Phytoplankton dynamics and implications for eutrophication management in an urban river with a series of rubber dams

Smartphone as an alternative to measure chlorophyll-a concentration in small waterbodies

Monitoring chlorophyll-a concentrations (Chl-a, μg·L-1) in aquatic ecosystems has attracted much attention due to its direct link to harmful algal blooms. However, there has been a lack of a cost-effective method for measuring Chl-a in small waterbodies. Inspired by the increase of smartphone photography, a Smartphone-based convolutional neural networks (CNN) framework (SCCA) was developed to estimate Chl-a in Aquatic ecosystem. To evaluate the performance of SCCA, 238 paired records (a smartphone image with a 12-color background and a measured Chl-a value) were collected from diverse aquatic ecosystems (e.g., rivers, lakes and ponds) across China in 2023. Our performance-evaluation results revealed a NS and R2 value of 0.90 and 0.94 in Chl-a estimation, demonstrating a satisfactory (NS = 0.84, R2 = 0.86) model fit in lower Chl-a (<30 μg L-1) conditions. SCCA had involved a realtime-update method with hyperparameter optimization technology. In comparison with the existing methods of measuring Chl-a, SCCA provides a useful screening tool for cost-effective measurement of Chl-a and has the potential for being an algal bloom screening means in small waterbodies, using Huajin River as a case study, especially under limited resources for water measurement. Overall, we highlight that the SCCA can be potentially integrated into a smartphone application in the future to diverse waterbodies in environmental management.

In-lake water turnover time shapes the distribution pattern of phytoplankton communities in a river-connected floodplain lake

In floodplains, phytoplankton communities are mainly shaped by environmental heterogeneity, hydrological connectivity, and habitat diversity. However, it remains unclear how hydrological connectivity drives phytoplankton biodiversity in floodplain lakes. This study was carried out in the Dongting Lake connected to the Yangtze River to ascertain the response mechanisms of phytoplankton communities to different hydrological connectivity gradients. We quantified the hydrological connectivity between lake and river habitats using in-lake water turnover time, and identified its relationship with phytoplankton community structure. Changes in hydrological connectivity can lead to different hydrodynamic and environmental conditions, which have a direct or indirect impact on phytoplankton community structure in water environments. The results showed that spatiotemporal changes in the hydrological connectivity and water environment led to distinct spatial variation in phytoplankton community structure across the study area. α and β diversity showed a consistent change law with the change of turnover time, and the diversity index gradually increased with the decrease of hydrological connectivity, reaching the maximum value at the moderate hydrological connectivity, and then gradually decreasing. The peak of β diversity occurs earlier than the peak of α diversity during the decline of hydrological connectivity. This study demonstrates that in-lake water turnover time has a non-negligible impact on phytoplankton community distribution in river-connected lakes. Phytoplankton can maintain the highest α diversity and possibly β diversity under moderate hydrological connectivity, which is crucial for maintaining aquatic biodiversity in floodplain lakes.

Analysis of water quality and the response of phytoplankton in the low-temperature environment of Majiagou Urban River, China

Majiagou River, a crucial urban river in Harbin, traverses densely populated areas including agricultural, suburban, and main urban areas, presenting highly intricate habitat characteristics. In recent years, urbanization has significantly intensified human interference, fundamentally reshaping the phytoplankton community. Understanding the response mechanism of phytoplankton to environmental factors is of paramount importance as they serve as primary producers in aquatic ecosystems. To investigate this, we established 25 sampling sites to analyze the phytoplankton community and 14 key physicochemical parameters, such as total phosphorus (TP) and total nitrogen (TN). Utilizing hierarchical clustering analysis (HCA) and One-way Analysis of Variance (ANOVA), we identified distinct river segments, revealing spatial distribution differences and environmental factor variations among phytoplankton species across segments. By adopting redundancy analysis (RDA), we pinpointed the primary environmental factors impacting phytoplankton communities and examined the correlation between phytoplankton and these factors to elucidate the driving mechanisms governing phytoplankton dynamics. The outcomes demonstrated that the phytoplankton community in Majiagou River was predominantly composed of Bacillariophyta and Chlorophyta, however, notable disparities in spatial distribution and species composition resulting from human interference were evident. Areas with intense human disturbance were dominated by diatoms and exhibited trends of homogenization and reduced biodiversity. RDA showed that pH, NH4+-N, NH3-N, chemical oxygen demand (COD), and TP were key environmental factors influencing phytoplankton communities. We have confirmed that due to variations in environment conditions and different levels of human disturbance, there will be some differences in the critical limiting factors affecting phytoplankton. Our study offers valuable insights for governing urban rivers during the low-temperature period.

Damming exacerbates the discontinuities of phytoplankton in a subtropical river in China

Phytoplankton is sensitive to changes in river ecosystems. Increasing dams disrupt the continuity of river ecosystems. However, the spatial impacts of dams on phytoplankton have not been well documented. In this study, using multiple statistical analyses, the relationships between environmental drivers and phytoplankton community structures in natural background reaches, reservoirs, and corresponding post-dam reaches were explored in the Jiulong River with multiple cascaded dams, which encountered eutrophication and algal blooms in the past 15 years. Results illustrated that damming exacerbated longitudinal discontinuities of phytoplankton communities. The relative abundance of phytoplankton varied in three types of river sections. The average phytoplankton abundance in the reservoirs (1.62 × 105 cell·L-1) was higher than those in the natural background reaches (5.15 × 104 cell·L-1) and the corresponding downstream reaches (4.55 × 104 cell·L-1). The total β diversity ranged from 0.38 to 0.89 with an average of 0.64 and dominated by species replacement and least by species richness. The water environmental factors and hydraulic parameters rather than nutrients were more attributable to phytoplankton community variability in three river sections. These findings facilitate the management of rivers with multiple cascade dams by releasing environmental flows, jointly operating cascade hydropower stations, and developing nutrient reduction schemes to mitigate the negative impacts of damming in the river.

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.

Microbial community structure and denitrification responses to cascade low-head dams and their contribution to eutrophication in urban rivers

Low-head dams are one of the most common hydraulic facilities, yet they often fragment rivers, leading to profound changes in aquatic biodiversity and river eutrophication levels. Systematic assessments of river ecosystem structure and functions, and their contribution to eutrophication, are however lacking, especially for urban rivers where low-head dams prevail. In this study, we address this gap with a field survey on microbial community structure and ecosystem function, in combination with hydrological, environmental and ecological factors. Our findings revealed that microbial communities showed significant differences among the cascade impoundments, which may be due to the environment heterogeneity resulting from the cascade low-head dams. The alternating lentic-lotic flow environment created by the low-head dams caused nutrient accumulation in the cascade impoundments, enhancing environmental sorting and interspecific competition relationships, and thus possibly contributing to the reduction in sediment denitrification function. Decreased denitrification led to excessive accumulation of nutrients, which may have aggravated river eutrophication. In addition, structural equation model analysis showed that flow velocity may be the key controlling factor for river eutrophication. Therefore, in the construction of river flood control and water storage systems, the location, type and water storage capacity of low-head dams should be fully considered to optimize the hydrodynamic conditions of rivers. To summarize, our findings revealed the cumulative effects of cascade low-head dams in an urban river, and provided new insights into the trade-off between construction and decommissioning of low-head dams in urban river systems.

Estimating the contribution of environmental variables to water quality in the postrestoration littoral zones of Taihu Lake using the APCS-MLR model

Water quality monitoring is one of the most important aspects of postrestoration assessments because it affects water pollution control and the development of sustainable management strategies. However, a comprehensive understanding of potential water pollution and source apportionment in restoration projects is still lacking. In this study, the water quality variables of three restored national wetland parks with different cofferdam systems (i.e., an eco-layered cofferdam, a fully enclosed cofferdam, and open water) in the littoral zone of Taihu Lake were monitored monthly for three years (2019-2021). Hydrochemical and meteorological variables were used as auxiliary parameters for multivariate statistics, including principal component analysis (PCA) and absolute principal component score-multiple linear regression (APCS-MLR), to accurately estimate the source apportionment of the potential factors influencing the water environment. PCA extracted three or four potential sources, accounting for 64.71 %, 65.40 %, and 63.85 % of the total variance. The APCS-MLR results showed that wind direction and volatile suspended solids were the primary sources affecting water quality in open water, with a sum of the mean source contributions of 40.7 %. In fully enclosed cofferdam systems, the dire state of endogenous pollution was the greatest potential source affecting water quality, with a mean contribution of 41.2 %. The eco-layered cofferdam alleviated the contributions of suspended solids (mean contribution of 23.7 %) and nutrients in the water column (mean contribution of 30.8 %); however, the contribution of organic matter in the cofferdam was relatively high (mean contribution of 13.4 %). Based on these results, eco-layered cofferdams play a positive role in eutrophication control and ecological restoration in the littoral zone of large shallow lakes. Meanwhile, adding meteorological variables to assist hydrochemical variables in multivariate statistics may improve the accuracy and certainty of pollution source apportionment and support decision-makers in developing water quality protection and management strategies for postrestoration projects in littoral zones.

Stationary Probability Density Analysis for the Randomly Forced Phytoplankton–Zooplankton Model with Correlated Colored Noises

In this paper, we propose a stochastic phytoplankton–zooplankton model driven by correlated colored noises, which contains both anthropogenic and natural toxins. Using Khasminskii transformation and the stochastic averaging method, we first transform the original system into an Itô diffusion system. Afterwards, we derive the stationary probability density of the averaging amplitude equation by utilizing the corresponding Fokker–Planck–Kolmogorov equation. Then, the stability of the averaging amplitude is studied and the joint probability density of the original two-dimensional system is given. Finally, the theoretical results are verified by numerical simulations, and the effects of noise characteristics and toxins on system dynamics are further illustrated.