Reyes-Pesaresi, Patrick

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    Dynamics of chromophoric dissolved organic matter, cdom, in coastal tropical waters
    (2010) Reyes-Pesaresi, Patrick; López-Díaz, José M.; College of Arts and Sciences - Sciences; Gilbes Santaella, Fernando; Corredor Garcia, Jorge E.; Otero Morales, Ernesto; Morell Rodríguez, Julio M.; Aponte Avellanet, Nilda E.; Department of Marine Sciences; Ramírez Martínez, Wilson
    Chromophoric Dissolved Organic Matter (CDOM ag355) distribution in surface waters of Mayagüez Bay, Puerto Rico can be related to riverine allochtonous sources. Inshore CDOM ag355 had six times (0.55 m-1), and offshore stations twice (0.22 m-1), the values found at Caribbean Time Series (CaTS) station surface waters (0.100 m-1). Spatial and temporal analysis of CDOM spectral slope values reveals no statistically significant difference (p > 0.05) between Mayagüez Bay and Caribbean surface waters (0.017 nm-1). Correlation analysis between Chl-a, vs. CDOM suggests that phytoplankton is not a dominant CDOM ag355 source in Mayagüez Bay waters. Bottom sediment resuspension appears to be an important source of CDOM ag355 during the dry season. At this time of minimal river water discharges, half the average values of CDOM ag355 and non-significant temporal differences in TSS, CDOM ag355 spectral slopes were prevalent. Photodegradation does not appear to be an important process in explaining variability of CDOM ag355 nor the modification of the spectral slope of CDOM ag355, locally. Non-conservative behavior of CDOM ag375 was demonstrated in a mixing experiment consisting in step-wise dilution of marine waters end-members with Añasco River water endmember, including and excluding its typical suspended sediments loads. At low salinity dilution (S < 8), CDOM ag375 was lost from the dissolved fraction between 8.7 and 18.6 % in the absence of suspended particulate. In the presence of particles (at typical river water TSS concentrations), simple dilution produced a decrease in CDOM ag375 of between 13.9 and 9.7% suggesting loss by sorption; but at intermediate salinities, rising values suggested desorption. Spectral slope (S) shifts, above theoretical conservative dilution line values, suggested that CDOM ag375 was removed from the dissolved fraction. The values of S were not significantly different regardless of level of dilution or whether suspended particles were absent or present in the mixture of river and marine water. At salinity values typical of Mayagüez Bay (33.20 - 35.91 psu), S values characteristic of the marine water end-member dominate. Far greater amounts of allochtonous CDOM than enters the Bay would be necessary to significantly change the resulting value of S in the mixed marine and river waters since the proportion of fresh river water is <7%. CDOM ag375 flocculation and adsorption onto clay-minerals can be important processes in the partition between dissolved and particulate phases. Three characteristic Remote Sensing Reflectance (Rrs) spectral signatures were identified in Mayagüez Bay: inshore, offshore and in the vicinity of an underwater sewage outfall. A unique phytoplankton assemblage and unique combination of water constituents can cause the characteristic Rrs spectral signature near the underwater treated sewage outfall. Empirical algorithms were successful in reaching the validated uncertainty threshold, Absolute Percent Difference (APD < 35 %), for three of the four evaluated band ratios of acceptable APD. The best band ratios were Rrs412/Rrs510 and Rrs510/Rrs555 (R2 > 0.80). The same band ratios were evaluated for Color Detrital Matter (aCDM412) with better R2 than obtained for CDOM for the first three band ratios (R2 > 0.80), making these suitable for the aCDM412 estimate. The empirical model may be enhanced through further validation.