Community assembly mechanism of phytoplankton in drinking water sources of tropical island, China

Phytoplankton community is a model for of monitoring aquatic systems and interpreting the environmental change in aquatic systems. The present study aimed to forecast environmental parameters that drive the change of phytoplankton community structure in the lake. The present study was carried out in Baghdad Tourist Island Lake (BTIL) for the period From October 2021 to May 2022. The study included the quality and quantity of phytoplankton, moreover, the highest and lowest value of the physical and chemical parameters were (Water temperature (13-30 °C), Light penetration (94-275cm), electric conductivity (837-1128 µS/cm), salinity (0.5-0.7 ‰), pH (7-8.2), total alkalinity (126-226 mg CaCO3/L), total Hardness (297-395 mg CaCO3/L), Calcium (62-98 mg/L), Magnesium (59-75mg/L), Sodium (45-77 mg/L), Potassium (4-9 mg/L), dissolved oxygen (6-9 mg/L), total dissolved solids (586-777 mg/L), total phosphorus (0.1-0.7 mg/L), total nitrogen (0.2-3 mg/L). Monthly samples were taken from the subsurface water from three sampling sites in the Lake and the results and presented as dry and wet seasons. A total of 127 algal taxa was identified in the dry and wet seasons with the dominancy of diatoms (Nitzschia, Gomphonema, Navicula, Ulnaria) and followed by Cyanophyceae (Oscillatoria, Merismopedia) and Chlorophyceae (Cladophora), while Dinophyceae (Ceratium, Gymnodinium), and Euglenophyceae (Euglena) were uncommon. The lowest total cell number of phytoplankton was 221cell*104 /L in the first site during the wet season, while the highest total cell number of phytoplankton was recorded at the third site with 323×104 cells/L in the dry season. The Canonical Correspondence Analysis (CCA) showed the impact of environmental parameters on phytoplankton community structure. Therefore, the changes in phytoplankton species were noticed in the present study in comparison with previous periods and this finding is a warning of alteration in the environmental condition of the lake.

Lake phytoplankton communities are affected by environmental and spatial factors. We studied the relative importance of environmental and spatial factors on the phytoplankton community assembly in Lake Wuchang across three hydrological seasons, which were divided into dry (December to March), normal (April to June, October to November) and wet seasons (July to September) based on the water level and depth. Spatial and temporal patterns of environmental factors and phytoplankton community composition and diversity were studied using Kruskal–Wallis test, Wilcoxon test and NMDS. CCA, Mantel and partial Mantel tests, and PLS-PM were used to investigate the effects of environmental and spatial factors on phytoplankton community characteristics. Results showed that phytoplankton assemblages at the eight study sites were composed of totally 244 species belonging to 9 phyla, which changed from Bacillariophyta and Chlorophyta to Cyanophyta across the whole hydrological period. There was significantly higher abundance and biomass in the normal and wet seasons than that in the dry season. Phytoplankton alpha diversity exhibited uniform temporal distribution patterns with higher values in the dry season than in the normal and wet seasons. The Mantel and partial Mantel tests revealed that environmental (physicochemical conditions of lake water) and spatial factors (geographic distances among sites) jointly affected the phytoplankton community structure and beta diversity across the hydrological seasons, while spatial factors were more important in the wet season. Partial least squares path models showed that spatial factors exhibited a significant positive correlation with the phytoplankton diversity with the path coefficients of 0.53 and 0.71 in the normal and wet seasons, respectively. Phytoplankton composition had significant correlation with on phytoplankton diversity with the path coefficient of −0.75 and 0.61 in the normal and wet seasons, respectively. Our findings revealed that both environmental and spatial factors affected the phytoplankton community assembly in Lake Wuchang. Environmental factors played a more important role in the dry season, while spatial factors were more important in the wet season. With the exception of the abiotic factors (environmental and spatial), the impacts of biotic factors on phytoplankton community cannot be ignored. Therefore, it is also necessary to strengthen further research on the top-down control over phytoplankton communities in Lake Wuchang.

Factors influencing the spatio-temporal dynamics of plankton communities in small tropical lakes are not well-understood. This study assessed plankton communities in response to spatial (six sampling sites) and seasonal (wet vs. dry seasons) changes in environmental variables in Lake Kanyaboli, a small satellite lake on the northern shores of Lake Victoria, Kenya. Water quality variables, including pH, conductivity (EC), dissolved oxygen (DO), temperature, Secchi depth (SD), nitrates (NO3-), nitrites (NO2-), ammonium (NH4+), soluble reactive phosphorus (SRP), total nitrogen (TN), total phosphorus (TP), and chlorophyll-a(Chl-a), were monitored monthly at six sites spread throughout the lake for 1 year. Phytoplankton and zooplankton samples were collected and analyzed for taxon composition and abundance. Two-way ANOVA showed no significant interaction between site and season for all variables. Likewise, there were no significant spatial differences for all variables except Chl-a. At-test showed significant seasonal differences in SD, DO,NH4+,NO3-,NO2-, and TN. Thirty phytoplankton genera were identified belonging to Bacillariophyceae, Chlorophyceae, Cryptophyceae, Cyanophyceae, Euglenoidae, Trebouxiophyceae, and Zygnematophyceae, with Chlorophyceae being the most dominant (42.30%). Zooplankton comprised of 15 genera, belonging to Copepoda (55.4%), Rotifera (27.9%), and Cladocera (16.7%). Two-way ANOVA for plankton abundance showed no significant interaction between site and season, but there were significant differences in community composition between the wet and dry seasons. Canonical correspondence analysis identified water clarity (Secchi depth) and concentrations of dissolved fractions of nitrogen and phosphorus as the major water quality variables driving variation in the composition of plankton communities in the lake. This study showed that seasonality was a major driver of changes in plankton community composition between dry and wet seasons through changes in the concentrations of nutrients (NH4+,NO3-,NO2-, TN, and TP). Lake Kanyaboli's phytoplankton community indicated a non-equilibrial state, perhaps due to short residence times of water, especially during the wet season, and dense macrophytes fringing the lake that increase nutrient uptake and limit the dominance of select phytoplankton species. This study shows the importance of long-term studies covering dry and wet seasons to understand the dynamics of plankton communities and their drivers in small tropical waterbodies to inform management and conservation.

The phytoplankton community structure is potentially influenced by both environmental and spatial processes. In addition, the relative importance of these two processes to phytoplankton assemblage will be affected by hydrological connectivity. However, the impacts of anthropogenic activities on phytoplankton beta diversity and the relative importance of these two processes to phytoplankton are still poorly understood, especially in water conservation areas. Here, we examined the relative importance of local and regional environmental control and spatial structuring of phytoplankton communities in five rivers with different degrees of disturbance during wet and dry seasons. We found that community structure and local environmental conditions varied greatly in seasons and rivers. The reference river (with minimum disturbance) had the highest homogeneity of environmental conditions and phytoplankton assemblage, while the excessive disturbance rivers (sand mining activities) had the greatest environmental heterogeneity and species dissimilarity between sites. Variation partitioning analysis showed that the phytoplankton community variation was mainly explained by the spatial variables in the wet season (summer and autumn) and winter, while the local environmental variables explained the largest variation of phytoplankton community in the dry season (spring). However, broad-scale variables were selected by redundancy analysis in both dry and wet seasons, which indicates that long-distance scales always have low river connectivity, regardless of whether the river is overflowing or drying up. Local environmental processes explained the most variation in phytoplankton community within all of the rivers, suggesting that deterministic processes usually work on relatively small spatial scales. However, this effect would be weakened by anthropogenic activities, especially sand mining activities. We inferred that sand mining activities increased the environmental heterogeneity and species dissimilarity between sites by causing watercourse habitat patches and obstructing river connectivity. On the other hand, as the excessive disturbance, sand mining activities significantly reduced the species richness and abundance of phytoplankton.

Environmental gradients regulate the spatial variations of phytoplankton biomass and community structure in surface water of the Pearl River estuary

Flood events can significantly affect the physical and biological processes of aquatic ecosystems in a short time, leading to rapid changes in phytoplankton community structure. The Huayanghe Lakes experienced extreme flooding in the summer of 2020, with the water level reaching 16.42 m. In order to understand the effects of flooding on phytoplankton diversity and community structure, eight samples were collected in the Huayanghe Lakes from 2019 to 2020. Water-level disturbance has a significant influence on lake-water quality and phytoplankton community structure. The results showed that the Secchi depth increased from 65.36 to 8.52 cm, while the concentration of total nitrogen (from 0.98 to 0.7 mg/L) and total phosphorus (from 0.04 to 0.031 g/L) decreased. In addition, flooding significantly increased the Shannon–Wiener diversity index, the Pielou index, and the Margalef richness index by an average of 43.5%, 36.7%, and 40.21%, respectively. The phytoplankton community structure in the Huayanghe Lakes changed due to the change of physicochemical environment caused by flood. While in the pre-flood period phytoplankton was composed of large diatoms (e.g., Aulacoseira granulata), cyanobacteria (e.g., Microcystis sp., Anabaena sp., and Aphanizomenon sp.) and other multicellular taxa, the flood period showed an increase in the proportion of chlorophytes and diatoms that quickly adapted to settle in new environments. Pearson correlations and redundancy analyses showed that water level fluctuation was the most significant environmental factor affecting the phytoplankton community between the regular hydrological cycle and flood periods. There are few studies on phytoplankton in the Huayanghe Lakes, and the present study provides basic data on phytoplankton diversity and community structure. In addition, it provides a theoretical basis for controlling water level change in the Yangtze River.

Freshwater phytoplankton communities are currently experiencing multiple global change stressors, including increasing frequency and intensity of storms. An important mechanism by which storms affect lake and reservoir phytoplankton is by altering the water column's thermal structure (e.g., changes to thermocline depth). However, little is known about the effects of intermittent thermocline deepening on phytoplankton community vertical distribution and composition or the consistency of phytoplankton responses to varying frequency of these disturbances over multiple years. We conducted whole‐ecosystem thermocline deepening manipulations in a small reservoir. We used an epilimnetic mixing system to experimentally deepen the thermocline via five short (24–72 hr) mixing events across two summers, inducing potential responses to storms. For comparison, we did not manipulate thermocline depth in two succeeding summers. We collected weekly depth profiles of water temperature, light, nutrients, and phytoplankton biomass as well as bottle samples to assess phytoplankton community composition. We then used time‐series analysis and multivariate ordination to assess the effects of intermittent thermocline deepening due to both our experimental manipulations and naturally occurring storms on phytoplankton community structure. We observed inter‐annual and intra‐annual variability in phytoplankton community response to thermocline deepening. We found that peak phytoplankton biomass was significantly deeper in years with a higher frequency of thermocline deepening events (i.e., years with both manipulations and natural storms) due to altered thermal stratification and more variable depth distributions of soluble reactive phosphorus. Furthermore, we found that the depth of peak phytoplankton biomass was linked to phytoplankton community composition, with certain taxa being associated with deep or shallow biomass peaks, often according to functional traits such as optimal growth temperature, mixotrophy, and low‐light tolerance. For example, Cryptomonas taxa, which are low‐light tolerant and mixotrophic, were associated with deep peaks, while the cyanobacterial taxon Dolichospermum was associated with shallow peaks. Our results demonstrate that abrupt thermocline deepening due to water column mixing affects both phytoplankton depth distribution and community structure via alteration of physical and chemical gradients. In addition, our work supports previous research that phytoplankton depth distributions are related to phytoplankton community composition at inter‐annual and intra‐annual timescales. Variability in the inter‐annual and intra‐annual responses of phytoplankton to abrupt thermocline deepening indicates that antecedent conditions and the seasonal timing of surface water mixing may mediate these responses. Our findings emphasise that phytoplankton depth distributions are sensitive to global change stressors and effects on depth distributions should be taken into account when predicting phytoplankton responses to increased storms under global change.

Background Elucidating phytoplankton community assembly processes is crucial for understanding the mechanisms of river bloom formation. The blooms in the Hanjiang River, the largest tributary of the Yangtze River, have garnered significant attention. However, research on the phytoplankton community assembly processes and their driving mechanisms in this river remains limited. Results Based on a comprehensive survey of phytoplankton communities in the Hanjiang River during both dry and wet seasons in 2023, this study employed statistical methodologies, including similarity and correlation analyses, to investigate these assembly processes. Furthermore, it quantified the relative contributions of stochastic and deterministic processes. The results indicated significant heterogeneity in the distribution and composition of phytoplankton communities. Phytoplankton community dissimilarity increased significantly with geographical distance. VPA results showed that spatial variables had a stronger effect on phytoplankton communities than environmental factors. Fitting results from the neutral community and null models revealed that stochastic processes played a more significant role in phytoplankton community construction in the Hanjiang River than deterministic processes, with undominated stochastic processes being predominant during both the dry (82%) and wet (64%) seasons. The roles of diatoms and cyanobacteria in community construction were systematically analyzed based on the dominant species in the Hanjiang River blooms. Compared to other phytoplankton species (such as diatoms), cyanobacteria were more influenced by deterministic factors, suggesting a risk of cyanobacteria blooms in the middle and lower reaches of the Hanjiang River during the wet season. Conclusions This study elucidated the phytoplankton community assembly processes and their influencing factors in the Hanjiang River, playing an important role in revealing bloom outbreak mechanisms and providing insights for future studies on the assembly processes of phytoplankton and other aquatic organisms.

High-fluoride (F-) deep groundwater in the vicinity of mining areas poses severe ecological risks. In this study, we aimed to characterize and reveal the seasonal distribution and influencing factors of elevated F- concentrations in the deep groundwater in the Shendong mining area, Shaanxi and Inner Mongolia province, China. In addition, the ecological risks associated with F- concentrations in irrigation water were assessed. During the wet and dry seasons, the F- concentrations in mine water samples ranged between 0.12 and 13.92 mg/L (mean: 4.24 mg/L) and between 0.20 and 17.58 mg/L (mean: 4.59 mg/L), respectively. The F- content of mine water was clearly higher during the dry season than that during the wet season. F- concentrations in deep groundwater exhibited consistent spatial distributions during both the dry and wet seasons, with an evident increase from southeast to northwest. The dissolution and precipitation of F--bearing and calcium minerals, cation exchange, competitive adsorption, evaporation, and anthropogenic activities during both the wet and dry seasons were identified as important factors influencing F- concentrations in deep groundwater. In addition, the ecological assessment revealed that 100% and 88.89% of low-F- deep groundwater samples were suitable for practices during the dry and wet seasons, respectively. In contrast 84.00% and 84.62% of high-F- deep groundwater samples were unsuitable for irrigation practices during the dry and wet seasons, respectively. This research provided useful prevention policies of deep groundwater extraction to mitigate environment problems associated with excessive F- irrigation.

The southwestern mountains of Hainan Island are the southernmost region with tropical karst landform in China. The frequent alternation of dry and wet seasons leads to the loss of the mineral nutrients of limestone, creating karst fissure habitats. Plants living in karst fissure habitats for long periods of time have developed local adaptation mechanisms correspondingly. In the paper, hydrogen–oxygen stable isotope technology was applied to determine the water-use sources of Impatiens hainanensis in the dry and wet seasons, hoping to expound the adaptation mechanism of I. hainanensis in karst fissure habitats to the moisture dynamics in the wet and dry seasons. In the wet season (May to October, 2018), the air humidity is relatively high in the I. hainanensis habitat; in the dry season (November 2018 to April 2019), there is a degree of evaporation. In the wet season, fine-root biomass increases with soil depths, while coarse-root biomass decreases with soil depths; in the dry season, fine-root biomass is lower and coarse-root biomass is higher compared with the wet season. It was found that the average rainfall reached 1523 mm and the main water-use sources were shallow (0–5 cm) and middle (5–10 cm) soil water, epikarst water, and shallow karst fissure water during the wet season; the average rainfall reached 528 mm, and the deep (10–15 cm) soil water and shallow karst fissure water were the main water-use sources during the dry season. Fog water has a partial complementary effect in the dry season. The differences in the distribution of root biomass and each source of water in the wet and dry seasons of I. hainanensis also reflect the different water-use strategies of I. hainanensis in the wet and dry seasons. In both dry and wet seasons, I. hainanensis formed a water-use pattern dominated by soil water and shallow fissure water (0–15 cm) under the influence of the “fissure-soil-plant” system in the karst region.

To explore the temporal and spatial variations in phytoplankton community in small estuaries, we collected surface water samples from Yongjiang River estuary during wet, normal, and dry seasons and determined the main driving factors of phytoplankton community. A total of 358 species belonging to nine phyla and 123 genera were identified in all seasons. During wet, normal, and dry seasons, species number was 276, 154 and 151, and the abundance was (170.45±225.43)×103, (51.92±30.28)×103 and (31.65±12.79)×103 cells·L-1, respectively. Diatoms dominated the phytoplankton community, and the main dominant species were Cyclotella meneghiniana, Skeletonema costatum, and Paralia sulcata. Shannon diversity and Pielou evenness indices decreased from inside mouth to outside mouth in wet season, but there was no obvious spatial difference in normal season or dry season. Results of non-metric multidimensional scaling analysis and analysis of similarities showed that phytoplankton community composition differed significantly among different regions (inside, at and outside mouth) and different seasons. In wet season, phytoplankton abundance was significantly positively correlated with temperature, dissolved inorganic nitrogen, and dissolved reactive phosphorus, but significantly negatively correlated with salinity. In normal season, phytoplankton abundance was significantly negatively correlated with temperature. In dry season, it was not significantly correlated with environmental factors. Results of redundancy analysis showed that temperature, salinity, ammonium and dissolved reactive phosphorus explained the variations in phytoplankton community by 19.5%, 11.9%, 9.4% and 8.2%, respectively. These results revealed high dominance of diatoms and the main driving factors (temperature, salinity and nutrients) of phytoplankton community in Yongjiang River estuary.

Biotic and abiotic properties most closely associated with subtropical forest soil respiration differ in wet and dry seasons: A 10-year in situ study

Previous studies have indicated certain environmental variables such as rainfall, nutrient concentrations and mixing events can be strong drivers of the phytoplankton community structure in reservoirs. Shifts in the phytoplankton community composition of Puerto Rican reservoirs, however, are not well understood. The present study identified the spatial and temporal changes in the phytoplankton community structure of two reservoirs in Puerto Rica, and which environmental parameters determined the structure. Whether or not the main environmental variables varied on the basis of the initial trophic status of the reservoirs was also explored. Two Puerto Rican reservoirs, one classified as mesotrophic and the other as eutrophic, were sampled on three sampling dates, with the reservoirs into three zones (dam; transition; riverine). Physical parameters were recorded at each sampling site, nutrient concentrations were measured, and the phytoplankton community biomass and diversity were determined. A canonical correspondence analysis (CCA) was run for each reservoir to explore which variables determined the phytoplankton community composition. Temperature, pH and total phosphorus concentration were the main drivers of the phytoplankton community composition for the mesotrophic reservoir, while specific conductance, pH, total phosphorus and nitrate were the main explanatory variables. Although the driving variables changed between the reservoirs, the temporal shifts observed in the phytoplankton community structure were consistent for both reservoirs regardless of their trophic status. The findings of the present study suggest that, although shifts in the phytoplankton community structure are similar in reservoirs with differing initial trophic status, the environmental variables driving community composition may nevertheless vary. Accordingly, tropical reservoirs managers should consider a reservoir's initial trophic status when evaluating what environmental parameters may lead to changes in its phytoplankton community structure. The present study represents a first step at examining the variables driving phytoplankton community structure in Puerto Rican reservoirs.

Soil quality is a critically important component of soil within ecosystems, and our understanding of soil quality variation and its driving factors in tropical forests is limited. This study collected soil samples at 0–20 cm depths from two typical tropical forests (tropical rainforest and rubber plantation) on Hainan Island, China's largest tropical island, during dry and rainy seasons. A soil quality index (SQI) using principal component analysis was applied to measure soil quality based on 21 soil properties. The soil properties' spatial and seasonal changes and their driving factors were simultaneously collected for a holistic analysis of the ecosystems. The results were summarized as follows: (i) The SQI of the rubber plantation decreased by 26.48% compared to the tropical rainforest on the island, while four soil properties investigated (soil pH, total phosphorus, cellulose decomposition, and actinomycetes) increased. (ii) The SQI of different soil types in the two tropical forests displayed an apparent discrepancy. Within the same soil type, the SQI of tropical rainforests showed no obvious differences, while there was a notable spatial discrepancy among rubber plantations in dispersed geographical positions belonging to varied climatic zones. (iii) The SQI of the tropical rainforest was significantly higher in the wet season than in the dry season. In contrast, there were no significant differences between the two seasons for rubber plantations, indicating that soil quality in the tropical rainforest was more sensitive to seasonal changes than soil quality in rubber plantations. Overall, we concluded that except for land use and soil type, spatial variability and seasonal pattern also played essential roles in soil quality on the tropical island. The results from the SQI are expected to provide novel information regarding the sustainable use of tropical forests on Hainan Island.

A study on seasonal phytoplankton abundance and composition in a mangrove estuary, Matang Mangrove Forest Reserve (MMFR), Malaysia, was carried out to determine the phytoplankton structure in this ecosystem, and to identify potential indicators of environmental changes. Phytoplankton samples were collected bimonthly from June 2010 to April 2011, to cover both dry (June to October) and wet (November to April) seasons, at four selected sampling sites along the river. Diatoms showed the highest number of species (50 species) from a total of 85 phytoplankton species from 76 genera. Diatoms contributed more than 90% of the total phytoplankton abundance during the dry season (southwest monsoon) and less than 70% during the wet season (northeast monsoon) as dinoflagellates became more abundant during the rainy season. Two diatoms were recorded as dominant species throughout the sampling period; Cyclotella sp. and Skeletonema costatum. Cyclotella sp. formed the most abundant species (62% of total phytoplankton) during the dry period characterized by low nutrients and relatively low turbidity. Skeletonema costatum contributed 93% of the total phytoplankton in October, which marked the end of the dry season and the beginning of the wet season, characterized by strong winds and high waves leading to the upwelling of the water column. Massive blooms of Skeletonema costatum occurred during the upwelling when total nitrogen (TN) and total phosphorus (TP) concentrations were highest (p < 0.05) throughout the year. The abundance of diatom species during the wet season was more evenly distributed, with most diatom species contributing less than 12% of the total phytoplankton. Autotrophic producers such as diatoms were limited by high turbidity during the northeast monsoon when the rainfall was high. During the wet season, Cyclotella and Skeletonema costatum only contributed 9% and 5% of the total phytoplankton, respectively, as dinoflagellates had more competitive advantage in turbid waters. This study illustrates that some diatom species such as Cyclotella sp. and Skeletonema costatum could be used as indicators of the environmental changes in marine waters.