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Transcript
10 Chemistry 10.1 Introduction The chemistry of saltwater wetlands is principally governed by local tidal inundation. The mixing of fresh and salt water in the estuary gives the wetlands unique chemical and biological properties, which help dictate the development of plant and animal communities. The main chemical factors that influence plant growth in saltwater wetland areas are salinity, oxygen availability and nutrients. Macroalgae within a coastal lake (DECC) 10.2 Salinity Salinity is the defining and most important feature of saltwater wetland chemistry. The high salt content of the tidal waters and of the soil affects species selection and the productivity of the plants and animals. With the exception of hypersaline areas, estuarine salinity ranges from 0.5 to 35 ppt. This range allows a number of different plants and animals to survive in particular niches to which they are specially adapted. Salinity is extremely important for the germination success and survival of many estuarine plant species and accounts for the zonation patterns seen commonly in these areas. The adaptations of estuarine species allow them to inhabit this zone with relatively little competition from terrestrial species. The salinity of intertidal soils is governed by several factors: • • • • Local tidal regime: The height of the preceding tide determines whether an area of wetland has been flooded or not. Lower elevations will be flooded more often, and their salinity often reflects that of the estuarine water. Higher elevations will dry out between inundations, and evaporation may cause salinity to increase in these areas. Drainage: The nature of the soil and the proximity of creeks affect the drainage of the wetland and the soil salinity. Fine silty mud retains more water and more salt than do sandy sediments. Gradient: The wetland slope influences the penetration of tidal waters and the rate at which the water can drain away. Vegetation: Plant cover reduces evaporation and lowers the salt accumulation at the surface of the wetland. 10 Chemistry 133 • • • Water table depth: An elevated water table may reduce soil salinity by dilution or may carry salt to the surface. Freshwater inputs: The inflow of fresh water from natural or anthropogenic sources tends to dilute the salinity within the wetland environment. Climate: Rates of evapotranspiration depend on temperature, and the relationship between seasonal rainfall and tides affects dilution of soil salinity. 10.3 Oxygen Plants and animals need oxygen. In the estuarine environment, frequent tidal inundation commonly leads to waterlogging of the soil that can deplete the available oxygen within the sediments, a situation referred to as anoxia. Water fills the soil pore spaces and reduces the exchange of oxygen between the air and soil. The oxygen remaining is consumed by microbial respiration within the soil. The lack of oxygen in the root zone poses particular difficulties for intertidal vegetation. All species that are able to live in this environment have specific adaptations, such as pneumatophores, to cope with the low soil oxygen in the waterlogged areas of the intertidal zone. Fauna that lives in these areas can improve the aeration of the soil by burrowing into the sediments. This in turn aids the survival of many plant species in this zone. The activity of crabs is particularly important. 10.4 Nutrients Nutrients are very important in the coastal environment, and nutrient cycling is probably the main ecological function that drives the processes within the saltwater wetland ecosystem. Owing to the substantially different chemical environment in saltwater wetlands, nutrient cycles have different characteristics from those in freshwater wetlands. The two most important nutrients to living organisms within estuaries are nitrogen and phosphorus, which are present in concentrations of three to four orders of magnitude lower than those in typical freshwater wetlands. However, excessive inputs of nitrogen and phosphorus from sewage, urban, agricultural and industrial effluents can lead to eutrophication, resulting in algal blooms if the wetland is not well flushed by the tides. Nutrient uptake and litter decomposition are important to the functioning of ecosystems because they provide essential components to sustain life in the estuary. A great number of animals found in saltwater wetland habitats can be classified as detritivores or decomposers (for example, protozoans and nematodes). These fauna convert the wealth of plant matter in wetlands to detrital food sources for a rich and diverse invertebrate community which may in turn support other marine and terrestrial species. Nitrogen Nitrogen (N) generally arrives in most intertidal areas via tidal water, precipitation, groundwater and sediments carried by the tide. Where tidal influence is infrequent (for example, in saltmarshes), groundwater often carries more N than any other source. The processes associated with N cycling are shown in Figure 10.1. Plants extract nutrients from the sediments via their roots and use them for growth and reproduction. The supply of N may not always be sufficient to maintain the productivity of these ecosystems after denitrification, the process whereby nitrate or nitrite is converted to gaseous nitrogen (N2). Denitrification (and thereby the loss of N) is generally enhanced in waterlogged anoxic soils that are typical of habitats constantly inundated by the tides. Consequently, other sources of N are important. In N fixation, bacteria convert atmospheric N2 to organic N (Figure 10.1). 134 Part 3 Characteristics and processes In nitrification, aerobic bacteria (Nitrosomas spp. and Nitrobacter spp.) convert ammonium, produced by proteolytic bacteria and fungi in the soil, to nitrate. Plant decomposition recycles N. The N cycle varies slightly in saltmarsh environments on account of the infrequency of tidal exchange. The infrequent wetting and drying of soils favours exchange with the atmosphere over exporting nutrients and plant matter to the estuary via the tide. Therefore, a relatively large proportion of plant material is consumed on the saltmarsh by respiratory or burial processes. Denitrification in the soil is limited, because the soils of saltmarshes are typically drier than those of nearby mangroves, thereby reducing the potential for N loss to the atmosphere. Furthermore, saltmarsh vegetation actively transfers oxygen to its roots and consequently to the soil. This background forms the basis for nitrification processes, whereby nitrogenous compounds are converted to biologically available ammonium and nitrate, which are used in plant growth. In addition, during decomposition, nutrients in the plant tissues are released and recycled into new plant growth. Figure 10.1 Depiction of the main processes of nitrogen cycling in saltwater wetland environments N fix in g ba ct er ia atmospheric N2 N- N groundwater N decomposition N uptake by plants ammonia NH3 Nitrosomonas spp. aerobic process nitrite NO2 Nitrobacter spp. anaerobic process nitrate NO3 denitrifying bacteria uptake by plants tide N gaseous N in soil N + NO2 decomposition 10.4.2 Phosphorus Phosphorus (P) is very important in the wetland environment, being one of the macronutrients needed for building living tissue. The P cycle in intertidal soils is much simpler than the N cycle: P is taken up by organisms as phosphate and is liberated by excretion or the decomposition of dead matter by microbial activity. By the processes of excretion, death and sedimentation, P bound to organic matter concentrates in sediments, where microbial decomposition converts organic P to inorganic forms, which may again be taken up by organisms. Phosphate ions are also adsorbed to silt and clay particles. Mobilisation of sediments can contribute considerably to P levels within the water. 10 Chemistry 135 Carbon Plant material is the most significant source of organic carbon in estuaries and also of atmospheric carbon dioxide (by respiration). The herbivore, decomposer and detritivore communities play a pivotal role in processing and mobilising this material (Figure 10.2). Saltwater wetlands usually support a substantial biomass of coastal macrophytes, most of which do not support a large number of grazers. Therefore, the majority of the plant material ends up as leaf litter and, eventually, detritus. This detrital material can either be consumed in situ by detritivores and decomposers or be exported with the tides to other parts of the estuary. Organic detritus in all states of decomposition is a very important source of particulate organic carbon in estuaries. Figure 10.2 The process of carbon cycling within saltwater wetlands, showing the importance of the detrital food chain CO 2 Photosynthesis Oxygen producers decomposers Respiration First-order oxygen consumers grazers detritivores Second-order oxygen consumers 136 Part 3 Characteristics and processes filter feeders Many of the most important chemical reactions occurring in sediments are associated with the decomposition of organic matter. Decomposition reactions affect the sediment environment and organic content. The process of decomposition involves the gradual breaking down of dead material to smaller and smaller particles and eventually to small molecules. It involves the action of physical factors, such as weathering and leaching, and the activities of microorganisms and detritivores. Litter within intertidal areas is derived from several sources. The largest source in terms of mass is usually the higher plants, such as mangroves and saltmarsh plants. Smaller detrital particles derived from microphytobenthos, epiphytic algae, phytoplankton and faecal material may also be significant. In addition, a substantial portion of the organic litter in intertidal areas may be of terrestrial origin, washed down by rivers or deposited by the tides. 10.5 Sulfur The water that inundates saltwater wetlands not only is saltier than terrestrial waters, but also contains relatively high levels of sulfur (S). The lack of oxygen within the soil allows anaerobic bacteria, which can operate on S respiration, to thrive. However, the microbial action and the reduction of sulfate to sulfide under anaerobic conditions creates the potential for ASS (Section 9.9) to form. On exposure to air, these soils can generate potentially harmful levels of sulfuric acid. In some estuarine environments, the availability of S combined with certain environmental conditions will lead to the generation of hydrogen sulfide (H2S), or rotten egg gas. H2S is produced naturally by sulfate-reducing bacteria within the sediments of all marineinfluenced environments. The bacteria convert sulfate and organic matter into H2S, CO2 and water. The conditions under which the release of H2S is triggered generally relate to the availability of oxygen. If there is insufficient oxygen available in the water or if the rate of oxygen input to the water (from photosynthesis by aquatic plants and atmospheric diffusion) is not fast enough, the H2S will not be completely oxidised, and it will be released to the atmosphere. While H2S can prove a nuisance because of its unpleasant odour, it is unlikely to cause health problems in open environments, where it is quickly dispersed into the atmosphere. 10 Chemistry 137