Quantile function based, optical characterisation of the Nelson River plume dispersion in Hudson Bay (Canada)

Abstract

<p>This study hypothesizes, that sediment-based optical estimation of plume extent underestimates the dispersion limit of river discharge in marine waters. Optically active Colour Dissolved Organic Matter (CDOM) has been used as a tracer along with suspended sediments to validate the hypothesis, for the Nelson River (NR) plume in southwest Hudson Bay (HB). Remote sensing reflectance (R<sub>rs</sub>) band ratio (R<sub>rs </sub>678nm/488nm) was tuned in to retrieve CDOM absorbance at 412nm: a<sub>CDOM</sub> (412nm) from Moderate Resolution Imaging Spectroradiometer (MODIS) images. Similarly, R<sub>rs </sub>(678nm) was used for TSS concentration retrieval. Plume dispersion characteristics were compared for spring and neap tidal periods during a high (2005) and a normal (2006) discharge year. Quantile function (QF) provides variable values (a<sub>CDOM</sub>, TSS concentration) for a given cumulative probability. It was assumed that 0.90 QF (10% of the data distribution) is a representative of the river plume function, while 0.05 QF (95% of the data distribution) represents the ocean end-member values. These threshold values were calculated for each of the cumulative areas of coastal waters of southwest HB with NR mouth as the origin, limited till 500 km radial distance. Thresholds averaged over 0 km to 50km was used as the reference for estimation of river plume dilution. A conditional approach of the plume extent limit was set to the point at which the QF dilution (0.90-0.50) equals to the QF dilution (0.90-0.05). This dilution conditionality was satisfied for CDOM but was never achieved for TSS, indicating the additional source of sediment influx. Plume discharge volume was proportional to the a<sub>CDOM</sub> (412nm) plume extent threshold for all tidal periods. This threshold value was observed at ~250km during neap ebb tide (NET) for both years. While a shorter extent (150 km) was observed during the 2005 spring ebb tide (SET) and ~300km for the same tidal period during 2006. The point of minimum variance within a<sub>CDOM</sub> (412nm) dilution (0.50 to 0.90 QF) was used as a representative of the freshwater-marine boundary. An extent of 400km (SET) and 350km (NET) for 2005 was recorded, with a 500km extent observed for both ebb tides in 2006.  A lower a<sub>CDOM </sub>(412nm) dispersion threshold was observed for the SET (0.39 m<sup>-1</sup>) than the NET (0.59 m<sup>-1</sup>) for 2005 and vice-versa for 2006. A weak, negative correlation of a<sub>CDOM </sub>(412nm) minimum variance threshold between 2005 and 2006 indicated a weaker influence of discharge beyond the maximum variance threshold. This dispersion is expected to be controlled by a cyclonic eddy in southwest HB. Modeled dilution slope of a<sub>CDOM </sub>(412nm), <200km showed a significant influence of discharge with higher slope values obtained for 2005 for both spring and neap ebb tide conditions. The minimum variance of TSS concentration dilution is limited within 200-250 km, while its maximum variance is limited within 100km. Thus, TSS underestimates the river plume dispersion extent, which is better represented by CDOM of terrestrial provenance.</p>