Tracing the sources, transport and dispersal of suspended sediment from the Burdekin River catchment into the Great Barrier Reef lagoon
Other Publication ResearchOnline@JCUAbstract
Increased turbidity and sedimentation associated with the delivery of greater quantities of fine sediments to the coast due to anthropogenic modification of catchments have seriously degraded many near-shore marine ecosystems around the world. Within the Great Barrier Reef (GBR), Australia, there is growing evidence that increased turbidity and sedimentation associated with agricultural development of the coastal catchments have negatively impacted valuable ecosystems, including coral reefs and seagrass meadows. Further, research over the past decade has determined fine (<63 μm) organic and nutrient-rich terrigenous sediments have the greatest negative effects on tropical marine ecosystems because they: a) efficiently adsorb and transport other contaminants; b) aggregate and form organic-rich flocs; and c) can remain in suspension within the water column where they impede light penetration and reduce photic depth. However, limited field studies have examined the composition and transformation of suspended sediment in flood plumes over space and time (e.g. into organic-rich sediment flocs) within the GBR, or determined the source and nature of the sediment delivered by flood plumes that is most widely dispersed across the GBR lagoon. The Burdekin River catchment (130,400 km²) is the largest discrete source of suspended sediment to the GBR, with an average annual export of 3.93 million tonnes. This accounts for ~30% of the total average annual load from the entire GBR catchment area (426,000 km²). Identifying major catchment source areas of this sediment and an improved understanding of how it is transported through the Burdekin catchment and dispersed within GBR coastal waters are required to better manage this threat. The overall aim of this research is to characterise and source suspended sediments discharged by Burdekin River flood plumes into the central GBR lagoon, that are most likely to negatively affect coral reef and seagrass ecosystems. Novel sediment budget and clay mineral-based tracing techniques were applied to this large, seasonally-dry tropical catchment to examine and quantify suspended sediment sources and transport across the catchment to marine continuum. These techniques have historically been applied only to smaller (i.e <10,000 km²) temperate river catchments. The transformation and dispersal of suspended sediments and associated nutrients carried by Burdekin coastal flood plumes within the GBR lagoon were also investigated. This study specifically focused on the sources, transport and dispersal of particular fine sediment size fractions that are not normally separated for attention, recognising the increased ecological risk of clay (<3.9 μm) and fine silts (3.9–15.6 μm) to downstream marine ecosystems. The research reported in chapter 2 examined the hydrodynamic, biological and chemical processes controlling the transformation and dispersal of suspended sediments and particulate nutrients carried in flood plumes discharged from the Burdekin River into the GBR lagoon. An examination of flood plume sediment dynamics from 2007/08 to 2010/11 found all sand (>63 μm) and the majority (>80%) of clay and silt (<63 μm) sized-sediment rapidly settle once floodwaters mix with seawater, where salinity can be as low as 0.1 psu, usually within 10 km of the coastline. Microphotographs of subsurface plume water within this zone revealed flocs of sediment particles were settling bound by organic matter, with floc sizes >100 μm in diameter. This is the first evidence of biologically-mediated flocculation processes occurring in the flood plumes of the large, sediment-laden dry tropical rivers discharging into the GBR. It is likely that particulate nutrients play a key role in driving this biologically-mediated flocculation and accelerated settling of river suspended sediment through heterotrophic bacteria production in this turbid zone, where low light and salinity conditions usually prevent marine phytoplankton blooms. The analyses in chapter 2 also identified clay and fine silt (<16 μm) sized-sediments to be preferentially transported in Burdekin flood plume waters during peak flood conditions, and were observed as discrete mineral particles or, once suspended sediment had reduced to <10 mg L⁻¹, as small flocs in plume waters adjacent to the coast. As light conditions improved within plume waters over following weeks and marine biological activity increased, these clay and fine silt particles were observed in microphotographs encased in biological matter, forming large, low-density floc aggregates (100–200 μm), with sampling indicating that they maintain this state after seaward propagation at least 100 km from the river. Hence, this study identified clay and fine silts to have the greater dispersal potential within the GBR lagoon, and these fine mineral particles are often dispersed within large, buoyant organic-rich flocs. These flocs pose a risk to benthic organisms (e.g. coral reefs and seagrass meadows) because they: a) increase turbidity; b) worsen smothering impacts due to their 'sticky' nature; and c) are more easily remobilised during dry season wind-driven resuspension events. Research reported in chapter 3 identified the major sources and spatial and temporal variability of suspended sediment yielded from Burdekin sub-catchments from a series of annual catchment-wide sediment source and transport budgets. These budgets incorporate suspended sediment loads (calculated from suspended sediment concentration and streamflow data) collected at seven strategic sub-catchment, reservoir outlet and end-of-river gauging station locations over five consecutive water years (Oct 1 to Sept 30: 2005/06 to 2009/10). The study confirmed that this budget approach of source identification in large, tropical catchments can reliably discriminate consistent, dominant sub-catchment sources of end-of-river suspended sediment export. Two major sub-catchments (Upper Burdekin and Bowen Rivers) distinguished by key geomorphic features including steep topography, erosive soils and wetter coastal climates generated sediment yields (147–530 t km² yr⁻¹) an order of magnitude higher than their inland, low relief and drier counterparts (<23 t km² yr⁻¹). Research examining the transport of specific sediment-size fractions within tropical catchments has been limited to date, and this study also quantified sub-catchment contributions of the clay (<4 μm), fine silt (4−16 μm) and coarse (i.e. >16 μm) sediment fractions. Sediment trapping within a reservoir (capturing 88% of the catchment) and the preferential transport of clays and fine silts downstream of this structure were also examined. The data reveal that the highest clay and fine silt loads, of most interest to environmental managers of the GBR, are not always sourced from areas that yield the largest total suspended sediment load (i.e. all size fractions). For example, the 'bulk' sediment loads to the end-of-river were dominated by the Bowen River source (3.76 million tonnes) compared to BFD overflow source (2.52 million tonnes), but the BFD overflow source contributed a higher clay-specific load than the Bowen sub-catchment (1.32 million tonnes and 1.03 million tonnes, respectively). However, the clay-specific yield from the smaller Bowen River source (145 t km⁻² y⁻¹) is 10-fold higher than the BFD overflow source (11 t km⁻² y⁻¹). The results demonstrate the importance of incorporating particle size into catchment sediment budget studies undertaken to inform management decisions to reduce downstream turbidity and sedimentation. Chapter 4 examined the potential of clay mineralogy as a sediment tracing technique for catchment studies with a specific focus on tracing terrigenous sediment in flood plumes back to a catchment origin. A comprehensive clay mineral dataset (231 samples) representing 31 river and upstream tributary sites over multiple streamflow events and water years found consistency in clay mineral relative abundances, with a ratio of common clay minerals (illite/illite+expandable clays) clearly distinguishing basaltic (ratio of 0–7), granitic (28) and sedimentary (42–52) geological sources. These ratios also clearly distinguished the Upper Burdekin-BFD reservoir source (34–35) from the expandable clays-rich Bowen River source (11), and were used in conjunction with the sediment budget approach to provide multiple lines of evidence to guide the remediation of fine sediment sources. Further, the I/I+E ratio provided evidence of relative enrichment of the expandable (smectite-rich) clays within remaining flood plume sediment after this bulk deposition near the river mouth, with increasing salinity. The distinctive geological source-related "fingerprints" found in this study validate the relative proportions of clay minerals as a valuable tracing tool in large and geologically complex catchments, and across freshwater-marine continuums. This study also found 1-2 samples from any given source area is sufficient to generate a reproducible signature, highlighting the efficacy of this technique and its potential application for similar sediment tracing and climatic studies in other catchments. This thesis has utilised complementary, multiple lines of evidence to trace the source and fate of fine sediments across a large, seasonally-dry tropical catchment and adjacent coastal waters, and has demonstrated the applicability of sediment budget and clay mineral-based tracing techniques rarely utilised at such scales. This approach to sediment source identification is suitable for broader application across the GBR catchment area and lagoon, and similar coastal settings. The importance of incorporating sediment particle size into sediment source investigations has been highlighted by this study, and should also guide further research examining the ecological effects of terrigenous sediment (and associated nutrients and other contaminants) on coral reefs, seagrass meadows and other marine ecosystems.
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226
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10.25903/qfkk-8735