The effect of European settlement on water quality in the Great Barrier Reef of Australia is a long-standing and controversial issue. Erosion and sediment transport in river catchments in this region have increased substantially since European settlement, but the magnitude of these changes remains uncertain. Here we report analyses of Ba/Ca ratios in long-lived Porites coral from Havannah Reef--a site on the inner Great Barrier Reef that is influenced by flood plumes from the Burdekin river--to establish a record of sediment fluxes from about 1750 to 1998. We find that, in the early part of the record, suspended sediment from river floods reached the inner reef area only occasionally, whereas after about 1870--following the beginning of European settlement--a five- to tenfold increase in the delivery of sediments is recorded with the highest fluxes occurring during the drought-breaking floods. We conclude that, since European settlement, land-use practices such as clearing and overstocking have led to major degradation of the semi-arid river catchments, resulting in substantially increased sediment loads entering the inner Great Barrier Reef.
"Therefore, it is the ideal place to clarify the interactions between trace elements and biological activity in surface water. Furthermore, trace elements in coral skeleton are proved to be good geochemical proxies for river runoff, upwelling or primary production (Shen et al., 1987; Lea et al., 1989; McCulloch et al., 2003; Inoue et al., 2014), as they can be incorporated into the aragonite lattice of coral skeletons at the concentrations that are controlled by those in seawater (Shen, 1986; Shen and Boyle, 1988). The full knowledge of trace elements biogeochemical cycles in reef ecosystem will contribute to the evaluation of these trace element geochemical proxies. "
[Show abstract][Hide abstract] ABSTRACT: Abstract Bioactive trace elements play an important role in metabolic processes and, ultimately, marine productivity. An understanding of the complex interactions between biogeochemical processes and trace elements in reef ecosystems is important for establishing geochemical proxies for marine productivity or biological activity. In an attempt to illustrate this relationship, concentrations of dissolved trace elements (V, As, Se, W, and Y), dissolved oxygen (DO), and carbonate system parameters were measured over diurnal cycles on the Luhuitou fringing reef, located in the northern South China Sea. Prominent diurnal variations in DO and carbonate system parameters indicate that biological activities, especially photosynthesis and respiration, are the primary controls on reef water chemistry. The trace elements V, W, and Y co-varied significantly with seawater pH and DIC, implying that they participate in community photosynthesis and respiration. In contrast, As and Se demonstrated divergent diurnal to semi-diurnal behaviors, and their concentrations were not correlated with carbonate system parameters, suggesting that they were involved in other biogeochemical processes. The biologically controlled behaviors of V, W, and Y suggest that their records could potentially serve as geochemical proxies for marine productivity.
"For example, trace element-to-calcium ratios (TE/Ca), such as Sr/Ca and Mg/ Ca, in coral skeletons exhibit strong correlation with sea surface temperature (SST) and are utilized as seawater thermometers or SST proxies (Mitsuguchi et al. 1996; Beck et al. 1997; McCulloch et al. 1999). In contrast, Ba/Ca in coral skeletons provides information on freshwater input and/or deepwater upwelling to the surface ocean (Fallon et al. 1999; Alibert et al. 2003; McCulloch et al. 2003). Boron isotopic ratios (d 11 B) were once thought to directly record seawater pH, assuming that only borate ion (B(OH) 4 -) was incorporated into coral aragonite (Hemming and Hanson 1992; Pelejero et al. 2005; Wei et al. 2009; Shinjo et al. 2013). "
[Show abstract][Hide abstract] ABSTRACT: The response of Acropora digitifera to ocean acidification is determined using geochemical proxy measurements of the skeletal composition of A. digitifera cultured under a range of pH levels. We show that the chemical composition (δ11B, Sr/Ca, Mg/Ca, and Ba/Ca) of the coral skeletons can provide quantitative constraints on the effects of seawater pH on the pH in the calcification fluid (pH CF) and the mechanisms controlling the incorporation of trace elements into coral aragonite. With the decline of seawater pH, the skeletal d 11 B value decreased, while the Sr/Ca ratio showed an increasing trend. The relationship between Mg/Ca and Ba/Ca versus seawater pH was not significant. Inter-colony variation of d 11 B was insignificant, although inter-colony variation was observed for Ba/Ca. The decreasing trend of pH CF calculated from d 11 B was from 8.5, 8.4, and 8.3 for seawater pH of 8.1, 7.8, and 7.4, respectively. Model calculations based on Sr/Ca and pH CF suggest that upregulation of pH CF occurs via exchange of H+ with Ca 2+ with kinetic effects (Rayleigh fractionation), reducing Sr/Ca relative to inorganic deposition of aragonite from seawater. We show that it is possible to constrain the overall carbonate chemistry of the calcifying fluid with estimates of the carbonate saturation of the calcifying fluid (Ω CF) being derived from skeletal Sr/Ca and pH CF (from δ11B). These estimates suggest that the aragonite saturation state of the calcifying fluid Ω CF is elevated by a factor of 5–10 relative to ambient seawater under all treatment conditions.
"Terrestrial runoff alters water quality in four ways: by introducing (1) inorganic nutrients and (2) particulate organic matter, (3) by reducing light owing to turbidity, and (4) by sediment deposition (e.g., Fortes 2000; Fabricius 2005). Human activity has generally increased sediment loading on coral reefs (e.g., McCulloch et al. 2003) and often results in chronic sediment stress to coral reef ecosystems (e.g., Jokiel et al. 2004; Cooper et al. 2007; Erftmeijer et al. 2012; DeMartini et al. 2013). Predicting effects of (Acevedo et al. 1989; Jokiel et al. 2014), or acclimatization to (Anthony and Larcombe 2002; Mallela and Perry 2007; Perry et al. 2012), chronic sediment stress is predicated on determining relationships between physical factors and physiological responses. "
[Show abstract][Hide abstract] ABSTRACT: Sediment has been shown to be a major stressor to coral reefs globally. Although many researchers have tested the impact of sedimentation on coral reef ecosystems in both the laboratory and the field and some have mea- sured the impact of suspended sediment on the photosynthetic response of corals, there has yet to be a detailed investigation on how properties of the sediment itself can affect light availability for photosynthesis. We show that finer-grained and darker-colored sediment at higher suspended-sediment concentrations attenuates photosynthetically active radiation (PAR) significantly more than coarser, lighter-colored sediment at lower concentrations and provide PAR attenuation coefficients for various grain sizes, colors, and suspended-sediment concentrations that are needed for biophysical modeling. Because finer-grained sediment particles settle more slowly and are more susceptible to resuspension, they remain in the water column longer, thus causing greater net impact by reducing light essential for photosynthesis over a greater duration. This indicates that coral reef monitoring studies investigating sediment impacts should concentrate on measuring fine- grained lateritic and volcanic soils, as opposed to coarser- grained siliceous and carbonate sediment. Similarly, coastal restoration efforts and engineering solutions ad- dressing long-term coral reef ecosystem health should focus on preferentially retaining those fine-grained soils rather than coarse silt and sand particles.
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