25° S; Brazil), were interpreted using a stoichiometric approach. Significant depletion of magnesium and sulphates were observed and were apparently due to magnesium-iron replacement in octahedral sheets of clay minerals, and sulphate reduction, respectively. Iron release from clays, after bacterial-mediated reduction, apparently reacted with sulphides produced by sulphate reduction to form nono- and di-sulphides. Production of 41.5% of FeS and 58.5% of FeS2satisfactorily explained all the stoichiometric relations between concentration changes of magnesium, sulphates, sulphides, iron and alkalinity expected by the model. As a consequence, the decomposition rate of organic matter in these prevailing anoxic conditions, would mainly stem from sulphate reduction (90% against 10% for ferrireduction). The simultaneous bacterial processes of uptake and release of ammonium included in the model would explain why the mangrove forest does not export significant amounts of nitrogen toward the adjacent lagoon.

Diurnal gas exchange characteristics were measured simultaneously in two mangrove species, Avicennia marina and Bruguiera gymnorrhiza, over 7 d in summer (February–March), to compare their productivity. The study was undertaken in the Beachwood Mangroves Nature Reserve, Durban, South Africa, using fully expanded leaves of young and mature trees at the top of the canopy. Gas exchange was strongly influenced by photosynthetic photon flux density (PPFD), leaf temperature and the accompanying leaf to air vapour pressure deficit (Δ w). Carbon dioxide exchange was saturated at a PPFD of about 600 μmol m-2s-1 in B. gymnorrhiza compared to 800 μmol m-2s-1 in A. marina. Maximal CO2 exchange occurred between 12h00 and 14h00 and was consistently greater in A. marina (8.8 μmol m-2s-1) than in B. gymnorrhiza (5.3 mu;mol m-2s-1). Mean internal CO2 concentrations ( ci) were 260 μl l-1 in A. marina and 252 μl l-1 in B. gymnorrhiza. Photorespiratory activity was 32% in A. marina and 30% in B. gymnorrhiza. Mean water use efficiency (WUE) was 8.0 μmol mmol-1 in A. marina and 10.6 μmol mmol-1 in B. gymnorrhiza. Diurnal leaf water potentials ranged from –0.8 to –3.5 MPa and were generally lower in A. marina.

Hinchinbrook Island, a wilderness National Park, lies closely adjacent to the Australian mainland within the boundaries of the Great Barrier Reef World Heritage area. The mangroves that occupy much of its shoreline and the neighbouring mainland intertidal are major features. Recent preliminary surveys have revealed considerable floristic variety at local scales and, over the total area, often intricate vegetal patterning (Bunt, 1997). It is the intention now to define the most conspicuous features in numeric and graphical terms. Particular attention has been given to differences in shore normal species sequencing within the study area and to the complexities associated with overlap in species distributions.