Increase In Water Transparency Research Articles

Effect of Climate and Socio-Economic Changes on the State of the Baltic Sea Ecosystems

In this study, calculations are performed to reproduce the main components of the Baltic Sea ecosystems based on the stationary, boxed SANBALTS model under various scenarios of climate change in combination with two scenarios of the development of socio-economic conditions. Under various warming scenarios, a decrease in the nutrient load will lead to a reduction in the area of hypoxic zones in the Baltic Sea, while the most significant decrease in the area is due to the scenario of a reduction in the agricultural sector and population. The phosphorus in the benthic layer will decrease, the concentration of inorganic nitrogen will increase, and nitrogen fixation will decrease. The most noticeable decrease in primary production and an increase in water transparency is associated with a moderate climate change scenario and with a significant decrease in the nutrient load. The results showed that under certain combinations of scenarios, an improvement in the ecological state of the sea can be expected even under the most unfavorable climatic scenario.

Breeding waterbird species as ecological indicators of shifts from turbid to clear water conditions in northwest European shallow eutrophic lakes

AbstractWe used biological and physical responses at 71 shallow waterbodies with contrasting nutrient levels undergoing recovery from eutrophication to predict potential changes in waterbird species abundance, an important component of lake ecosystems. These general predictions were tested using 28 years of breeding waterbird data from three Danish shallow eutrophic lakes, comparing species-specific responses to improved nutrient and water transparency in two lakes with a third where conditions remained constantly suitable for breeding waterbirds. We predicted positive responses to improved water quality from pursuit diving predators (three grebe species), a specialist zooplankton feeder (northern shoveler Anas clypeata) and waterbirds feeding on (common pochard Aythya ferina) or within (tufted duck A. fuligula) submerged macrophyte underwater canopies. These species were characterised by positive waterbird community composition changes (using Principal Components Analysis) associated with decreasing nutrient loading and increasing water transparency at two lakes, with no change in breeding waterbird community at the third. Secchi depth explained 73–95% of variance in both PC axes at both restored lakes, but not at the third, suggesting water transparency was the major factor driving waterbird community composition. These examples show predicting waterbird species-specific responses to management can usefully direct the use of breeding waterbirds as indicator species.

Application Research of Fish-Plant Symbiosis Technology in a Urban Lake: A Case Study of the Environment Remediation Project with Machine Learning Application

This research examines the Jiefang Park Water Environment Remediation Project, a pioneering initiative in urban lake rehabilitation. The project is anchored in a fish-plant symbiosis system, which has significantly improved water quality, upgrading it from Category V to Category III, and in certain zones, achieving the commendable Category II. This enhancement is quantitatively marked by a notable reduction in nitrogen and phosphorus levels, and a marked increase in water transparency, with submerged plants covering over 70% of the lakebed.A key innovation is the utilization of a machine learning model to predict chlorophyll-a concentration, a vital water quality metric. The model’s accuracy is underscored by its R² values, ranging from 0.23 to 0.99, and RMSE values between 6.921 and 0.237, with the best performance at point 1. Additionally, comprehensive measurement data highlight the project’s effectiveness. Water transparency significantly improved from baseline levels, as evidenced by increased dissolved oxygen levels at various sampling sites, indicative of restored ecological equilibrium in the lake. This study represents an exemplary fusion of ecological restoration and advanced data analytics, signifying a substantial advancement in urban lake restoration. It underscores the potential of combining synergistic ecological approaches with technological innovation to further sustainable urban environmental management.

Reduced genetic diversity and the success of the invasive peacock bass (Cichliformes: Cichlidae).

Several species of Cichla successfully colonized lakes and reservoirs of Brazil, since the 1960's, causing serious damage to local wildlife. In this study, 135 peacock bass were collected in a reservoir complex in order to identify if they represented a single dominant species or multiple ones, as several Cichla species have been reported in the basin. Specimens were identified by color pattern, morphometric and meristic data, and using mitochondrial markers COI, 16S rDNA and Control Region (CR). Overlapping morphological data and similar coloration patterns prevented their identification using the taxonomic keys to species identification available in the literature. However, Bayesian and maximum likelihood from sequencing data demonstrated the occurrence of a single species, Cichla kelberi. A single haplotype was observed for the 16S and CR, while three were detected for COI, with a dominant haplotype present in 98.5% of the samples. The extreme low diversity of the transplanted C. kelberi evidenced a limited number of founding maternal lineages. The success of this colonization seems to rely mainly on abiotic factors, such as increased water transparency of lentic environments that favor visual predators that along with the absence of predators, have made C. kelberi a successful invader of these reservoirs.

Environmental Factors Affecting the Efficiency of Water Reservoir Restoration Using Microbiological Biotechnology

Aquatic ecosystems are often subject to degradation due to various environmental stressors. The accumulation of an organic sediment layer causes shallowing, algal blooms, and hypertrophy in water reservoirs. The processes of overgrowth and shallowing lead to a reduction in the ecosystem services provided by the reservoir as well as potentially causing the disappearance of the water body. To address these challenges and restore the ecological balance of water reservoirs, effective and sustainable revitalisation methods are essential. In recent years, biotechnological approaches, particularly utilizing microbiological interventions, have emerged as promising strategies for water reservoir revitalization. Microorganisms, with their remarkable ability to degrade pollutants and enhance nutrient cycling, offer great potential in remediating environmental issues in a natural and eco-friendly manner. This article presents the results of a study of 33 Polish reservoirs subjected to reclamation with microbial biopreparations from 2014 to 2023. The results of changes in bottom sediment reduction, water transparency, dissolved oxygen concentration, and water turbidity are presented. Reduction in morphological changes in the fraction of soft organic sediments, an improvement in the oxygen profile of the bottom and surface water layers, and an increase in water transparency were observed after reclamation with the use of biopreparations.

Sequential Application of Different Types of Coagulants as an Innovative Method of Phosphorus Inactivation, on the Example of Lake Mielenko, Poland

The process of accelerated eutrophication forces the search for innovative, effective methods to restore the quality of surface waters. This study was conducted on shallow, urban Lake Mielenko (Maximum depth 1.9 m; Mean depth 1.3 m) in the context of implementing a new, sustainable method of lake restoration, i.e., phosphorus inactivation by sequential application of two types of coagulants. Approximately 9.9 tons of polyaluminium chloride (trade name PAX 18) were introduced into the profundal zone of Lake Mielenko, and 9.0 tons of iron chloride (trade name PIX 111) in the coastal area. The applications were divided into two spring and two autumn stages. Before restoration, the mean Pmin. concentration in Lake Mielenko water was 0.031 mg P/L, and TP was in the range of 0.091 to 0.346 mg P/L. After restoration, the average content of Pmin. was 0.007 mg P/L (a decrease of 80%), and the average value of TP was 0.096 mg P/L (a decrease of 72%). The obtained results indicate that phosphorus inactivation does not change nitrogen compounds’ content. However, due to the application of coagulants, P content decreased, i.e., the main factor limiting photosynthesis, which resulted in a significant decrease in primary production in Lake Mielenko. Before restoration, the average content of N-NH4 was 0.100 mg N/L, N-NO3—0.145 mg N/L, Norg. 1.70 mg N/L, and TN—1.86 mg N/L. After restoration, the average content of N-NH4 was 0.096 mg N/L, N-NO3—0.123 mg N/L, Norg. 1.28 mg N/L, and TN—1.50 mg N/L. This also resulted in a reduction in the Norg. content, as well as a reduction in the amount of chlorophyll a (from 30.51 to 13.41 mg/m3), organic compounds (BOD5 from 8.9 to 4.6 mg O2/L), and an increase in water transparency to the bottom (up to 1.45 m on average). The results obtained in Lake Mielenko indicate that the innovative method of phosphorus inactivation, which involves the sequential application of two types of phosphorus-binding preparations, is an excellent solution that ensures higher ecological safety in the coastal areas of the reservoir and also allows for a significant reduction in restoration costs.

Water turbidity dynamics using random forest in the Yangtze River Delta Region, China

Turbidity is a water quality indicator that is essential for the sustainable development of aquatic ecosystems and the protection of biodiversity. The turbidity of different water surfaces and its response mechanisms to regional climatic factors and human activities in the Yangtze River Delta Region (YRDR), an important rapid economic development region in China, remain poorly understood. To enhance the knowledge of turbidity variations and dominant drivers of YRDR water surfaces, a complete long-term turbidity series was obtained using Landsat images from 1990 to 2020. The results show that the turbidity trend differed from −1.3 NTU/yr to 0.7 NTU/yr in different water surfaces. Turbidity decreased significantly in the mainstream of the Yangtze River (MYR), aquaculture ponds (AP) and other water bodies, whilst increasing significantly in the medium lakes (ML) and mainstream of the Qiantang River (MQR). Meanwhile, no significant changes in turbidity were observed in the great lakes (GL) and small lakes (SL). Rather than climatic factors, urbanisation and decreasing wastewater discharge were the dominant drivers of turbidity trends during the study period. In addition, ecological engineering in AP increased water transparency. The Three Gorges Dam also decreased turbidity in MYR. Increasing turbidity in the downstream of MQR was driven by increasing seasonal water surfaces and reclamation projects near Hangzhou Bay. GL faced no significant increase in turbidity due to the offset of afforestation to urbanisation-induced turbidity increase. These findings provide important information for government decision-making for subsequent aquatic environmental protection and restoration in the YRDR.

Configuration of Submerged Plants for Plain River Networks Based on Transparency Improvement

The configuration of submerged plants plays an important role in the restoration of river networks. In this study, the Sudong River, a typical plain river network in southern China, was selected for submerged plant configuration research to improve water transparency. Analysis of historical and field research data of Sudong River showed that most of the water was in a state of eutrophication. The main factors affecting transparency were water temperature, ammonia nitrogen, chlorophyll a, total phosphorus, and permanganate index, which were identified through correlation analysis using SPSS. Submerged plants are important for water ecological restoration, and a plant selection evaluation system was constructed to screen six suitable local submerged plants in Suzhou for ecological restoration based on secondary indicators divided into five segments. Each plant was evaluated and scored. Finally, a combination of three submerged plants, Ceratophyllum demersum, Watermifoil, and Foxtail algae, was selected based on their scores. This combination effectively improved the eutrophication status of water and increased water transparency while adhering to the laws of submerged plant succession and facilitating the construction of a stable aquatic ecosystem.

Sustainable Restoration as a Tool for the Improvement of Water Quality in a Shallow, Hypertrophic Lake

Sustainable restoration treatments were implemented with the simultaneous application of pro-ecological methods that complement each other to improve water quality in the shallow and heavily polluted Raczyńskie Lake. Phosphorus inactivation with magnesium chloride and Phoslock® was introduced along with biomanipulation. Physico-chemical and biological parameters were studied in 2015 (before restoration) and throughout 2018 and 2019 (during restoration). Water quality improved in the first year of treatment. An increase in water transparency, oxygen concentration above the bottom, a decrease of chlorophyll-a concentration and a reduction in cyanobacteria were observed. In the second year of treatment, a slight deterioration of water quality was recorded, probably caused by fewer phosphorus inactivation treatments and a shortened period of application. However, the deterioration of conditions is also characteristic of sustainable restoration at the beginning of the restoration process. The obtained results showed that sustainable restoration requires more than two years, and its scope and intensity should strictly depend on the pace of changes, determined on the basis of monitoring. In addition, improved water quality will have a positive impact on the recreational use of this reservoir.

Fish eradication in freshwater ecosystems by repeated fishing

The removal of planktivorous fish results in ecosystem changes that increase water transparency and decrease the risk of harmful algal blooms. In many situations, complete eradication of fish is necessary to improve water quality, which can also protect natural populations. In this work, we construct a generic fish-zooplankton (FZ) model and we determine conditions to eradicate the fish population through repeated fish removal. The model accounts for the weight and age of fish, and is described by a system of ordinary differential equations with impulsive effects representing the reproduction process and fish removal. We demonstrate that the survival and extinction of fish can be determined from the roots of a polynomial, whose coefficients depend on maximal fecundity, mortality rates, and catching effort. If all roots of the polynomial lie within the unit circle, then eradication is ensured. To illustrate our results, we show the importance of removing young fish, and not only adults, to achieve eradication.

Исследования влияния расхода воды на протяжённость нерестовой миграции байкальского омуля Coregonus migratorius (Coregonidae) в реке Селенге на базе многолетних данных

The Selenga River, the main tributary of Lake Baikal, is 1,453 km long and accounts for 50 % of its surface runoff. It is a large foothill, free-flowing, unregulated watercourse flowing through Mongolia (2/3 of the length) and Russia (1/3). Global climatic changes in recent decades have been particularly evident in the Baikal region: since the mid-1990s, there has been a significant decrease in water and sediment runoff in the Selenga basin. Omul spends most of its life cycle in the lake; spawning takes place in the river system. In total, there are 22 spawning rivers of Baikal omul in the Lake Baikal basin, among which 47 % of its spawning grounds are concentrated in Selenga. Omul spawns in the Selenga channel within 20–580 km upstream of the lake, and in some years it crosses the border between Russia and Mongolia. Based on long-term (1936–2018) data of the Baikal omul spawning migration in the Selenga River, the relationship between the average migration distance of the spawning herd core and the water discharge at the beginning of the spawning migration (September) was analyzed. Statistical analysis has established an approximate threshold for water discharge, which marks the shortening of the spawning path in «high-water» years and a noticeable extension of it upstream in “low-water” years. The role of other cumulative factors affecting the spawning distance of omul is discussed. Under conditions of a long subglacial period, omul eggs are successfully incubated in large areas for five to six months (November–April). Stability of habitat characteristics in such a shelter is ensured by a significant decrease in flow velocity and temperature, as well as increased water transparency and a relatively favorable oxygen regime. Ice cover formation can also be attributed to protective factors: it reliably shelters eggs from external atmospheric impact: average ice thickness on the river during the study period was 0,5 m (0,1–1,5 m, maximum by spring it often reaches 2 m near the shores). Hydraulic construction in the upper Selenga Watershed in Mongolia could have a significant negative impact on the spawning grounds of the Baikal omul, reducing the survival of its eggs during the winter months. Such impacts, combined with global warming and other anthropogenic disturbances, may even lead to the extinction of this cold-loving endemic of the Selenga Basin, which will undoubtedly affect the entire lake ecosystem.

Effects of two submerged macrophyte species on microbes and metazoans in rooftop water-storage ponds with different labile carbon loadings

Nature-based solutions including rooftop-water storage ponds are increasingly adopted in cities as new eco-designs to address climate change issues, such as water scarcity and storm-water runoff. Macrophytes may be valuable additions for treating stored rooftop waters and provisioning other services, including aquaponics, esthetic and wildlife-conservation values. However, the efficacy of macrophyte treatments has not been tested with influxes of different labile carbon loadings such as those occurring in storms. Moreover, little is known about how macrophytes affect communities of metazoans and microbes, including protozoans, which are key players in the water-treatment process. Here, we experimentally investigated the effectiveness of two widely distributed macrophytes, Ceratophyllum demersum and Egeria densa, for treating drained rooftop water fed with two types of leaf litter, namely Quercus robur (high C lability) and Quercus rubra (low C lability). C. demersum was better than E. densa at reducing water conductivity (by 10 ̶ 40 µS/cm), TDS (by 10–18 mg/L), DOC (by 4–5 mg/L) and at increasing water transparency (by 4–9%), water O2 levels (by 19–27%) and daylight pH (by 0.9–1.3) compared to leaf-litter only microcosms after 30 days. Each treatment developed a different community of algae, protozoa and metazoa. Greater plant mass and epiphytic chlorophyll-a suggested that C. demersum was better at providing supporting habitat than E. densa. The two macrophytes did not differ in detritus accumulation, but E. densa was more prone to develop filamentous bacteria, which cause sludge bulking in water-treatment systems. Our study highlights the superior capacity of C. demersum and the usefulness of whole-ecosystem experiments in choosing the most adequate macrophyte species for nature-based engineered solutions.

Diversity Variation of Silica-Scaled Chrysophytes Related to Differences in Physicochemical Variables in Estuaries of Rivers in an Arctic Watershed

The present study examined the taxonomic diversity of silica-scaled chrysophytes in the estuaries of the Arctic watershed of Yakutia in the context of global climate change, as these aquatic organisms are highly sensitive to environmental changes. Previously, 41 species of silica-scaled chrysophytes were recorded in the waters of Yakutia. In the present study, we supplemented this list with 55 species. We observed a high species richness (82 taxa) of silica-scaled chrysophytes in the study area. Of these, eight species were recorded in the waters of Russia for the first time. At present, the study area represents the northernmost habitat for most species observed during the study. The diversity of silica-scaled chrysophytes in Arctic rivers is comparable to or even greater than the diversity of chrysophycean flora at the middle latitudes. Most of these chrysophytes are represented by polyzonal and ubiquitous species. During 2008–2010, we noted increased relative numbers of boreal species in northern waters compared with data obtained during the previous 30 years. Overall, the species richness of silica-scaled chrysophytes in the studied rivers increased with increase in water transparency, temperature, and magnesium ion concentration, but decreased with increase in surfactant concentration. The origin of samples from a certain river; distance of the sampling site from the estuary; water temperature, transparency, and colour; and concentration of carbon dioxide, magnesium ions, total iron, surfactants, and oil products affect the species composition of silica-scaled chrysophytes in the study area. Our findings are fundamental to evaluate the current status of Arctic aquatic microflora and its further monitoring in the context of anthropogenic and climatic impacts.

Restoration of aquatic macrophytes with the seed bank is difficult in lakes with reservoir-like water-level fluctuations: A case study of Chaohu Lake in China

Aquatic vegetation in lakes along the middle and lower reaches of the Yangtze River has been seriously degraded by human disturbances such as river–lake disconnection and water eutrophication. Chaohu Lake is a typical lake with reservoir-like water-level fluctuations (WLFs). Since a sluice was built in 1962, the coverage of aquatic vegetation in Chaohu Lake has been very low (0.77%). In this study, field investigations of the aquatic vegetation and seed bank of Chaohu Lake were conducted, and aboveground vegetation diversity was found to be low. Forty-eight species were recorded, of which submerged, floating-leaved, and emergent macrophytes were 1, 5, and 10, respectively. Currently, artificially planted Phragmites australis and Salix communities have become the main community types. A total of 18 aquatic macrophytes were identified in the seed bank, of which the number of submerged, floating-leaved, and emergent macrophytes and hygrophytes were 2, 2, 3, and 11, respectively; the seed density was 2.05, 2.05, 16.93, and 9.30 ind./m2, respectively. The seed density of aquatic macrophytes was much lower in Chaohu Lake than in the lakes with quasi-natural WLFs, and the seeds were mainly distributed in the estuary area. Only two emergent macrophyte species (Typha) germinated in the open water area. Spearman correlation analysis showed that the distance to bank was the main environmental factor that affected aquatic macrophyte diversity and seed density in the seed bank. It is not feasible to promote the natural reconstruction and restoration of aquatic macrophytes in Chaohu Lake by decreasing the water level or increasing water transparency, but the area near the estuary can be identified as a priority restoration area (PRA) for aquatic macrophytes. Artificial transformation of the micro-topography can be used to guide some hydrochorous seeds in the tributaries to the PRA and restore aquatic vegetation in the local area.

Response of the phytoplankton community to seasonal and spatial environmental conditions in the Haldia port ecosystem located in the tropical Hooghly River estuary.

The phytoplankton community structure exhibits seasonal and spatial variations in response to the environmental conditions, which aids in understanding the ecosystem's health. Given this, four samplings were conducted between October 2013 to April 2015, encompassing the monsoon, post-monsoon, and pre-monsoon seasons, from the Haldia port ecosystem of India. The samples were collected from the flowing estuary and an extended semi-enclosed dock. We hypothesized that the seasonal phytoplankton community (diversity, abundance, and carbon biomass) response will differ based on the environmental and hydrographical characteristics of the study site. Picophytoplankton and nano-microphytoplankton dominated the phytoplankton community in terms of numbers and biomass, respectively. Bacillariophytes dominated the nano-microphytoplankton abundance and total biomass, except during the monsoon when Dinophytes contributed (inner-zone). The dominance of Bacillariophytes and Chlorophytes in the outer-zone with picophytoplankton and Dinophytes in the inner-zone indicated group-specific hydrographic preferences that supported the hypothesis. The positive correlation of the picophytoplankton abundance (pre-monsoon) with Secchi disc depth and the negative correlation of diatoms (post-monsoon) with temperature signify the seasonal role of light and temperature, respectively. The highest nano-microphytoplankton species diversity (Shannon-Wiener's index) during the pre-monsoon (inner-zone) with more rare species indicated the probable influence of stable waters with increased water transparency. However, the community was unevenly distributed in the estuary due to the high abundance of the diatom, Aulacoseira granulata. Although harmful algal blooms were not detected, the higher temperature and nutrient concentrations could have favoured potentially harmful species (Pseudonitzschia delicatissima, Dinophysis acuta) during the monsoon. The system ranged from oligo- to mesotrophic state with moderate pollution levels (Carlson's Trophic State Index and Shannon's Index), indicating a reduction of the nutrient accumulation effects by the existing water renewal frequency. This study recommends incorporating qualitative and quantitative phytoplankton assessment in ecological monitoring of the stable coastal sites to prevent future harmful algal episodes.

Ecological engineering in a eutrophic lake: A case study of large aquatic macrophyte enclosures in Baima Lake, China

Ecological restoration of eutrophic lakes using aquatic macrophytes is an important and practical technology. Here, we investigated the response of phytoplankton and zooplankton to a large-scale 2015-built aquatic macrophyte enclosure (AME, 200,000 m2) screened of by a PVC net in Baima Lake, a eutrophic lake, from spring to autumn of 2019. AME significantly improved water quality by increasing water transparency, and reducing total nitrogen, total phosphorus, and chlorophyll-a content during the growing season. AME significantly decreased phytoplankton abundance and biomass and marginally increased zooplankton abundance and biomass. Phytoplankton and zooplankton communities were closely related to environmental factors, such as water temperature, conductivity, total phosphorus, chemical oxygen demand, and chlorophyll-a inside and outside the AME. The zooplankton:phytoplankton biomass ratio inside was slightly higher than outside the AME. Zooplankton and phytoplankton biomass were significantly positively correlated inside and outside the AME, as were chlorophyll-a and total phosphorus. We found phosphorus to be a key factor limiting primary productivity in Baima Lake, and that bottom-up effects were the main driver to control phytoplankton in the AME. Using aquatic macrophytes to reduce nutrient loads is an effective way to manage eutrophication in Baima Lake.

Changes of phosphorus delivery from Yangtze River to Dongting Lake under new water and sediment conditions

Impounding the Three Gorges Reservoir (TGR) has altered the water-sediment transportation regime in the Yangtze River (YR) downstream, with inevitable effects on phosphorus (P) transport behavior in river-connected lakes. In this study, we investigate the characteristics of water, suspended sediment (SS), and P transportation, adsorption, and bioavailability at the three inlets along YR diverted to Dongting Lake through field surveys, adsorption experiments, and historical data analysis. The results showed that P transportation was significantly correlated with sediment (p < 0.01) and fine sediment particles dominated the transportation of particulate phosphorus (PP) and bioavailable phosphorus (Bio-P). Before the TGR impoundment, the dissolved inorganic phosphorus (DIP) concentration (0.03 mg/L) at the three inlets maintained a low level despite high P input, which was mainly due to the P adsorption and removal effect by sediment with high concentration and fine-grained texture. In addition, we reveal two changes at the three inlets caused by the impoundment: (1) SS content and discharge were reduced by an order of magnitude and the particle size was coarsened by three times; (2) the PP flux and bioavailability significantly decreased, whereas the transportation proportion of DIP increased. Consequently, the P buffering and adjusting capacity by sediment in the pristine river ecosystem from dam upstream supplies and downstream resuspension were notably weakened. Currently, the SS content at the three inlets (0.07 g/L) is far below the lower critical threshold (0.20 g/L) obtained through experiments that can effectively remove phosphate. Additionally, the decline of water and sediment supply from YR to Dongting Lake increased water transparency and prolonged water retention time in lake. With continuous TGR regulation and construction of upstream cascade reservoirs, the risk of algal blooms in river-connected lakes is significantly exacerbated.

Survey on the roles and impacts of seaweeds and aquatic plants in the improved extensive black tiger shrimp farms in Bac Lieu province

A survey was conducted to assess the roles and impacts of seaweeds and aquatic plants on the production and financial efficiency of black tiger shrimp in improved extensive farms through direct interviews with 60 farmers from Dong Hai district, Bac Lieu province. The householders were divided into two groups based on the presence of natural seaweed and aquatic plant, including group 1: the presence of different seaweeds and aquatic plants but the absence of red seaweed (mixed vegetation) and group 2: the presence of red seaweed dominates over the mixed vegetation. The surveyed results showed that group 2 had an average shrimp yield of 268.6 kg/ha/year with a profit of 58.4 million VND/ha/year, which was higher than that of group 1 (shrimp yield of 203.1 kg/ha/year and profit of 43.0 million VND/ha/year). Almost all farmers agreed that seaweeds and aquatic plants acted as important roles in improving water quality, providing natural food and shelter for shrimp, crab, and fish. Farmers, on the other hand, claimed that their excessive growth (high biomass) increased water transparency, limited living space, and hampered the activities of shrimp, crab, and other organisms, as well as reduced shrimp growth and production and polluted water after seaweed died off. Notably, the farm that maintained vegetation coverage ≤50% obtained higher shrimp production and profit than pond with coverage &gt;50%. Moreover, 46.7% of farmers preferred red seaweed over other seaweed and aquatic plant species in their farms.

Long-Term Oxbow Lake Trophic State under Agricultural Best Management Practices

A key principle of agricultural best management practices (BMPs) is to improve water quality by reducing agricultural-sourced nutrients and associated eutrophication. Long-term (1998–2016) lake summer trophic state index (TSI) trends of an agricultural watershed with agricultural best management practices (BMPs) were assessed. Structural BMPs included vegetative buffers, conservation tillage, conservation reserve, a constructed wetland, and a sediment retention pond. TSI included Secchi visibility (SD), chlorophyll a (Chl), total phosphorus (TP), and total nitrogen (TN). Summer TSI 1977 was &gt;80 in 1998–1999 (hypertrophic) and decreased over the first 10 years to TSI 1977 ≈ 75 (eutrophic). TSI 1977 decrease and changing TSI deviations coincided with vegetative buffers, conservation tillage, and conservation reserve. The TSI(SD) decrease (&gt;90 to &lt;70) coincided with vegetative buffers and TSI(TP) decrease (&gt;90 to &lt;75) coincided primarily with conservation tillage and the sediment retention pond. TSI(Chl) increase (&lt;60 to &gt;70) coincided with conservation tillage and vegetative buffer. Results indicate watershed-wide BMPs can modestly decrease summer trophic state through increased water transparency and decreased TP, but these changes are off-set by increases in chlorophyll a to reach a new stable state within a decade. Future research should assess algal nutrient thresholds, internal nutrient loading, and climate change effects.

Niche differentiation between a native and an invasive species of submersed macrophyte in a subtropical reservoir

Submersed macrophytes have important ecological roles but non-native invasive species may affect biodiversity and water uses. We investigated the native macrophyte Egeria najas and the invasive Hydrilla verticillata and measured their maximum colonization depth and its relationship with Secchi disk depth, their biomass along the depth gradient and their preferred depths of occurrence. The Itaipu Reservoir was monitored for seven years, during which maximum colonization depth and Secchi disk depth were measured. During a separate sampling, plants were collected to determine biomass along the depth gradient. Ancova showed that the maximum colonization depth of both species increased with increasing Secchi disk depth, but the maximum colonization depth of H. verticillata increased faster with increasing water transparency than did that of E. najas. Quadratic regression revealed that the biomass of each species peaks at intermediate depths. Hydrilla verticillata colonizes deeper regions than does E. najas. The patterns found in the present study can be explained by underwater light and, probably, wave disturbances. The preference of H. verticillata for deeper sites indicates that the ecological niches of the two macrophytes differ, and that H. verticillata has great potential to spread and accumulate biomass in reservoirs.