Effect of rainfall in shaping microbial community during Microcystis bloom in Nakdong River, Korea.

Various environmental factors play a role in the formation and collapse of Microcystis blooms. This study investigates the impact of heavy rainfall on cyanobacterial abundance, microbial community composition, and functional dynamics in the Nakdong River, South Korea, during typical and exceptionally rainy years. The results reveal distinct responses to rainfall variations, particularly in cyanobacterial dominance and physicochemical characteristics. In 2020, characterized by unprecedented rainfall from mid-July to August, Microcystis blooms were interrupted significantly, exhibiting lower cell densities and decreased water temperature, compared to normal bloom patterns in 2019. Moreover, microbial community composition varied, with increases in Gammaproteobacteria and notably in genera of Limnohabitans and Fluviicola. These alterations in environmental conditions and bacterial community were similar to those of the post-bloom period in late September 2019. It shows that heavy rainfall during summer leads to changes in environmental factors, consequently causing shifts in bacterial communities akin to those observed during the autumn-specific post-bloom period in typical years. These changes also accompany shifts in bacterial functions, primarily involved in the degradation of organic matter such as amino acids, fatty acids, and terpenoids, which are assumed to have been released due to the significant collapse of cyanobacteria. Our results demonstrate that heavy rainfall in early summer induces changes in the environmental factors and subsequently microbial communities and their functions, similar to those of the post-bloom period in autumn, leading to the earlier breakdown of Microcystis blooms.

Development of a basin-scale total nitrogen prediction model by integrating clustering and regression methods.

Nutrient runoff into rivers caused by human activity has led to global eutrophication issues. The Nakdong River in South Korea is currently facing significant challenges related to eutrophication and harmful algal blooms, underscoring the critical importance of managing total nitrogen (T-N) levels. However, traditional methods of indoor analysis, which depend on sampling, are labor-intensive and face limitations in collecting high-frequency data. Despite advancements in sensor allowing for the measurement of various parameters, sensors still cannot directly measure T-N, necessitating surrogate regression methods. Therefore, we conducted T-N predictions using a water quality dataset collected from 2018 to 2022 at 157 observatories within the Nakdong River basin. To account for the water quality characteristics of each location, we employed a clustering technique to divide the basin and compared a Gaussian mixture model with K-means clustering. Moreover, optimal regressor for each cluster was selected by comparing multiple linear regression (MLR), random forest, and XGBoost. The results showed that forming four clusters via K-means clustering was the most suitable approach and MLR was reasonably accurate for all clusters. Subsequently, recursive feature elimination cross-validation was used to identify suitable parameters for T-N prediction, thus leading to the construction of high-accuracy T-N prediction models. Clustering was useful not only for improving the regressors but also for spatially analyzing the water quality characteristics of the Nakdong River. The MLR model can reveal causal relationships and thus is useful for decision-making. The results of this study revealed that the combination of a simple linear regression model and clustering method can be applied to a wide watershed. The clustering-based regression model showed potential for accurately predicting T-N at the basin level and is expected to contribute to nationwide water quality management through future applications in various fields.

Phytoplankton functional groups as indicators of environmental changes in weir and non-weir sections of the lower Nakdong River, Republic of Korea.

The Nakdong River underwent water impoundment after eight weirs were constructed as part of South Korea's Four Major River Restoration Project from 2009 to 2012. In this study, we aimed to confirm whether the assemblage of phytoplankton based on phytoplankton functional groups (PFGs), could indicate environmental changes in the weir section (WS) and non-weir section (NWS) of the lower Nakdong River after the construction of the weir. Thus, we examined the relationships between PFGs and gradients in environmental drivers, such as physicochemical, meteorological, and hydrological variables. Environmental gradients were observed between the WS and NWS in dissolved oxygen (DO), electric conductivity (EC), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), dissolved total nitrogen (DTN), dissolved total phosphorus (DTP), ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), and phosphorus (PO4-P), which were relatively higher in the WS. Seventeen PFGs were identified (A, B, C, D, E, F, G, H1, J, LM, LO, MP, P, T, W1, X1, and X2). Additionally, the LM and P groups, preferring an enriched lentic system more than other groups, were found to be the dominant PFGs that led the succession of assemblages. Traditional nutrients (N, P) and organic pollutants (BOD, COD) primarily affected the autochthonous growth of the most dominant PFGs in the WS as HRT (hydraulic retention time) increased. Furthermore, the hydrological variables associated with meteorological conditions have a synergistic effect on the composition of the major PFGs and chemical and physical variables in the WS. In other words, the WS may be a new source of inoculum that primarily determines the occurrence and maintenance of phytoplankton in the immediate downstream region (NWS). In particular, group LM (mainly potentially toxic Microcystis) developing in the upper weir impoundment is transported downstream, resulting in a high inoculation effect on further growth in the NWS during the summer monsoon season.

Proposal for priority emerging pollutants in the Nakdong river, Korea: Application of EU watch list mechanisms.

The Nakdong River, the longest in Korea, has received numerous pollutants from heavily industrialized and densely populated areas while being used as a drinking water source. A number of research have reported occurrences of emerging pollutants (EPs) in the river. The results requested efficient monitoring and systematic management strategies such as EU watch list under Water Framework Directive. The aim of this study is to propose a watch list through preliminary monitoring of the river and risk-based prioritization approach. As candidates for monitoring target, 632 substances were selected based on literature and database searches. Among them, 175 substances were subjected to target screening method whereas 457 were evaluated via suspect screening. A risk-based prioritization was applied to substances quantified through target screening based on concentrations, and a scoring-based prioritization was applied to substances tentatively identified through suspect screening. Sampling campaigns (n = 12) were conducted from October 2020 to September 2021, at 8 sampling sites along the river. As a result, 130 target substances were quantified above the LOQ. Among the 21 substances whose priority score was assigned through risk-based prioritization, telmisartan and iprobenfos were identified with very high environmental risk while candesartan, TBEP, imidacloprid, azithromycin and clotrimazole were classified with high or intermediate risk. As result of the scoring system for 39 tentatively identified substances, 6 substances (benzophenone, caprolactam, metolachlor oxanilic acid, heptaethylene glycol, octaethylene glycol and pentaethylene glycol), which were then confirmed with reference standards, showed a potential environmental risk. Those substances prioritized through target and suspect screening followed by scoring systems can be a subset for the watch list and potential targets for nationwide water quality monitoring program in the future.

Microbial communities in aerosol generated from cyanobacterial bloom-affected freshwater bodies: an exploratory study in Nakdong River, South Korea.

Toxic blooms of cyanobacteria, which can produce cyanotoxins, are prevalent in freshwater, especially in South Korea. Exposure to cyanotoxins via ingestion, inhalation, and dermal contact may cause severe diseases. Particularly, toxic cyanobacteria and their cyanotoxins can be aerosolized by a bubble-bursting process associated with a wind-driven wave mechanism. A fundamental question remains regarding the aerosolization of toxic cyanobacteria and cyanotoxins emitted from freshwater bodies during bloom seasons. To evaluate the potential health risk of the aerosolization of toxic cyanobacteria and cyanotoxins, the objectives of this study were as follows: 1) to quantify levels of microcystin in the water and air samples, and 2) to monitor microbial communities, including toxic cyanobacteria in the water and air samples. Water samples were collected from five sites in the Nakdong River, South Korea, from August to September 2022. Air samples were collected using an air pump with a mixed cellulose ester membrane filter. Concentrations of total microcystins were measured using enzyme-linked immunosorbent assay. Shotgun metagenomic sequencing was used to investigate microbial communities, including toxic cyanobacteria. Mean concentrations of microcystins were 960 µg/L ranging from 0.73 to 5,337 µg/L in the water samples and 2.48 ng/m3 ranging from 0.1 to 6.8 ng/m3 in the air samples. In addition, in both the water and air samples, predominant bacteria were Microcystis (PCC7914), which has a microcystin-producing gene, and Cyanobium. Particularly, abundance of Microcystis (PCC7914) comprised more than 1.5% of all bacteria in the air samples. This study demonstrates microbial communities with genes related with microcystin synthesis, antibiotic resistance gene, and virulence factors in aerosols generated from cyanobacterial bloom-affected freshwater body. In summary, aerosolization of toxic cyanobacteria and cyanotoxins is a critical concern as an emerging exposure route for potential risk to environmental and human health.

The geochemistry and isotopic compositions of the Nakdong River, Korea: weathering and anthropogenic effects.

The Nakdong River is the longest river in South Korea, and flows through various geological terrains with different land use characteristics; therefore, the geochemistry of its water is expected to be influenced by many factors. In this work, the geochemical characteristics of the Nakdong River were examined, and its chemical compositions, δD, δ18O, and δ13CDIC values, and 87Sr/86Sr ratios were determined to investigate the geological and anthropogenic effects on the geochemistry of the Nakdong River water. The obtained concentrations of major ions were strongly affected by both the anthropogenic activity and weathering of the rocks. With increasing the flow distance, the ion concentrations slightly increased; and after the inflow of the Kumho River, which was the largest tributary running through Daegu (the fourth largest city in South Korea), the concentrations of Na and SO4 ions abruptly increased and decreased again, suggesting the existence of strong anthropogenic effects caused by sewage treatment plants and dyeing industrial complex. Other activities such as agricultural ones also increased the NO3 concentration. In July, the high precipitation level from tropical cyclones and downpours decreased the ion concentrations as well as the δD and δ18O values. The δ13CDIC magnitudes showed that the dissolved inorganic carbon mainly originated from mineral weathering upstream, while the oxidation of soil organic materials influenced by agricultural activity became more important downstream. The 87Sr/86Sr ratios revealed that in the upstream regions, the weathering of granite and gneiss complex was dominant, while in the downstream regions, the weathering of sedimentary rocks became more important. The weathering and anthropogenic effects on the river water chemistry were also demonstrated using statistical analysis, which revealed that the water geochemistry was mostly influenced by the anthropogenic sources, including industrial complex, represented by Na, Cl, and SO4. The obtained results show that, as compared to the geochemistry of the Han River (which is also a major river in Korea), the geochemistry of the Nakdong River is more influenced by anthropogenic activities (including agriculture and the industrial complex) due to the different land use.

Growth characteristics of lytic cyanophages newly isolated from the Nakdong River, Korea.

Cyanophages are primary regulators of cyanobacterial harmful algal blooms (CyanoHABs), and they control host cyanobacterial dynamics, frequency, and diversity in the aquatic environment. This study deals with growth characteristics of three lytic cyanophages, Myoviridae AGM-1, Myoviridae NGM-1, and Podoviridae NDP-1, newly isolated from the Nakdong River in South Korea. These isolates are capable of infecting Amazoninema brasiliense, Nododsilinea nodulosa, and Nostoc sp. The results showed that abiotic parameters such as water temperature and pH balance significantly affect the growth of a cyanophage and the interaction with its host in the aquatic environment. The optimal growth conditions of the newly isolated cyanophages are less than 37 °C and pH 9, whereas optimal conditions are 25-30 °C and pH 7 for the cyanobacteria used as hosts. However, each cyanophage was found to have significantly different growth characteristics in phage titer, latent period, and burst size, depending on the characteristics of the species. Among the three cyanophages, Podoviridae NDP-1 showed the highest burst size and infection activity. The lower the designed multiplicity of infection (MOI) ratio (0.01 to 10), the longer it takes to lyse the host cells. The minimum MOI value for sustainable biocontrol of CyanoHABs is proposed as MOI=1. These results can be used as basic information in further studies, such as pyophage control of CyanoHABs and enrichment of cyanophages with high activity.

Present and potential future critical source areas of nonpoint source pollution: a case of the Nakdong River watershed, South Korea.

Identifying critical source areas (CSAs) is the first step to effectively managing nonpoint source (NPS) pollution. Increasing variability in climate can affect identification of CSAs. In this study, we identified present and future CSAs of NPS pollution in the Nakdong River watershed and examined how climate change will influence the identification of CSAs. Nine NPS pollution-related factors affecting the watershed environment and water quality were considered. These factors were rescaled through a min-max normalization to propose an index system that ranks basins based on the sensitivity of basins to climate change on identifying CSAs. For analyses, past rainfall was replaced with future rainfall under two RCP scenarios, RCP 2.6 and RCP 8.5. Results showed insignificant differences in the spatial distribution of CSAs between the present and the future and between the future scenarios. Basins that are on or adjacent to the Nakdong River mainstream were mainly identified as CSAs, in addition to many basins of the Geumho and Nam rivers. Highly ranked CSAs including the level 1 CSAs, were mainly distributed in the mid- and downstream areas of the Nakdong River, indicating high need of NPS pollution management. This study can provide a foundation for the effective management of NPS pollution in the present and the future.

Influence of Abiotic Factors on the Growth of Cyanobacteria Isolated from Nakdong River, South Korea1.

Changes in physico-chemical factors due to natural climate variability and eutrophication could affect the cyanobacterial growth patterns in aquatic systems that may cause environmental health problems. Based on morphological and 16S rRNA gene analysis, three cyanobacterial species isolated for the first time from the Nakdong River water sample in South Korea were identified as Amazoninema brasiliense, Microcystis elabens, and Nododsilinea nodulosa. The variations in temperature, pH, nitrogen, or phosphorus levels significantly impacted the cyanobacterial growth patterns. The optimal temperature range for the growth of isolates was from 25-30°C. A neutral or weak alkaline environment favored growth; however, A. brasiliense resulted in 44.2-87.5% higher biomass (0.75 g · L-1 as dry solids, DS) and growth rate (0.24 · d-1 ) at pH 7 than the other isolates (0.4-0.52 g DS · L-1 , 0.16-0.19 · d-1 ). The increased nitrate-nitrogen (NO3 -N) concentrations significantly (P < 0.05) favored biomass production and growth rate for A. brasiliense and M. elabens, respectively, and the maximum growth rate was observed for A. brasiliense at 3.5 mg NO3 -N · L-1 . The orthophosphate concentration (PO4 -P) from 0.1 to 0.5 mg PO4 -P · L-1 increased the growth of the isolates. These observations suggest that isolate growth rates in water bodies can vary depending on different physico-chemical parameters. This study contributes to the further understanding of the growth of microalgae in natural freshwater bodies under fluctuating environmental conditions and aquatic ecosystem stability.

Deciphering the key factors determining spatio-temporal heterogeneity of cyanobacterial bloom dynamics in the Nakdong River with consecutive large weirs.

Following the construction of eight large weirs in a 200-km section of the Nakdong River, which is a major water source for the region, harmful cyanobacterial blooms have been occurring annually, causing severe problems with water quality. The present study investigated the community structure of harmful cyanobacteria and identified temporal and spatial patterns in harmful cyanobacterial blooms and their dynamic relationships with physicochemical, hydrological, and meteorological variables in the eight weir sections for 6 years from 2013 to 2018. The dominant harmful cyanobacteria in the eight weir sections were Aphanizomenon and Microcystis spp. There was a successional phenomenon wherein Aphanizomenon spp. first bloomed in spring, and then Microcystis spp. bloomed as water temperatures increased. Additionally, the initiation and duration of the blooms of both genera were affected by the timing and volume of heavy rainfall that caused flushing of cyanobacterial biomass, resulting in direct reduction of cyanobacterial growth in all sections. The harmful cyanobacteria of upstream weirs did not affect the biomass of downstream weirs in terms of either growth initiation or time taken to reach peak biomass, despite being physically connected. Owing to the long retention time during the dry season, similar to what occurs in separate reservoirs, the water quality of each weir section, particularly regarding nutrient characteristics and retention time, were the major factors determining the harmful cyanobacterial abundance, resulting in heterogeneous spatial distribution of harmful cyanobacterial blooms in the Nakdong River.

Invasion and toxin production by exotic nostocalean cyanobacteria (Cuspidothrix, Cylindrospermopsis, and Sphaerospermopsis) in the Nakdong River, Korea.

The extent and frequency of harmful cyanobacterial blooms are increasing, owing to the climate change caused by global warming, and some harmful filamentous cyanobacteria that were first reported in the tropics are spreading to temperate regions, such as North America, Europe, and Northeast Asia. Although these exotic invasive cyanobacteria have a high toxigenic potential, they are not targeted in management plans in many countries. This study analyzed the occurrence of and potential toxin and off-flavor secondary metabolite production by invasive nostocalean cyanobacteria in the Nakdong River in Korea, which is a temperate region. The occurrence of four species belonging to three genera of cyanobacteria was confirmed in the Nakdong River. The quantities of cyanobacteria in the Nakdong River were mostly low, fewer than 1,000 cells mL-1. Twenty-four strains belonging to four species in three genera of cyanobacteria were isolated from the Nakdong River. Analysis revealed no off-flavor secondary metabolite production by any of the isolates, and those belonging to Cylindrospermopsis raciborskii, Sphaerospermopsis aphanizomenoides, and S. reniformis were identified as nontoxic strains. However, anatoxin-a production was observed in two of the eleven isolates of Cuspidothrix issatschenkoi. Given the sites and the timing of its occurrence, C. issatschenkoi had the highest potential for toxin production among the invasive nostocalean cyanobacteria appearing in the Nakdong River.

Tracing river water versus wastewater sources of trace elements using rare earth elements in the Nakdong River estuarine waters.

The concentrations of dissolved rare earth elements (REEs) and trace elements (Al, V, Mn, Fe, Co, Ni, Cu, Zn, Mo, Cd, and Pb) were measured along the Nakdong River Estuary. In general, REE concentrations presented negative correlations with salinity, except for the sampling sites close to the wastewater treatment plant (WWTP), where the concentrations were approximately two orders of magnitude higher. In this study, we attempted to utilize REEs as tracers for river versus WWTP sources of trace elements. The main sources of trace elements can be attributed as follows: the seawater for Mo and Cd, the seawater and WWTP for V, the river and WWTP for Ni and Cu, and the WWTP for Al, Mn, Fe, Co, Zn, and Pb. Our results suggest that REEs can serve as powerful tracers for WWTP sources, particularly in coastal waters where various trace element sources are present.

Distribution of metal contamination and grain size in the sediments of Nakdong River, Korea.

To assess distribution of metal contamination and grain size in the sediments of Nakdong River (South Korea), surface sediments were collected from 21 sites and analyzed. Within the study area, sand was typically the dominant grain size. However, because of the reduced flow rate and flow velocity, sites adjacent to weirs were composed of relatively fine sediments. A comparison of sediment metal concentrations with sediment quality guidelines proposed by the USA, Canada, and South Korea revealed that sites adjacent to weirs had concentrations that exceeded the standard values. The enrichment factor, index of geo-accumulation, and pollution load index calculation results that the sites adjacent to weirs showed high contamination, with Cd accounting for the highest contamination levels. The metals in the study area varies due to the effect of fine sediments; therefore, high concentrations of metals accumulated adjacent to weirs where fine sediments were distributed in greater proportions. Furthermore, Cd exhibited the greatest contribution to metal contamination in the study area and the highest contamination levels were found at NS19 (adjacent to the Haman weir). Thus, the accumulation of fine sediment increased due to the influence of the weirs, thereby increasing the overall amount of metal contamination.

Processes and characteristics of hydrogeochemical variations between unconfined and confined aquifer systems: a case study of the Nakdong River Basin in Busan City, Korea.

This study is to assess the hydrogeochemical characteristics of groundwater at the deltaic region of the Nakdong River Basin in the Busan Metropolitan City of Korea. The study area is covered by the Quaternary sedimentary deposits and the Cretaceous granites associated with unconformity. The thick sedimentary deposits consists of two aquifers, i.e., unconfined and confined aquifers on the basis of clay deposit. Groundwater samples were collected from seven boreholes: two from unconfined aquifer and five from confined aquifer systems during the wet season of 2017 year. ORP and DO indicates that the groundwater of the unconfined aquifer exists in the oxidization condition and that of the confined aquifer pertains in the reduction condition. Piper's trilinear diagram shows CaSO4 type for groundwater of the unconfined aquifer, and NaCl type for that of the confined aquifer. Ionic concentrations of groundwater increase in the confined aquifer because of direct and reverse ion exchange processes. Carbonate weathering and evaporation are other mechanisms in the water-rock interaction. Saturation indices of dolomite and calcite are observed as oversaturated, while halite reveals undersaturation. Hierarchical cluster analysis (HCA) exhibits that cluster 1 and cluster 2 represents the properties of groundwater in unconfined and confined aquifers, respectively. Factor analysis shows that groundwater of the confined aquifer is much influenced by seawater, and includes heavy metals of iron and aluminum. Groundwater samples in unconfined and confined aquifers are located at the rock weathering and evaporation zones in the Gibbs diagram. Inverse geochemical modeling of PHREEQC code suggests that carbonate dissolution and ion exchange of major ions are the prevailing geochemical processes. This comprehensive research provides the distinguished hydrogeochemical characteristics of groundwater in confined and unconfined aquifer systems of the Nakdong River Basin in Busan City, Korea.

Limnobaculum parvum gen. nov., sp. nov., isolated from a freshwater lake.

A Gram-stain-negative, rod-shaped, facultatively anaerobic bacterium, designated as strain HYN0051T, was isolated from lake water. 16S rRNA gene sequence analyses indicated that the isolate shares the highest sequence similarity with the genus Pragia (97.1 %) of the family Enterobacteriaceae. Strain HYN0051T did not form a rigid clade with the genus Pragia in any of the phylogenetic trees, demonstrating the novel generic status of the isolate. The genome of strain HYN0051T (CP029185) is a single circular chromosome of 3.84 Mb. The major fatty acids detected were C14 : 0, C16 : 0, summed feature 2 (C12 : 0 aldehyde and/or unknown 10.928), summed feature 3 (C16 : 1ω6c and/or C16 : 1ω7c) and summed feature 8 (C18 : 1ω6c and/or C18 : 1ω7c). The isoprenoid quinones were Q-8, MK-8 and DMK-8. The major polar lipids were phosphatidylglycerol, phosphatidylethanolamine and one unidentified phospholipid. The genome size, G+C content and fatty acid profile were very similar to that of the genus Pragia, but the composition of isoprenoid quinones and polar lipids, and numerous phenotypic properties also supported the distinctiveness of the new isolate from the genus Pragia. Thus, based on phylogenetic and phenotypic data, a novel species of a new genus, Limnobaculum parvum gen. nov., sp. nov., is proposed. The type strain of Limnobaculum parvum is HYN0051T (=KACC 19186T=NBRC 112742T).

Phenylobacterium parvum sp. nov., isolated from lake water.

A Gram-stain-negative, rod-shaped and aerobic bacterium, designated HYN0004T, was isolated from lake water. The strain grew at 15-35 °C and pH 7.0-9.0 on R2A. The isoprenoid quinone was Q10 and major polar lipids were phosphatidylglycerol and one unidentified glycolipid. The genome was 2.83 Mb with a DNA G+C content of 69.9 mol%. 16S rRNA gene sequence analyses revealed that HYN0004T represented a member of the genus Phenylobacterium and shared sequence similarities with Phenylobacterium conjunctum (97.8 %), Phenylobacterium koreense (97.5 %), Phenylobacterium aquaticum (97.2 %), and Phenylobacteriumheamatophilum (97.0 %). In addition to the low sequence similarities, the phylogenetic tree shapes indicated that HYN0004Trepresents an independent species of this genus. The genomic and phenotypic properties, including small genome size, inability to carry out numerous enzymatic reactions and high ratio of C18 : 1ω6c and/or C18 : 1ω7c in fatty acids, verified the differentiation between HYN0004T and related species. Thus, we propose a novel species of the genus Phenylobacterium, named as Phenylobacterium parvum sp. nov. The type strain is HYN0004T (=KACC 19185T=NBRC 112736T).

Depositional environment classification based on environmental variables of South Korea's Nakdong River Estuary barrier-lagoon system.

Anthropogenic activities have altered the geomorphological and ecological conditions of the Nakdong River Estuary (NRE) dramatically over the last century. The objectives of this study were to classify NRE sub-environments and to identify their unique ecological functions. The first step in classification was the establishment of 14 a priori sub-environmental groups based on landscape factors. Surface sediments obtained for these groups were analyzed for factors related to grain size and organic matter. Based on the results, the NRE was divided into two primary estuarine environment divisions: (1) a mixed marine and terrestrial environment influenced primarily by land; and (2) a principally marine environment influenced primarily by the ocean. Using multivariate analysis, we subdivided these primary estuarine divisions into six sub-environments, including Sub-1 and -2 in the former and Sub-3, -4, -5, and -6 in the latter.

Algal Bloom Prediction Using Extreme Learning Machine Models at Artificial Weirs in the Nakdong River, Korea.

In this study, we design an intelligent model to predict chlorophyll-a concentration, which is the primary indicator of algal blooms, using extreme learning machine (ELM) models. Modeling algal blooms is important for environmental management and ecological risk assessment. For this purpose, the performance of the designed models was evaluated for four artificial weirs in the Nakdong River, Korea. The Nakdong River has harmful annual algal blooms that can affect health due to exposure to toxins. In contrast to conventional neural network (NN) that use backpropagation (BP) learning methods, ELMs are fast learning, feedforward neural networks that use least square estimates (LSE) for regression. The weights connecting the input layer to the hidden nodes are randomly assigned and are never updated. The dataset used in this study includes air temperature, rainfall, solar radiation, total nitrogen, total phosphorus, N/P ratio, and chlorophyll-a concentration, which were collected on a weekly basis from January 2013 to December 2016. Here, upstream chlorophyll-a concentration data was used in our ELM2 model to improve algal bloom prediction performance. In contrast, the ELM1 model only uses downstream chlorophyll-a concentration data. The experimental results revealed that the ELM2 model showed better performance in comparison to the ELM1 model. Furthermore, the ELM2 model showed good prediction and generalization performance compared to multiple linear regression (LR), conventional neural network with backpropagation (NN-BP), and adaptive neuro-fuzzy inference system (ANFIS).

Molecular Verification of Bloom-forming Aphanizomenon flos-aquae and Their Secondary Metabolites in the Nakdong River.

Aphanizomenon spp. have formed harmful cyanobacterial blooms in the Nakdong River during spring, autumn, and now in winter, and the expansion of blooming period and area, associated with the global warming is predicted. The genus Aphanizomenon has been described to produce harmful secondary metabolites such as off-flavors and cyanotoxins. Therefore, the production of harmful secondary metabolites from the Aphanizomenon blooms in the Nakdong River needs to be monitored to minimize the risk to both water quality and public health. Here, we sampled the cyanobacterial blooms in the Nakdong River and isolated ten Aphanizomenon strains, morphologically classified as Aphanizomenon flos-aquae Ralfs ex Bornet et Flahault 1888. Phylogenetic analysis using 16S rRNA and internal transcribed spacer (ITS) region nucleotide sequences confirmed this classification. We further verified the harmful secondary metabolites-producing potential of A. flos-aquae isolates and water samples containing cyanobacterial blooms using PCR with specific primer sets for genes involved in biosynthesis of off-flavor metabolites (geosmin) and toxins (microcystins, saxitoxins and cylindrospermopsins). It was confirmed that these metabolite biosynthesis genes were not identified in all isolates and water samples containing only Aphanizomenon spp. Thus, it is likely that there is a low potential for the production of off-flavor metabolites and cyanotoxins in Aphanizomenon blooms in the Nakdong River.

Physical and biological control of aragonite saturation in the coastal waters of southern South Korea under the influence of freshwater.

We investigated the aragonite saturation state (Ωarag) during all four seasons in a coastal region of southern Korea that receives considerable freshwater input. The surface Ωarag values were higher during productive seasons with enhanced freshwater influences, likely due to an increased net removal of dissolved inorganic carbon (DIC) from the water column (i.e., biological control). In addition, during the productive seasons, enhancement of Ωarag was observed with decreasing salinity within a linear mixing zone present between river-influenced surface and saltier bottom waters. DIC appeared to be effectively sequestered from the warmer, less salty surface water by downward flux of organic matter, but not significantly affected by the relatively DIC-rich, cooler and saltier bottom waters under strong stratification conditions during these seasons (i.e., physical control). Low phytoplankton productivity and seasonal breakdown of the stratification caused reduced saturation in other seasons and made the study area a weak sink for atmospheric CO2.