Evolution and dynamics of tropical river plumes in the Great Barrier Reef: An integrated remote sensing and in situ study

Abstract

[1] The short-lived but intense discharge of freshwater from tropical rivers into the Great Barrier Reef (GBR) Lagoon and the associated salinity reductions are a critical consideration in marine research and management of the ecologically sensitive GBR World Heritage Area. Salinity provides a unique tracer that gives clues to the origin of river-borne contaminants and allows the influences of storm-induced resuspension and river discharge on turbidity to be clearly distinguished. We describe a field investigation of the evolution and dynamics of the Herbert River plume in the central GBR. Its primary goals were to use an airborne salinity mapper and in situ instruments to study the three-dimensional structure and evolution of the plume and to lay a foundation for numerical modeling studies of its dynamics. The aircraft surveys provided a rapid assessment of the plumes spatial extent, while the in situ data revealed details of its subsurface structure. The Herbert River plume was produced by heavy rainfall associated with tropical storms during the La Nina-dominated 1999/2000 monsoon season. In the near field, the surface expression of the plume boundaries was indicated by sharp color and salinity fronts that were clearly visible from the air and sea surface. In the far field and middle Lagoon, the plume was more dispersed and ultimately merged with the larger-scale salinity gradients and with the remnant plume of the more distant, and larger, Burdekin River. The plume location and structure evolved in response to changing river flows, tidal and subtidal circulation, and wind. Using Garvine's Kelvin number-based scheme, the plume was classified as intermediate in dynamical character and thus is subject to a variety of forcings. The plume evolved in response to changes in the relative intensity of tidal currents and low-frequency circulation due to wind and western boundary current forcing. It also displayed a characteristic ''hook-shaped'' structure, which has been identified previously in numerical plume model studies. This structure appeared in the presence of accelerating along-shelf current flow and horizontal shear and it indicates that the plume circulation had a strongly three-dimensional character. The approach demonstrates the efficacy of combining airborne and in situ methods to observe rapidly evolving coastal salinity structure and dynamics and sets the stage for future satellite-borne studies of larger-scale features showing contrasting salinity distributions.

Environmental and spatial predictors of species richness and abundance in coral reef fishes

“…Most environmental variables we examined on the Great Barrier Reef displayed strong cross‐shelf gradients such as salinity (Burrage et al. , 2002), temperature and chlorophyll (Liston et al.…”

“…Most environmental variables we examined on the Great Barrier Reef displayed strong cross-shelf gradients such as salinity (Burrage et al, 2002), temperature and chlorophyll (Liston et al, 1992) that are influenced by freshwater inputs and terrigenous influences near the coast, and by oligotrophic oceanic waters at the shelf edge. Indeed, the distance between the coast and the ocean increases from northern to southern sectors on the Great Barrier Reef; therefore, for northern sites both distances would be short, whereas for southern sites at least one of these distances would be long.…”

“…, 1998) or cross‐shelf gradients in salinity and chlorophyll (Liston et al. , 1992; Burrage et al. , 2002).…”

“…Under the mid-domain model, random placement of species ranges is expected to result in the greatest species richness occurring in the middle of the spatial domain considered. Spatial gradients in the structure of tropical reef fish assemblages can also correlate closely with environmental gradients such as latitudinal variation in temperature (Roy et al, 1998) or cross-shelf gradients in salinity and chlorophyll (Liston et al, 1992;Burrage et al, 2002). Therefore, both theory and empirical observations suggest that latitudinal and/or cross-shelf position might provide a cost-effective surrogate for the combined effects of different environmental variables when predicting the assemblage structure of coral reef fishes at broader spatial scales.…”

Have droughts and increased water extraction from the Murray River (Australia) reduced coastal ocean productivity?

“…Although a limited number of studies reflect on potential implications of a complete lack of river outflows on marine environments, it is generally understood that changing the scale, frequency and seasonality of river outflows can have severe impacts on species that require food pulses for successful reproduction or larval survival (Drinkwater and Frank 1994;Whitfield and Marais 1999). Several studies have identified important relationships between outflow-stimulated productivity and populations of marine species and habitats as a result of these controls (Burrage et al 2002;De Robertis et al 2005;Burla et al 2010;Brookes et al 2015;Black et al 2016). There is also a critical period at the point of first feeding by larvae where the presence of suitable amounts and types of food determine year-class strength (Hjort 1914).…”