Phytoplankton, dissolved oxygen and nutrient patterns along a eutrophic river-estuary continuum: Observation and modeling.

Transport and fate of phytoplankton blooms and excessive nutrients along salinity and turbidity gradients of a river-estuary continuum could determine when and where impaired water quality occurs. However, the general spatiotemporal patterns, underlying mechanisms and their implication for water quality management are not well understood. This study reveals typical seasonal variations and longitudinal patterns of phytoplankton, dissolved oxygen (DO) and nutrients (C, N, and P) in the lower St. Johns River estuary in Florida based on 23 years of data and a model which spans 3 years. Evident declines in freshwater phytoplankton and DO concentrations were observed in the freshwater-saltwater transition zone and the estuarine turbidity maxima along the river-estuary continuum. Observations show that most cyanobacteria blooms originating from upstream lakes collapsed in the freshwater-saltwater transition zone where salinity was greater than 1 ppt, but data analysis and model simulation both indicate salinity stress was not the sole reason, other factors such as changes in hydrodynamics and river morphology also contributed to the bloom crashes. Inorganic nutrients (ammonium, nitrate, and phosphate) exhibited inverse longitudinal patterns with phytoplankton. Due to algal uptake, summer concentrations of inorganic nutrients were low in the freshwater, but substantially elevated in the marine reach as a result of large point source inputs and nutrient regeneration from organic detritus. However, because of strong river-ocean mixing, the dramatic increase in nutrients did not promote a phytoplankton bloom in the local marine reach. The nutrients were eventually transported into coastal waters or oceans where they fueled phytoplankton blooms. Our findings highlight that strategies for nutrient reduction and phytoplankton bloom management should be developed beyond local reaches and across a river-estuary-ocean continuum, exploring the possibility that freshwater phytoplankton blooms and excessive nutrients may be transported to downstream estuaries, coastal waters and even oceans that are vulnerable to poor water quality.

Low flows and downstream decline in phytoplankton contribute to impaired water quality in the lower Minnesota River.

The underlying physical and biogeochemical mechanisms associated with low dissolved oxygen (DO) levels below 5 mg L-1 were examined through field data analyses and water quality modeling of the lower 40 miles of the Minnesota River. Insights into flow and water quality data of nineteen years (1999-2017) at five sites demonstrate that low DO levels parallel the obvious longitudinal (upstream-to-downstream) decline in phytoplankton biomass and increase in ammonium nitrogen (NH4) and dissolved orthophosphate (PO4) in the last 22-mile river reach (i.e., navigation channel) during late summer low flow conditions. River discharge is inversely related to the magnitude of the longitudinal change in DO, phytoplankton biomass, NH4 and PO4, indicating that the late summer low flow hydrodynamics in the navigation channel with a longer residence time, deeper water and slower velocity provide an extended opportunity for the biogeochemical reactions involving phytoplankton, DO and nutrients. Moreover, the ratio of the longitudinal decline in DO versus the longitudinal increase in NH4 is particularly close to the Redfield O:N ratio, suggesting that the decline in DO and increase in nutrients most likely result from the decomposition of phytoplankton detritus under aerobic conditions. This is further proved by the water quality modeling of the lower Minnesota River. The primary reasons for impaired water quality are substantially elevated sediment oxygen consumption and nutrient release derived from the decomposition of settled phytoplankton detritus in the navigation channel. Therefore, we recommend that active prevention of abrupt phytoplankton blooms and collapses through regulation of river discharge and local hydrodynamics may assist in maintaining acceptable water quality in eutrophic rivers with a high level of phytoplankton biomass.

Mercury transport and fate models in aquatic systems: A review and synthesis.

Mercury contamination in aquatic systems has been an issue to the natural ecosystem and human health. Environmental models have become a valuable decision-making tool and play a significant role in mercury pollution control and management. This paper gives an overview of currently available models for simulating mercury transport and fate in aquatic systems. The mercury transformation mechanisms included in these models were identified, as well as data limitations in the models' application. Future advances in understanding mercury transport, cycling, and biogeochemistry in both water column and sediment will improve the robustness of current modeling applications. Moreover, additional field data are critically needed to better predict the concentrations of multi-phase mercury species in various aquatic systems, including measurements in the water column, benthic sediments, and organisms. Field data are also crucial for model calibration and validation. Without this information it will not be possible to adequately understand the environmental factors controlling mercury fate in aquatic systems. The insufficient quantity of adequate measurements and the unsatisfactory accuracy of mercury models are, in numerous cases, supplemented by mass balances since they diminish the unreliability of models. Mercury science evolves gradually with the advancement of science and technology, which requires that mathematical modeling of mercury transport and transformation should be consistently updated.

Biomass production in the Lower Mississippi River Basin: Mitigating associated nutrient and sediment discharge to the Gulf of Mexico.

A watershed model was developed using the Soil and Water Assessment Tool (SWAT) that simulates nitrogen, phosphorus, and sediment loadings in the Lower Mississippi River Basin (LMRB). The LMRB SWAT model was calibrated and validated using 21 years of observed flow, sediment, and water-quality data. The baseline model results indicate that agricultural lands within the Lower Mississippi River Basin (LMRB) are the dominant sources of nitrogen and phosphorus discharging into the Gulf of Mexico. The model was further used to evaluate the impact of biomass production, in the presence of riparian buffers in the LMRB, on suspended-sediment and nutrient loading discharge from the Mississippi River into the Gulf of Mexico. The interplay among land use, riparian buffers, crop type, land slope, water quality, and hydrology were anlyzed at various scales. Implementing a riparian buffer in the dominant agricultural region within the LMRB could reduce suspended sediment, nitrogen, and phosphorus loadings at the regional scale by up to 65%, 38%, and 39%, respectively. Implementation of this land management practice can reduce the suspended-sediment content and improve the water quality of the discharge from the LMRB into the Gulf of Mexico and support the potential production of bioenergy and bio-products within the Mississippi River Basin.