Land use change increases contaminant sequestration in blue carbon sediments.

Coastal blue carbon habitats perform many important environmental functions, including long-term carbon and anthropogenic contaminant storage. Here, we analysed twenty-five 210Pb-dated mangrove, saltmarsh, and seagrass sediment cores from six estuaries across a land-use gradient to determine metal, metalloid, and phosphorous sedimentary fluxes. Cadmium, arsenic, iron, and manganese had linear to exponential positive correlations between concentrations, sediment flux, geoaccumulation index, and catchment development. Increases in anthropogenic development (agricultural or urban land uses) from >30 % of the total catchment area enhanced mean concentrations of arsenic, copper, iron, manganese, and zinc between 1.5 and 4.3-fold. A ~ 30 % anthropogenic land-use was the threshold in which blue carbon sediment quality begins to be detrimentally impacted on an entire estuary scale. Fluxes of phosphorous, cadmium, lead, and aluminium responded similarly, increasing 1.2 to 2.5-fold when anthropogenic land-use increased by at least 5 %. Exponential increases in phosphorus flux to estuary sediments seem to precede eutrophication as observed in more developed estuaries. Overall, multiple lines of evidence revealed how catchment development drives blue carbon sediment quality across a regional scale.

Using multispectral drones to predict water quality in a subtropical estuary.

Drones are revolutionising earth system observations, and are increasingly used for high resolution monitoring of water quality. The objective of this research was to test whether drone-based multispectral imagery could predict important water quality parameters in an ICOLL (intermittently closed and opened lake or lagoon). Three water quality sampling campaigns were undertaken, measuring temperature, salinity, pH, dissolved oxygen (DO), chlorophyll (CHL), turbidity, total suspended sediments (TSS), coloured dissolved organic matter (CDOM), green algae, crytophyta, diatoms, bluegreen algae and total algal concentrations. DistilM statistical analyses were conducted to reveal the bands accounting for the most variation across all water quality data, then linear correlations between specific band/band ratios and individual water quality parameters were performed. DistilM analyses revealed the NIR band accounted for most variation in March, the Green band in April and the RE band in May, and showed that the most important contributors varied significantly among campaigns and variables. Significant linear correlations with R2 > 0.4 were obtained for eleven of the water quality parameters tested, with the strongest correlation obtained for CHL and the green band (R2 = 0.72). The relative importance of predictor bands and observed water quality parameters varied temporally. We conclude that drones with a multispectral sensor can produce useful 'snapshot' prediction maps for a range of water quality parameters, such as chlorophyll, bluegreen algae and dissolved oxygen. However, a single model was insufficient to reproduce the temporal variation of water parameters in dynamic estuarine systems.

Land use and episodic rainfall as drivers of nitrogen exports in subtropical rivers: Insights from δ15N-NO3-, δ18O-NO3- and 222Rn.

Ongoing land-use intensification in subtropical catchments is expected to release more inorganic nitrogen to downstream coastal waters similar to historical changes in temperate ecosystems. Here, we examined spatial and temporal drivers of stream nitrogen loads across a subtropical land-use gradient using the isotopic compositions of nitrate (NO3--N) and radon (222Rn), a natural groundwater tracer. We investigated eleven subtropical creeks/rivers over contrasting hydrological conditions in Australia. NOx-N (nitrite (NO2--N) + nitrate (NO3--N)) accounted for 13.1%, 34.0%, and 42.6% of total dissolved nitrogen (TDN-N) in forest, peri-urban and agricultural creeks, respectively. Following an 80 mm rain event, loads of dissolved inorganic nitrogen (DIN-N) from agriculture catchments reached 368 mg N m-2 catchment area day-1. Forest and peri-urban catchments had aquatic TDN-N loads 17.8% and 31.1% of loads from agricultural catchments. Radon observations suggest that nitrogen and phosphorus loads were driven primarily by surface runoff rather than groundwater discharge. The δ15N-NO3- and δ18O-NO3- values in the agriculture, forest and peri-urban catchments indicate fertilisers and soil nitrogen as the main sources of NO3--N. However, one of the catchments (Double Crossing Creek) received a mixture of recirculated greywater and chemical nitrogen fertilisers. Isotopic signatures imply significant NO3--N losses via denitrification during dry conditions. Groundwater discharge played a minor role because regional aquifers were not contaminated by nitrogen. Overall, intensive agricultural land use and episodic rainfall events were the major spatial and temporal drivers of nitrogen loads.