River systems around the world have been degraded by agricultural land use, land clearing and other human impacts. While the impacts on rivers have been well documented in many temperate regions, the cumulative impacts of land use change on water quality in tropical regions are less well understood. Changes in suspended sediment or nutrient flux have not been accurately quantified for many tropical rivers, particularly in more remote regions globally. This is largely due to the lack of long-term monitoring and uncertainties associated with measuring and modelling discharge and loads. This study addresses this deficiency by analysing nutrient and sediment concentrations during baseflow and flood event conditions across the Normanby Basin in Cape York Peninsula, northern Australia. The Basin, comprised of the inter-connected Normanby and North Kennedy Rivers, is the fourth largest river system flowing into the Great Barrier Reef lagoon, with a watershed (“catchment”) area of 24,550 km2. Data were collected from 13 sites over 12 years to describe the supply and fate of nutrients and sediments from upper catchment tributaries to coastal flood plumes and to gain an understanding of potential drivers of spatial and temporal variation in water quality in a remote tropical river system. The Basin is located in the wet-dry tropics, characterised by intense monsoonal rains during the summer “wet season” and periods of low or intermittent flow during the winter “dry season”. The Basin and receiving marine environment at Princess Charlotte Bay (PCB) are recognized for their diverse and rich freshwater, estuarine and marine ecosystems. Indigenous clan groups rely upon these waters for fish and other foods. Although half the Basin is now designated for conservation and/or cultural land uses, around 80% of the land has been historically grazed by cattle. A small area (≈30 km2) at Lakeland Downs in the upper Normanby River catchment is used for horticulture and cropping (including bananas, sorghum, pineapples and watermelon). The Basin also has a history of mining, and alluvial mining leases currently cover 6 km2 in the upper catchment. Previous research has shown that the rates of gully erosion have increased since the introduction of cattle to the Basin. Erosion rates are highest in upper Normanby River sub-catchments which are dominated by highly erosive sodic soils. Not surprisingly, these upper Normanby River tributaries were found to supply most nutrients and sediment to the lower catchment and estuary, primarily during annual monsoon-driven flood events. Suspended sediment concentrations were highest in tributaries with the highest density of gully erosion (the Normanby River at Battlecamp Crossing and West Normanby). Maximum concentrations occurred during the rising and peak stages of flood events, particularly during “first flush” events. Annual and event suspended sediment loads were estimated for four sites with gauging stations in the upper tributaries of the Normanby River and one lower Normanby River gauge (Kalpower Crossing, ≈70 km upstream from the river mouth). Differences between the combined upper catchment load estimates and Kalpowar Crossing revealed that between 65 to 80% of the upper catchment sediment load was either deposited in the mid-catchment area or bypassed the Kalpower gauge by overbank flow and channel diversions. On average, only 27% of the estimated sediment load from the combined upper catchment tributaries was delivered to the Kalpowar Crossing gauge. Preliminary estimates from a small sample set over one wet season indicate that the upper catchment to Kalpowar gauge delivery ratio was higher for fine sediments, roughly estimated as 37% for fine silts (4 – 16 μm) and 46% for clays (<4 μm). Estimated volumes of suspended sediment from the upper Normanby catchment deposited in the mid-catchment area over an average discharge wet season (2015-16) were 8 kt of sand, 76 kt of silt, and 55 kt of clay. During the highest discharge water year (2013-14), an estimated total of 545 kt of sediment was deposited in the mid-catchment above Kalpower Crossing. While acknowledging the many uncertainties associated with both the loads estimates and quantification of land use impacts, it is estimated that more than half of this load is anthropogenic. The deposition of sediments occurs largely in the region of Rinyirru National Park/ Cape York Peninsula Aboriginal Land, where rivers and freshwater lagoons support a rich diversity of flora and fauna, including several vulnerable and endangered species (Environment Australia 2001, Howley et al. 2013). This diversity is threatened by the in-filling of waterholes and the smothering of benthic habitats that inevitably must accompany the deposition of large sediment loads. Both measured and anecdotal reports of rapid river and wetland infilling support these estimates of significant sediment deposition within the Basin. In addition to the anthropogenic sediment supplied from the upper catchment, this study suggests that farms in the Lakeland region supply nutrients to the Laura River and Boggy Creek (a tributary of the West Normanby River). Nutrient concentrations and nitrogen and oxygen isotopes in nitrate were analysed from six sites located downstream from Lakeland farms and four upstream reference sites. Dissolved inorganic nutrients (phosphorus and nitrogen) were significantly elevated downstream from Lakeland farms compared to reference sites. In particular, concentrations of nitrate + nitrite (NO3 + NO2) in the Laura River were elevated by as much as 50 times above background. Groundwater was found to be an important pathway to deliver anthropogenic NO3 + NO2 to the rivers and river concentrations were highest during baseflow periods. Nutrient concentrations were elevated at least 30 km downstream from Lakeland, supporting phytoplankton blooms over the dry season. These findings show that rivers in the upper Normanby Basin are most vulnerable to impacts from horticultural land use during periods of low flow, highlighting the need to maintain adequate discharge during the dry season. High δ15N values measured downstream from Lakeland farms (mean 10‰) compared to reference sites (mean 5‰), suggest that compost fertilisers made from chicken manure and applied to banana farms may be a dominant source of dissolved inorganic nitrogen (DIN) to the rivers. This study has shown that sediment and nutrient concentrations in the lower catchment and estuary are controlled by complex interactions between river flows from the upper catchment, sediment deposition and nutrient uptake in the mid- to lower catchment, coastal sources, and re-suspension processes in the estuary. Although the upper catchment supplied most materials to the coast during flood events, the coastal zone, which is largely unimpacted by human development, was also found to supply sediments and nutrients to the estuary and Princess Charlotte Bay. Salt flats, covering 266 km2 in the Normanby Basin coastal zone, are likely to be a significant source of sediments and nutrients to the estuary based on the results of several salt flat run-off samples and regular observations of turbid tidal plumes flowing into the estuary from adjacent salt flats. Sediment re-suspension also contributes to high ambient suspended sediment concentrations (mean 50 mg L-1) within the estuary as has now been documented in several Normanby estuary studies (Croswell et al. 2020, Shellberg and Howley 2018). Most notably, concentrations of DIN in the lower estuary were double the concentrations measured upstream at Kalpowar Crossing during both baseflow and flood periods. This provides evidence that undisturbed coastal zones can provide a significant natural source of DIN to tropical river flood plumes. The concentrations of nutrients, sediment and chlorophyll-a in Normanby and North Kennedy River flood plumes were assessed over three flood events to determine the fate of materials delivered to PCB from the upper catchment and estuary. Preliminary particle size analysis suggested that only fine silt and clay (<16μm) were transported beyond the estuary during flood events. Most of the fine sediments were deposited in the low salinity zone within 6 km of the river mouths. Beyond the low salinity, high turbidity zone, dissolved nutrients within flood plumes stimulated increases in phytoplankton biomass. Phytoplankton species composition varied significantly between the three events. Plumes from average magnitude flood events inundated mid-shelf coral reefs, with the area of influence determined by flood magnitude, wind direction and wind speed. Until recent coral bleaching events (2016-2017) in the Great Barrier Reef resulting from warmer than average water temperatures, reefs in the PCB region were generally assessed as being in good to moderate condition (Australian Institute of Marine Science surveys). This is likely due to the buffering effect of the catchment, trapping at least half of anthropogenic nutrients and sediments, including the fertiliser-derived nitrogen and phosphorus in the Laura River. However, a percentage (roughly estimated at 46% for clays) of these materials from the upper catchment is transported to the estuary and PCB during flood events. Therefore, future increases in anthropogenic loads of nutrients and sediments from the upper Normanby catchment as a result of catchment clearing, alluvial mining and expanding agricultural activity could increase phytoplankton growth and sediment deposition in PCB flood plumes. Careful management and monitoring of future development in the upper Normanby catchment are required to maintain the health and resilience of both freshwater and marine aquatic ecosystems.

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