Impacts of tracer type, tracer selection, and source dominance on source apportionment with sediment fingerprinting.

Sediment fingerprinting estimates the proportional contribution of fine sediment from distinct catchment sources delivered to downstream receiving environments. Increased attention has focused on assessing the accuracy of source contribution estimates, particularly in relation to tracer selection and statistical un-mixing procedures. However, no studies have systematically tested the impact of source combination or dominance on the accuracy of source estimates. Here, we assess sensitivity to tracer type, selection, and number of sources, and examine how variations in the dominant sediment source affect the accuracy of source apportionments using numerical mixtures. Sources were sampled according to erosion process and land cover from a New Zealand catchment. Topsoil and subsoil (landslide) samples were collected from pasture, harvested pine, kanuka scrub, and native forest, while banks were sampled along the main channel. Samples were analysed for bulk geochemistry, fallout radionuclides, and compound specific stable isotopes (CSSIs). Source apportionment accuracy tended to decrease as source number increased, which reflected decreasing source discrimination. Tracer selection showed variations in accuracy but exhibited no clear pattern overall. Source combination and particularly the dominant source had the largest impact on accuracy, reflecting the level of discrimination for each source. Notably, channel bank was frequently identified as the dominant source when using CSSI tracers. While this partly reflected lower levels of discrimination, the CSSI apportionment was particularly sensitive to the use of post-unmixing corrections routinely applied to derive soil proportional contributions from isotopic proportions. This sensitivity likely related to the low organic carbon content in bank material and the assumption that source apportionments based on isotopic proportions can be corrected using a linear relationship with organic carbon content. These results indicate that the use of CSSI tracers in catchments where erosion sources exhibit large differences in soil organic carbon content may introduce significant unquantified error in source apportionments.

Transition in the isotopic signatures of fatty-acid soil biomarkers under changing land use: Insights from a multi-decadal chronosequence.

The effects of climate warming on soil erosion in upland ecosystems will be disproportionately higher than for lowlands due to steeper topography and higher predicted rainfall. Soil erosion may be enhanced by climate warming and upslope shifts in agriculture as conditions for plant growth improve. Identification of eroded-soil sources will inform land management practices that mitigate soil loss and impacts on aquatic receiving environments. Isotopic signatures of plant-derived fatty acid (FA) soil biomarkers can discriminate sediment sources and will detect shifts in land use and natural vegetation toposequences. Accounting for these isotopic shifts requires knowledge of the magnitude and time scale for transition in biomarker signatures. We examined a 30-year chronosequence to quantify the transition in isotopic values of bulk nitrogen, carbon and FA biomarkers following a change from pine forestry to pastoral agriculture in the central North Island of New Zealand. We found the transition in soil biomarker isotopic values was complete within 6 years, with substantial increases in both organic carbon (1% yr-1) and total N (0.13% yr-1) of top soils. Subsequent changes were negligible (i.e., <0.04% yr-1), indicative of a new steady state. Similar patterns were observed in the isotopic signatures of bulk δ13C and δ15N values and FA δ13C values (i.e., ±0.5-0.6‰ yr-1). Bulk C and N properties and the FAs C14:0, C16:0, C18:2, C24:0 and C26:0 displayed clear transitions from harvested pine to mature pasture. We found evidence that mycorrhizal fungi could disperse and influence soil FA isotopic signatures. This highlights the need to consider both harvested and mature forests in source-tracing studies. Finally, our study shows that near-instantaneous changes in land use associated with agriculture can alter the isotopic signatures of plant biomarkers in soils. This produces a step change that can be readily detected in sedimentary records.

Night and day: Shrinking and swelling of stems of diverse mangrove species growing along environmental gradients.

Tree stems swell and shrink daily, which is thought to reflect changes in the volume of water within stem tissues. We observed these daily patterns using automatic dendrometer bands in a diverse group of mangrove species over five mangrove forests across Australia and New Caledonia. We found that mangrove stems swelled during the day and shrank at night. Maximum swelling was highly correlated with daily maxima in air temperature. Variation in soil salinity and levels of tidal inundation did not influence the timing of stem swelling over all species. Medium-term increases in stem circumference were highly sensitive to rainfall. We defoliated trees to assess the role of foliar transpiration in stem swelling and shrinking. Defoliated trees showed maintenance of the pattern of daytime swelling, indicating that processes other than canopy transpiration influence the temporary stem diameter increments, which could include thermal swelling of stems. More research is required to understand the processes contributing to stem shrinking and swelling. Automatic Dendrometer Bands could provide a useful tool for monitoring the response of mangroves to extreme climatic events as they provide high-frequency, long-term, and large-scale information on tree water status.

The vulnerability of Indo-Pacific mangrove forests to sea-level rise.

Sea-level rise can threaten the long-term sustainability of coastal communities and valuable ecosystems such as coral reefs, salt marshes and mangroves. Mangrove forests have the capacity to keep pace with sea-level rise and to avoid inundation through vertical accretion of sediments, which allows them to maintain wetland soil elevations suitable for plant growth. The Indo-Pacific region holds most of the world's mangrove forests, but sediment delivery in this region is declining, owing to anthropogenic activities such as damming of rivers. This decline is of particular concern because the Indo-Pacific region is expected to have variable, but high, rates of future sea-level rise. Here we analyse recent trends in mangrove surface elevation changes across the Indo-Pacific region using data from a network of surface elevation table instruments. We find that sediment availability can enable mangrove forests to maintain rates of soil-surface elevation gain that match or exceed that of sea-level rise, but for 69 per cent of our study sites the current rate of sea-level rise exceeded the soil surface elevation gain. We also present a model based on our field data, which suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070.

Light attenuation - a more effective basis for the management of fine suspended sediment than mass concentration?

Fine sediment continues to be a major diffuse pollution concern with its multiple effects on aquatic ecosystems. Mass concentrations (and loads) of fine sediment are usually measured and modelled, apparently with the assumption that environmental effects of sediment are predictable from mass concentrations. However, some severe impacts of fine sediment may not correlate well with mass concentration, notably those related to light attenuation by suspended particles. Light attenuation per unit mass concentration of suspended particulate matter in waters varies widely with particle size, shape and composition. Data for suspended sediment concentration, turbidity and visual clarity (which is inversely proportional to light beam attenuation) from 77 diverse New Zealand rivers provide valuable insights into the mutual relationships of these quantities. Our analysis of these relationships, both across multiple rivers and within individual rivers, supports the proposition that light attenuation by fine sediment is a more generally meaningful basis for environmental management than sediment mass. Furthermore, optical measurements are considerably more practical, being much cheaper (by about four-fold) to measure than mass concentrations, and amenable to continuous measurement. Mass concentration can be estimated with sufficient precision for many purposes from optical surrogates locally calibrated for particular rivers.