The spatial variability of elemental and isotopic compositions of carbon and nitrogen within sediments in a subtropical mangrove forest

Field: Environmental Science (Physical Sciences)

Mangrove forests are coastal habitats, which provide numerous ecosystem services in tropical and temperate regions, including burial and storage of Carbon and Nitrogen in the sediments. In New Zealand mangrove coverage is rapidly expanding throughout muddy coastlines and estuaries, in direct contrast to the current global trend of declining mangrove coverage. In this thesis, we explore the spatial distribution of Carbon and Nitrogen content and isotopic compositions in the sediments of a mangrove forest in New Zealand. \n \nThe field site is the Firth of Thames mangrove forest, which has rapidly expanded since around 1950, and now reaches 1 km in width over the southern Firth. Sediment cores with lengths ranging from 34 to 40 cm were collected from twelve across-shore transects and two along-shore transects. At each location, samples were taken from the surface and at depth, resulting in a total of 175 samples, providing substantial spatial coverage across the forest. A number of samples of mangrove vegetation were also collected. Elemental and isotopic analyses were performed on the sediment and vegetation samples. Additionally, grain size, fluorometric determination of Chlorophyll-a pigments and loss on ignition analyses were made on a selected subset of sediment samples. The elemental analyses in sediments showed mean concentrations of 2.09%, 1.59%, and 0.23% of total carbon, organic carbon and nitrogen, respectively, with generally larger concentrations at the surface. The C:N ratios were much larger from tree tissue samples (mean 38.01) relative to sediments (mean of 6.83). The mean δ13C and δ15N values in sediments were -23.04‰ and 8.20‰, respectively, while the mangrove samples were 13C-depleted (mean of -26.89‰) and exhibited a wide range in the Nitrogen isotope composition (values from 4.89-11.94‰). \n \nSurface sediment samples showed a progressive increase of N, C and OC towards the interior of the mangrove forest, a decrease in δ13C values, but no clear spatial pattern was discerned in the depth samples. Despite some scatter, δ15N values were relatively constant in the across-shore direction. The elemental and isotopic concentration in the sediments did not appear to be affected on a large scale by the freshwater discharge, which locally influenced only the concentrations within the sediments on the adjacent riverbanks. Relatively high δ13C values and low C: N ratios indicate the predominance of an allochthonous marine source for carbon in FoT mangrove sediments. Neither algal biomass nor grain size were found to be correlated with OC content in the Firth of Thames sediments. A gross estimate (integrated over the top 10 cm) of C and N stocks was 13.26 t C ha-1 and 1.47 t N ha-1, respectively, thus adding to the recognition of these forests as efficient carbon and nitrogen sinks. \n \nThis research contributes to the knowledge of C and N content, sources and burial patterns in subtropical mangrove sediments. Such information is required to underpin environmental management decisions, for example prior to undertaking restoration or removal projects.