Coastal wetlands dominated by mangroves provide important ecological services to the coastal zone, many of which are associated with tidal inundation . In this study I investigated whether all coastal wetlands provide similar ecosystem services, or whether these varied depending on their geomorphological setting and the eutrophication level of the floodwater. Sampling was conducted over two years with significantly different levels of rainfall over six estuaries in Southeast Queensland, Australia. Sediment, nutrient and carbon exchange were investigated in tidal and riverine estuaries dominated by mangrove forests. The estuaries selected also comprised a gradient from low mesotrophic to low eutrophic. Spatial variation within the coastal wetlands was also investigated, comparing nutrient exchange in the low and high intertidal cyanobacteria mat . To assess sediment exchange, I measured retention of total suspended solids and sedimentation rates. Additionally, in order to assess the origin of sediment deposited in mangroves, glomalin, a novel terrestrial soil carbon (C) tracer, was used. Nutrient and C exchange were quantified by comparison of concentrations of soluble reactive phosphorus (SRP), nitrogen oxides (NOx--N), ammonium (NH4+) total organic carbon (TOC) and dissolved organic C (DOC) in water entering and leaving the mangroves during complete tidal cycles (3 tidal cycles in 6 estuaries for 2 years). Finally, the biogeochemical function of the cyanobacteria mat was measured through experiments investigating nitrogen fixation (N) and nutrient exchange in the extensive mats in the Exmouth Gulf, Western Australia. My results show that the seaward fringe mangrove retains the majority of sediment entering the wetland during a tidal cycle accounting for 52.5 ± 12.5 % of the total sedimentation (fringe mangrove + scrub mangrove + saltmarsh/ cyanobacteria mat). Geomorphological setting had a stronger influence on spatial patterns of deposition than on sedimentation rates. Riverine mangroves had more homogeneous distribution of sediments across the intertidal zone than tidal mangroves, where most sedimentation occurred in the seaward fringe mangrove zone. The presence of glomalin in sediments, and thus the relative importance of terrigenous sediment, was strongly influenced by geomorphological setting. Glomalin was primarily delivered to riverine mangroves and deposited within the scrub mangrove zone, while tidal mangroves received less glomalin during tidal inundation and most of it was deposited within the fringe mangrove zone. Overall, NOx--N concentrations decreased in the floodwater after flooding the coastal wetland, suggesting that these ecosystems act as sinks of dissolved NOx--N during tidal inundation. In average, NOx--N concentrations in the floodwater decreased 28 %. Additionally, during periods of high rainfall the decrease in nutrient concentrations was more pronounced, and not only NOx--N but also SRP and NH4+ concentrations decreased to up to 51 % and 83 %, respectively. My results suggest that rainfall enhances nutrient removal by coastal wetlands in the region. Geomorphological setting also affected nutrient removal. Riverine mangroves received stronger nutrient pulses, which resulted in strongest rates of nutrient removal during tidal inundation. Nutrient removal was closely related to the nutrient concentration of the floodwater: high nutrient removal occurred when floodwater was rich in nutrients. The C entering the wetland in the floodwater was mainly composed of DOC and its exchange did not vary among sites with differing geomorphological setting. However, DOC exchange was strongly affected by the water quality of floodwater inundating the coastal wetland. DOC concentrations were higher in the flood compared to the ebb tide in sites flooded by water high in C, NH4+ and SRP, suggesting DOC import. Contrary, DOC concentrations were lower in the flood compared to the ebb tide in sites flooded by water high in C, NH4+ and SRP, suggesting DOC export. The high intertidal cyanobacteria mat was important in regulating N fluxes in coastal wetlands. In the arid Exmouth Gulf, where cyanobacteria mats are abundant, nitrogen fixation rates were 4.9 ± 3.2 nmol cm-1 h-1. Cyanobacteria mats also removed N from the floodwater in the form of NOx--N (0.47 ± 0.45 g m-2 h-1) and NH4+ (0.31 ± 0.02 g m-2 h-1). N fixation and nutrient removal from the floodwater was highly variable spatially and temporally. N fixation rates were highest during the day in the mat situated at low tidal elevations. Overall, I found that the material exchange in coastal wetlands is variable within the coastal zone as a result of natural factors, such as geomorphology, vegetation composition and rainfall. But material exchange in wetlands is also affected by anthropogenic factors, particularly eutrophication. From all these factors, eutrophication of the floodwater appears to be the most critical, shifting the mangrove function from a DOC source to a nutrient and DOC sink. Approximate thresholds of nutrient and C concentrations in the floodwater that are likely to trigger shifts in ecosystem function in coastal wetlands in Southeast Queensland, and thus in the ecosystem services they provide, are 0.02 mg L-1 of SRP, 0.04 mg L-1 of NH4+ and 7.5 mg L-1 of DOC.
| Identifer | oai:union.ndltd.org:ADTP/287467 |
| Creators | Maria Adame Vivanco |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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