Automatic area estimation of algal blooms in water bodies from UAV images using texture analysis

Algal blooms,the spread of algae on the surface of water bodies,have adverse effects not only on aquatic ecosystems but also on human life.The adverse effects of harmful algal blooms(HABs)necessitate a convenient solution for detection and monitoring.Unmanned aerial vehicles(UAVs)have recently emerged as a tool for algal bloom detection,efficiently providing on-demand images at high spatiotemporal resolutions.This study developed an image processing method for algal bloom area estimation from the aerial images(obtained from the internet)captured using UAVs.As a remote sensing method of HAB detection,analysis,and monitoring,a combination of histogram and texture analyses was used to efficiently estimate the area of HABs.Statistical features like entropy(using the Kullback-Leibler method)were emphasized with the aid of a gray-level co-occurrence matrix.The results showed that the orthogonal images demonstrated fewer errors,and the morphological filter best detected algal blooms in real time,with a precision of 80%.This study provided efficient image processing approaches using on-board UAVs for HAB monitoring.

Impacts of algal blooms on sinking carbon flux and hypoxia off the Changjiang River estuary

Based on 10 multidisciplinary investigations conducted from February 2015 to January 2016,the phytoplankton community and its association with ambient seawater physicochemical parameters in the Changjiang(Yangtze)River estuary(CE)and its adjacent waters were comprehensively examined.In total,265 taxa were identified,belonging to 5 phyla and 94 genera.Diatoms(63.78%)and dinoflagellates(33.21%)were the dominant groups.The variation of diatom abundance showed a positive relationship with the nutrient concentrations while the dinoflagellate abundance showed a negative relationship.Two algal bloom events occurred during the investigation period.The Changjiang Diluted Water(CDW)induced environmental gradients in the upper layer,favoring the diatom bloom in July.The invasion of the nearshore Kuroshio branch current could affect the formation of a bloom of Prorocentrum donghaiense.With the blooming and senescence of phytoplankton,low dissolved oxygen(DO)and hypoxia occurred in the bottom waters.The bottom DO concentration displayed a significantly negative correlation with phytoplankton carbon flux.The present study provides straightforward evidence for the source of organic matter for oxygen consumption in the CE and its adjacent waters.

Monitoring cyanobacteria-dominant algal blooms in eutrophicated Taihu Lake in China with synthetic aperture radar images

Monitoring algal blooms by optical remote sensing is limited by cloud cover.In this study,synthetic aperture radar(SAR) was deployed with the aim of monitoring cyanobacteria-dominant algal blooms in Taihu Lake in cloudy weather.The study shows that dark regions in the SAR images caused by cyanobacterial blooms damped the microwave backscatter of the lake surface and were consistent with the regions of algal blooms in quasi-synchronous optical images,confirming the applicability of SAR for detection of surface blooms.Low backscatter may also be associated with other factors such as low wind speeds,resulting in interference when monitoring algal blooms using SAR data alone.After feature extraction and selection,the dark regions were classified by the support vector machine method with an overall accuracy of 67.74%.SAR can provide a reference point for monitoring cyanobacterial blooms in the lake,particularly when weather is not suitable for optical remote sensing.Multi-polarization and multi-band SAR can be considered for use in the future to obtain more accurate information regarding algal blooms from SAR data.

Temporal and spatial characteristics of harmful algal blooms in Qingdao Waters,China

Qingdao waters,including both the semi-enclosed Jiaozhou Bay(JB) and the adjacent water out of JB(OJB),have been the areas that are most frequently affected by harmful algal blooms(HABs) in the western Yellow Sea(west of 124°E).In this research,HAB occurrences in Qingdao waters from 1990 to 2009 were investigated using spatial tools in geographic information system(GIS) and are discussed in terms of their connection to temporal variation.Additionally,the effects of each HAB occurrence were further evaluated using a simple model.The calculated results were then visualized using a GIS software to indicate the effects of HABs in Qingdao waters during the entire period.As a result,the OJB was proven to be responsible for the frequent HAB occurrences in Qingdao waters after 2000,although JB was traditionally believed to be the principle source of HAB occurrences in Qingdao waters.In addition,increasing nitrogen and N/P structure imbalance were essential for increasing HAB occurrences in Qingdao waters throughout the entire period,especially for the recent HAB occurrences in the OJB.The results of this research would improve the current understanding on HAB occurrences in Qingdao waters,which would benefit HAB monitoring and the implementation of a control strategy in China as well.

Sustainable Utilization of Agricultural Straw for Harmful Algal Blooms Control:A Review

The use of agricultural straw for algal bloom control has been studied for more than 30 years.In this article,we have reviewed the promising potentials of using agricultural straw as source of anti-algal agents,including the effectiveness of each major straw type so far used in this regard,and the investigated algal species.Various pre-treatment methods have also been widely reviewed.Significant progress has been made in natural product chemistry and molecular biology with regards to agricultural straw,especially in relation to the extraction of antialgal allelochemicals,degradation processes of agricultural straws and the mechanisms through which these inhibitions occur.The development of biotechnologies using agricultural straw to successfully inhibit growth of bloom forming algae has been generally accepted as environmentally friendly.The current research status and that of the future should include isolation and discovery of antialgal allelochemicals,development of models that would illustrate the sequence of physiologic events that match the species-specific inhibitor phenomenon,and products fit in the field applications.

The impact of the accumulation of algal blooms on reed wetlands in the littoral zones of Chaohu Lake

In a large eutrophic lake,the littoral zone is normally an area with high-density elevated aquatic plant life,including algal blooms,where the presence of reed wetlands allows the accumulation of algae.In this study,the impact of accumulated algal blooms in reed wetlands in the littoral zone s of Chaohu Lake was investigated seasonally from 2018 to 2019.The concentrations of chlorophyll a(Chl a),total nitrogen(TN),and total phosphorus(TP)were much higher in the reed-covered littoral zones(RCLZ)than in the unvegetated littoral zones(ULZ),indicating that more algal biomass was trapped and accumulated in the RCLZ.Algal biomass could be horizontally transported to downwind littoral zones under low wind speeds,favoring the establishment of blooms.Algal accumulation levels were highest in summer due to high water temperatures and algal biomasses.Likewise,the northern littoral zones were conducive to the development of large algal blooms because of the wind pattern.The values of TN,TP,Chl a,and loss on ignition in surface sediments were higher in the RCLZ than in the ULZ.Moreover,the diffusive fluxes of ammonium and soluble reactive pho sphorus were also higher in the RCLZ than in the ULZ.Considering the capability of reed wetlands to trap algae,mechanical salvage and other physical methods should be adopted to eliminate algal biomass when massive blooms accumulate in the RCLZ.

Does reduced sediment load contribute to increased outbreaks of harmful algal blooms off the Changjiang Estuary?

Harmful algal blooms（HABs） have been increasingly frequent in coastal waters around the world over the last several decades. Accelerated coastal eutrophication, resulting from the increased anthropogenic loadings of nutrients, is commonly assumed to be the primary cause of this increase. However, although important,accelerated coastal eutrophication may not be the only explanation for the increasing blooms or toxic outbreaks in estuarine waters. Changes in riverine material fluxes other than nutrients, such as sediment load, may significantly affect biological activities and HAB incidence in estuarine and coastal waters. A case study off the Changjiang（Yangtze River） Estuary indicated that with the increasing riverine loadings of nutrients, the sediment load from the Changjiang River has been reduced by 70% over the past four decades. A comparison of long-term data revealed that the phytoplankton biomass maximum has expanded to a region of much lower salinity due to the drastic reduction in riverine sediment load and the subsequent improvement in light penetration in the Changjiang River plume. Furthermore, there was an apparent mirror-image relationship between the sediment load from the Changjiang River and the HAB incidence off the Changjiang Estuary over the past four decades, and the number of HAB incidents was significantly negatively correlated with the sediment load. Therefore, it is argued that the drastic decline in sediment load from the Changjiang River reduced turbidity in the Changjiang Estuary and thus contributed to the increased frequency of HABs in the buoyant discharge plumes.

Increased diversity and environmental threat of harmful algal blooms in the Southern Yellow Sea,China

Harmful algal blooms(HABs)in the Southern Yellow Sea(SYS)have shown a trend of increasing diversity and detrimental ef fects.While the Bohai Sea,East China Sea,and South China Sea have experienced a high incidence of HABs since the 1980s,the Yellow Sea provides a relatively healthy ecological environment in which fewer HABs have been documented before the 21s t century.Yet largescale blooms of the green macroalga Ulva prolifera(so-called“green tides”)have occurred annually since 2007 in the Yellow Sea.Six people were poisoned and one person died in Lianyungang in 2008 due to ingestion of algal toxins.Moreover,the Yellow Sea experienced co-occurrence of harmful red tides,green tides,and golden tides in 2017.This combination of events,rare worldwide,indicates the potential for further deterioration of the marine environment in the Yellow Sea,which may be related to climate change,aquaculture,and other human activities.Using the SYS as an example,we collected data of the frequency and scale of HABs over the years,as well as that of marine algal toxins,and analyzed the trend in the diversity of HABs in the SYS,to explore the causes and impacts of HABs,as well as the interrelationships among dif ferent types of HABs,including harmful red tides,green tides,and golden tides.We also attempted to improve our understanding of HAB evolution under the influence of global climate change and intensified human activities.

Differentiation of Algal Blooms and Aquatic Vegetation in Chinese Lakes Using Modified Vegetation Presence Frequency Index Method

Algal blooms in lakes have become a common global environmental problem. Nowadays, remote sensing is widely used to monitor algal blooms in lakes due to the macroscopic, fast, real-time characteristics. However, it is often difficult to distinguish between algal blooms and aquatic vegetation due to their similar spectral characteristics. In this paper, we used modified vegetation presence frequency index(VPF) based on Moderate-resolution Imaging Spectroradiometer(MODIS) imagery to distinguish algal blooms from aquatic vegetation, and analyzed the spatial and temporal variations of algal blooms and aquatic vegetation from a phenological perspective for five large natural lakes with frequent algal bloom outbreaks in China from 2019 to 2020. We simplified the VPF method to make it with a higher spatial transferability so that it could be applied to other lakes in different climatic zones. Through accuracy validation, we found that the modified VPF method can effectively distinguish between algal blooms and aquatic vegetation, and the results vary from lake to lake. The highest accuracy of 97% was achieved in Hulun Lake, where the frequency of algal outbreaks is low and the extent of aquatic vegetation is stable, while the lowest accuracy of 76% was achieved in Dianchi Lake, which is rainy in summer and the lake is small. Analyses suggests that the time period when algal blooms occur most frequently might not coincide with that when they have the largest area. However, in most cases these two are close in terms of time period. The modified VPF method has a broad scope of application, is easy to implement, and has a high practical value. Furthermore, the method could be established using only a small amount of measured data, which is useful for water quality monitoring on large spatial scales.

Overview of harmful algal blooms(red tides)in Hong Kong during 1975–2021

Hong Kong has a long historical record of harmful algal blooms(HABs).In the 1980s-1990s,HABs were mainly pollution-related and most of the events happened in estuaries and enclosed embayment such as Tolo Harbour and Port Shelter.The major cause of HABs in 1980s-1990s was closely related to nutrients enrichment;included but not limited to changes in the concentration and ratio of soluble N and P in seawater.The major causative organisms of HABs in Hong Kong by then were meso-plankton such as selected species of diatoms and dinofl agellates.Sometimes,zooplankton was also a cause of red tides(the common name of HABs).There has been gradual change after the 2000s.It is attributed to higher and higher influences from the region,namely the Zhujiang(Pearl)River delta.There are increasing influences from Chinese mainland due to extraordinary social and economic growths during the past 20 years.In the past10 years,HABs in Hong Kong was mainly subject to regional impacts in the Zhujiang River delta.Both the duration period and covered areas have been enlarged which overwhelmed the localized influences of stream pollution and self-contamination of aquaculture zones.More flagellates of uncommon happening become dominant species in HAB now.Nevertheless,the seasonal impact of Noctiluca scintillans in late winter to mid spring remained.

A numerical model study on multi-species harmful algal blooms coupled with background ecological fields

Based on systematized physical, chemical, and biological modules, a multi-species harmful algal bloom （HAB） model coupled with background ecological fields was established. This model schematically embod-ied that HAB causative algal species and the background ecological system, quantified as total biomass, were significantly different in terms of the chemical and biological processes during a HAB while the inter-action between the two was present. The model also included a competition and interaction mechanism between the HAB algal species or populations. The Droop equation was optimized by considering tempera-ture, salinity, and suspended material impact factors in the parameterization of algal growth rate with the nutrient threshold. Two HAB processes in the springs of 2004 and 2005 were simulated using this model. Both simulation results showed consistent trends with corresponding HAB processes observed in the East China Sea, which indicated the rationality of the model. This study made certain progress in modeling HABs, which has great application potential for HAB diagnosis, prediction, and prevention.

Cladophora as ecological engineer:A new test from the largest lake of Qinghai-Tibet plateau with filamentous algal blooms

Filamentous algae blooms(FABs)have been increasing globally in recent years,and their presence can have both harmful and beneficial effects on aquatic ecosystems.As one of the most common FABs,Cladophora blooms have been reported in the lakes of the Qinghai-Tibet Plateau during the past few years.However,there have been few studies focused on how FABs impact other aquatic organisms,especially in alpine lakes since these are at the forefront of responding to global climate change.In this study,the phytoplankton communities in different regions of Qinghai Lake were profiled in different seasons using meta-barcode sequencing.The phytoplankton assemblages in areas with Cladophora blooms were compared to those without Cladophora.The phytoplankton community structure correlated with physicochemical properties including water temperature,electrical conductivity,nitrate,and the presence or absence of Cladophora blooms.The relative abundance of Bacillariophytes was found to be higher in zones with Cladophora blooms than in other regions.Significant seasonal changes in phytoplankton biomass andβdiversity were observed in zones with Cladophora blooms.Growth and microbial degradation of Cladophora can change the pH,dissolved oxygen,secchi depth,and nitrate.Together with seasonal temperature and electrical conductivity changes,Cladophora growth can significantly impact the phytoplankton biomass,community dissimilarity and assembly process.These results showed that Cladophora plays a key role in littoral aquatic ecosystem ecology.

Variations of sediment organic phosphorus and organic carbon during the outbreak and decline of algal blooms in Lake Taihu,China

In this study,sediment organic phosphorus(OP)and organic carbon(OC)in Lake Taihu,China,as well as their relationships,were analyzed during the outbreak and decline of algal blooms(ABs)over a five-month field study.The results showed synchronous temporal changes in the sediment OP and OC contents with the development of ABs.In addition,there was a significant positive correlation between the sediment OP and OC(p<0.01),suggesting simultaneous deposition and consumption during the ABs outbreak.The sediment OP and OC contents decreased significantly at the early and last stages of the ABs outbreak and increased at the peak of the ABs outbreak and during the ABs decline.These temporal variation patterns suggest that the sediment OC and OP contents did not consistently increase during the ABs outbreak,even though algae are an important source of organic matter in sediments.The depletion or enrichment of OC and OP in sediments may also depend on the scale of the ABs outbreak.The obtained results revealed significant differences in the sediment OC and OP contents between the months(p<0.05).In addition,OP in the sediments was dominated by orthophosphate diester(phospholipids and DNA-P)and orthophosphate monoester during the ABs outbreak and decline,respectively.The active OC contents and proportions in the sediments in the ABs outbreak were significantly lower than those observed in the ABs decline period,demonstrating the significant impacts of the ABs outbreak and decline on the sediment OC and OP in Lake Taihu.

Snow algal blooms in Antarctic King George Island in 2017-2022 and their future trend based on CMIP6 projection

Snow algal blooms have a remarkable climatic or environmental effect through influencing the snow-albedo feedback,accelerating the melting of surface snow,and amplifying global warming.Snow algal blooms occurred frequently on King George Island,Antarctic,during the recent six austral summers(December to next February)through 2017-2022.Based on an assessment of satellite images,this study found that the range and amount of snow algal blooms in the summers of 2018,2020,2021 and 2022 are relatively larger than in the summers of 2017 and 2019.Whether meteorological conditions have shaped the year-to-year variation of algal bloom intensities is analyzed through observational composite.The results suggest that during the strong bloom summers there exist prevailing northerly or northwesterly wind anomalies which advect warm and humid airmass from the southern ocean into the island,increasing surface air temperature and humidity;the warmer and more humid surface favors melting of snow and an increase of low cloud cover,subsequently enhancing the atmospheric downward long-wave radiation and amplifying surface warmth;the increased low cloud cover reflects more ultraviolet rays back to space and weakens the short-wave radiation reaching the surface.All these factors together favor to a stronger bloom.In comparison,during 2017 and 2019 there exist weak southerly wind anomalies which induce the northward advection of cold and dry air from the Antarctic Continent and favor the cooler surface.Consequently,it is unfavorable for the snow algal bloom.Based on these results,a snow algal bloom potential index(API)integrating the meteorological conditions is constructed,and its future trend is projected based on the EC-Earth3 run attending the CMIP6 under SSP245 and SSP585.A significant increasing trend is projected especially under SSP585.Thus snow algal bloom on King George Island will become more frequent and stronger in future.This implies a potential accelerate melting of ice shelf over Antarctic Peninsula.

Impact Assessment of Climate Change on Algal Blooms by a Parametric Modeling Study in Han River

The potential impact of climate change on international and domestic concern. This study aims water eutrophication and ecosystems is of great to analyze the impact of climate change on algal bloom problems in large river systems by utilizing a parametric river eutrophication model that is established using indicators of climate change, hydrological regimes, water quality and nutrient loads. Specifically, the developed parametric modeling method is based on statistical and simulation methods including： Multiple Linear Regressions （MLR）, Multiple Non-linear Regressions （MNR）, Artificial Neural Network （ANN） based on Back-propagation （BP） algorithms, as well as an integrated river eutrophication model. The developed model was applied to Han River, which is one of the major sources of fresh water in Wuhan City, China. The impacts of climate change and human activities on the occurrence mechanisms of algal blooms in the Hart River were identified by scenarios analysis. The individual assessment result indicates that the waste nutrient P load has the most significant impact （14.82%）, followed by the flow rate （5.56%） and then by temperature （3.7%）. For the integrated climate change assessment, it has been found that there is a significant impact （20.37%） when waste load increases and flow rate decreases at the same time. This is followed by increases but flow rate decreases, increase of both waste load and the impact is predicted to be 11 temperature （15.82%）. If temperature 11%. The final results point to human activities as a significant influence on water quality and the Han River ecosystem, temperature is also one of the main factors which directly contribute to algal blooms in Han River. The results in present study are expected to give theoretical supports for further relevant research on water eutrophication.

Impacts of algal blooms removal by chitosan-modified soils on zooplankton community in Taihu Lake,China

It is important to assess the effect on zooplankton when perform the environmental protection or restoration technology, especially removing algal blooms, because algae were the major primary producer in algal lakes. The influence on zooplankton community after half a year of algal blooms removed by chitosan-modified soils in Taihu Lake was assessed and the rationality of carrying out the process semiannually was evaluated in the present study. Morphological composition and genetic diversity of zooplankton community were investigated by microscope checkup and polymerase chain reaction-denatured gradient gel electrophoresis （PCR-DGGE）. A total of 44 zooplankton taxa （23 protozoa, 17 rotifers, 3 copepoda and 1 cladocera） were detected by microscope checkup, and a total of 91 bands （28 bands amplified by primers F1427-GC and R1616, 63 bands amplified by primers Fung-GC and NS1） were detected by PCR-DGGE. The results of cluster analysis or detrended correspondence analysis indicated that there was no considerable difference in morphological composition of zooplankton and DGGE profiles between experimental and control sites, and DGGE profiles could represent the biologic diversity. The study showed that zooplankton community could recover original condition after half year of algal blooms removed by chitosan-modified soils and it was acceptable to apply this process semiannually. In addition, the results revealed that PCR-DCJGE could be applied to investigate the impacts of the environmental protection or restoration engineering on zooplankton community diversity.

Selection of water source for water transfer based on algal growth potential to prevent algal blooms

Water transfer is becoming a popular method for solving the problems of water quality deterioration and water level drawdown in lakes. However, the principle of choosing water sources for water transfer projects has mainly been based on the effects on water quality, which neglects the influence in the variation of phytoplankton community and the risk of algal blooms. In this study, algal growth potential(AGP) test was applied to predict changes in the phytoplankton community caused by water transfer projects. The feasibility of proposed water transfer sources(Baqing River and Jinsha River) was assessed through the changes in both water quality and phytoplankton community in Chenghai Lake, Southwest China. The results showed that the concentration of total nitrogen(TN) and total phosphorus(TP) in Chenghai Lake could be decreased to 0.52 mg/L and 0.02 mg/L respectively with the simulated water transfer source of Jinsha River. The algal cell density could be reduced by 60%, and the phytoplankton community would become relatively stable with the Jinsha River water transfer project, and the dominant species of Anabaena cylindrica evolved into Anabaenopsis arnoldii due to the species competition. However, the risk of algal blooms would be increased after the Baqing River water transfer project even with the improved water quality. Algae gained faster proliferation with the same dominant species in water transfer source. Therefore, water transfer projects should be assessed from not only the variation of water quality but also the risk of algal blooms.

Factors inducing morphological transition in harmful algal bloom-forming species Phaeocystis globosa with emphasis on predator chemical cue

The microalga Phaeocystis globosa is recognized as a harmful alga and also one of the few keystone phytoplankton genera that shape the structure and function of marine ecosystems.P.globosa possess a complex polymorphic life cycle,exhibiting phase alternation between free-living cells of approximately 3-6μm in diameter and gelatinous colonies(palmelloid stage)reaching several millimeters.The knowledge on the factors that induced the morphological transition of P.globosa in the last two decades was reviewed.Emphasis is given to infochemicals,an additional biological factor induced by predator,with the attempt to reveal a relevant mechanism of induced morphological defense.

Eutrophication and algal blooms in channel type reservoirs:A novel enclosure experiment by changing light intensity

To explore eutrophication and algal bloom mechanisms in channel type reservoirs, a novel enclosure experiment was conducted by changing light intensity （LI） in the Daning River of the Three Gorges Reservoir （TGR）. Square enclosures （side 5.0 m） were covered on the surface with shading materials of different thickness, and with their bases open to the river. Changes and characteristics of the main eutrophication factors under the same water quality and hydrodynamic conditions but different LI were evaluated. All experimental water samples were neutral and alkalescent, with high nitrogen and phosphate concentrations, low potassium permanganate index, stable water quality, and different LI. At the same water depth, LI decreased with increasing shade material, while dissolved oxygen and water temperature were both stable. The growth peak of phytoplankton was with light of 345-4390 lux underwater or 558-7450 lux above the water surface, and water temperature of 25.6--26.5℃. Algae were observed in all water samples, accounting for 6 phylum and 57 species, with algal density changing frequently. The results showed that significantly strong or weak light was unfavorable for phytoplankton growth and the function together with suitable temperature and LI and ample sunshine encouraged algal blooms under the same water quality and hydrodynamic conditions. Correlation analysis indicated that algae reduced gradually lengthwise along water depth in the same enclosure while pH became high. The power exponent relationship between chlorophyll a （Chl-a） and LI was found by curve fitting, that is Chl-a = K（LI）n.

Impacts of water release operations on algal blooms in a tributary bay of Three Gorges Reservoir

Water release operation is crucial for water quality in large reservoirs such as Three Gorges Reservoir(TGR),because it determines the hydrodynamics and hence the self-purification capability.As algal blooms were often observed in some tributary bays of TGR during the release periods,high frequency field observations were carried out in a typical eutrophic tributary bay(Xiangxi River) from February 9 to May 10,2009.In this paper we assess the hydrodynamic behaviour,density stratification,and trophic status in this bay,respectively using a series of observations for flow,physical,chemical and biological parameters.Then,we analyze the effects of reservoir release operations on algal blooms using correlation analysis method.An empirical prognosis is concluded for the likelihood of algal bloom occurrence as a function of daily fluctuation of water level(DFWL) and water temperature.Our results indicate that during the release period,the algal bloom occurrences are closely tied to the DFWL in that if the ratio of DFWL to total water depth ranges from -10×10-4 to 0,the possibility of algal blooms may reach up to 70%,and if the ratio is less than -10×10-4,then that risk can be significantly reduced to less than 10%.This paper finally suggests that a wave-type water release operation should be beneficial in reducing bloom frequencies in the tributary water bodies,which is helpful for TGR water quality management,especially for the water release operation optimization.