River algal blooms are well predicted by antecedent environmental conditions

Authors

Rui Xia, Gangsheng Wang, Yuan Zhang, Peng Yang, Zhongwen Yang, Sen Ding, Xiaobo Jia, Chen Yang, Chengjian Liu, Shuqin Ma, Jianing Lin, Xiao Wang, Xikang Hou, Kai Zhang, Xin Gao, Pingzhou Duan, Chang Qian

River algal blooms have become a challenging environmental problem worldwide due to strong interference of human activities and megaprojects (e.g., big dams and large-scale water transfer projects). Previous studies on algal blooms were mainly focused on relatively static water bodies (i.e., lakes and reservoirs), but less on the large rivers. As the largest tributary of the Yangtze River of China and the main freshwater source of the South-to-North Water Diversion Project (SNWDP), the Han River has experienced frequent algal blooms in recent decades. Here we investigated the algal blooms during a decade (2003–2014) in the Han River by two gradient boosting machine (GBM) models with k-fold cross validation, which used explanatory variables from current 10-day (GBMc model) or previous 10-day period (GBMp model). Our results advocate the use of GBMp due to its higher accuracy (median Kappa = 0.9) and practical predictability (using antecedent observations) compared to GBMc. We also revealed that the algal blooms in the Han River were significantly modulated by antecedent water levels in the Han River and the Yangtze River and water level variation in the Han River, whereas the nutrient concentrations in the Han River were usually above thresholds and not limiting algal blooms. This machine-learning-based study potentially provides scientific guidance for preemptive warning and risk management of river algal blooms through comprehensive regulation of water levels during the dry season by making use of water conservancy measures in large rivers.

The role of phosphorus and nitrogen on chlorophyll a: Evidence from hundreds of lakes

Authors

Zhongyao Liang, Patricia A. Soranno, Tyler Wagner

The effect of nutrients on phytoplankton biomass in lakes continues to be a subject of debate by aquatic scientists. However, determining whether or not chlorophyll a (CHL) is limited by phosphorus (P) and/or nitrogen (N) is rarely considered using a probabilistic method in studies of hundreds of lakes across broad spatial extents. Several studies have applied a unified CHL-nutrient relationship to determine nutrient limitation, but pose a risk of ecological fallacy because they neglect spatial heterogeneity in ecological contexts. To examine whether or not CHL is limited by P, N, or both nutrients in hundreds of lakes and across diverse ecological settings, a probabilistic machine learning method, Bayesian Network, was applied. Spatial heterogeneity in ecological context was accommodated by the probabilistic nature of the results. We analyzed data from 1382 lakes in 17 US states to evaluate the cause-effect relationships between CHL and nutrients. Observations of CHL, total phosphorus (TP), and total nitrogen (TN) were discretized into three trophic states (oligo-mesotrophic, eutrophic, and hypereutrophic) to train the model. We found that although both nutrients were related to CHL trophic state, TP was more related to CHL than TN, especially under oligo-mesotrophic and eutrophic CHL conditions. However, when the CHL trophic state was hypereutrophic, both TP and TN were important. These results provide additional evidence that P-limitation is more likely under oligo-mesotrophic or eutrophic CHL conditions and that co-limitation of P and N occurs under hypereutrophic CHL conditions. We also found a decreasing pattern of the TN/TP ratio with increasing CHL concentrations, which might be a key driver for the role change of nutrients. Previous work performed at smaller scales support our findings, indicating potential for extension of our findings to other regions. Our findings enhance the understanding of nutrient limitation at macroscales and revealed that the current debate on the limiting nutrient might be caused by failure to consider CHL trophic state. Our findings also provide prior information for the site-specific eutrophication management of unsampled or data-limited lakes.

Evaluation of algal chlorophyll and nutrient relations and the N:P ratios along with trophic status and light regime in 60 Korea reservoirs

Authors

Md Mamun, Seokcheol Kwon, Jeong-Eun Kim, Kwang-Guk An

The present study aimed to determine the spatial and temporal variations in trophic state and identify potential causes for these variations in 60 Korean reservoirs. Empirical models were developed using the relations of nutrients (total phosphorus, TP, and total nitrogen, TN) with chlorophyll-a (CHL-a) for efficient lake managements. The empirical models indicated that TP was the key regulating factor for algal growth in agricultural (R² = 0.69) and power generation (R² = 0.50) reservoirs. The CHL-a:TP and TN:TP ratios, indicators of phosphorus limitation, were used to validate the phosphorus reduction approach. The mean CHL-a:TP ratio of agricultural reservoirs was 0.60, indicating that algal chlorophyll is potentially limited by TP than any other factors. Agricultural, multipurpose, and power generation reservoirs, based on the N:P ratios, were more P- limited systems than natural lakes and estuarine reservoirs. The trophic state index (TSI) of Korean reservoirs varied between mesotrophy to hypereutrophy based on values of TSI (TP), TSI (CHL-a), and TSI (SD). Agricultural reservoirs were hypereutrophic using the criteria of TSI (CHL-a) and blue-green algae dominated the algal community. Analysis of trophic state index deviation (TSID) indicated that agricultural reservoirs were primarily P limited and other factors had minor effect. In contrast, the trophic status of estuarine and power generation reservoirs and natural lakes was largely modified by non-algal turbidity. Our outcomes may be effectively used for Korean lakes and reservoirs management.

Variable Oxygen Levels Lead to Variable Stoichiometry of Benthic Nutrient Fluxes in a Hypertrophic Estuary

Authors

Marco Bartoli, Sara Benelli, Marta Lauro, Monia Magri, Irma Vybernaite-Lubiene & Jolita Petkuviene

Harmful blooms of cyanobacteria may extend over long time spans due to self-sustaining mechanisms. We hypothesized that settled blooms may increase redox-dependent P release and unbalance the stoichiometry of benthic nutrient regeneration (NH₄⁺:SiO₂:PO₄³⁻ ratios). We tested this hypothesis in the hypertrophic Curonian Lagoon, the largest in Europe. During summer, at peak chlorophyll and water temperatures, sediment cores were collected over 19 stations representing all the lagoon sedimentary environments. Sediment organic content, granulometry, aerobic respiration, and oxic and anoxic fluxes of dissolved inorganic nutrients and metals—Fe²⁺ and Mn²⁺—were measured. Loads and stoichiometry of regenerated nutrients were compared with those from the watershed. Analyzed sediments had elevated oxygen demand (−1.90 to −5.66 mmol O₂ m⁻² h⁻¹), generally uncoupled to their variable organic matter content (1–23%) and median particle size (30–300 μm). Under oxic conditions, summer internal recycling equaled (SiO₂) or exceeded, by a factor of ~66 and ~ 2, external loads of NH₄⁺ and PO₄³⁻, respectively. Transient anoxia produced a general decrease of NH₄⁺ and SiO₂ regeneration, likely due to decreased macrofauna activity or inefficient mineralization, whereas it doubled average PO₄³⁻ fluxes. In sandy, well-flushed areas, anoxia had a minor effect on PO₄³⁻, but stimulated a large production of Mn²⁺. Muddy sediments in lagoon areas with slow water renewal displayed large redox-dependent PO₄³⁻ mobility, coupled to Fe²⁺ release. Settled algal blooms and hypoxic conditions might unbalance benthic regeneration stoichiometry and sustain blooms. The sedimentary pool of Mn⁴⁺ may represent a natural buffer preventing iron reduction and PO₄³⁻ mobility.

Role of algal accumulations on the partitioning between N2 production and dissimilatory nitrate reduction to ammonium in eutrophic lakes

Authors

Xingyu Jiang, Guang Gao, Lu Zhang, Xiangming Tang, Keqiang Shao, Yang Hu, Jian Cai

Cyanobacterial blooms change benthic nitrogen (N) cycling in eutrophic lake ecosystems by affecting organic carbon (OC) delivery and changing in nutrients availability. Denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are critical dissimilatory nitrate reduction pathways that determine N removal and N recycling in aquatic environments. A mechanistic understanding of the influence of algal accumulations on partitioning among these pathways is currently lacking. In the present study, a manipulative experiment in aquarium tanks was conducted to determine the response of dissimilatory nitrate reduction pathways to changes in algal biomass, and the interactive effects of OC and nitrate. Potential dinitrogen (N₂) production and DNRA rates, and related functional gene abundances were determined during incubation of 3–4 weeks. The results indicated that high algal biomass promoted DNRA but not N₂ production. The concentrations of dissolved organic carbon were the primary factor affecting DNRA rates. Low nitrate availability limited N₂ production rates in treatments with algal pellets and without nitrate addition. Meanwhile, the AOAamoA gene abundance was significantly correlated with the nrfA and nirS gene abundances, suggesting that coupled nitrification-denitrification/DNRA was prevalent. Partitioning between N₂ production and DNRA was positively correlated with the ratios of dissolved organic carbon to nitrate. Correspondingly, in Lake Taihu during summer to fall, the relatively high organic carbon/nitrate might favorably facilitate DNRA over denitrification, subsequently sustaining cyanobacterial blooms.

Trophic control changes with season and nutrient loading in lakes

Authors

Tanya L. Rogers Stephan B. Munch Simon D. Stewart Eric P. Palkovacs Alfredo Giron‐Nava Shin‐ichiro S. Matsuzaki Celia C. Symons

Experiments have revealed much about top‐down and bottom‐up control in ecosystems, but manipulative experiments are limited in spatial and temporal scale. To obtain a more nuanced understanding of trophic control over large scales, we explored long‐term time‐series data from 13 globally distributed lakes and used empirical dynamic modelling to quantify interaction strengths between zooplankton and phytoplankton over time within and across lakes. Across all lakes, top‐down effects were associated with nutrients, switching from negative in mesotrophic lakes to positive in oligotrophic lakes. This result suggests that zooplankton nutrient recycling exceeds grazing pressure in nutrient‐limited systems. Within individual lakes, results were consistent with a ‘seasonal reset’ hypothesis in which top‐down and bottom‐up interactions varied seasonally and were both strongest at the beginning of the growing season. Thus, trophic control is not static, but varies with abiotic conditions – dynamics that only become evident when observing changes over large spatial and temporal scales.

Tools for successful proliferation: diverse strategies of nutrient acquisition by a benthic cyanobacterium

Authors

H. S. Tee, D. Waite, L. Payne, M. Middleditch, S. Wood & K. M. Handley

Freshwater cyanobacterial blooms have increased worldwide, channeling organic carbon into these systems, and threatening animal health through the production of cyanotoxins. Both toxic and nontoxic Microcoleus proliferations usually occur when there are moderate concentrations of dissolved inorganic nitrogen, but when phosphorus is scarce. In order to understand how Microcoleus establishes thick biofilms (or mats) on riverbeds under phosphorus-limiting conditions, we collected Microcoleus-dominated biofilms over a 19-day proliferation event for proteogenomics. A single pair of nitrogen-dependent Microcoleus species were consistently present in relatively high abundance, although each followed a unique metabolic trajectory. Neither possessed anatoxin gene clusters, and only very low concentrations of anatoxins (~2 µg kg⁻¹) were detected, likely originating from rarer Microcoleus species also present. Proteome allocations were dominated by photosynthesizing cyanobacteria and diatoms, and data indicate biomass was actively recycled by Bacteroidetes and Myxococcales. Microcoleus likely acquired nutrients throughout the proliferation event by uptake of nitrate, urea, and inorganic and organic phosphorus. Both species also harbored genes that could be used for inorganic phosphate solubilization with pyrroloquinoline quinone cofactors produced by cohabiting Proteobacteria. Results indicate that Microcoleus are equipped with diverse mechanisms for nitrogen and phosphorus acquisition, enabling them to proliferate and out-compete others in low-phosphorus waters.

Nutrients and Heavy Metals in Legacy Sediments: Concentrations, Comparisons with Upland Soils, and Implications for Water Quality

Authors

Alyssa Lutgen, Grant Jiang, Nathan Sienkiewicz, Katie Mattern, JinJun Kan, Shreeram Inamdar

Concentrations of nutrients and heavy metals in streambank legacy sediments are needed to estimate watershed exports and to evaluate against upland inputs. Concentrations of nutrients and heavy metals were determined for legacy sediments in 15 streambanks across northeastern Maryland, southeastern Pennsylvania, and northern Delaware. Samples were collected from multiple bank depths from forested, agricultural, urban, and suburban sites. Analyses were performed for fine (<63 μm) and coarse sediment fractions. Nutrient and heavy metal concentrations were significantly higher in fine than coarse legacy sediments and water extractable nutrient concentrations were significantly greater for fine sediments. Nutrient and heavy metal concentrations were highest in streambank legacy sediments associated with urban land use, but few differences were found with bank depth. Total N (40–3,970 mg/kg) and P (25–1,293 mg/kg) and bioavailable P (0.25–48.8 mg/kg) concentrations for legacy sediments were lower than those for reported for upland soils. This suggests that legacy sediments could serve as sink or source of N and P depending on the redox conditions and stream water nutrient concentrations. However, despite low concentrations, caution should be exercised since streambank erosion and legacy sediment mass loadings could be high, these sediments are in immediate proximity of aquatic ecosystems, and biogeochemical transformations could result in release of the nutrients.

Quantifying the dependence of cyanobacterial growth to nutrient for the eutrophication management of temperate-subtropical shallow lakes

Authors

Wei Zou, Guangwei Zhu, Yongjiu Cai, Hai Xu, Mengyuan Zhu, Zhijun Gong, Yunlin Zhang, Boqiang Qin

The increasing global occurrence of cyanobacterial blooms, attributed primarily to human-induced nutrient enrichment, significantly degrades freshwater ecosystems and poses serious risks to human health. The current study examined environmental variables and cyanobacterial biovolume (B_Cyano) of 28 shallow lakes in the eastern China plains during the spring and summer of 2018. We used a 95% quantile regression model to explore season-specific response of B_Cyano to total nitrogen (TN), or total phosphorus (TP), and robust linear relationships were observed between log(B_Cyano+0.001) and log(TN), or log(TP) in both spring and summer periods. Based on these regressions, regional-scale and season-specific TN and TP thresholds are proposed for these lakes to ensure the safety for recreational waters and drinking water source. However, actual B_Cyano for a given concentration of TN (or TP) for many observations were considerably lower than the results of the 95% regression model predict, indicating that other factors significantly modulated nutrient limitation of B_Cyano. Generalized additive model and quantile regression model were used together to explore potentially significant modulating factors, of which lake retention time, macrophytes cover and N: P ratio were identified as most important. Thus, it is necessary to develop type-specific nutrient thresholds with the consideration of these significant modulating factors. Furthermore, nutrient-B_Cyano relationships of our studied lakes with lake retention time>100 days and no macrophyte were further explored and nutrient thresholds of this lake type were proposed. Nutrient thresholds proposed in this study may play an essential role in achieving a cost-effective eutrophication management for shallow lakes both in the eastern China plains and elsewhere with similar climatic background. On a broader scale, the approaches and findings of this study may provide valuable reference to formulate reasonable nutrient reduction targets for other ecoregions with different climatic conditions.

Landscape Drivers of Dynamic Change in Water Quality of U.S. Rivers

Authors

Edward G. Stets, Lori A. Sprague, Gretchen P. Oelsner, Hank M. Johnson, Jennifer C. Murphy, Karen Ryberg, Aldo V. Vecchia, Robert E. Zuellig, James A. Falcone, Melissa L. Riskin

Water security is a top concern for social well-being, and dramatic changes in the availability of freshwater have occurred as a result of human uses and landscape management. Elevated nutrient loading and perturbations to major ion composition have resulted from human activities and have degraded freshwater resources. This study addresses the emerging nature of streamwater quality in the 21st century through analysis of concentrations and trends in a wide variety of constituents in streams and rivers of the U.S. Concentrations of 15 water quality constituents including nutrients, major ions, sediment, and specific conductance were analyzed over the period 1982–2012 and a targeted trend analysis was performed from 1992 to 2012. Although environmental policy is geared toward addressing the long-standing problem of nutrient overenrichment, these efforts have had uneven success, with decreasing nutrient concentrations at urbanized sites and little to no change at agricultural sites. Additionally, freshwaters are being salinized rapidly in all human-dominated land use types. While efforts to control nutrients are ongoing, rapid salinity increases are ushering in a new set of poorly defined issues. Increasing salinity negatively affects biodiversity, mobilizes sediment-bound contaminants, and increases lead contamination of drinking water, but its effects are not well integrated into current paradigms of water management.