An Advanced Laser Fluorometer (ALF) capable of discriminating several phytoplankton pigment types was utilized in conjunction with microscopic data to map the distribution of phytoplankton communities in the Amazon River plume in May-June-2010, when discharge from the river was at its peak. Cluster analysis and Non-metric Multi-Dimensional Scaling (NMDS) helped distinguish three distinct biological communities that separated largely on the basis of salinity gradients across the plume. These three communities included an ''estuarine type'' comprised of a high biomass mixed population of diatoms, cryptophytes and green-water Synechococcus spp. located upstream of the plume, a ''mesohaline type'' made up largely of communities of Diatom-Diazotroph Associations (DDAs) and located in the northwestern region of the plume and an ''oceanic type'' in the oligotrophic waters outside of the plume made up of Trichodesmium and Synechococcus spp. Although salinity appeared to have a substantial influence on the distribution of different phytoplankton groups, ALF and microscopic measurements examined in the context of the hydro-chemical environment of the river plume, helped establish that the phytoplankton community structure and distribution were strongly controlled by inorganic nitrate plus nitrite (NO 3 + NO 2) availability whose concentrations were low throughout the plume. Towards the southern, low-salinity region of the plume, NO 3 + NO 2 supplied by the onshore flow of subsurface ($80 m depth) water, ensured the continuous sustenance of the mixed phytoplankton bloom. The large drawdown of SiO 3 and PO 4 associated with this ''estuarine type'' mixed bloom at a magnitude comparable to that observed for DDAs in the mesohaline waters, leads us to contend that, diatoms, cryptophytes and Synechococcus spp., fueled by the offshore influx of nutrients also play an important role in the cycling of nutrients in the Amazon River plume.
Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from 238 U:234 Th disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C·m) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front was mechanistically linked to Fe-stressed diatoms and high mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional ∼225 mg C·m −2 ·d −1 was exported as subduction of particlerich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.particle flux | particulate organic carbon | plankton | carbon cycle | biological carbon pump
IntroductionActive fluorescence analysis of natural aquatic environments including oceanic, estuarine, and fresh waters is based on the measurements of the laser-induced water emission to retrieve qualitative and quantitative information about the insitu fluorescent constituents. In vivo fluorescence of chlorophyll a (Chl a) and accessory phycobiliprotein (PBP) pigments is broadly used as an index of Chl a concentration and phytoplankton biomass (e.g., Falkowski and Kiefer 1985;Wirick 1994;Dandonneau and Neveux 1997) and provides useful information for structural (Yentsch and Yentsch 1979;Exton et al. 1983b;Phinney 1984, 1985; Oldham and Warner 1987;Hilton et al. 1989;Cowles et al. 1993;Poryvkina et al. 1994;Seppala and Balode 1998; Hoge et al. 1998, Beutler et al. 2002) and photophysiological (e.g., Falkowski and Kolber 1995Kolber et al. 1998;Schreiber et al. 1993;Olson et al. 1999 Olson et al. , 2000 characterization of the mixed algal populations. The broadband colored dissolved organic matter (CDOM) fluorescence emission can be used for assessment of CDOM abundance and its qualitative characterization (e.g., Del Castillo et al. 2000;Hudson et al. 2007).Early studies have revealed significant spectral complexity of the actively excited emission of natural waters due to the overlap between water Raman (WR) scattering and the fluorescence bands of the aquatic constituents (Exton et al. 1983a,b;Babichenko et al. 1993;Chekalyuk et al. 1995). As pointed out by Exton et al. (1983a), not accounting for the spectral complexity may lead to severe problems in interpretation of the fluorescence measurements and compromise the accuracy of the fluorescence assessments. To address this issue, they proposed (i) to use blue and green narrow-band laser excitation to selectively stimulate the constituent fluorescence and simplify the overlapped spectral patterns, (ii) to conduct broadband spectral measurements of laser-stimulated emission (LSE), and (iii) to develop spectral deconvolution analysis of the LSE signatures to retrieve information about the aquatic fluorescent constituents AbstractThe Advanced Laser Fluorometer (ALF) provides spectral deconvolution (SDC) analysis of the laser-stimulated emission (LSE) excited at 405 or 532 nm for assessment of chlorophyll a, phycoerythrin, and chromophoric dissolved organic matter. Three spectral types of phycoerythrin are discriminated for characterization of cyanobacteria and cryptophytes in mixed phototrophic populations. The SDC analysis is integrated with measurements of variable fluorescence, F v /F m , corrected for the SDC-retrieved background fluorescence, B NC , for improved photophysiological assessments of phytoplankton. The ALF deployments in the Atlantic and Pacific Oceans, and Chesapeake, Delaware, and Monterey Bays revealed significant spectral complexity of LSE. Considerable variability in chlorophyll a fluorescence peak, 673-685 nm, was detected. High correlation (R 2 = 0.93) was observed in diverse water types between chlorophyll a concentration and fluoresce...
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