Uncovering the processes of microbial community assembly in the near-surface sediments of a climate-sensitive glacier-fed lake.

Glacier-fed lakes are characterized by cold temperatures, high altitudes, and nutrient-poor conditions. Despite these challenging conditions, near-surface sediments of glacier-fed lakes harbor rich microbial communities that are critical for ecosystem functioning and serve as a bridge between aquatic ecology and the deep subsurface biosphere. However, there is limited knowledge regarding the microbial communities and their assembly processes in these sediments, which are highly vulnerable to climate change. To fill this knowledge gap, this study systematically analyzed environmental variables, microbial communities, diversity, co-occurrence relationships, and community assembly processes in the near-surface sediments of a glacier-fed lake in the Tibetan Plateau. The results revealed distinct vertical gradients in microbial diversity and subcommunities, highlighting the significant influence of selection processes and adaptive abilities on microbial communities. Specifically, specialists played a crucial role within the overall microbial communities. Microbial assembly was primarily driven by homogeneous selection, but its influence declined with increasing depth. In contrast, homogenizing dispersal showed an opposite pattern, and the bottom layer exhibited heterogeneous selection and undominated processes. These patterns of microbial assembly were primarily driven by environmental gradients, with significant contributions from processes associated to ammonium and organic matter deposition, as well as chemical precipitation in response to a warming climate. This study enhances our understanding of the microbial communities and assembly processes in the near-surface sediments of glacier-fed lakes and sheds light on geo-microbiological processes in climate-sensitive lacustrine sediments.

Sensitivity Analysis of Leakage Correction of GRACE Data in Southwest China Using A-Priori Model Simulations: Inter-Comparison of Spherical Harmonics, Mass Concentration and In Situ Observations.

The Gravity Recovery and Climate Experiment (GRACE) level-2 spherical harmonic (SH) solutions are noisy and thus require filtering. Filtering reduces noise but affects signal quality via signal leakage. Generally, a leakage correction is required for GRACE applications to remove leakage signal and recover the true signal. Forward modelling based on some a priori information is a widely used approach for leakage correction of GRACE data. The a priori information generally relies on global hydrological model simulations. There are many global hydrological models and therefore it is of interest to explore how different global hydrology model simulations influence leakage correction results. This study investigated the sensitivity of three leakage correction methods (additive method, scaling factor method and multiplicative method) to five global hydrology model simulations (four models from the Global Land Data Assimilation System (GLDAS) and the WaterGAP Global Hydrology Model (WGHM)). The sensitivity analysis was performed with observational data in Southwest China and one sub-region, Guangxi. Results show that although large differences were identified among the five global model simulations, the additive and scaling factor methods are less affected by the choice of a priori model in comparison to the multiplicative approach. For the additive and scaling factor methods, WGHM outperforms the other four GLDAS models in leakage correction of GRACE data. GRACE data corrected with the multiplicative method shows the highest amount of error, indicating this method is not applicable for leakage correction in the study area. This study also assessed the level-3 mascon (mass concentration) solutions of GRACE data. The mascon-based results are nearly as good as the leakage corrected results based on SH solutions.

Abundance and Diversity of Aerobic/Anaerobic Ammonia/Ammonium-Oxidizing Microorganisms in an Ammonium-Rich Aquitard in the Pearl River Delta of South China.

Natural occurring groundwater with abnormally high ammonium concentrations was discovered in the aquifer-aquitard system in the Pearl River Delta, South China. The community composition and abundance of aerobic/anaerobic ammonia/ammonium-oxidizing microorganisms (AOM) in the aquitard were investigated in this study. The alpha subunit of ammonia monooxygenase gene (amoA) was used as the biomarker for the detection of aerobic ammonia-oxidizing archaea (AOA) and bacteria (AOB), and also partial 16S rRNA gene for Plantomycetes and anaerobic ammonium-oxidizing (anammox) bacteria. Phylogenetic analysis showed that AOA in this aquitard were affiliated with those from water columns and wastewater treatment plants; and AOB were dominated by sequences among the Nitrosomonas marina/Nitrosomonas oligotropha lineage, which were affiliated with environmental sequences from coastal eutrophic bay and subtropical estuary. The richness and diversity of both AOA and AOB communities had very little variations with the depth. Candidatus Scalindua-related sequences dominated the anammox bacterial community. AOB amoA gene abundances were always higher than those of AOA at different depths in this aquitard. The Pearson moment correlation analysis showed that AOA amoA gene abundance positively correlated with pH and ammonium concentration, whereas AOB amoA gene abundance negatively correlated with C/N ratio. This is the first report that highlights the presence with low diversity of AOM communities in natural aquitard of rich ammonium.

Investigation on bacterial community and diversity in the multilayer aquifer-aquitard system of the Pearl River Delta, China.

Bacteria play an important role in groundwater chemistry. The groundwater resource in the Pearl River Delta (PRD) is responsible for 50 million people's water requirement. High amount of ammonium, arsenic and methane had been reported in groundwater of the PRD, which was considered as the result of intensive bacterial metabolism in the multilayer aquifer-aquitard system. To investigate bacterial community in this system and its relation with groundwater chemistry, sediment and groundwater samples were taken from representative locations in the PRD at different lithological units. Bacterial 16S rRNA gene clone libraries were constructed for microbial identifications and community structures in different strata. Canonical correlation analysis between bacterial linages and environment variables (Cl(-), PO4(3-), SO4(2-), NH4(+)) showed that community structures were significantly modified by geological conditions. Higher bacterial diversity was observed in samples from the Holocene aquitard M1 and aquifer T1, while in the older aquitard M2 and basal aquifer T2, bacterial diversity was much lower. Chloroflexi, γ-proteobacteria and δ-proteobacteria were the dominant phyla in the aquitard sediment. ß-proteobacteria was the dominant phylum in sediment which was strongly influenced by fresh water. The results of this study demonstrated that bacterial community contains information of geological events such as sea transgression and deltaic evolution, and microbes in the aquitards have great potential in dominating groundwater quality in aquifers.

Synergistic controls of water column stability and groundwater phosphate on coastal algal blooms.

Algal blooms have been identified as one major threat to coastal safety and marine ecosystem functioning, but the dominant mechanism regulating the formation of algal blooms remains controversial, ranging from physical control (via water column stability), the chemical control (via coastal nutrients) to joint control. Here we leveraged the unique data collected in the Hong Kong water over the annual cycle and past three decades, including direct observations of algal blooms and coastal nutrients and process model output of water column stability, and evaluated the differential competing hypotheses in regulating algal blooms. Our results demonstrate that the joint mechanism rather than the single mechanism effectively predicts all algal blooms. Meanwhile, we observed that the adequate nutrients (phosphate, PO43-) significantly originate from coastal groundwater. The production and fluctuation of PO43- in beach aquifers are primarily governed by groundwater temperature, leading to a sustained and sufficient supply of PO43- in a low groundwater temperature environment. Furthermore, along with submarine groundwater discharge (SGD), the ongoing release of PO43- in groundwater enters coastal waters and serves as sufficient nourishment for promoting algal blooms in coastal areas. These results highlight the importance of both physical and chemical mechanisms, as well as SGD, in regulating coastal algal blooms. These findings have practical implications for the prevention of coastal algal blooms and provide insights into mariculture, water security, and the sustainability of coastal ecosystems.

Lacustrine groundwater discharge-derived carbon and nitrogen to regulate biogeochemical processes as revealed by stable isotope signals a large shallow eutrophic lake.

Eutrophic shallow lakes are hotspots of carbon (C) and nitrogen (N) accumulation and transformation, and are increasingly recognized as important sources of greenhouse gases (GHGs: CO2, CH4 and N2O). Lacustrine groundwater discharge (LGD) is a crucial component of the water budget and terrestrial material delivery for lakes, but its interplays with intrinsic CN biogeochemical processes remain less tackled. In this study, C and N ingredients and multiple stable isotopes (δ2H, δ18O, δ13C, and δ15N) were measured seasonally in groundwater, river water and lake water of a large eutrophic shallow lake in eastern China. The results revealed that groundwater is enriched with various forms of C and N that have similar sources and pathways as surface water in lake and rivers. The isotope balance model also indicated that LGD derived C and N contribute significantly to lake inventories in addition to river runoff. These allochthonous C and N provide extra substrates for related biogeochemical processes, such as algae proliferation, organic matter degradation, methanogenesis and denitrification. Simultaneously, the excess oxygen consumption leads to depletion and hypoxia in the lake, further facilitating the processes of methanogenesis and denitrification. LGD functions not only as an external source of C and N that directly increases GHG saturations, but also as a mediator of internal CN pathways, which significantly affect hypoxia formation, GHG productions and emissions in the eutrophic lake. This study highlights the unrevealed potential regulation of LGD on biogeochemical processes in the eutrophic lake, and underscores the need for its consideration in environmental and ecological studies of lakes both regionally and globally.

Disentangling external loadings, hydrodynamics and biogeochemical controls on the fate of nitrate in a coastal embayment.

Nitrogen, as an essential nutrient, largely contributes to the coastal eutrophication. However, the accurate depiction and evaluation of how external loadings, hydrodynamics, and biogeochemical reactions mediate the occurrence, transport, and transformation of nitrate (NO3-) within coastal embayment still pose ongoing challenges to date. In this study, we took advantage of dual isotopes of NO3- to track external NO3- loadings, radium and dual isotopes of H2O to characterize the influences of hydrodynamic on NO3- transport, δ18O-NO3- and δ18O-H2O along with microbial analysis to explore major NO3- biogeochemical reactions in Tolo Harbour, Hong Kong. The multiple isotopic evidence showed that NO3- in surface harbour water was predominantly contributed by precipitation in wet season and its impact was strengthened by stratification. In dry season, NO3- in the surface harbour water became largely influenced by benthic input and biogeochemical reactions due to intensified vertical mixing. Based on NO3- mass balance model, biogeochemical reaction, especially nitrification, was found to be the major process to secure the closure of NO3- budget and increase NO3- inventory from wet to dry season. Hydrodynamics redistributed the external NO3- loadings and mediated nitrogen biogeochemical reactions, both of which further synergistically regulated the fate of NO3- in the embayment.

Evolution of groundwater system in the Pearl River Delta and its adjacent shelf since the late Pleistocene.

Our extensive field studies demonstrate that saline groundwater inland and freshened groundwater offshore coexist in the same aquifer system in the Pearl River delta and its adjacent shelf. This counterintuitive phenomenon challenges the commonly held assumption that onshore groundwater is typically fresh, while offshore groundwater is saline. To address this knowledge gap, we conduct a series of sophisticated paleo-hydrogeological models to explore the formation mechanism and evolution process of the groundwater system in the inland-shelf systems. Our findings indicate that shelf freshened groundwater has formed during the lowstands since late Pleistocene, while onshore saline groundwater is generated by paleo-seawater intrusion during the Holocene transgression. This reveals that terrestrial and offshore groundwater systems have undergone alternating changes on a geological timescale. The groundwater system exhibits hysteresis responding to paleoclimate changes, with a lag of 7 to 8 thousand years, suggesting that paleoclimatic forcings exert a significantly residual influence on the present-day groundwater system.

The changing nature of groundwater in the global water cycle.

In recent decades, climate change and other anthropogenic activities have substantially affected groundwater systems worldwide. These impacts include changes in groundwater recharge, discharge, flow, storage, and distribution. Climate-induced shifts are evident in altered recharge rates, greater groundwater contribution to streamflow in glacierized catchments, and enhanced groundwater flow in permafrost areas. Direct anthropogenic changes include groundwater withdrawal and injection, regional flow regime modification, water table and storage alterations, and redistribution of embedded groundwater in foods globally. Notably, groundwater extraction contributes to sea level rise, increasing the risk of groundwater inundation in coastal areas. The role of groundwater in the global water cycle is becoming more dynamic and complex. Quantifying these changes is essential to ensure sustainable supply of fresh groundwater resources for people and ecosystems.

Storm accelerated subsurface Escherichia coli growth and exports to coastal waters.

Storm significantly deteriorates coastal water fecal pollution now and beyond. Questions relating to storm exerting on coastal water safety are often intertwined with both surface water and subsurface processes. Stormwater runoff is a vital metric for coastal water fecal pollution under current cognition, while the controls of subsurface system remain unclear. Here, this study leveraged two time-series field data collected in a sandy beach during storm and non-storm periods to probe subsurface Escherichia coli (E. coli) growth and exports to coastal waters under storm events. Results demonstrated that storm events can not only stimulate subsurface E. coli growth, but also accelerate subsurface E. coli exports into the receiving water. Storm-intensified rainfall injected more oxygenous rainwater in the shallow groundwater, subsequently stimulating subsurface E. coli growth. Storm-strengthened wave energy was responsible for accelerating subsurface E. coli exports through enhanced wave-induced recirculated seawater. This study proposes a new insight for the stress of storm events on microbial pollution in coastal waters. The findings are constructive to the prevention of beach ecosystem pollution and can pave the way for coastal safety management to future extreme weather.

Spatiotemporal-aware machine learning approaches for dissolved oxygen prediction in coastal waters.

Coastal waters face increasing threats from hypoxia, which can have severe consequences for marine life and fisheries. This study aims to develop a machine learning approach for hypoxia monitoring by investigating the effectiveness of four tree-based models, considering spatiotemporal effects in model prediction, and adopting the SHapley Additive exPlanations (SHAP) approach for model interpretability, using the long-term climate and marine monitoring dataset in Tolo Harbour (Zone 1) and Mirs Bay (Zone 2), Hong Kong. The LightBoost model was found to be the most effective for predicting dissolved oxygen (DO) concentrations using spatiotemporal datasets. Considering spatiotemporal effects improved the model's bottom DO prediction performance (R2 increase 0.30 in Zone1 and 0.68 in Zone 2), although the contributions from temporal and spatial factors varied depending on the complexity of physical and chemical processes. This study focused not only on error estimates but also on model interpretation. Using SHAP, we propose that hypoxia is largely influenced by hydrodynamics, but anthropogenic activities can increase the bias of systems, exacerbating chemical reactions and impacting DO levels. Additionally, the high relative importance of silicate (Zone 1:0.11 and Zone 2: 0.19) in the model suggests that terrestrial sources, particularly submarine groundwater discharge, are important factors influencing coastal hypoxia. This is the first machine learning effort to consider spatiotemporal effects in four dimensions to predict DO concentrations, and we believe it contributes to the development of a forecasting tool for alarming hypoxia, combining real-time data and machine learning models in the near future.

Offshore freshened groundwater in the Pearl River estuary and shelf as a significant water resource.

Large-river deltaic estuaries and adjacent continental shelves have experienced multiple phases of transgressions and regressions to form interlayered aquifer-aquitard systems and are expected to host vast paleo-terrestrial groundwater hundreds of kilometres offshore. Here, we used offshore hydrogeology, marine geophysical reflections, porewater geochemistry, and paleo-hydrogeological models, and identified a previously unknown offshore freshened groundwater body with a static volume up to 575.6 ± 44.9 km3 in the Pearl River Estuary and adjacent continental shelf, with the freshwater extending as far as 55 km offshore. An integrated analysis of stable isotopic compositions and water quality indices reveals the meteoric origins of such freshened groundwater and its significance as potential potable water or raw water source for desalination. Hotspots of offshore freshened groundwater in large-river deltaic estuaries and adjacent continental shelves, likely a global phenomenon, have a great potential for exploitable water resources in highly urbanized coastal areas suffering from freshwater shortage.

Dominance of evaporation on lacustrine groundwater discharge to regulate lake nutrient state and algal blooms.

As global threats to freshwater lakes, eutrophication and harmful algal blooms (HABs) are governed by various biogeochemical, climatological and anthropogenic processes. Groundwater is key to join these processes in regulating HABs, but the underlying mechanisms remain unclear. Here, we leveraged basin-wide field data of Lake Taihu (China's largest eutrophic lake) and global archives, and demonstrate the dominance of evaporation on lacustrine groundwater discharge (LGD) in shallow lakes. We extrapolated decadal LGD and the derived nutrient loadings and found that HABs promptly consume ubiquitous groundwater borne nutrients, leading lake water N: P ratios 2-3 months time lagged behind LGD N: P ratios. We conclude that evaporation dominated LGD is an unraveled but crucial regulator of nutrient states and HABs in shallow lakes, which advocates synergistical studies from both climatological and hydrogeological perspective when restoring lake ecosystems.

Delineating E. coli occurrence and transport in the sandy beach groundwater system by radon-222.

Fecal pollution poses a global threat to environmental safety and ecosystem, but the mechanism of microbial transport and occurrence in the beach groundwater system is still poorly explored. Here, we leveraged one-year field data of Escherichia coli (E. coli) and radon-222 (222Rn) and found that E. coli occurrence and transport in the sandy beach groundwater system can be delineated by 222Rn. The underlying mechanism behind this phenomenon is due to similar half-lives of 222Rn and E. coli in the sandy beach groundwater system. Thus, the unique relationship between 222Rn and E. coli can provide additional critical context to the microbial water quality assessments and ecosystem resilience. Also, the beach aquifer in this study is found to be a vital compartment for E. coli removal. The net E. coli removal/production capacity is identified to be highly impacted by submarine groundwater discharge. Finally, a conceptual model is constructed for a better understanding of the occurrences and characteristics of E. coli and 222Rn at multiple spatial scales. These findings are constructive to mitigate the hazardous influences of microbe on water quality, especially in recreational sandy beaches and mariculture zones.

Effective coastal Escherichia coli monitoring by unmanned aerial vehicles (UAV) thermal infrared images.

Coastal Escherichia coli (E. coli) significantly influence ocean safety and public health, thus requiring an effective E. coli pollution monitoring. However conventional detection relying on manual field sampling is time-consuming. Here, this study established an E. coli estimation model based on thermal remote sensing of unmanned aerial vehicles (UAV). This model was developed against one-year comprehensive field work in a representative sandy beach and further validated against 50 beaches in Hong Kong to evaluate its applicability. The estimated E. coli concentrations were in a reliable agreement with direct measurements. For this model, this study deployed the radon-222 (222Rn) as a bridging tracer to couple UAV thermal images and coastal E. coli concentrations. Coastal 222Rn can be reflected on the UAV thermal images, and there was a good positive correlation between the 222Rn activity and coastal E. coli concentration via one-year field data. Hence, coupling the 222Rn activity estimated from UAV thermal images and the relationship between 222Rn and E. coli, this study can readily monitor coastal E. coli by UAV. These findings highlighted that UAV technology is an effective approach to measure the E. coli concentrations and can further pave the way for an efficient coastal E. coli monitoring and public health risk warning.

Subglacial Meltwater Recharge in the Dongkemadi River Basin, Yangtze River Source Region.

Glaciers on the Tibetan Plateau play an important role in the local hydrological cycle. However, there are only few studies on groundwater in the alpine basins in the Tibetan Plateau which considered the effects of glaciers. Glaciers are extensively distributed in the Dongkemadi River Basin, which is a representative alpine basin in the Yangtze River source region. This study focuses on building a numerical groundwater flow model with glaciations using HydroGeoSphere (HGS) to simulate subglacial meltwater recharge to groundwater in the Dongkemadi River Basin in response to future climate changes. Effects of hydraulic conductivity, precipitation, and temperature on subglacial meltwater recharge to groundwater were discussed. Glacier changes in the future 50 years were predicted under different climate change scenarios. Results show that: (1) the average thickness of the glacier will change significantly; (2) the simulated rate of annual mean subglacial meltwater recharge to groundwater is 4.58 mm, which accounts for 6.33% of total groundwater recharge; and (3) hydraulic conductivity has the largest influence on subglacial meltwater recharge to groundwater, followed by temperature and precipitation. Results of this study are also important to sustainable water resource usage in the Yangtze River source region.

Microbial community assembly and co-occurrence relationship in sediments of the river-dominated estuary and the adjacent shelf in the wet season.

In the estuarine ecosystem, microbial community plays a vital role in controlling biogeochemical processes. However, there is currently limited comprehensive study on the deterministic and stochastic processes that drive the microbial community assembly in the estuaries and adjacent shelves. In this study, we systematically investigated the co-occurrence relationship and microbial community assembly in the sediments along a large river-dominated estuary to shelf in the northern South China Sea during the wet season. The sampling sites were divided into estuary, transection, and shelf sections based on their salinity values. The microbial co-occurrence networks, hierarchical partitioning-based canonical analysis, null model, neutral community model, and the Mantel test were used to investigate the community assembly. Results suggested that microbial community in the estuary section exhibited more interactions and a higher positive interaction ratio than those in the transition and shelf sections. Stochastic processes dominated community assembly in the study, with homogenizing dispersal contributing the most. The estuary exhibited a higher degree of heterogeneous selection than the transition and shelf sections, whereas homogeneous selection showed an opposite trend. Only the estuary section showed dispersal limitation and undominated processes. The river inflow and the resulting environmental heterogeneity were believed to be the key regulators of the community assembly in the studied area. Our study improved the understanding of how microbial community assembly in estuaries and adjacent shelves.

Abnormally high ammonium of natural origin in a coastal aquifer-aquitard system in the Pearl River Delta, China.

High-nitrogen loadings of rivers and aquifers systems are a major concern because of potential effects on human health and water quality impacts such as eutrophication of lakes and coastal zones. This nitrogen enrichment is commonly attributed to anthropogenic sources such as sewage and agricultural and industrial wastes. The aims of this study were to delineate spatial distribution of groundwater ammonium in the coastal aquifer system in Pearl River Delta (PRD), China and to identify the origin of the abnormally high ammonium. A total of 40 boreholes were drilled to collect core samples of the aquitard and groundwater samples in the basal aquifer. The core samples were used for extraction of pore water for centrifugation and bulk chemical analyses in laboratory. Unlike previous studies which focused mainly on the aquifer, this study treated the aquifer-aquitard system as a hydrogeochemical continuum. The results show that the aquifer-aquitard system contains an exceptionally large total ammonium mass. Ammonium occurred at concentrations up to 390 mg/L in the basal sand Pleistocene aquifer 20-50 m deep, the largest concentration reported for groundwater globally. This ammonium was natural, areally extensive (1600 km(2)) and originated in the overlying Holocene-Pleistocene aquitard and entered the aquifer by groundwater transport and diffusion. Total ammonium in the aquifer (190 × 10(6) kg) was exceeded by total ammonium in the aquitard (8600 × 10(6) kg) by a factor of 45. Much organic nitrogen remained in the aquitard available for conversion to ammonium. This natural ammonium in the aquifer was slowly transported into the PRD river channels and the estuary of the South China Sea. The rate of this contribution will likely be greatly increased by sand dredging in the river channels and estuary. Although the ammonium in PRD groundwater occurred in the largest concentrations and mass reported globally, the literature shows no reports of other delta aquitards having been examined for ammonium occurrence and therefore abundant ammonium formed in aquitards rich in organic matter may not be uncommon and this "geologic" source of ammonium may present a large and hitherto unappreciated source of nitrogen discharging to surface waters.

Two-decade variations of fresh submarine groundwater discharge to Tolo Harbour and their ecological significance by coupled remote sensing and radon-222 model.

Although submarine groundwater discharge (SGD) comprises an insignificant proportion of the global hydrologic cycle, it contributes significantly to chemical fluxes into the coastal waters due to concentrated constituents in coastal groundwater. Large nutrient loadings derived from SGD can lead to a series of environmental and ecological problems such as algal blooms, resulting in water discoloration, severe dissolved oxygen depletion, and eventually beach closures and massive fish kills. Previous studies have demonstrated the relationship between algal blooms and SGD obtained from direct measurement with seepage meters or from geo-tracer (i.e., radon and radium) based models; these traditional methods are time-consuming, laborious and point monitoring, and can hardly achieve a high spatiotemporal resolution SGD estimation, which is vital in revealing the effects of SGD to algal blooms over a long period. Alternatively, remote sensing methods for high spatiotemporal resolution SGD localization and quantification are applicable and effective. The temperature difference or anomaly between groundwater and coastal water extracted from satellite thermal images can be used as the indicator to localize and detect SGD especially its fresh component (or fresh SGD). In this study, multi-year (2005, 2011 and 2018) radon samples in Tolo Harbour were used to train regression models between in-situ radon (Rn) activity and the temperature anomaly by Landsat satellite thermal images. The models were used to estimate two-decade variations of fresh SGD in Tolo Harbour. The synergistic analysis between the time series of fresh SGD derived from regression models and high spatiotemporal resolution ecological metrics (chlorophyll-a, algal cell counts, and E.coli) leads to the findings that the increase of the fresh SGD associated with high nutrient concentrations is witnessed 10-20 days before the observations of algal bloom events. This study makes the first attempt to demonstrate the strong relation between the SGD and algal blooms over a vicennial span, and also provides a cost effective and robust technique to estimate SGD on a bay scale.

The dynamics of dissolved inorganic nitrogen species mediated by fresh submarine groundwater discharge and their impact on phytoplankton community structure.

Submarine groundwater discharge (SGD)-driven nutrient inputs have long been speculated to sustain the high frequency of red tide occurrence in Tolo Harbour, Hong Kong, for its larger flux and higher nutrient loadings than river discharge. Based on analysis of high resolution time series biogeochemical and climatological data from 2000 to 2015, fresh SGD-derived dissolved inorganic nitrogen (DIN) is found to be a significant regulator of the annual cycle of phytoplankton community structure in the harbour. In the wet season, fresh SGD supplies nutrients with NH4+:NO3- ratio < 1 to the seawater, meanwhile creates an intensive vertical stratification environment. As a result, diatom which is a NO3- specialist, is prone to be the major group in the harbour. Fresh SGD delivers a same orders of magnitude of DIN as river and precipitation, but it is more important to phytoplankton community structure dynamics because fresh groundwater has smaller NH4+:NO3- ratio that significantly changes the ratio in the harbour. In the dry season, with the decline of fresh SGD and the ease of stratification, vertical mixing uplifts the nutrient (NH4+:NO3- ratio > 1) released from the bottom sediment leading to a NH4+ dominant environment in water column. Dinoflagellate and other groups then become dominant species of phytoplankton in the harbour. Fresh SGD has a major influence on the NH4+:NO3- ratio in the seawater compared to tide-driven SGD, even though the latter contributes a larger proportion SGD. Tide-driven SGD also produces NH4+ and NO3-, but NH4+:NO3- ratio are mainly subject to the beach environment (bare/mangrove beach), which does not change much seasonally, thus dominant DIN species do not change significantly throughout a year. In a conclusion, fresh SGD plays the most important role among all the endmembers in regulating the DIN composition in Tolo Harbour and its fluctuation mediates the phytoplankton community structure.