Ecosystem analysis of water column processes in the York River estuary, Virginia: Historical records, field studies and modeling analysis

Analyses of EPA long-term datasets (1985--1994) combined with field studies and ecosystem model development were used to investigate phytoplankton and nutrient dynamics in the York River estuary. Analysis of the EPA dataset showed that algal blooms occurred during winter-spring followed by smaller summer blooms. Peak phytoplankton biomass during the winter-spring blooms occurred in the mid reach of the mesohaline zone whereas during the summer bloom it occurred in the tidal fresh-mesolialine transition zone. River discharge appears to be the major factor controlling the location and timing of the winter-spring blooms and the relative degree of potential nitrogen (N) and phosphorus (P) limitation. Phytoplankton biomass in tidal fresh water regions was limited by high flushing rates. Water residence time was less than cell doubling rate during seasons of high river flow. Positive correlations between PAR at 1m depth and chlorophyll a suggested light limitation of phytoplankton in the tidal fresh-mesohaline transition zone. A significant relationship between the delta of salinity between surface and bottom water and chlorophyll a distribution suggested the importance of tidal mixing for phytoplankton dynamics in the mesohaline zone. Accumulation of phytoplankton biomass in the mesohaline zone was generally controlled by N with the nutrient supply provided by benthic or bottom water remineralization. In general, phytoplankton dynamics appear controlled to a large extent by resource limitation (bottom-up control) rather than zooplankton grazing (top-down control). The dynamics of phytoplankton size structure were investigated in the freshwater, transitional and estuarine reaches of the York River over an annual cycle. The contribution of large cells (micro-plankton, \u3e20 mum) to total biomass increased downstream during winter whereas that of small cells (nano-, 3--20 mum) pico-plankton, \u3c3 mum) increased downstream during summer. I conclude from these studies that spatial and seasonal variations in size structure of phytoplankton observed on the estuarine scale are determined both by the different preferences of micro-, nano-, and picoplankton for nutrients and by their different light requirements. Analyses of phytoplankton size structure are, thus, necessary to better understand phytoplankton dynamics and to better manage water quality in estuarine systems. An ecosystem model was developed to integrate these data and to investigate mechanisms controlling the size-structured phytoplankton dynamics in the mesohaline zone of the York River estuary. The model developed in Fortran90 included 12 state variables describing the distribution of carbon and nutrients (nitrogen, phosphorus) in the surface mixed layer. Forcing functions included incident radiation, temperature, wind stress, mean flow and tide including advective transport and turbulent mixing. Model results supported the general view that phytoplankton dynamics are controlled by abiotic mechanisms (i.e. bottom-up control) rather than biotic, trophic interactions in the York River estuary. Model sensitivity tests showed that small cells (pico-, nano-sized) are more likely regulated by temperature and light whereas large cells (micro-sized) are regulated by physical processes such as advection, and tidal mixing. Microphytoplankton blooms during winter- pring resulted from a combination of longitudinal advection and vertical diffusion of phytoplankton cells rather than in-situ production

Effects of Humic Acids on Size and Species Composition of Phytoplankton in a Eutrophic Temperate Estuary

The Yeongsan River estuary was divided into freshwater and seawater zones by a sea dike constructed at its mouth in 1981. The freshwater zone, which flows through a metropolitan area, is eutrophic, causing frequent algal blooms with an expected increase in the concentration of refractory organic compounds such as humic substances (HS). Herein, the in situ freshwater zone phytoplankton community size and taxonomic composition were investigated in response to the addition of humic acids (HA) using seasonal mesocosm experiments. Phytoplankton (chlorophyll a) were fractionated into nano-(20 µm) classes and identified by species or genus. Their response to HA treatment was examined by repeated measures analysis of variance (RM-ANOVA). With the addition of HA, the concentrations of total and nanosized chlorophyll a increased significantly (p a did not change significantly through the seasons. The abundance of Stephanodiscus sp. (diatoms) also increased significantly when this genus dominated the phytoplankton community. This suggests that the management of HS may be crucial in mitigating algal blooms in estuaries, such as in the Yeongsan River estuary, that are subjected to anthropogenic disturbances by engineered structures

Artificial Neural Network (ANN) Modeling Analysis of Algal Blooms in an Estuary with Episodic and Anthropogenic Freshwater Inputs

The Youngsan River estuary, located on the southwest coast of South Korea, has transitioned from a natural to an artificial estuary since dike construction in 1981 separated freshwater and seawater zones. This artificial transition has induced changes in the physical properties and circulation within the estuary, which has led to hypoxia and algal blooms. In this study, an artificial neural network (ANN) model was employed to simulate phytoplankton variations, including algal blooms and size fractions based on chlorophyll a, using data obtained by long-term monitoring (2008–2018) of the seawater zone of the Youngsan River estuary. The model was validated through statistical analyses, and the validated model was used to determine the contribution of the environmental factors on size-fractionated phytoplankton variations. The statistical validation of the model showed extremely low sum square error (SSE ≤ 0.0003) and root mean square error (RMSE ≤ 0.0173) values, with R2 ≥ 0.9952. The accuracy of the model predictions was high, despite the considerable irregularity and wide range of phytoplankton variations in the estuary. With respect to phytoplankton size structure, the contribution of seasonal environmental factors such as water temperature and solar radiation was high for net-sized chlorophyll a, whereas the contribution of factors such as freshwater discharge and salinity was high for nano-sized chlorophyll a, which includes typical harmful algae. Notably, because the Youngsan River estuary is influenced by a monsoon climate—characterized by high precipitation in summer—the contribution of freshwater discharge to harmful algal blooms is predicted to increase during this period. Our results suggest that the ANN model can be an important tool for understanding the influence of freshwater discharge, which is essential for managing algal blooms and maintaining the ecosystem health of altered estuaries

Spatial and Temporal Dynamics of Potentially Toxic Cyanobacteria in the Riverine Region of a Temperate Estuarine System Altered by Weirs

The effects of weirs on fish and other biological communities have garnered considerable study, whereas the effects of weirs on community composition of toxic cyanobacteria have not yet been well documented. In this study, temporal and spatial variations in species composition and the abundance of potentially toxic cyanobacteria were investigated in the riverine regions of the temperate Youngsan River estuary, where two weirs have recently been constructed. Four stations were sampled 0.5 m below the surface monthly along the channel of the upper river from May 2014 to April 2015 to explore cyanobacterial composition and abundance, while physicochemical and biological parameters were measured to elucidate possible mechanisms controlling these dynamics. Two stations were located upstream at free-flowing sites, and the other stations were located downstream at impounded sites near the weirs. Twenty-eight cyanobacterial species were identified, seven of which were potentially toxic: Microcystis sp., M. aeruginosa, M. flos-aquae, Dolichospermum sp., Aphanocapsa sp., Oscillatoria sp. and Phormidium sp. Microcystis sp. was the most abundant in June 2014 at the lowest station near the weir. Meanwhile, Phormidium sp. occurred at low abundance throughout the study period, except during the winter months, when its abundance was elevated. The interactive forward selection method highlighted dissolved inorganic nitrogen and zooplankton abundance as explanatory variables for this observed variation, but their effects on cyanobacterial growth are unclear. However, temperature was the major determinant for the temporal variation in cyanobacterial populations. Cluster analysis showed that the downstream stations near the weirs had a high similarity of potentially toxic cyanobacteria. Significantly higher abundance, especially of Microcystis sp., was also recorded at the impounded sites suggesting that the presence of weirs might affect variations in toxic cyanobacterial communities

Kohn-Sham Time-Dependent Density Functional Theory on the Massively Parallel Graphics Processing Units

We report a high-performance multi graphics processing unit (GPU) implementation of the Kohn-Sham time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation. Our newly developed GPU algorithm on massively parallel computing systems using multiple parallel models in tandem scales optimally with material size, considerably reducing the computational wall time. A benchmark TDDFT study was performed on a green fluorescent protein complex composed of 4,353 atoms with 40,518 atomic orbitals represented by Gaussian-type functions. As the largest molecule attempted to date to the best of our knowledge, the proposed strategy demonstrated reasonably high efficiencies up to 256 GPUs on a custom-built state-of-the-art GPU computing system with Nvidia A100 GPUs. We believe that our GPU-oriented algorithms, which empower first-principles simulation for very large-scale applications, may render deeper understanding of the molecular basis of material behaviors, eventually revealing new possibilities for breakthrough designs on new material systems