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.

Riley encased methodology (REM) was developed for the purpose of establishing mangroves along high‐energy shorelines, revetments, and bulkheads where natural recruitment no longer occurs and where conventional planting methods are ineffective. The principles of REM include the processes of individual seedling isolation within tubular encasements and adaptation of the juvenile plant to the external environment of the restoration site. The success of REM results from specifications for encasement preparation, propagule or seedling selection, and positioning of both encasements and seedlings according to elevation and tidal regimes.

Riley encased methodology (REM) was developed for the purpose of establishing mangroves along high‐energy shorelines, revetments, and bulkheads where natural recruitment no longer occurs and where conventional planting methods are ineffective. The principles of REM include the processes of individual seedling isolation within tubular encasements and adaptation of the juvenile plant to the external environment of the restoration site. The success of REM results from specifications for encasement preparation, propagule or seedling selection, and positioning of both encasements and seedlings according to elevation and tidal regimes.

Despite an undeserved reputation for being dull and homogenous systems, mangal and saltmarsh in Australia have highly complex patterns and processes. Their role as key ‘edge’ systems between land and sea has implications for many species which have larval stages in mangal and saltmarsh, but spend adult life as benthic, pelagic or demersal species. Many such species are also important commercially. Mangal and saltmarsh are both highly dynamic systems, reacting rapidly to changes in hydrological condition and sedimentation. In many areas of the world mangal and saltmarsh are threatened systems, especially near human habitation. Appropriate management strategies for mangal and saltmarsh are therefore critical for both conservation and sustainable use, the two key objectives of the Convention on Biological Diversity. Clearing and associated development, invasion of alien species, pollution effects and poor management are the key threats to these systems. Management at a bioregional level, including the development of a comprehensive system of protected areas, is identified as the key management strategy which will ensure an adequate future for these dynamic systems.

During the last two decades a decrease of salt marsh area of at least 6% was found along the mainland coast of the Dutch Wadden Sea. However, it was not clear what determined the seaward boundary of the pioneer vegetation in this area. In the period 1993–1995, abiotic and biological variables were monitored along a transect from the mudflat to the low salt marsh in two sites, the Negenboerenpolder and the Noordpolder. At the Negenboerenpolder site the pioneer zone extended further from the dike and had a dense cover with vegetation when compared to the Noordpolder site. The observed difference in extension of the pioneer zone could not be attributed to differences in tidal frequency, sedimentation rate or nutrient availability. During the winter a dramatic loss in seed numbers was found at both sites. This loss decreased from mudflat to low marsh. The mud/sand ratio, on the other hand, increased from mudflat to low marsh and was higher in the Negenboerenpolder. The shear strength of the top soil layer measured with an in‐situ erosion flume was correlated to the mud/sand ratio and increased with an increasing mud content of the soil. The strength of the soil seems to be the key factor for the settlement and survival of Salicornia dolichostachya in the pioneer zone.

210Pb, 234Th and 7Be activities were measured to establish sediment accumulation rates, estimate sediment mixing rates, and determine the depth of the sediment mixed layer in the Sepetiba Bay mangrove ecosystem near Rio de Janeiro City, Brazil. Three sediment cores were collected from Enseada das Garças, a typical exposed tidal flat region with a sequence of sedimentary features. The seaward edge of this sequence is a mud flat with the landward portion covered with Spartina alterniflora followed by mangrove vegetation. An additional core was collected on an overwash island near Barra de Guaratiba, which is cover with mangroves without a mud flat or Spartina alterniflora sequence. Sediment accumulation rates were determined to range up to 1.8 cm/yr with the Spartina alterniflora having the maximum rate. Mixing rates were estimated for the Spartina alternifloracore at 40 cm2/yr based on 210Pb and 7Be from the upper mixed region of the core. The 234Th activity in this core suggested that either mixing or the input of 234Th were not in steady state. The sediment mixed region depth ranged from 4 cm to greater than 30 cm. At the Enseada das Garças site the mixing depth decreased in the landward direction (i.e. mud flats > 30 cm, Spartina alterniflora 11 cm, mangroves 4 cm). Along with this decrease in sediment mixing depth was a shift from physical to biological mixing. The Barra de Guaratiba core had a sediment mixed layer of 13 cm as a result of physical and intense biological activity.

Methane fluxes from sediments in different zones of a Bruguiera sexangula mangrove wetland were determined by closed static chamber techniques during one whole year period, at Changning River estuary, northeast of Hainan Island, China. Methane productions were also measured by anaerobically incubating sediment samples. Impacts of salinity, sulphate and temperature on methane production rates were studied in vitro. Great differences of annual methane fluxes were observed in three zones, with the values of 0.39, 0.20 and 0.12 g m−2 in the outer zone, middle zone and inner zone, respectively, in part due to the differences of sediment water contents and crab bioturbation. The highest fluxes in each zone occurred in autumn and the lowest in winter. Large diurnal fluctuations in fluxes were caused by the changes of tidal conditions rather than the changes of air or sediment temperatures. The temporal and spatial patterns of methane production differed somewhat from those of methane flux. There was great seasonality for methane production and the highest productions were found in autumn and the lowest in spring. Different horizontal and vertical patterns occurred in different seasons and different zones, suggesting the complexity of factors controlling methane production. The in vitro control experiments indicated that salinity and sulphate had negative effects whereas temperature (20–50°C) had positive effects on methane production rates. However, there were different sensitivities for the different levels of the three factors.

This paper reports on a study of accretion rate and sedimentological variability in a estuarine mangrove swamp. One hundred and sixteen stations were monitored for 2 years. In addition, surface sediment samples were collected at 52 stations during the months of May, June and July (1994) to represent the non‐monsoon sediments and November, December (1994) and January (1995) to represent the monsoon sediments. Results show that the accretion rate for the first year was 1.46±0.13 cm/yr and 0.66±0.04 cm/yr for the second year thus making the average accretion for the 2 years period to be 1.06 cm/yr. The average accretion rate for the monsoon season (0.26 ± 0.04 cm/month) was found to be significantly higher than the non‐monsoon season (0.12 ± 0.03 cm/month). Nevertheless, the same is not true for the surface sediment characteristics. The sedimentological characteristics between the monsoon and the non‐monsoon sediments were not significantly different.

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.