The recent trend of declining winter and spring snow cover over Eurasia is causing a land-ocean thermal gradient that is particularly favorable to stronger southwest (summer) monsoon winds. Since ...1997, sea surface winds have been strengthening over the western Arabian Sea. This escalation in the intensity of summer monsoon winds, accompanied by enhanced upwelling and an increase of more than 350% in average summertime phytoplankton biomass along the coast and over 300% offshore, raises the possibility that the current warming trend of the Eurasian landmass is making the Arabian Sea more productive.
Coastal oceans link terrestrial and marine carbon cycles. Yet, carbon sources and sinks in these biomes remain poorly understood. Here, we explore the dynamics of dissolved organic matter (DOM) along ...the Amazon River-to-ocean continuum from the lower mainstem at Óbidos to the open ocean of the western tropical North Atlantic. We molecularly characterized DOM via ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), determined DOM stable carbon isotopes, and interpreted the data in the context of bacterial abundance and production, phytoplankton biomass and composition. Multivariate analysis revealed that the DOM molecular variability in the plume was mainly influenced by the input of terrigenous DOM. Incubation experiments with water from close to the river mouth showed that photo- and bio-degradation preferentially removed 13C-depleted and 13C-enriched terrigenous DOM, respectively. However, there was no significant quantitative change in the total amount of dissolved organic carbon (DOC) over five days. This result suggests that most of the reactive DOM had already been bio-degraded upstream within the river and that photo-degradation was diminished in the turbid plume close to the river mouth. Terrigenous DOM therefore appeared to be relatively non-reactive nearshore. In the less turbid offshore plume, enhanced light penetration stimulated growth of phytoplankton and increased bacterial production. Although marine DOM compounds became relatively enriched, bulk DOC concentrations were 9 to 30% below levels expected from conservative mixing of river and ocean endmembers suggesting that quantitative removal of terrigenous DOM was not compensated by marine DOM production. We propose that removal of terrigenous DOM in the outer plume may be enhanced by (i) bio-degradation primed by reactive algal DOM, (ii) photo-degradation, which may further break down DOM into more bio-available forms, and possibly (iii) sorption of DOM to sinking particles.
•DOM was characterized using ultrahigh-resolution mass spectrometry from the lower Amazon River to the Caribbean Sea•DOM molecular variability in the plume was primarily influenced by terrigenous river DOM input•In the intermediate plume, phytoplankton biomass and bacterial activity were significantly correlated to DOM composition•9 - 30% of initial DOC was lost along the plume: removal of terrigenous DOM was not compensated by in situ new production•Molecular DOM patterns suggest bio- and photo-degradation as DOM sinks
Constraining the fate of dissolved organic matter (DOM) delivered by rivers is a key to understand the global carbon cycle, since DOM mineralization directly influences air‐sea CO2 exchange and ...multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet the fate of this material in the ocean remains unclear. Here we investigate the molecular composition and transformations of DOM in the Amazon River‐ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50–76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially derived organic carbon from the continental margin, where it can be entrained in the large‐scale circulation and potentially contribute to the long‐term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean.
Key Points
There is a strong gradient in source and composition of DOM in the continuum
Phytoplankton inputs and bacterial/photochemical transformations are significant
A large fraction of the Amazon River DOM is exported from the continental margin
Cochlodinium polykrikoides formed large blooms in the coastal waters of Oman from October 2008 through mid-January 2009, and satellite images from Aqua-MODIS and region-wide reports suggest that this ...bloom was found throughout the Arabian Gulf and Sea of Oman for more than 10 months. The unusual occurrence of this species appears to have supplanted the more regularly occurring bloom species, Noctiluca scintillans, in 2008–2009. For the first 2 weeks of the coastal Omani bloom, C. polykrikoides abundance was near monospecific proportions, with cell densities ranging from 4.6 × 10³ to 9 × 10⁶ cells L⁻¹ and very high levels of chlorophyll a (78.0 μg L⁻¹) were also recorded. The regional progression of the bloom likely began with stronger than normal upwelling along the Iranian and northern Omani coasts during the southwest monsoon in late summer, followed by discharge of unusually warm coastal plume water along the coast of Oman with the reversal of monsoonal winds in late October. The occurrence and persistence of high densities of C. polykrikoides in Oman coastal water were also significantly influenced by an elevated nutrient load and warmer than normal temperatures. Concentrations of nutrients, especially NH₄ ⁺, urea, PO₄ ³⁻, and organic nitrogen and phosphorus, were manyfold higher than observed in the year prior or since. These findings suggest that mesoscale features were important in bloom dynamics more regionally, but locally the bloom was sustained by nutrient enrichment supplemented by its mixotrophic capabilities.
Airborne hyperspectral data play an important role in remote sensing of coastal waters. However, before their application, atmospheric correction is required to remove or reduce the atmospheric ...effects caused by molecular and aerosol scattering and absorption. In this study, we first processed airborne hyperspectral CASI-1500 data acquired on 4 May 2019 over the Uljin coast of Korea with Polymer and then compared the performance with the other two widely used atmospheric correction approaches, i.e., 6S and FLAASH, to determine the most appropriate correction technique for CASI-1500 data in coastal waters. Our results show the superiority of Polymer over 6S and FLAASH in deriving the Rrs spectral shape and magnitude. The performance of Polymer was further evaluated by comparing CASI-1500 Rrs data with those obtained from the MODIS-Aqua sensor on 3 May 2019 and processed using Polymer. The spectral shapes of the derived Rrs from CASI-1500 and MODIS-Aqua matched well, but the magnitude of CASI-1500 Rrs was approximately 0.8 times lower than MODIS Rrs. The possible reasons for this difference were time difference (1 day) between CASI-1500 and MODIS data, higher land adjacency effect for MODIS-Aqua than for CASI-1500, and possible errors in MODIS Rrs from Polymer.
The Amazon River has the largest discharge of all rivers on Earth, and its complex plume system fuels a wide array of biogeochemical processes, across a large area of the western tropical North ...Atlantic. The plume thus stimulates microbial processes affecting carbon sequestration and nutrient cycles at a global scale. Chromosomal gene expression patterns of the 2.0 to 156 μm size-fraction eukaryotic microbial community were investigated in the Amazon River Plume, generating a robust dataset (more than 100 million mRNA sequences) that depicts the metabolic capabilities and interactions among the eukaryotic microbes. Combining classical oceanographic field measurements with metatranscriptomics yielded characterization of the hydrographic conditions simultaneous with a quantification of transcriptional activity and identity of the community. We highlight the patterns of eukaryotic gene expression for 31 biogeochemically significant gene targets hypothesized to be valuable within forecasting models. An advantage to this targeted approach is that the database of reference sequences used to identify the target genes was selectively constructed and highly curated optimizing taxonomic coverage, throughput, and the accuracy of annotations. A coastal diatom bloom highly expressed nitrate transporters and carbonic anhydrase presumably to support high growth rates and enhance uptake of low levels of dissolved nitrate and CO2. Diatom-diazotroph association (DDA: diatoms with nitrogen fixing symbionts) blooms were common when surface salinity was mesohaline and dissolved nitrate concentrations were below detection, and hence did not show evidence of nitrate utilization, suggesting they relied on ammonium transporters to aquire recently fixed nitrogen. These DDA blooms in the outer plume had rapid turnover of the photosystem D1 protein presumably caused by photodegradation under increased light penetration in clearer waters, and increased expression of silicon transporters as silicon became limiting. Expression of these genes, including carbonic anhydrase and transporters for nitrate and phosphate, were found to reflect the physiological status and biogeochemistry of river plume environments. These relatively stable patterns of eukaryotic transcript abundance occurred over modest spatiotemporal scales, with similarity observed in sample duplicates collected up to 2.45 km in space and 120 minutes in time. These results confirm the use of metatranscriptomics as a valuable tool to understand and predict microbial community function.
The accurate retrieval of chlorophyll-a concentration (Chl-a) from ocean color satellite data is extremely challenging in turbid, optically complex coastal waters. Ariake Bay in Japan is a turbid ...semi-enclosed bay of great socio-economic significance, but it suffers from serious water quality problems, particularly due to red tide events. Chl-a derived from the MODerate resolution Imaging Spectroradiometer (MODIS) sensor on satellite Aqua in Ariake Bay was investigated, and it was determined that the causes of the errors were from inaccurate atmospheric correction and inappropriate in-water algorithms. To improve the accuracy of MODIS remote sensing reflectance (Rrs) in the blue and green bands, a simple method was adopted using in situ Rrs data. This method assumes that the error in MODIS Rrs(547) is small, and MODIS Rrs(412) can be estimated from MODIS Rrs(547) using a linear relation between in situ Rrs(412) and Rrs(547). We also showed that the standard MODIS Chl-a algorithm, OC3M, underestimated Chl-a, which was mostly due to water column turbidity. A new empirical switching algorithm was generated based on the relationship between in situ Chl-a and the blue-to-green band ratio, max(Rrs(443), Rrs(448)/Rrs(547), which was the same as the OC3M algorithm. The criterion of Rrs(667) of 0.005 sr−1 was used to evaluate the extent of turbidity for the switching algorithm. The results showed that the switching algorithm performed better than OC3M, and the root mean square error (RMSE) of estimated Chl-a decreased from 0.414 to 0.326. The RMSE for MODIS Chl-a using the recalculated Rrs and the switching algorithm was 0.287, which was a significant improvement from the RMSE of 0.610, which was obtained using standard MODIS Chl-a. Finally, the accuracy of our method was tested with an independent dataset collected by the local Fisheries Research Institute, and the results revealed that the switching algorithm with the recalculated Rrs reduced the RMSE of MODIS Chl-a from 0.412 of the standard to 0.335.
Abstract The mixoplankton green Noctiluca scintillans (gNoctiluca) is known to form extensive green tides in tropical coastal ecosystems prone to eutrophication. In the Arabian Sea, their recent ...appearance and annual recurrence have upended an ecosystem that was once exclusively dominated by diatoms. Despite evidence of strong links to eutrophication, hypoxia and warming, the mechanisms underlying outbreaks of this mixoplanktonic dinoflagellate remain uncertain. Here we have used eco-physiological measurements and transcriptomic profiling to ascribe gNoctiluca’s explosive growth during bloom formation to the form of sexual reproduction that produces numerous gametes. Rapid growth of gNoctiluca coincided with active ammonium and phosphate release from gNoctiluca cells, which exhibited high transcriptional activity of phagocytosis and metabolism generating ammonium. This grazing-driven nutrient flow ostensibly promotes the growth of phytoplankton as prey and offers positive support successively for bloom formation and maintenance. We also provide the first evidence that the host gNoctiluca cell could be manipulating growth of its endosymbiont population in order to exploit their photosynthetic products and meet critical energy needs. These findings illuminate gNoctiluca’s little known nutritional and reproductive strategies that facilitate its ability to form intense and expansive gNoctiluca blooms to the detriment of regional water, food and the socio-economic security in several tropical countries.
We have examined the seasonality of phytoplankton in the western and northern Bay of Bengal using shipboard data collected during three seasons as well as ocean colour imagery from OCTS and SeaWiFS. ...Seasonal changes in the hydrography of the bay observed during these seasons gave rise to striking differences in biomass and primary productivity. Heavy fresh water influx from rivers and the resulting vertical stratification impeded vertical transfer of nutrients. Although such a nutrient regime resulted in an oligotrophic environment, chlorophyll
a and primary production were substantially enhanced by physical processes that could erode the strong halocline. In March–April (pre-southwest monsoon), the poleward flowing East India Coastal Current brought to the surface, nutrient laden cooler waters that enriched the coastal region, but highest biomass (Chl
a, 53
mg
m
−2) and productivity (4.5
g
C
m
−2
d
−1) were in the region of an eddylike structure along the coast and in the region between 13° and 16°N lat. Its appearance in satellite images of two consecutive years suggests the structure to be an annual feature. Wind driven coastal upwelling and increased river runoff during the following season, the southwest monsoon (July–August), increased phytoplankton biomass dramatically (92
mg
m
−2) but productivity averaged only 0.3
g
C
m
−2
d
−1 suggesting light limitation due to intense cloud cover. With a reduction in cloud cover and enhanced irradiance during the following season, the northeast monsoon (January–February), primary production increased especially in the northern part of the bay where phytoplankton appeared to benefit from both improved light conditions and nutrient inputs from estuarine mechanisms and river runoff.
The absorption coefficients of phytoplankton (aPHY(λ)), non-algal particles (NAP) (aNAP(λ)) and colored dissolved organic matter (CDOM) (aCDOM(λ)) were investigated and parameterized in the ...southeastern Bering Sea during July 2008. The absorption coefficients were well structured with respect to hydrographic and biogeochemical characteristics of the shelf. The highest values of aPHY(443) were observed offshore and the lowest values of aPHY(443) were found in the coastal domain, a low productivity region associated with limited macronutrients. Values of aDG(λ) (aCDOM(λ)+aNAP(λ)) revealed an east–west gradient pattern with higher values in the coastal domain, and lower values in the outer domain. Lower chlorophyll specific aPHY(λ) (a*PHY(λ)) observed relative to middle and lower latitude waters indicated a change in pigment composition and/or package effect, which was consistent with phytoplankton community structure. aCDOM(λ) was the dominant light absorbing coefficient at all wavelengths examined except at 676nm. Modeling of remote-sensing reflectance (Rrs(λ)) and the diffuse attenuation coefficient (Kd(λ)) from inherent optical properties revealed the strong influence of aCDOM(λ) on Rrs(λ) and Kd(λ). Good optical closure was achieved between modeled and radiometer measured Rrs(λ) and Kd(λ) with average percent difference of less than 25% and 19% respectively, except at red wavelengths. The aCDOM(λ) accounted for >50% of Kd(λ) which was vertically variable. Chlorophyll-a calculated by the NASA standard chlorophyll-a algorithm (OC4.v6) was overestimated due to higher aCDOM(λ) and underestimated due to lower a*PHY(λ) at low and high concentrations of chlorophyll-a, respectively.
•An absorption budget for the southeastern Bering Sea waters is proposed•Large variability observed in specific phytoplankton absorption (a*PHY(λ))•Strong influence of CDOM absorption (aCDOM(λ)) on Rrs(λ) and Kd(λ)•Chlorophyll-a at low concentrations overestimated due to higher aCDOM(λ) using OC4•Chlorophyll-a at high concentrations underestimated due to lower a*PHY(λ) using OC4
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