Caracterização bio-óptica das águas do estuário da Lagoa dos Patos, RS - Brasil.

Abstract

Neste estudo das águas estuarinas da Lagoa dos Patos (ELP), especial ênfase é dada na caracterização das propriedades ópticas e suas relações com constituintes opticamente ativos. Para tanto, foram coletados dados físicos (salinidade, temperatura e velocidade da corrente) e ópticos (absorção, atenuação, retroespalhamento, fluorescência, radiância e irradiância) em conjunto com amostra das águas de superfície para determinação das concentrações do material total em suspensão (TSM), material orgânico dissolvido colorido (CDOM) e clorofila-a (CHLa) em 10 estações distribuídas ao longo do canal do ELP,em 7 cruzeiros realizados entre fevereiro e outubro de 2012. o período amostrado foi caracterizado por baixa descarga de água doce com relativa alta carga de TSM proveniente da re-suspensão do fundo, especialmente durante os cruzeiros de verão-outono. O TSM, composto principalmente por minerais ou partículas inorgânicas controlou a variabilidade das propriedades ópticas em quase todo perido amostrado. A concentração de TSM foi mais variável (3.75 – 68.00 mg.L-1 , C.V. = 68.8%) que CDOM (9.74 – 41.41 ppb QSDE, C.V. = 31.4%) e CHLa (0.82 – 4.57 mg.m-3 , C.V. = 46.1%). TSM e CDOM mostrou correlação com a salinidade (r=0.73 and r=-0.58, N=32, p<0.05) que o CDOM teve origem de águas fluvial, e TSM foi maior na entrada do estuário devido à re-suspensão do material fundo. Os espectros de IOPs e reflectância do sensoriamento remoto (Rrs) foram tipicamente Caso II assinatura, com predominância de águas túrbidas e ricas em CDOM. Não houve diferença estatística em propriedades ópticas foram encontrados dentro de zonas estuarinas ou estações do ano. Análise de agrupamento hierárquico mostrou espectros de Rrs distintos para baixo e moderado níveis de descarga de água doce, mas sem evidência clara de coexistência de diferentes tipos de água em EPL. O TSM mostrou boa correlação com VI coeficientes de retroespalhamento do particulado, a 600 e 715 nm, e com coeficiente de absorção do particulado e dissolvido em 660nm. CDOM se correlacionou melhor com a diferença de coeficientes de absorção do particulado e dissolvido a 412 e 440 nm (r = 0,72, N = 28, p <0,05). A influência das IOPs e constituintes opticamente ativos sobre a variabilidade da Rrs(λ) foi avaliada por análise de EOF. Os resultados mostraram que o primeiro modo de EOF responsável por quase toda a variabilidade dos dados (92,4%), enquanto o segundo e terceiro modos explicam 4,7% e 2,3%, respectivamente. Os fatores de amplitude do primeiro modo de EOF foram correlacionados com o TSM (r = 0,39, n = 32, p <0,05) e com todas as propriedades ópticas inerentes medidos.

The Patos Lagoon, the world’s largest choked lagoon, has a very dynamic estuary in which circulation, forced by freshwater discharge and local and remote wind, regulates the spatial and temporal distribution of physical and biogeochemical properties in the estuary. In this work, we present a baseline study on the variability of inherent (IOP) and apparent optical properties (AOP) in the Patos Lagoon Estuary (PLE) with emphasis on their relationships with optically active water constituents. The dataset consists of physical (salinity, temperature and current velocity), optical (absorption, attenuation, backscattering, fluorescence, and remote sensing reflectance) and biogeochemical (total suspended matter (TSM), chlorophyll-a concentration (CHL), and colored dissolved organic matter (CDOM) properties measured at 10 fixed stations during 7 cruises conducted between February and October 2012. The sampling period was characterized by low river discharge but with relatively high load of TSM. associated with bottom resuspension in the estuary entrance, especially during the summer-autumn period. The TSM, mainly composed by minerals or inorganic particles, controlled the variability of optical properties in almost the entire sampling period. The total TSM was more variable (3.75 – 68.00 mg.L-1 , C.V. = 68.8%) than CDOM (9.74 – 41.41 ppb QSDE, C.V. = 31.4%) and CHLa (0.82 – 4.57 mg.m-3 , C.V. = 46.1%). TSM and CDOM were correlated with salinity (r=0.73 and r=-0.58, N=32, p<0.05) indicating that CDOM originated from riverine water, and TSM was higher at the entrance of the estuary due to bottom resuspension of material. The IOPs and remote sensing reflectance (Rrs) spectra were typically of Case 2 signature, with dominance of turbid and CDOMenriched waters. No statistical differences in optical properties were found within estuarine zones or seasons in this work. Hierarchical clustering analysis has shown distinct Rrs for low to moderate freshwater discharge, but with no clear evidence of coexistence of different water types in PLE. The TSM correlated well with backscattering coefficients, at 600 and 715 nm, and with particulate absorption coefficient at 660nm. CDOM correlates better with the difference in absorption coefficients at 412 and 440 nm (r= 0.72, N= 28, p<0.05). The influence of IOPs and optical constituents on the variability of Rrs(λ) was assessed by EOF analysis. The results showed that the first EOF mode accounted for almost all data variability (92.4%), while the second and third modes explained 4.7% and 2.3%, respectively. The 45 amplitude factors of first EOF mode were correlated with TSM (r= 0.39, N=32, p<0.05) and with all the measured inherent optical properties

The Patos Lagoon, the world’s largest choked lagoon, has a very dynamic estuary in which circulation, forced by freshwater discharge and local and remote wind, regulates the spatial and temporal distribution of physical and biogeochemical properties in the estuary. In this work, we present a baseline study on the variability of inherent (IOP) and apparent optical properties (AOP) in the Patos Lagoon Estuary (PLE) with emphasis on their relationships with optically active water constituents. The dataset consists of physical (salinity, temperature and current velocity), optical (absorption, attenuation, backscattering, fluorescence, and remote sensing reflectance) and biogeochemical (total suspended matter (TSM), chlorophyll-a concentration (CHL), and colored dissolved organic matter (CDOM) properties measured at 10 fixed stations during 7 cruises conducted between February and October 2012. The sampling period was characterized by low river discharge but with relatively high load of TSM. associated with bottom resuspension in the estuary entrance, especially during the summer-autumn period. The TSM, mainly composed by minerals or inorganic particles, controlled the variability of optical properties in almost the entire sampling period. The total TSM was more variable (3.75 – 68.00 mg.L-1 , C.V. = 68.8%) than CDOM (9.74 – 41.41 ppb QSDE, C.V. = 31.4%) and CHLa (0.82 – 4.57 mg.m-3 , C.V. = 46.1%). TSM and CDOM were correlated with salinity (r=0.73 and r=-0.58, N=32, p<0.05) indicating that CDOM originated from riverine water, and TSM was higher at the entrance of the estuary due to bottom resuspension of material. The IOPs and remote sensing reflectance (Rrs) spectra were typically of Case 2 signature, with dominance of turbid and CDOMenriched waters. No statistical differences in optical properties were found within estuarine zones or seasons in this work. Hierarchical clustering analysis has shown distinct Rrs for low to moderate freshwater discharge, but with no clear evidence of coexistence of different water types in PLE. The TSM correlated well with backscattering coefficients, at 600 and 715 nm, and with particulate absorption coefficient at 660nm. CDOM correlates better with the difference in absorption coefficients at 412 and 440 nm (r= 0.72, N= 28, p<0.05). The influence of IOPs and optical constituents on the variability of Rrs(λ) was assessed by EOF analysis. The results showed that the first EOF mode accounted for almost all data variability (92.4%), while the second and third modes explained 4.7% and 2.3%, respectively. The 45 amplitude factors of first EOF mode were correlated with TSM (r= 0.39, N=32, p<0.05) and with all the measured inherent optical properties.