Chikugo River Estuary Research Articles

Relationship between hydro-environmental variables and Coilia nasus catch in a highly turbid macrotidal estuary in Japan

The effect of estuarine hydro-environmental parameters and tidal phases on the fish catch in the macrotidal Chikugo River estuary was investigated. This study focuses on the catch data of Etsu (Coilia nasus) fish, obtained during their spawning season from 2009 to 2020, employing traditional fishing techniques at a site situated 14.6–16 km upstream from the river mouth. It analyses various hydro-environmental parameters and identifies optimal ranges of environmental parameters. The optimal range of salinity (0.04–0.2) and suspended sediment concentration (SSC, 16–118 mg/L) indicated a higher fish catch at the saltwater-freshwater interface and upstream of the estuarine turbidity maximum (ETM) zone. By developing a generalized additive model (GAM) to analyze the relationship between CPUE and environmental parameters, the study assesses the relative importance of each variable in explaining CPUE variance. GAM exhibited a reasonable predictive capability (R2 = 0.509) and indicated the relative importance of each variable (SSC-24.69%, salinity-21.81%, temperature-24.41%, discharge-9.16%, and MTR-19.91%) in explaining the CPUE variance. Additionally, it aims to verify the maturity status and spawning migration patterns of C. nasus fish through standard-length distribution analysis during the spawning season. The standard-length distribution and comparison with previous literature confirmed that fishes caught by the fisherman belong to the category of matured fishes aged 2–3 years. The results indicated that the high CPUE of matured C. nasus at the upstream of ETM and freshwater-saltwater interface was related to the spawning migration and not to their feeding behaviour at the ETM. This research contributes to fisheries management strategies and conservation efforts in the Chikugo River estuary by providing valuable guidance for habitat restoration, informing fishing regulations, and understanding the behaviour of C. nasus.

Analysing the performance of the NARX model for forecasting the water level in the Chikugo River estuary, Japan

Estuaries are dynamic environments which are driven by various natural processes like river discharge, tides, waves, influx of saline water and sediments, etc. These ecosystems are the most sensitive to sea level rise and fluctuations in river discharge associated with climate change. A direct response of sea level rise and river discharge can be observed in the water level of estuaries. However, existing models have not considered these parameters for forecasting water level. This paper focuses on developing a water level forecast model for the Chikugo River estuary in Japan using Nonlinear Autoregressive with Exogenous inputs (NARX Model). NARX neural network was used to do the one-step-ahead prediction of water level considering the various parameters that can very well be influenced by climate change: previous water level, river discharge, and salinity. Accordingly, three models were developed: (i) Model I considering previous water level; (ii) Model II additionally considering river discharge; and (iii) Model III additionally considering salinity. All the models showed appreciable performance in forecasting the water level. Model III had the best correlation with the water level with a cross-correlation value of 0.6030, while the river discharge had only a cross-correlation of 0.1113 indicating that the Chikugo River estuary is tide-dominated. The model was trained using different combinations of available data - previous water level, river discharge, and salinity. Cross-correlation results showed a better correlation between water level and salinity than various other combinations trained. Therefore, tidal intrusion influences the water level in the estuary, thereby depicting that sea level rise can affect the water level, and its influence can be well predicted by the developed model. The water level significantly affects the flora and fauna and the predictability of future estuarine floods can help in taking necessary mitigation strategies.

Phytoplankton habitats and size distribution during a neap-spring transition in the highly turbid macrotidal Chikugo River estuary

The effect of neap-spring transition on the spatial and temporal changes in the cell size distribution of different phytoplankton species and their size-based habitats distribution were investigated in the highly turbid macrotidal Chikugo River estuary, Japan in 2021. The estuarine mixing changed from stratified to well-mixed over the transition from neap to spring tides and saltwater (1–5) intruded until 17 km with negligible effect from river discharge. The suspended sediment concentration (SSC) was low (<40 mg L−1) during neap tide and highest (1000 mg L−1) during spring tide associated with strong mixing, which led to the formation of an estuarine turbidity maximum (ETM) zone between 8 and 12 km. A total of 159 phytoplankton species were identified in the estuary and are classified into seven groups. The diatoms (freshwater and marine habitats) were found as significant groups (79–86 %). Principal component analysis (PCA) results showed that salinity (p < 0.05) and mixing (p < 0.05) were the major controlling factors in the phytoplankton habitats distribution. The diatoms abundance started to increase from neap tide and reached the maximum abundance (174.714 cells/mL) two days after neap tide, then decreased towards spring tide (69.257 cells/mL). Additionally, the cell sizes of dominant diatoms species (Skeletonema costatum and Nitzschia acicularis) increased from neap to intermediate tides and continued increasing until the spring tide. Spatially, the abundance of diatoms was lower in the middle estuary (67–86 cells/mL) than in other parts because of ETM formation, although larger size distribution occurred. This study concludes that the abundance and size distribution of phytoplankton are influenced by estuarine mixing, and the neap-spring transition phase is more essential for phytoplankton growth than individual neap and spring tides in tide-dominated estuaries. Furthermore, size-increasing patterns play an important role in sustaining their survival strategy under the strong turbulent conditions in addition to saltwater intrusion and ETM.

Riverbed fluctuation and erosion property of cohesive sediment based on long-term topographic surveys in a macrotidal estuary

The feature of large scouring process of cohesive sediment and the long term seasonal riverbed fluctuation at the estuarine turbidity maximum (ETM) zone of the macrotidal Chikugo River estuary, Japan were studied using the intensive periodical transverse surveys of riverbed topography and sediment sampling along with continuous monitoring of water level, salinity and turbidity during 2005–2017. The results revealed that substantial erosion occurs (increase in channel capacity) when the river discharge exceeds 2,000 m3 s−1 while deposition occurs (decrease in channel capacity) when the flow falls under 100 m3 s−1. During the normal flows, the bed shear stress (values ranging from 0.2–4.0 N m−2) showed a harmonious behavior driven by the fortnightly tidal variations. Conversely, rapid variations in bed shear stress values (2.0–22.7 N m−2) were observed during the flood events. Further, a critical shear stress value of 1.8 N m−2 was estimated as the erosion threshold of mud deposit in the estuary. Finally, an erodibility coefficient of 0.0015 s2 m−2 was determined based on the reasonable relationship (R2=0.767) between erosion depth and accumulated friction velocity from the actual field data. This value is several times lower than those reported in previous studies due to the differences in deposition and consolidation properties of sediments between actual and experimental channels. This indicates that the erosion of cohesive sediments in estuaries is not only influenced by the magnitude of shear velocity, but also by its consolidation characteristics.

Diverse downstream migration patterns of the anadromous Japanese grenadier anchovy Coilia nasus in the Chikugo River estuary and Ariake Sea, Japan

Variations in the downstream migration patterns of the anadromous Japanese grenadier anchovy Coilia nasus in the Chikugo River estuary and the inner part of the Ariake Sea, Japan were investigated using otolith Sr:Ca ratios over life history transects. Although most individuals migrated from freshwater or low salinity areas (i.e., upper estuary; salinities <10) to brackish water or sea water areas (i.e., lower estuary or sea), the timing of the downstream migration was considerably different among individuals. Based on the otolith Sr:Ca profiles, four migratory groups were distinguished and their mean standard length (SL) at the initial downstream migration were inferred: (1) the Late Migration Group (LMG), in which individuals remain within freshwater or low salinity areas, where the estuarine turbidity maximum (ETM) is well developed at salinities <10, during the larva and juvenile stages until initial downstream migration at a size of 154 mm in SL; (2) the Early Migration Group (EMG), in which individuals immediately migrate from freshwater or low salinity areas to relatively high salinity brackish water areas at the early juvenile stage (45 mm in SL); (3) the Middle Migration Group (MMG), in which the initial downstream migration of the fish occurs at 90 mm in SL; and (4) the Brackish-Sea water Resident Group (BSRG), in which individuals are hatched in relatively high salinity brackish water areas and spend time there during the early life period. The relative contribution of the LMG and BSRG to adult females were low, whereas adult females were dominated by the EMG and MMG (72.4% of n = 58). Based on previous studies and our current results, C. nasus may greatly depend on the ETM zones during the early life stage, but there are significant differences in the timing and downstream migration patterns of C. nasus.

A three-dimensional environmental hydrodynamic model, Fantom-Refined: Validation and application for saltwater intrusion in a meso-macrotidal estuary

Many three-dimensional hydrodynamic models were developed to investigate saltwater intrusion in estuaries. However, the horizontal grid discretization remains a challenge to achieve high accuracy as well as to reduce the cost in grid generation and computation. A three-dimensional hydrodynamic simulator, Fantom-Refined was developed to address this issue. A structured gridding technique which enables a flexible local mesh refinement was used for the horizontal gridding to analyze the multi-resolution phenomena from coastal bay to river upstream. The model is based on a collocated finite-volume method with second or higher-order spatial and temporal numerical schemes. The model was calibrated and validated using the data measured in a meso-macrotidal Chikugo River estuary, Japan. The simulated water level and vertical and longitudinal distribution of salinity were reasonably matched with the measured data. The salinity intrusion length (SIL) and salinity interface gradient (SIG) calculated from measured and simulated results were in good agreement with R2 value of 0.99 and 0.98 respectively. Although there were limited studies that reproduced continuous change in mixing condition from well mixed to salt wedge via partially mixed type, the present study successfully verified the change with intensive observations. The effect of channel meanderings and width on horizontal salinity distribution in the estuary was explained using the model results. Furthermore, the effect of horizontal grid resolution on saltwater intrusion and mixing conditions was investigated with three different grid resolutions and three different bathymetry resolutions. The results showed that both of the horizontal grid and bathymetry resolutions have significant effect on the prediction of the salinity distribution as well as the surface level in the estuary.

Seasonal morphodynamic evolution in a meandering channel of a macrotidal estuary

The spatio-temporal morphodynamics in relation to the fortnightly tidal variations and seasonal river discharge variations in a meandering estuarine channel were investigated. Intensive topographic surveys (longitudinal and transverse) every 3–4 months using a digital sonar system equipped with a differential global positioning system and continuous monitoring of salinity and turbidity with moored instruments were carried out during 2009–2012 in the upstream section of the highly turbid macrotidal Chikugo River estuary, Japan. The results revealed that the tidal flow dominated in the estuary for most of the year except for the rainy season in which river flow dominates. During the low flow season, tidally induced net upward sediment transport dominates and deposition takes place in the upstream section especially in the inner part of the meander. It contributes towards the strengthening of the existing estuarine turbidity maximum (ETM) in the upstream section of the estuary, and the channel capacity reduced gradually during this period. However, large flood events led to the breakdown of this ETM zone in the upstream during the rainy season and exported the sediments to the downstream tidal flat which resulted in the rapid increase of channel capacity. This further exhibited that the ETM zone was migrated to the downstream part of the estuary during the rainy season.There were significant differences in the seasonal trends of morphological evolution in the estuary which further greatly influenced the channel capacity. The drastic changes in channel morphology due to the sediment export by the high river flow during the flood season was approximately recovered through the sediment import by the fortnightly tidal cycle during the dry season over a seasonal basis. The study concludes that the morphological changes in the tide dominated estuaries are strongly influenced by the fortnightly tidal variations as well as the seasonal river discharge variations.

Unravelling habitat use of Coilia nasus from the Rokkaku River and Chikugo River estuaries of Japan by otolith strontium and calcium

The migratory history of the engraulid fish Coilia nasus in the Rokkaku and Chikugo River estuaries of the Ariake Sea, Japan was assessed using otolith strontium (Sr) X-ray intensity maps and strontium:calcium (Sr:Ca) ratio life history transects by an electron probe microanalyzer (EPMA). The results showed that seven of the ten specimens from the Rokkaku River Estuary (LJC) and all 15 specimens collected in the Chikugo River Estuary (ZHC) had low Sr:Ca ratios (⩽3) at the central otolith area, indicating their riverine origin and initial freshwater residence. After the first regime shift adjacent to natal regions, the Sr level mapping displayed a wide variety of color patterns, and the Sr:Ca ratios obtained by the line transect analysis could be divided into one to six significantly different phases indicative of gradual life history transition. The other three specimens from the Rokkaku River Estuary had high Sr:Ca ratios (3–6.7) at the central otolith area but showed alternating changes between low and high values outside the natal region, suggesting that estuarine-origin individuals occurred in the Rokkaku River Estuary. The two-dimensional maps of the Sr level and average of the otolith Sr:Ca ratios along the life-history transects could be used as effective tools for reconstruction of past habitat use of the tapertail anchovy in estuaries of the Ariake Sea, Japan.

Sediment transport and fluid mud layer formation in the macro-tidal Chikugo river estuary during a fortnightly tidal cycle

The erosion and deposition dynamics of fine sediment in a highly turbid estuarine channel were successfully surveyed during the period from August 29 to September 12, 2009 using an echo sounder in combination with a high-resolution acoustic Doppler current profiler. Field measurements were conducted focusing on the tide driven dynamics of suspended sediment concentration (SSC), and fluid mud at the upstream of the macrotidal Chikugo river estuary during semidiurnal and fortnightly tidal cycles. Morphological evolution was observed especially during the spring tide over a period of two weeks. The elevation of the channel bed was stable during neap tide, but it underwent fluctuations when the spring tide occurred owing to the increase in the velocity and shear stress. Two days of time lag were observed between the maximum SSC and peak tidal flow, which resulted in the asymmetry between neap-to-spring and spring-to-neap transitions. During the spring tide, a hysteresis loop was observed between shear stress and SSC, and its direction was different during flood and ebb tides. Although both fine sediments and flocs were dominant during flood tides, only fine sediments were noticed during ebb tides. Hence, the net elevation change in the bed was positive, and sedimentation took place during the semilunar tidal cycle. Finally, a bed of consolidated mud was deposited on the initial bed, and the height of the channel bed increased by 0.9 m during the two-week period. The observed hysteretic effect between shear stress and SSC during the spring tides, and the asymmetrical neap-spring-neap tidal cycle influenced the near-bed sediment dynamics of the channel, and led to the formation of a fluid mud layer at the bottom of the river.

Spatio-temporal variability of phytoplankton (Chlorophyll-a) in relation to salinity, suspended sediment concentration, and light intensity in a macrotidal estuary

The influences of environmental gradients on the spatio-temporal variability of phytoplankton (Chlorophyll-a) in the macrotidal Chikugo River Estuary were studied during a two-week period of September 2010. Vertical profiles of salinity, turbidity, and light intensity were measured at 18 stations separated by a 1-km interval. Water samples for the determination of suspended sediment concentration (SSC), concentration of Chlorophyll-a (Chl-a) and Pheophytin-a (Pheo-a) were collected from the surface layer at all stations. The estuarine water column was vertically well mixed with high SSC (100–2000mgL−1) during spring tide and the photic depth (zp) was less than 0.2m. The mixing depth (zm) was more than 10 times the photic depth for the major part of the estuary. The estuary gradually changed to partially mixed with decrease in SSC (≤400mgL−1) during the intermediate tide. The estuary became stratified with low SSC (20–50mgL−1) during neap tide and the zp reached 4m. The zm was less than 0.5 times the zp for the whole estuary. Light attenuation was dominated by SSC and the zp varied according to semidiurnal and semilunar tidal cycle. The zp: zm ratio did not show any relationship with Chl-a in the Chikugo river estuary. This is because the Chl-a concentration reached maximum two to three days after the neap tide. The peak concentration of Chl-a was located near the low salinity region and that of Pheo-a was located in the Estuarine Turbidity Maximum (ETM) zone. The Pheo-a concentration reached maximum during the spring tide. A good relation between zp: zm ratio and Pheo-a indicates that the increase in Pheo-a was caused by the light limitation due to suspended sediment and the responses of the Pheo-a on the light condition was instantaneous. These phenomena were remarkably found in the interface between freshwater and saltwater. Light availability driven by mixing and ETM process during semidiurnal and semilunar tidal cycle is the controlling factor of the phytoplankton dynamics in the Chikugo River Estuary.

Long term variation in transparency in the inner area of Ariake Sea

This study analyzed the long time record of transparency (Secchi disk depth) in the inner area of Ariake Sea and determined the reason for the change. The data observed by the Saga Prefectural Ariake Fisheries Research and Development Center for 38 years at 10 or 11 stations was used. The Secchi disk depth increased significantly in autumn and winter. The analysis based on the 1 dimensional model of surface salinity and the SS concentration showed that the increase in Secchi disk depth would be caused by the decline of the SS supply at the source. It also suggested that there would have been an additional SS source existed in the bay head before 1984. It is coincident with the construction of the Chikugo Ozeki weir and the channel bed dredging. The channel bed depth of the Chikugo River estuary increased largely from 60's to 80's by dredging and became almost stable after 1985. Provided that the Chikugo River estuary was an additional SS source, the decline of the SS supply would be generated by the upstream displacement of the turbidity maximum induced by the increase in channel bed depth.

Temporal and Spatial Variation of Mixing and Movement of Suspended Sediment in the Macrotidal Chikugo River Estuary

ABSTRACT Azhikodan, G. and Yokoyama, K., 2015. Temporal and spatial variation of mixing and movement of suspended sediment in the macrotidal Chikugo River estuary. The effect of mixing on the movement of the estuarine turbidity maximum (ETM) was examined in the highly turbid Chikugo River estuary, Japan, in consideration of four significant issues: semidiurnal and semilunar tidal variations, spatial distribution, and synchronism of data acquisition. Vertical profiles of salinity and turbidity were measured for 2 weeks at 1 km intervals from the river mouth (0 km) to 17 km upstream, with a short duration for every cruise. Spring-neap transition and the spatial variation of mixing, and suspended sediment concentration (SSC) was found during the semilunar tidal cycle. The estuary gradually changed from a vertically well-mixed condition during spring tide to a stratified condition during the neap tide via a partially mixed condition during the intermediate half tides. The relationship between the flow ratio a...

Estuarine Mixing and Spatial Distribution of Phytoplankton in the Chikugo River Estuary

Estuarine Mixing and Spatial Distribution of Phytoplankton in the Chikugo River Estuary

Microdistribution and feeding dynamics of Coilia nasus (Engraulidae) larvae and juveniles in relation to the estuarine turbidity maximum of the macrotidal Chikugo River estuary, Ariake Sea, Japan

AbstractTo evaluate the nursery value of the estuarine turbidity maximum (ETM) for the anadromous engraulid fish Coilia nasus, physical and biological samples were collected along the macrotidal Chikugo River estuary, southwestern Japan, from June 2005 to January 2006. Generally, eggs were distributed upstream of the ETM, whereas larvae were distributed in the ETM. The first cohort was washed out of the estuary by a large flood in early July. The second cohort appeared after the flood, gradually extending its distributional area through the estuary and growing to 100 mm in standard length (SL) by December. Larvae and juveniles (20–100 mm SL) fed selectively on copepods, specifically large calanoid species, whereas more developed juveniles fed on mysids. Prey organisms in guts were significantly more abundant and present in greater weights in the ETM (salinity &lt;10) than in the lower estuary (salinity &gt;10). The better feeding conditions were primarily attributable to consistently high densities of two large calanoid species, Sinocalanus sinensis and Pseudodiaptomus inopinus, in the ETM. In contrast, the poor feeding conditions in the lower estuary were likely due to a seasonal decrease in copepod density and the dominance of small copepods such as Oithona spp. Given that better feeding conditions contribute positively to the growth and survival of larvae and juveniles, we conclude that the ETM has a significantly higher nursery value for C. nasus compared with other habitats in the Chikugo River estuary.

Spatiotemporal dynamics of stable carbon isotope ratios in two sympatric oligohaline copepods in relation to the estuarine turbidity maximum (Chikugo River, Japan): implications for food sources

To improve our understanding of high copepod productivity in the estuarine turbidity maximum (ETM) of the macrotidal Chikugo River estuary in southwestern Japan, we determined stable carbon isotope ratios (δ 13 C) in the sympatric oligohaline copepods Sinocalanus sinensis and Pseudodiaptomus inopinus from 2005 to 2006. Terrestrial-plant and phytoplankton detritus always accumulated in the ETM (salinity 0.1 - 3), whereas outside the ETM phytoplankton dominated especially in the warm season (>20°C). In contrast to the year-round concentration of S. sinensis in the ETM, P. inopinus occurred widely along the upper estuary under phytoplankton-dominated conditions. Terrestrial-plant detritus was characterized by relatively constant δ 13 C (~ — 24‰), suggesting that significant spatiotemporal variability in copepod δ 13 C was attributable to the feeding of copepods on phytoplankton and/or its detritus. Both copepods had relatively depleted δ 13 C values in the ETM, reflecting δ 13 C in freshwater/oligohaline phytoplankton ( - 24‰) associated with meso/polyhaline phytoplankton downstream from the ETM were found only in P. inopinus, Although the contribution of terrestrial-plant detritus to copepod production remains to be determined, our results indicate that both copepods selectively utilize freshwater/oligohaline phytoplankton and/or its detritus in the ETM, whereas only P. inopinus utilizes meso/polyhaline phytoplankton downstream from the ETM.

筑後川感潮域における浮遊物質の構成に関する研究

筑後川感潮域における浮遊物質の構成に関する研究

Different patterns of stage-specific horizontal distribution between two sympatric oligohaline copepods along a macrotidal estuary (Chikugo River, Japan): implications for life-history strategies

Stage-specific horizontal distribution was compared between two sympatric oligohaline copepods to elucidate their life-history strategies related to reproduction: free-spawning Sinocalanus sinensis and egg-carrying Pseudodiaptomus inopinus. Copepods were sampled by vertical hauls of a 100-mm mesh plankton net along the macrotidal Chikugo River estuary in southwestern Japan from 2005 to 2006. Sinocalanus sinensis dominated throughout the year except for the warm season, when P. inopinus outnumbered S. sinensis. The spatiotemporal correspondence of peak densities with the estuarine turbidity maximum was evident for S. sinensis but ambiguous for P. inopinus. Nauplii of S. sinensis were always distributed on average downstream from copepodids and adults, whereas P. inopinus usually showed the opposite pattern of stage-specific horizontal distribution. During large freshwater discharges, S. sinensis was more subject to washout from the estuary. In addition, the potential vulnerability of S. sinensis to droughts was suggested by low tolerance for high salinities. Consequently, egg-carrying P. inopinus has higher flexibility to live under varying estuarine conditions especially during the warm season. In contrast, ecological specializations, such as tidal vertical migration and detritivorous feeding, are considered to underlie the dominance of the free-spawning S. sinensis under the conditions prevailing in the macrotidal environment.

Year-round accumulation of particulate organic matter in the estuarine turbidity maximum: comparative observations in three macrotidal estuaries (Chikugo, Midori, and Kuma Rivers), southwestern Japan

The inner part of the Ariake Sea is one of the most productive estuarine systems in Japan. To examine potential food items for estuarine organisms, we conducted monthly observations of the dynamics of particulate organic matter along the macrotidal Chikugo River estuary in 2005 and 2006. In the neighboring macrotidal Midori and Kuma River estuaries, comparative observations were made. High turbidity and strong vertical mixing were observed only at low salinities (<10) in the Chikugo River estuary. In contrast, the Midori and Kuma River estuaries were characterized by less turbid and less mixed waters. Concentrations of particulate organic carbon often exceeded 5 mg l−1 in or close to the estuarine turbidity maximum (ETM) of the Chikugo River estuary. However, such high concentrations were rarely observed in the other two estuaries. The observed differences could be attributable to different hydrodynamic processes related to the different lengths of tidal reaches: 23, 8, and 6 km in the Chikugo, Midori, and Kuma Rivers, respectively. In the Chikugo River estuary, spatiotemporal changes of chlorophyll a suggested that phytoplankton occurred abundantly up- and/or downstream from the ETM especially during the warm season. In contrast, pheophytin (i.e., plant detritus) always accumulated in or close to the ETM. Carbon stable isotope ratios and carbon to nitrogen ratios indicated that the plant detritus was derived from phytoplankton and terrestrial plants. The Chikugo River estuary has a high potential to support the production of estuarine organisms through abundant plant detritus in the well-developed ETM all the year round.

Seasonal and spring-neap tidal changes in ocean structure of the lower Chikugo river estuary based on monitoring data from oceanographic observation tower

Seasonal and spring-neap tidal changes in ocean structure of the lower Chikugo river estuary based on monitoring data from oceanographic observation tower

Settlement Properties of the Suspended Sediment using Micro Videoscope in the Chikugo River Estuary

Under water micro video scope was developed to measure floc settling velocity and applied to the suspended sediment floc in the Chikugo River estuary. The effective densities of the flocs were measured successfully. The floc density distribution function was proposed from the floc settling speed and their diameters. The floc density distribution function varied with tide. In ebb tide, floc densites were smaller than in flood tide. This fact shows that the flocs in flood tide mainly consists of the mud which was suspended from the river bed. Mass settling flux was also calculated. When suspended sediment contains algae or algal residuals, the settling velocity showed lower value. On the other hand, containing high concentration of extracellular polymeric substances (EPS) made settlement velocity high.