Biogeochemical Zones Within a Macrotidal, Dry-Tropical Fluvial-Marine Transition Area: A Dry-Season Perspective

Abstract

The Fitzroy River delivers large amounts of nutrients and fine sediments to Keppel Bay (contiguous with the Great Barrier Reef Lagoon) during intermittent flow events. This study explores sources, forms and transformations of nutrients in Keppel Bay, and develops a functional process zonation that integrates seabed geochemistry and water column nutrient characteristics which are controlled by suspended sediment. The water column and seabed properties were investigated over two dry seasons, with supplementary core incubations taken to measure carbon decomposition rates and nutrient fluxes. Keppel Bay can be divided into three zones, the: zone of maximum resuspension (ZMR); coastal transitional zone (CTZ); and blue water zone (BWZ). Mineralisation of predominantly terrestrial organic matter occurs in the ZMR where nutrient uptake by phytoplankton is light limited. The CTZ and BWZ had higher light penetration and phytoplankton growth was likely limited by N and P, respectively. The identified zones conform to the bathymetry and hydrodynamic characteristics of the bay, allowing for the development of an integrated conceptual model accounting for the benthic and pelagic biogeochemical processes. Recognition of these different zones shows that considerable variation in benthic and water column properties is possible within a small system with the bathymetric and hydrodynamic characteristics of the fluidized bed reactor.

An assessment of residence times of land-sourced contaminants in the Great Barrier Reef lagoon and the implications for management and reef recovery

“…These conclusions refute the suggestion that pollutant residence times are strongly linked to water residence times and that solutes and pollutants are quickly flushed from the GBR lagoon as Hancock et al (2006), Luick et al (2007), Wang et al (2007) and Choukroun et al (2010) Coarse sediment 1000's of years Larcombe and Carter (2004), Fielding et al (2006), Bostock et al (2007), Ryan et al (2007), Brooke et al (2008) Fine sediment 10's of years in bioactive form Lambeck and Woolfe (2000), Wolanski and Spagnol (2000a), Duke and Wolanski (2001), Larcombe and Carter (2004), Orpin et al (2004), Ryan et al (2007), Smith et al (2008), Wolanski et al (2008), Radke et al (2010), Lambrechts et al (2010) and Webster and Ford (2010) Reactive Nitrogen Years -decades Ayukai (1995), Furnas et al (1997), Furnas et al (2005), Furnas et al (2011), Suzuki et al (1995), Bronk, (2002, Alongi and McKinnon (2005), Davies and Eyre (2005), Devlin and Brodie (2005), O 'Neil and Capone (2008), Devlin and Schaffelke (2009) (2004), Wyndham et al (2004), Lewis et al (2007);2012;Brunskill (1996, 1997) and Mortimer (2000).…”

“…Additionally 50% of the modern fine sediment delivered by the Fitzroy River has been shown to be trapped in the river floodplain and estuary . The finer sediments are exported at least 30 km offshore and possibly as far as the GBR mid-shelf Smith et al, 2008;Radke et al, 2010). It is important to quantify correctly the residence time of these pollutants in the GBR lagoon when evaluating the impact of human activities in the adjoining catchments, as well as the effectiveness of remediation measures.…”

Terrestrial pollutant runoff to the Great Barrier Reef: An update of issues, priorities and management responses

“…In plumes from the Burdekin River, suspended solids concentrations can drop from more than 500 mg L À1 in the river at zero salinity close to the river mouth to less than 10 mg L À1 at salinities around 5-10 psu . Fine benthic sediment is continuously resuspended in shallow waters (<10 m) by the prevailing south east wind regime and tidal currents and transported north along the coast (Larcombe et al, 1995;Radke et al, 2010). Coarser sediments are mostly retained near the coast in sand bars, beach ridges and subaqueous dunes (Ryan et al, 2007).…”

Review: Sediment‐Related Controls on the Health of the Great Barrier Reef

“…The greatest abundance of phytoplankton occurs where fresh surface water mixes with seawater at the outer edge of the plume, where turbidity is relatively low compared with the river sediment plume behind it. Radke et al (2010) termed this zone the coastal transition zone , where settling of flocculated fine sediment coincides with increased availability of nutrients and light penetration, resulting in a large increase in the abundance of phytoplankton.…”

“…Radke et al (2010) presented a detailed examination of the biogeochemistry of the estuary and adjacent zones surrounding the mouth of the Fitzroy River. Although the sampling of this study was confined to the dry season, it elucidates the nutrient cycling processes in the estuary that cause part of the nutrients carried by the river to become available to the food web in the lagoon.…”