Combining in-situ water quality and remotely sensed data across spatial and temporal scales to measure variability in wet season chlorophyll-a: Great Barrier Reef lagoon (Queensland, Australia)

Abstract

Introduction: Combining in-situ data from single-point time series with remotely sensed spatial data allowed a greater elucidation of changes in chlorophyll-a concentrations through wet season conditions in the Great Barrier Reef coastal waters. Methods: Single-point time-series data were collected from 2006 to 2012 during high river flow conditions to assess changes in phytoplankton biomass (measured as chlorophyll-a). Additionally, three flood plume water types, derived from classified true-colour Aqua moderate resolution imaging spectroradiometer (MODIS) images, were used to group single-point time-series data for the phytoplankton biomass assessment. Results: Survey data illustrate the heterogeneity of chlorophyll-a distribution over seasonal and inter-annual cycles and the difficulty in describing community responses through the wet season. The spatial data demonstrate distinct regional differences throughout the Great Barrier Reef. The high chlorophyll-a concentrations measured in flood plume waters immediately adjacent to the inshore, highly turbid 'inner' flood plume are a product of sufficient light, given most of the suspended solids have settled from the plume, and the availability of sufficient nutrients, which drive higher phytoplankton production and characterise the formation of secondary stage flood plumes. The formation and extent of these secondary flood plumes were mapped using MODIS true-colour satellite imagery. The distance and the location of the secondary plume water are reliant on flow, coastal hydrodynamics, and biological activity. Conclusions: The combination of in-situ data and remotely sensed data provides information on the complexity of these coastal processes during the wet season and offers managers a more comprehensive understanding of the extent of nutrient enrichment in the Great Barrier Reef coastal area and the potential influence of flood plumes on coastal marine ecosystems.

Estimating the Exposure of Coral Reefs and Seagrass Meadows to Land-Sourced Contaminants in River Flood Plumes of the Great Barrier Reef: Validating a Simple Satellite Risk Framework with Environmental Data

“…The mean salinity linearly decreased from the CC6 to the CC1 water type (Figure 6a and the supplementary material (Table S5)) and the variation of water concentrations across the six water types followed the expected gradients over multi-annual time-scales [13,15,90]. Most of the water quality parameters, including K d (PAR), TSS, DIN and DIP, decreased across the six plume water types ( Figure 6a and the supplementary material (Table S5)).…”

“…High TSS in the CC1 plume water type is expected to inhibit phytoplankton productivity and Chl-a concentrations [70,90]; therefore, freshwater phytoplankton or detritus transported by GBR Rivers, could explain the relatively high Chl-a concentrations. With the exception of the CC2 and CC3 plume water types, which scored the same (RS CC2 and RS CC3 = 5) as a result of very similar Chl-a and TSS concentrations, risk scores generally decreased from CC1-CC6, supporting the assumption that the magnitude of risk decrease across the river plume gradient [16].…”

“…The maximum area of primary water was recorded in 2011 (69,745 km 2 , or 20% of the GBR). The extent of the secondary (CC5) and tertiary (CC6) flood plume frequency is rarely attributed to an individual river and is usually merged into one heterogeneous plume [90]. On average, the secondary and tertiary plume waters covered 25,000 km 2 of the GBR lagoon and 43,000 km 2 , respectively.…”

Water Quality and River Plume Monitoring in the Great Barrier Reef: An Overview of Methods Based on Ocean Colour Satellite Data

“…The onset and duration of river plumes into the GBR has been reported over several decades [20,[23][24][25][26][27][28][29][30] and monitoring of river plumes [31] now forms an integral component of the MMP. However, it has been difficult to evaluate the complex responses of the seagrass and coral communities to changing water quality based on in situ water quality data only due to the limitations of the monitoring time frames and the uncertainty associated with the time lag between exposure and impact.…”

“…For example, river plumes have been mapped through a combination of aerial and satellite imagery and the coverage of GBR ecosystems visually assessed using satellite imagery [30][31][32]. More recently, RS studies using quasi-true colour (hereafter true colour) satellite images ( Figure 1) and derived water quality level-2 products have been utilised to map and characterise the spatial and temporal distribution of GBR river plumes [28,[33][34][35][36][37].…”