Runoff-mediated seasonal oscillation in the dynamics of dissolved organic matter in different branches of a large bifurcated estuary-The Changjiang Estuary (original) (raw)
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Journal of Marine Systems, 2014
Hypoxia off the Changjiang Estuary has been frequently reported using short time duration field data. However, its evolution was unknown because of a lack of long-term data and its associated dominant factor. A 104-day long dataset was collected with a bottom mounted system off the Changjiang Estuary in summer 2009. The monitored parameters were bottom dissolved oxygen (DO), temperature, pressure and current. Two hypoxia events were identified, showing that hypoxia was severe and lasted for more than a half month. The first event appeared on July 18 and lasted 17 days. During this hypoxia period, the minimum DO was down to 0.17 mg/L, which broke the historical record. The second hypoxia event appeared on August 30 and lasted 18 days with a minimum DO of 1.29 mg/L. The variation of bottom DO was closely related to that of stratification. The monitored data showed that almost every increase/decrease of DO was associated with a weakening/enhancing of stratification, which were recorded as many as 12 times during the monitoring period. Wind mixing modulated or broke the stratification, which affected the variation of bottom DO and hypoxia events. Using a lagged correlation analysis, the stratification and wind mixing were significantly correlated with a coefficient of determination r 2 = 0.72, and stratification lagged wind by 35 h. The bottom DO and wind mixing were significantly correlated with a coefficient of determination r 2 = 0.65, and DO lagged wind by 33 h. The formation periods of two hypoxia events estimated from monitored data were 20 and 15 days, which were much shorter than that from on-board experiments. Strong wind mixing played a dual role on hypoxia. It could relieve hypoxia conditions by supplying DO through mixing. It accelerated the formation of hypoxia afterward as a result of the enhanced phytoplankton bloom induced by wind mixing and high organic decomposition rates consuming more DO.
This study examined the seasonality of dissolved organic matter (DOM) sources and transformations within the Neuse River estuary (NRE) in eastern North Carolina between March 2010 and February 2011. During this time, monthly surface and bottom water samples were collected along the longitudinal axis of the NRE, ranging from freshwater to mesohaline segments. The monthly mean of all surface and bottom measurements made on collected samples was used to clarify larger physical mixing controls in the estuary as a whole. By comparing monthly mean trends in DOM and chromophoric dissolved organic matter (CDOM) properties in surface and bottom waters during varying hydrological conditions, we found that DOM and CDOM quality in the NRE is controlled by a combination of discharge, wind speed, and wind direction. The quality of DOM was assessed using C:N ratios, specific ultraviolet absorption at 254 nm (SUVA 254 ), the absorption spectral slope ratio (S R ), and the humification (HIX) and biological (BIX) indices from fluorescence. The NRE reflects allochthonous sources when discharge and flushing time are elevated at which times SUVA 254 and HIX increased relative to base flow. During periods of reduced discharge and long flushing times in the estuary, extensive autochthonous production modifies the quality of the DOM pool in the NRE. This was evidenced by falling C:N values, and higher BIX and S R values. Lastly, a combination of increased wind speed and shifts in wind direction resulted in benthic resuspension events of degraded, planktonic OM. Thus, the mean DOM characteristics in this shallow micro-tidal estuary can be rapidly altered during episodic mixing events on timescales of a few weeks.
Performance and Applicability of Water Column Correction Models in Optically Complex Coastal Waters
Remote Sensing
Maps of submerged aquatic vegetation (SAV) are of primary importance for the sustainable management of coastal areas and serve as a basis for fundamental ecological studies. Various water column correction (WCC) models are successfully applied in clear Case-1 waters to compensate for the variable water depth effect. The performance of the WCC in less clear Case-2 waters is rarely assessed. In this study, the performance and applicability of model-based WCC algorithms in the complex Baltic Sea were analyzed. The bottom reflectance was retrieved from the Compact Airborne Spectrographic Imager (CASI) water surface reflectance by applying the Maritorena and Lee WCC algorithms. The Maritorena model retrieved bottom spectra that showed large variations in reflectance magnitudes. The Lee model was more successful in retrieving reasonable spectral magnitudes, although only in a rather narrow wavelength region (550–600 nm). Shorter and longer spectral regions were significantly overcorrected...
Hydrology and Earth System Sciences, 2011
The estuarine parameter ν is widely accepted as describing the relative contribution of the tide-driven and density-driven mixing mechanism of salt transport in estuaries. Van der Burgh's coefficient K is another parameter that also determines the relative strength of two mechanisms. However, a single value of K , which has been considered in previous studies, can not represent the spatial variation of these mechanisms in an estuary. In this study, the spatially varying K has been determined from the ν value calculated using intensively observed longitudinal salinity transects of the Sumjin River Estuary with exponential shape. The spatially varying K describes the spatial variation of these mechanisms reasonably well and is independent of the river discharge downstream of the estuary during spring tide where the strong tides cause well mixed conditions. However, K values increase upstream and are found to depend on the freshwater discharge, with suppressing vertical mixing. The K value has been scaled on the basis of the ν value and ranges between 0 and 1. If K is <0.4, the up-estuary salt transport is entirely dominated by tide-driven mixing during spring tide near the mouth. If 0.4 < K < 0.8, both tide-driven and density-driven mixing contribute to transporting salt in the central regimes. If K > 0.8, the salt transport is almost entirely by density-driven circulation in the upper most regimes during both spring and neap tides. In addition, another K-based dispersion equation has been solved by using this spatially varying K. The spatially varying K demonstrates density-driven circulation more prominently at the strong salinity gradient location compared with a single K value.
Wind-Driven Dissolved Organic Matter Dynamics in a Chesapeake Bay Tidal Marsh-Estuary System
Estuaries and Coasts, 2017
Controls on organic matter cycling across the tidal wetland-estuary interface have proved elusive, but high-resolution observations coupled with process-based modeling can be a powerful methodology to address shortcomings in either methodology alone. In this study, detailed observations and three-dimensional hydrodynamic modeling are used to examine biogeochemical exchanges in the marsh-estuary system of the Rhode River, MD, USA. Analysis of observations near the marsh in 2015 reveals a strong relationship between marsh creek salinity and dissolved organic matter fluorescence (fDOM), with wind velocity indirectly driving large amplitude variation of both salinity and fDOM at certain times of the year. Three-dimensional model results from the Finite Volume Community Ocean Model implemented for the wetland system with a new marsh grass drag module are consistent with observations, simulating sub-tidal variability of marsh creek salinity. The model results exhibit an interaction between wind-driven variation in surface elevation and flow velocity at the marsh creek, with northerly winds driving increased freshwater signal and discharge out of the modeled wetland during precipitation events. Wind setup of a water surface elevation gradient axially along the estuary drives the modeled local sub-tidal flow and thus salinity variability. On sub-tidal time scales (>36 h, <1 week), wind is important in mediating dissolved organic matter releases from the Kirkpatrick Marsh into the Rhode River.
Frontiers in Marine Science, 2022
Understanding shallow water biogeochemical dynamics is a challenge in coastal regions, due to the presence of highly variable land-water interface fluxes, tight coupling with sediment processes, tidal dynamics, and diurnal variability in biogeochemical processes. While the deployment of continuous monitoring devices has improved our understanding of high-frequency (12-24 hours) variability and spatial heterogeneity in shallow regions, mechanistic modeling of these dynamics has lagged behind conceptual and empirical models. The inherent complexity of shallow water systems is represented in the Corsica River estuary, a small basin within the Chesapeake Bay ecosystem, where abundant monitoring data have been collected from long-term monitoring stations, continuous monitoring sensors, synoptic sensor surveys, and measurements of sediment-water fluxes. A state-of-the-art modeling system, the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), was applied to the Corsica domain with a high-resolution grid and nutrient loads from the most recent version of the Chesapeake Bay watershed model. The Corsica SCHISM model reproduced observed highfrequency variability in dissolved oxygen, as well as seasonal variability in chlorophyll-a and sediment-water fluxes. Time-series signal analyses using Empirical Model Decomposition and spectral analysis revealed that the diurnal and M2 tide frequencies are the dominant high-frequency modes and physical transport contributes a larger share to dissolved oxygen budgets than biogeochemical processes on an hourly time scale. Heterogeneity and patchiness in dissolved oxygen resulting from phytoplankton distributions and geometry-driven eddies amplify the physical transport effect, and on longer time scales oxygen is controlled more by photosynthesis and Frontiers in Marine Science frontiersin.org 01
Aquatic Ecosystem Health & Management
The present study aims to investigate the temporal variability of chromophoric dissolved organic matter through remote sensing and to determine its influence factors in the Pearl River Estuary. A medium resolution imaging spectrometer, chromophoric dissolved organic matter product was evaluated with in situ absorption coefficient (R2 = 0.9605, RMS = 0.1672 and MRE = 0.3930). L2 daily products were then averaged into monthly data to analyze variability in the specified area from January 2003 to December 2009. Accumulated anomaly analysis and the wavelet analysis showed that the variability had a significant period of one year from 2003 to 2009, accompanied with 3 and 6 month periods in 2004 and 2008 separately. Cross wavelet transform was used to analyze the relationships between chromophoric dissolved organic matter and its influence factors including rainfall and phytoplankton in time frequency space. Relationships with salinity and light were also analyzed. It was found that photo...
2014
The Río de la Plata (RdP) estuary, located at 35°S on the southwestern Atlantic Ocean, is a shallow and large-scale plain, which drains the second largest basin in South America. The RdP river carries high amounts of nutrients, suspended particulate and dissolved organic matter to the adjacent shelf waters and is considered among the most turbid estuaries in the world. A turbidity maximum and a sharp surface front defining its seaward edge is a distinctive feature of this estuary. Such high sediment loads represent a challenge to atmospheric correction algorithms which usually rely on the assumption of zero waterleaving reflectance in the near infrared (NIR) or short wave infrared (SWIR) parts of the spectrum. Uncertainties of the primary remote sensing products have never been quantified in RdP before due to lack of in situ measurements. In February and April 2013 two field campaigns were performed in the turbidity maximum zone where water reflectance was measured and surface water...
Ocean Optics Conference XIX, 2008
Estimating the light propagation across the air-water interface and under water accurately is critical in biogeochemical models, as this parameter controls processes such as photosynthesis, heat flux and biological growth, in the ecosystem. Better estimates of under water light propagation in coastal waters can be achieved when diurnal variations are represented in the above surface downwelling irradiance (E tot ). In this work we have used MODIS derived instantaneous Photosynthetically Available Radiation (PAR) and a semi-analytical model to represent the diurnal variations in the above surface E tot . CSIRO Environmental Modelling Systems (EMS) is coupled with an inherent optical property based diffuse attenuation kd model to simulate the light propagation. This version of EMS is applied to Fitzroy Estuary and Keppel Bay. Comparison of model output in terms of Chlorophyll a concentration and Total Phosphorus in phytoplankton cell are compared in relation to model output which used standard daily averaged PAR. Results show that the chlorophyll a concentration modelled using daily par data deviates to a value which is 6 times higher than estimates made when MODIS iPAR data is used with diurnal representation.