Controls on Carbon, Nutrient, and Sediment Cycling in a Large, Semiarid Estuarine System; Princess Charlotte Bay, Australia

Abstract

Semiarid estuaries are characterized by pronounced seasonal variability, and a functional understanding of these systems requires constraint of coupled biogeochemical processes and relevant temporal and spatial scales. Here, we integrate 2 years of spatial surveys and time-series measurements to quantify physical, chemical, and biological drivers in the largest estuarine system in the Great Barrier Reef region. During wet season, freshwater inputs of nutrients and sediment to estuaries were dominated by flood pulses, whereas carbonate input was also influenced by groundwater discharge. This carbonate input counteracted the minimum buffering zone that would otherwise occur at low salinities, thereby decreasing system-wide air-water CO 2 fluxes. Sediment resuspension was a major control on the transformation and transport of material over tidal and seasonal scales. During wet season, tidal resuspension of benthic algae in nearshore mixing zones acted as an autotrophic filter, removing most bioavailable nutrients from the brackish plume. During dry season, upstream transport combined with hypersaline conditions trapped material in upper estuaries where denitrification and net heterotrophy were high. However, the role of sediment transport varied depending on tidal asymmetry and density-driven circulation. Estuarine regions with large intertidal areas were dominated by salt flat erosion, which showed a diagenetic signature associated with mid-Holocene swamp sediments. Tidal resuspension of these organic-rich sediments appeared to be the dominant control on biogeochemical cycling in coastal waters. This study demonstrates that a holistic understanding of coastal ecosystem connectivity and function requires resolution of both along-axis and water-column gradients as well as a range of timescales from tidal to geological cycles.

The origin of suspended particulate matter in the Great Barrier Reef

“…In this study sediment deposited near the river mouths (within the IEMZ) of the flood plume has not been sampled during and after flood periods to examine how the terrestrial OM is degraded over time. However, previous studies on the processing of terrestrial OM in river estuaries of the GBR have documented the rapid microbial degradation in such locations including high tidally driven estuarine zones 41 , 42 . According to our detailed fingerprinting analysis, there is a rapid change in the isotopic, structural and genetic fingerprints of the SPM between the IEMZ and the OEMZ, which highlights that terrestrial OM inputs are almost exclusively deposited near river mouths with very little (if any) even reaching our inshore monitoring sites.…”

Functional analysis of (pristine) estuarine marine sediments

“…Sediment particle size analysis was also consistent for the region (Supplementary Figure S2 and Table S3). The distinct wet-dry seasonality of the semi-arid Cape York estuaries has been shown to trap and redistribute sediments over tidal and seasonal cycles, leading to an extreme, but highly variable, turbidity [50,51]. As such, turbidity above 20 NTU for all estuary sites was expected and may account for elevated chlorophyll-a and primary nutrients.…”

Impact of a Tropical Cyclone on Terrestrial Inputs and Bio-Optical Properties in Princess Charlotte Bay (Great Barrier Reef Lagoon)

“…The adjacent catchments (Normanby and Kennedy basins) have particular economic, social and cultural values (land of the Bakanambia and Jeteneru people). The Normanby-Kennedy estuaries are the largest estuarine system in the GBR, and a major source of sediments, dissolved organic material and nutrients to the northern GBR lagoon [32,33]. The northern GBR estuaries have been mostly poorly studied due to logistical limitations associated with their remote location [32,33].…”

“…The Normanby-Kennedy estuaries are the largest estuarine system in the GBR, and a major source of sediments, dissolved organic material and nutrients to the northern GBR lagoon [32,33]. The northern GBR estuaries have been mostly poorly studied due to logistical limitations associated with their remote location [32,33]. The in situ biogeochemical and optical datasets collected were used in combination with ocean color satellite imagery to examine the biogeochemical inputs and transformation processes within PCB, the impact on the phytoplankton community and the implications for remote sensing bio-optical algorithms during flood events.…”