Because of the well-established influence of the Burdekin River on the central GBR , recent studies have implicated its influence ( among other factors ) on declines in coral biodiversity ( i . e . reductions in Acropora , Pavona and Porites sp .) in the Palm Island Group including Pelorus Island , Havannah Island and Pandora Reef ( Roff et al . 2012 ; Clark et al . 2014 ). Other coral reefs in the path of the Burdekin flood plume influence appear to have seen little recent changes in diversity including Middle Reef ( off Townsville : Browne et al . 2010 ; Perry et al . 2012 ), Nelly Bay , Magnetic Island ( Lewis et al . 2012 ) and Paluma Shoals ( Palmer et al . 2010 ) with reef growth largely associated with available accommodation space and the sedimentary regime ( Browne et al . 2012 ).
Identification of pollutants of most ecological concern for prioritised on-ground management There have been a number of studies that have examined the concentration of suspended sediments ( and particulate nutrients ) through several flood plume transects from the mouth of the Burdekin River ( Wolanski and Jones , 1981 ; Wolanski and van Senden , 1983 ; Devlin et al . 2001 ; Devlin and Brodie , 2005 ). However , only recently has the composition of this suspended sediment been further characterised through particle size measurements , organic matter contents and microscopy of the sediment flocs ( Bainbridge et al . 2012 ). The results show that the entire ‘ coarse ’ sediment fraction (> 16 µ m ) is deposited near the river mouth (< 10 km ) and as the flood plume disperses away from the river mouth , the suspended sediments form organic-rich floc aggregates around fine-grained (< 16 µ m ; clay to fine silt fraction ) ‘ mineral ’ sediment ( Bainbridge et al . 2012 ). Importantly , these organicrich suspended sediments in the flood plume were shown to impinge on coral reefs and seagrass meadows ( Bainbridge et al ., 2012 ) and similar material has been shown to be particularly detrimental to corals under laboratory conditions ( Weber et al . 2006 , 2012 ). Hence it is the fine-grained sediment (< 16 µ m ) that is discharged from the Burdekin River that should be targeted for management as this material appears to largely fuel the organic matter production in the plume that forms these floc aggregates . This fraction also travels longer distances within river flood plumes and likely has direct impacts on water clarity and sedimentation in the coastal and inshore GBR lagoon .
The fate of particulate nitrogen and phosphorus loads delivered to the GBR are poorly understood . While a review of particulate nitrogen in the GBR show that a large proportion is likely to be highly bio-available for mineralisation ( Brodie et al . 2015 ), the flood plume monitoring data show that much of the particulate nitrogen delivered to the coast is deposited close to the river mouth ( Bainbridge et al . 2012 supplement ). Hence in terms of total nutrient delivery to the ‘ far-field ’ GBR lagoon , the dissolved nitrogen component forms the majority of the nutrient load , at least in the short term . Whilst the particulate nutrient load decreases with distance , the remaining fraction appears to play an important role in forming / fuelling floc development ( Bainbridge et al . 2012 ), warranting further investigation . Furthermore , preliminary data suggest that particulate nitrogen has the potential for mineralisation to dissolved inorganic nitrogen within the freshwater reaches of rivers and hence contribute to the end-of-river nitrate load ( Burton et al . 2015 ); this finding suggests that the management of erosion and sediment yield should be a priority in the catchment for controlling nitrogen in coastal waters , and warrants further investigation to develop catchment-specific nutrient budgets for surface and sub-surface soils . This is also consistent with catchment models which show that a large proportion of total Burdekin River DIN is derived from grazing lands . Finally , studies of sediments on the GBR shelf suggest that ~ 50 % of the particulate nitrogen delivered to the coast is denitrified ( i . e . becomes ‘ inert ’ nitrogen , e . g . Alongi and McKinnon , 2005 ; Alongi et al . 2007 ), although a fraction will be mineralised and contribute to the reactive nitrogen loading in the GBR lagoon . The spatial influence of this mineralised particulate N fraction in the GBR is lesser known and should be the subject of future study .
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