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Water turbidity and suspended sediment concentration (SSC) are commonly used as part of marine monitoring and water quality plans. Current management plans utilise threshold SSC values derived from mean-annual turbidity concentrations. Little published work documents typical ranges of turbidity for reefs within open coastal waters. Here, time-series turbidity measurements from 61 sites in the Great Barrier Reef (GBR) and Moreton Bay, Australia, are presented as turbidity exceedance curves and derivatives. This contributes to the understanding of turbidity and SSC in the context of environmental management in open-coastal reef environments. Exceedance results indicate strong spatial and temporal variability in water turbidity across inter/intraregional scales. The highest turbidity across 61 sites, at 50% exceedance (T50) is 15.3 NTU and at 90% exceedance (T90) 4.1 NTU. Mean/median turbidity comparisons show strong differences between the two, consistent with a strongly skewed turbidity regime. Results may contribute towards promoting refinement of water quality management protocols.

[Extract] Kroon (2012) makes first-order estimations of the reduction in sediment and nitrogen loads from north Queensland rivers to the coastal waters of the Great Barrier Reef lagoon (GBRL) that would be required to reduce the concentrations of sediment (TSS) and dissolved inorganic nitrogen (DIN) in coastal waters to values less than water quality targets outlined in the Great Barrier Reef Water Quality Action Plan (Reef Water Quality Protection Plan Secretariat, 2009). Kroon (2012) makes the explicit assumption that TSS and Chl a values measured within the coastal waters are directly proportional to the river loads of TSS and DIN respectively. The modeling approach of Kroon (2012) assumes that rivers are the primary source of sediment and nutrients to the water column within the lagoon. In this comment we suggest that (a) the references cited by Kroon (2012) to support the assumptions in the model give scant justification, and are in some respects contradictory, and that (b) Kroon (2012) has ignored a large body of literature on both sediment and nitrogen fluxes and storage, which indicate that river discharge is not the sole contribution into sediment and nutrient budgets for the GBRL.

We present a method to estimate the spatial distribution of dredged material disposed of at sea. Using both dredged sediments and samples of sea-bed sediment from near the Rame Head disposal site, Plymouth, UK, we applied entropy analysis to the