Bio-Optical Properties of Two Neigboring Coastal Regions of Tropical Northern Australia: The Van Diemen Gulf and Darwin Harbour

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 nonalgal particles (a NAP ; >45%) during the dry season (May-October) and colored dissolved organic matter (a CDOM ; 60%) during the wet season (November-April). The combined absorption by a NAP and a CDOM generally exceeded ∼80% of the total absorption budget from 400 to 620 nm, with phytoplankton, a Phy , accounting for <20%. In Darwin Harbour, where only the dry season conditions were sampled, the total absorption budget was dominated by an equivalent contribution of a CDOM , a NAP, 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 [b p (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 a CDOM subjected to photobleaching during the dry season. Key bio-optical relationships derived from this comprehensive Blondeau-Patissier et al. Northern Australian Waters' Optical Properties set of parameters, the first ever to be collected in this tropical coastal environment, were successfully used for a region-specific seasonal parameterization a regionspecific seasonally parameterized ocean color algorithm. Challenges related to the parameterization, and the use, of ocean color remote sensing algorithms for these optically complex waters are discussed.

Seasonal variability in bio-optical properties along the coastal waters off Cochin

“…9 showing the triangular classification of coastal waters based on the three optical components CDOM, phytoplankton and detritus at three different wavebands (443, 555, and 670 nm). During the study period, highest contribution to the absorption budget was from detritus at 555 nm; this has also reported by (Blondeau-patissier et al, 2017;Spyrakos et al, 2017). Similarly, detritus absorption showed a slightly decreasing trend at 670 nm while compared to 443 and 555 nm.…”

“…However, it required more efforts to characterize the bio-optical properties of coastal waters (Devred et al, 2013;Garaba et al, 2014;Mouw et al, 2015;Han et al, 2016). The coastal waters are influenced not only by phytoplankton and its byproducts, but also by other optically active substances (Moore et al, 2014;Barnes et al, 2014;Zhang et al, 2014aZhang et al, , 2014bSun et al, 2014) that vary independently of phytoplankton, like particles in suspension and CDOM (Blondeau-patissier et al, 2017;Menon et al, 2011;Tilstone et al, 2012;Brezonik et al, 2015;Pérez et al, 2016). CDOM is a part of the dissolved organic carbon (DOC) pool composed essentially fulvic and humic acid (Tiwari and Shanmugam, 2011;Organelli et al, 2014;Xing et al, 2014).…”

Bio-optical properties of the Brazilian Abrolhos Bank’s shallow coral-reef waters

“…The Chl-a and a phy values observed in this study were lower than expected for coastal waters (Costa et al, 2006;Blondeau-Patissier et al, 2017;Russell et al, 2019), although comparable with average values reported for Case-1 waters (Bricaud et al, 2004(Bricaud et al, , 2010. The distinctive oceanographic conditions in the study region might be responsible for such low phytoplankton biomass as the influence of the nutrient-poor Brazil Current, which also promotes a nutrient wash with its flow, and the water column stratification in summer.…”

“…Since all the stations presented Sr values higher than 1 (i.e., S 275-295 ≥ S 350-400 ), there is probably a dominance of low molecular weight CDOM. Additionally, the exponential decrease found between S 275-295 and a CDOM (350) indicates possible effects of photobleaching for ABR samples (Fichot and Benner, 2012;Blondeau-Patissier et al, 2017). This study documented a spatial variability of the bio-optical properties in the Abrolhos Bank region but was limited by the relatively small number of in situ stations.…”

“…The Chl-a, a CDOM (440) and a NAP (440) values in NA were higher than in ABR. The NA is influenced by a monsoonal climate, with the tidal currents driving the water dynamics (Blondeau-Patissier et al, 2017). During the wet season, the monsoon rainfall (~1,700 mm yr −1 ) associated with six rivers is responsible for an increase in river discharge, which delivers a large amount of nutrients and land-sourced CDOM to the Northern Australia coastal water, enhancing primary productivity and increasing the number of particles in the water.…”