Abstract
This study focuses on the seasonal and spatial characterization of inherent optical properties and biogeochemical concentrations in the Van Diemen Gulf and Darwin Harbour, two neighboring tropical coastal environments of Northern Australia that exhibit shallow depths (~20 m), large (> 3 m) semi-diurnal tides and a monsoonal climate. To gain insight in the functioning of these optically complex coastal ecosystems, a total of 23 physical, biogeochemical and optical parameters were sampled at 63 stations during three field campaigns covering the 2012 wet and dry seasons, and the 2013 dry season. The total light absorption budget in the Van Diemen Gulf was dominated by non-algal particles (aNAP; >45%) during the dry season (May-October) and colored dissolved organic matter (aCDOM; 60%) during the wet season (November-April). The combined absorption by aNAP and aCDOM generally exceeded 70% of the total absorption budget from 400 to 620 nm, with phytoplankton, aPhy, accounting for less than 20%. In Darwin Harbour, where only the dry season conditions were sampled, the total absorption budget was dominated by an equivalent contribution of aCDOM, aNAP and phytoplankton. The major processes explaining the seasonal variability observed in the Van Diemen Gulf are resuspension from seasonal south-easterly trade winds in combination with the tidal energy and shallow bathymetry during the dry season months, and mostly terrestrial river runoff during the monsoon which discharge terrestrial CDOM from the surrounding wetlands. Due to light-limited conditions all year round, the particulate scattering coefficient (bp(555)) contributed significantly (90%) to the beam attenuation coefficient c(555), thus strongly limiting phytoplankton growth (Chlorophyll a~1 mg.m-3). Spatially, the Van Diemen Gulf had higher total suspended solids and nutrient concentrations than Darwin Harbour, with dissolved organic carbon and aCDOM subjected to photobleaching during the dry season. Key bio-optical relationships derived from this comprehensive set of parameters, the first ever to be collected in this tropical coastal environment, were successfully used for a region-specific seasonal parameterization of a Linear Matrix Inversion-based ocean color algorithm. Challenges related to the parameterization, and the use, of ocean color remote sensing algorithms for these optically complex waters are discussed.
Highlights
Tropical coastal systems represent
To date, the lack of in situ optical datasets in the NMR has limited the derivation of regional ocean color satellite algorithms for this region
The dataset presented in this study fills this gap and is the first collected in the tropical, optically complex coastal waters of VDG and DH
Summary
Recent studies suggest that tropical marine regions will experience less drastic environmental changes than temperate regions in the future (Acevedo-Trejos et al, 2014), they remain largely understudied. This is true in the southern hemisphere where very few datasets that capture the seasonal variations in bio-optical properties exist. Recommendations for future applications include the implementation of ocean color algorithms regionally tuned to the NMR using in situ optical measurements collected in this region for their parameterization The success of such approach has been previously shown in other coastal regions (e.g., Brando et al, 2012; Tilstone et al, 2012; Roy et al, 2013). The optical properties and concentrations presented in this study are the first to have been collected in the region, and may be used as a baseline in any future bio-optical assessments guiding environmental management
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