Seasonal cycles of phytoplankton biomass and primary production in a tropical temporarily open-closed estuarine lagoon - The effect of an extreme climatic event

Effect of the ten-year fishing ban on change of phytoplankton community structure: Insights from the Gan River

The Yangtze River is one of the largest riverine ecosystems in the world and is a biodiversity hotspot. In recent years, this river ecosystem has undergone rapid habitat deterioration due to anthropogenic disturbances, leading to a decrease in freshwater biodiversity. Owing to these anthropogenic impacts, the Chinese government imposed a "Ten-year fishing ban" (TYFB) in the Yangtze River and its tributaries. Ecological changes associated with this decision have not been documented to evaluate the level of success. This study evaluates the success of the TYFB by analyzing the changes in phytoplankton communities and comparing them to periods before the TYFB was imposed. A total of 325 phytoplankton species belonging to 7 phyla and 103 genera dominated by Chlorophyceae and Bacillariophyceae were identified. Species diversity according to the Shannon-Wiener ranged between 1.19 and 3.00. The results indicated that phytoplankton diversity increased, while both density and biomass decreased after the TYFB. The health of the aquatic ecosystem appeared to have improved after the TYFB, as indicated by the phytoplankton-based index of biotic integrity. Significant seasonal and spatial differences were found among the number of species, density, and biomass of phytoplankton, where these differences were correlated with pH, water depth, chlorophyll-a, permanganate index, transparency, copper, ammonia nitrogen, and total phosphorus based on redundancy analysis. Results from this study indicate that the TYFB played an important role in the restoration of freshwater ecosystem in the Yangtze River and its tributaries.

Impact of anthropogenic accumulation on phytoplankton community and harmful algal bloom in temporarily open/closed estuary

Spatio-temporal variation in phytoplankton community dynamics in a temporarily open/closed Swarnamukhi river estuary (SRE), located on the South East coast of India was investigated and correlated to that of the adjacent coastal waters. Understanding the seasonal variability of the phytoplankton community and influencing factors are essential to predicting their impact on fisheries as the river and coastal region serve as the main source of income for the local fishing communities. Downstream before the river meets the sea, an arm of the Buckingham Canal (BC), carrying anthropogenic inputs empties into the Swarnamukhi River (SR1). The impact of anthropogenic effects on the phytoplankton community at BC was compared to other estuarine stations SR2 (upstream), SR1 (downstream), SRM (river mouth) and coastal station (CS). In BC station, harmful algal blooms (HABs) of Chaetoceros decipiens (2940 × 103 cells L-1) and Oscillatoria sp. (1619 × 103 cells L-1) were found during the southwest monsoon and winter monsoon, respectively. These HABs can be linked to the anthropogenic input of increased nutrients and trace metals. The HABs of Oscillatoria sp. were shown to be induced by elevated concentrations of nitrate (10.18 µM) and Ni (3.0 ppm) compared to ambient, while the HABs of C. decipiens were caused by elevated concentrations of silicate (50.35 µM), nitrite (2.1 µM), and phosphate (4.37 µM). Elevated nutrients and metal concentration from the aquaculture farms, and other anthropogenic inputs could be one of the prime reasons for the recorded bloom events at BC station. During this period, observed bloom species density was found low at other estuarine stations and absent at CS. The formation of bloom events during the closure of the river mouth could be a major threat to the coastal ecosystem when it opens. During the Osillatoria sp. bloom, both the Cu and Ni levels were higher at BC. The elevated concentration of nutrients and metals could potentially affect the coastal ecosystem and in turn fisheries sector in the tropical coastal ecosystem.

Linking centennial scale anthropogenic changes and sedimentary records as lessons for urban coastal management

Coastal eutrophication and urban flooding are increasingly important components of global change. Although increased seawater renewal by barrier openings and channelizing are common mitigation measures in coastal lagoons worldwide, their effects on these ecosystems are not fully understood. Here, we evaluated the relationships between human interventions in the watershed, artificial connections to the sea, and the sediment burial rates in an urban coastal lagoon (Maricá lagoon, Southeastern Brazil). Sediment accretion along with nutrient and carbon burial rates were determined in two sediment cores representing the past ∼120 years (210Pb dating) and associated with anthropogenic changes as indicated by historical records and geoinformation analyses. Lagoon infilling and eutrophication, expressed by the average sediment accretion, TP, TN, and OC burial rates, respectively, increased ∼9-18, 13-15, 11-14 and 11-12-fold from the earliest (<1950) to the most recent (2000-2017) period. These multi-proxy records confirm mechanistic links between deforestation, urbanization, and untreated sewage discharges. In addition, our findings reveal artificial connections to the sea may contribute to lagoonal eutrophication and infilling, particularly when not integrated with sewage treatment and forest conservation or reforestation in the watershed. Therefore, increased seawater renewal by physical interventions commonly considered as mitigation measures may in contrast cause severe degradation in coastal lagoons, causing harmful consequences that should be not neglected when implementing management practices.

Trophic status of a coastal lagoon - marine harbor system: Potential outwelling rates to the Mesoamerican Barrier Reef southern region

Eutrophication is still a serious problem in many coastal areas, including the tropics, where river discharges of nutrients is usually high. The ecological stability and ecosystem services of the Mesoamerican Barrier Reef System (MBRS), the world's second-largest coral reef system, suffer a generalized impact by riverine discharge of sediment and organic and inorganic nutrients, which may lead to coastal eutrophication and a coral-macroalgal phase shift. However, few data exist on the MRBS coastal zone status, particularly in Honduras. Here, two in situ sampling campaigns were carried out (May 2017 and January 2018) in the Alvarado Lagoon and Puerto Cortés Bay (Honduras). Measurements included water column nutrients, chlorophyll-a (Chla), particulate organic and inorganic matter and net community metabolism, completed with satellite images analysis. The lagoon and bay environments are ecologically different systems and present different sensitivities to seasonal changes in precipitation as shown by the multivariate analysis. Nonetheless, net community production and respiration rates were neither different spatially, nor seasonally. In addition, both environments were highly eutrophic as shown by the TRIX index. Thus, the Puerto Cortés system represents an important source of dissolved nutrients and particulate matter to the coastal zone. Even though offshore, water quality, based on estimated outwelling rates from the Puerto Cortés system to the coastal waters of the southern MRBS region, improved considerably, concentrations of Chla and nutrients remained higher than those typically measured in non-polluted coral reefs in the Caribbean region and the suggested threshold values. In situ monitoring and assessment of these aspects are crucial to evaluate the ecological functioning of and threats on the MBRS, and elaborate and implement adequate policies for integrated management given its regional and global importance.

Seasonal variation of phytoplankton community assembly processes in Tibetan Plateau floodplain

Uncovering the mechanisms underlying phytoplankton community assembly remains a major challenge in freshwater ecology. The roles of environmental filtering and spatial processes in shaping phytoplankton metacommunity in Tibetan floodplain ecosystems under various hydrological conditions are still unclear. Here, multivariate statistics and a null model approach were used to compare the spatiotemporal patterns and assembly processes of phytoplankton communities in the river-oxbow lake system of Tibetan Plateau floodplain between non-flood and flood periods. The results showed that phytoplankton communities had significant seasonal and habitat variations, with the seasonal variations being more remarkable. Phytoplankton density, biomass, and alpha diversity were distinctly lower in the flood than non-flood period. The habitat differences (rivers vs. oxbow lakes) in phytoplankton community were less pronounced during the flood than non-flood period, most likely due to the increased hydrological connectivity. There was a significant distance-decay relationship only in lotic phytoplankton communities, and such relationship was stronger in the non-flood than flood period. Variation partitioning and PER-SIMPER analysis showed that the relative role of environmental filtering and spatial processes affecting phytoplankton assemblages varied across hydrological periods, with environmental filtering dominating in the non-flood period and spatial processes in the flood period. These results suggest that the flow regime plays a key role in balancing environmental and spatial factors in shaping phytoplankton communities. This study contributes to a deeper understanding of ecological phenomena in highland floodplains and provides a theoretical basis for floodplain ecosystem maintenance and ecological health management.

Spatial and temporal variations of estuarine stratification and flushing time across the continental U.S

Estuarine circulation attributes such as stratification and flushing time significantly influence estuarine ecological processes. Stratification reflects how much vertical mixing occurs in an estuary, while flushing time can describe the exchange rate of pollutants between the estuary and ocean. A recently developed estuarine characterization framework used estuarine geophysical attributes and water exchange datasets to characterize estuarine circulation for 360 estuaries in the continental U.S. between 1950 and 2015. The estuaries were grouped into nine ecoregions according to the Marine Ecoregions of the World. In the Gulf of Mexico and along the East Coast, most estuaries were well-mixed (63-93%), with 3-5% strongly-stratified estuaries. Along the West Coast, strongly-stratified estuaries dominated (46-63%), with the exception of the Puget Trough basin and the southern CA ecoregion with 83% and 75% well-mixed estuaries. The stratification type of some estuaries varied seasonally. Generally, they were more stratified winter through spring, then mixed during the summer, with the exception of southern FL, which had a reverse pattern due to the positive correlation between the stratification parameter and freshwater inflow (97% estuaries with R 2 > 0.9). The flushing times of the 300 well-mixed and partially-stratified estuaries were estimated using Tidal Prism (TPM) and Freshwater Fraction Methods (FFM). Flushing time seasonal variation exhibited a negative correlation with freshwater inflow (R 2 > 0.8 for 50% of estuaries using TPM). Generally, estuarine flushing times were short in winter and long in summer (reversed in FL and a portion of the Gulf of Mexico). On the West Coast, estuaries tended to flush quickly compared with estuaries in other regions, even though they usually had low freshwater inflows, since other factors, e.g., the estuarine volume, affected the flushing time as well. To ensure appropriate interpretation of responses to change in nutrient loading, the significant intra- and interannual variations in stratification and flushing time need to be incorporated into management and assessment of estuaries.

Spatial Differences in Zooplankton Community Structure between Two Fluvial Lakes in the Middle and Lower Reaches of the Yangtze River: Effects of Land Use Patterns and Physicochemical Factors

The zooplankton community composition in shallow lakes is influenced by numerous factors, such as environmental factors and the land use patterns around the lake. To investigate the interaction between the spatial differences in the zooplankton community structure, aquatic parameters, and land use patterns in the Lake Chen Yao complex (Lake Chen Yao and Lake Feng Sha), we assessed them in four seasons from October 2020 to August 2021. The results showed that the zooplankton density and biomass of Lake Chen Yao were higher than the latter. The results of Pearson correlation and RDA analysis revealed that electrical conductivity (EC), Chlorophyll a (Chl.a), dissolved oxygen (DO), and pH were the main environmental factors affecting the zooplankton community structure in the two lakes. The nutrient content of nitrogen (N) and phosphorus (P) were significantly higher in Lake Chen Yao, and there was a considerable relationship with the distribution of land use patterns around the two lakes. The land use patterns were the main reason for the difference in water quality and thus the spatial variation in the characteristics of the zooplankton communities in the two lakes.

Spatiotemporal Distribution Pattern of Phytoplankton Community and Its Main Driving Factors in Dongting Lake, China—A Seasonal Study from 2017 to 2019

As it is the second-largest freshwater lake downstream of the Three Gorges Dam and an important international wetland for migratory birds, there have been concerns about the ecological water health of Dongting Lake for a long time. In the present study, we studied the evolutionary characteristics of water quality in Dongting Lake in three recent years. Moreover, the evolution rules and dominant groups of the phytoplankton community were explored, and the major influencing factors of phytoplankton and their distribution were assessed based on the field survey and detection data from 2017 to 2019. The results indicated that the water quality of Dongting Lake improved in recent years. The concentration of dissolved oxygen (DO) increased by 6.91%, whereas the concentrations of the five-day biochemical oxygen demand (BOD5), chemical oxygen demand (CODCr), ammonia nitrogen (NH4+–N), total phosphorus (TP), and total nitrogen (TN) decreased by 17.5%, 13.0%, 33.8%, 7.6%, and 13.3%, respectively. The mean phytoplankton density reached 4.15 × 105 cells·L−1 in September 2017, whereas it was only 1.62 × 105 cells·L−1 in December 2018. There were 15 dominant species belonging to Cyanobacteria, Chlorophyta, Bacillariophyta, Cryptophyta, and Miozoa. Moreover, Fragilaria radians (Kützing) D.M.Williams & Round and Aulacoseiragranulata (Ehrenberg) Simonsen were the dominant populations in all seasons. The Pearson and linear regression analysis also indicated that the composition and distribution of phytoplankton in Dongting Lake were mainly affected by electrical conductivity (Cond), BOD5, potassium permanganate (CODMn), and CODCr, especially in Eastern Dongting Lake. Of course, NH4+–N, TN, and TP were also the main factors affecting the density and species of the phytoplankton community, especially in Western Dongting Lake. Finally, we suggested that local government could take “The relationship between Yangtze River and Dongting Lake”, “The relationship between the seven fed rivers and Dongting Lake”, and “The relationship between human activities and Dongting Lake” as the breakthrough points to guarantee the ecological flow, water environment, and ecological quality of Dongting Lake.