Sierra Nevada mountain lake microbial communities are structured by temperature, resources and geographic location

Authors

Marika A. Schulhof Andrew E. Allen Eric E. Allen Natalie Mladenov John P. McCrow Natalie T. Jones Jessica Blanton Hamanda B. Cavalheri Drishti Kaul Celia C. Symons Jonathan B. Shurin

Warming, eutrophication (nutrient fertilization) and brownification (increased loading of allochthonous organic matter) are three global trends impacting lake ecosystems. However, the independent and synergistic effects of resource addition and warming on autotrophic and heterotrophic microorganisms are largely unknown. In this study, we investigate the independent and interactive effects of temperature, dissolved organic carbon (DOC, both allochthonous and autochthonous) and nitrogen (N) supply, in addition to the effect of spatial variables, on the composition, richness, and evenness of prokaryotic and eukaryotic microbial communities in lakes across elevation and N deposition gradients in the Sierra Nevada mountains of California, USA. We found that both prokaryotic and eukaryotic communities are structured by temperature, terrestrial (allochthonous) DOC and latitude. Prokaryotic communities are also influenced by total and aquatic (autochthonous) DOC, while eukaryotic communities are also structured by nitrate. Additionally, increasing N availability was associated with reduced richness of prokaryotic communities, and both lower richness and evenness of eukaryotes. We did not detect any synergistic or antagonistic effects as there were no interactions among temperature and resource variables. Together, our results suggest that (a) organic and inorganic resources, temperature, and geographic location (based on latitude and longitude) independently influence lake microbial communities; and (b) increasing N supply due to atmospheric N deposition may reduce richness of both prokaryotic and eukaryotic microbes, probably by reducing niche dimensionality. Our study provides insight into abiotic processes structuring microbial communities across environmental gradients and their potential roles in material and energy fluxes within and between ecosystems.

 

Mitigating the global expansion of harmful cyanobacterial blooms: Moving targets in a human- and climatically-altered world

Authors

Hans W. Paerl, Malcolm A. Barnard

Cyanobacterial harmful algal blooms (CyanoHABs) are a major threat to human and environmental health. As global proliferation of CyanoHABs continues to increase in prevalence, intensity, and toxicity, it is important to identify and integrate the underlying causes and controls of blooms in order to develop effective short- and long-term mitigation strategies. Clearly, nutrient input reductions should receive high priority. Legacy effects of multi-decadal anthropogenic eutrophication have altered limnetic systems such that there has been a shift from exclusive phosphorus (P) limitation to nitrogen (N) limitation and N and P co-limitation. Additionally, climate change is driving CyanoHAB proliferation through increasing global temperatures and altered precipitation patterns, including more extreme rainfall events and protracted droughts. These scenarios have led to the “perfect storm scenario”; increases in pulsed nutrient loading events, followed by persistent low-flow, long water residence times, favoring bloom formation and proliferation. To meet the CyanoHAB mitigation challenge, we must: (1) Formulate watershed and airshed-specific N and P input reductions on a sliding scale to meet anthropogenic and climatic forcings. (2) Develop CyanoHAB management strategies that incorporate current and anticipated climatic changes and extremes. (3) Make nutrient management strategies compatible with other physical-chemical-biological mitigation approaches, such as altering freshwater flow and flushing, dredging, chemical applications, introduction of selective grazers, etc. (4) Target CyanoHAB toxin production and developing management approaches to reduce toxin production. (5) Develop broadly applicable long-term strategies that incorporate the above recommendations.

Effects of lake warming on the seasonal risk of toxic cyanobacteria exposure

Authors

Nicole M. Hayes Heather A. Haig Gavin L. Simpson Peter R. Leavitt

Incidence of elevated harmful algal blooms and concentrations of microcystin are increasing globally as a result of human‐mediated changes in land use and climate. However, few studies document changes in the seasonal and interannual concentrations of microcystin in lakes. Here, we modeled 11 yr of biweekly microcystin data from six lakes to characterize the seasonal patterns in microcystin concentration and to ascertain if there were pronounced changes in the patterns of potential human exposure to microcystin in lakes of central North America. Bayesian time series analysis with generalized additive models found evidence for a regional increase in microcystin maxima and duration but recorded high variation among lakes. During the past decade, warmer temperatures, but not nutrient levels, led to a marked increase in the number of days when concentrations exceeded drinking and recreational water thresholds set by the World Health Organization and United States Environmental Protection Agency.

 

Methane emission offsets carbon dioxide uptake in a small productive lake

Authors

Dominic Vachon Timon Langenegger Daphne Donis Stan E. Beaubien Daniel F. McGinnis

Here, we investigate the importance of net CH₄ production and emissions in the carbon (C) budget of a small productive lake by monitoring CH₄, CO₂, and O₂ for two consecutive years. During the study period, the lake was mostly a net emitter of both CH₄ and CO₂, while showing positive net ecosystem production. The analyses suggest that during the whole study period, 32% ± 26% of C produced by net ecosystem production was ultimately converted to CH₄ and emitted to the atmosphere. When converted to global warming potential, CH₄ emission (in CO₂ equivalents) was about 3–10 times higher than CO₂ removal from in‐lake net ecosystem production over 100‐yr and 20‐yr time frames, respectively. Although more work in similar systems is needed to generalize these findings, our results provide evidence of the important greenhouse gas imbalance in human‐impacted aquatic systems.

 

Responses of lake macrophyte species and functional traits to climate and land use changes

Authors

Ji Yoon Kim, Jun Nishihiro

Aquatic plants are essential components in the regulation of microhabitat complexity and physico-chemical parameters in lake ecosystems. Increased eutrophication, land use change, modification of hydrological regimes, and expansion of invasive species are expected to impact aquatic plant community composition; however, historical pathways and response patterns are not well understood at the national scale. We analyzed temporal changes in aquatic plant communities in Japan from the early 1900s to the 2000s using field survey records from 248 lakes. Relationships of species associations with climate, land use, and lake characteristics were described using a joint species distribution model. The mean variation attributable to lake characteristics was 25.4%, followed by climate (14.0%), and land use (10.5%). Among the 13 functional traits used in our analysis, sexual and pollination traits showed marked responses to precipitation and land use. Hypohydrophily increased with precipitation, whereas monoecious aquatic plants increased in lakes surrounded by urbanized area. The relative ratio of floating to submerged plants has increased over time. Our results provide insight into long-term changes in aquatic plant communities and identify functional traits sensitive to environmental change.

Scientists’ Warning to Humanity: Rapid degradation of the world’s large lakes

Authors

Jean-Philippe Jennya, Orlane Anneville, Fabien Arnaud, Yoann Baulaz, Damien Bouffard, Isabelle Domaizon, Serghei A. Bocaniov, Nathalie Chèvre, Maria Dittrich, Jean-Marcel Dorioz, Erin S. Dunlop, Gaël Dur, Jean Guillard, Thibault Guinald, iStéphan Jacquet, Aurélien Jamoneau, Zobia Jawed, Erik Jeppesen, Gail Krantzberg, John Lenters, Barbara Leoni, Michel Meybeck, Veronica Nava, Tiina Nõges, Peeter Nõges, Martina Patelli, Victoria Pebbles, Marie-Elodie Perga, Serena Rasconi, Carl R. Ruetz III, Lars Rudstam, Nico Salmaso, Sharma Sapna, Dietmar Straile, Olga Tammeorg, Michael R. Twiss, Donald G. Uzarskia, Anne-Mari Ventelä, Warwick F. Vincent, Steven W. Wilhelm, Sten-Åke Wängberg, Gesa A. Weyhenmeyer

Large lakes of the world are habitats for diverse species, including endemic taxa, and are valuable resources that provide humanity with many ecosystem services. They are also sentinels of global and local change, and recent studies in limnology and paleolimnology have demonstrated disturbing evidence of their collective degradation in terms of depletion of resources (water and food), rapid warming and loss of ice, destruction of habitats and ecosystems, loss of species, and accelerating pollution. Large lakes are particularly exposed to anthropogenic and climatic stressors. The Second Warning to Humanity provides a framework to assess the dangers now threatening the world’s large lake ecosystems and to evaluate pathways of sustainable development that are more respectful of their ongoing provision of services. Here we review current and emerging threats to the large lakes of the world, including iconic examples of lake management failures and successes, from which we identify priorities and approaches for future conservation efforts. The review underscores the extent of lake resource degradation, which is a result of cumulative perturbation through time by long-term human impacts combined with other emerging stressors. Decades of degradation of large lakes have resulted in major challenges for restoration and management and a legacy of ecological and economic costs for future generations. Large lakes will require more intense conservation efforts in a warmer, increasingly populated world to achieve sustainable, high-quality waters. This Warning to Humanity is also an opportunity to highlight the value of a long-term lake observatory network to monitor and report on environmental changes in large lake ecosystems.

Assessing the origin of a massive cyanobacterial bloom in the Río de la Plata (2019): Towards an early warning system

Authors

Luis Aubriot, Bernardo Zabaleta, Facundo Bordet, Daniel Sienra, Jimena Risso, Marcel Achkar, Andrea Somma

The Río de la Plata estuary drains the second largest river basin of South America. The occurrence of frequent cyanobacterial blooms of the Microcystis and Dolichospermum complex in the Uruguayan coast are associated with high flows of Uruguay River due to rainy years. In summer 2019, a massive cyanobacterial bloom reached up to the Uruguayan Atlantic coast. This study seeks to unveil the origin and the environmental conditions that favored the occurrence of the last cyanobacterial bloom in the Río de la Plata, and to contribute with the development of an early warning system of cyanobacterial scum on Montevideo beaches. A complementary approach was applied with Sentinel-2 imagery, environmental data of monitoring programs of Salto Grande Reservoir and Montevideo beaches, hydro-meteorological information, and hydroelectric dam operation. Images were analyzed with the Normalized Difference Chlorophyll Index (NDCI), which allowed evaluating several water bodies within the same ranges. Positive anomalous rainfall increased river flows, particularly that of Uruguay and Negro rivers, which caused the opening of the dam spillways. NDCI maps showed that areas with high values (NDCI_>0.06) in Salto Grande reservoir kept a similar surface area before and after the prolonged overflow period (8.7–7.8 km², before and after). In the Río Negro reservoirs, however, NDCI_>0.06 coverage remarkably changed (62.5 km², Palmar reservoir), with a subsequent 56-fold reduction in the post-discharge of surface water. Twenty days after opening the spillways, Montevideo beaches were closed to swimming and the NDCI_>0.06 surface reached 51.7 km² in the Río de la Plata coast. The dynamics of NDCI areas, the downstream bloom discharge, and the predicted Río de la Plata residual currents, suggest that the cyanobacterial bloom originated in the Negro River (Palmar reservoir). This bloom event was one of the worst that occurred in the Río de la Plata in last 20 years, circulated along the Uruguayan sub-corridor to the Atlantic coast along 690 km from its origin, and lasted three months on Montevideo coast. This is the first study that addresses the impact of cyanobacterial blooms from the Negro River reservoirs on the Río de la Plata estuary. Therefore, the Negro River basin is where the main efforts should be directed to mitigate massive cyanobacterial blooms.

Harmful algae at the complex nexus of eutrophication and climate change

Author

Patricia M.Glibert

Climate projections suggest–with substantial certainty–that global warming >1.5 °C will occur by mid-century (2050). Population is also projected to increase, amplifying the demands for food, fuel, water and sanitation, which, in turn, escalate nutrient pollution. Global projections of nutrient pollution, however, are less certain than those of climate as there are regionally decreasing trends projected in Europe, and stabilization of nutrient use in North America and Australia. In this review of the effects of eutrophication and climate on harmful algae, some of the complex, subtle, and non-intuitive effects and interactions on the physiology of both harmful and non-harmful taxa are emphasized. In a future ocean, non-harmful diatoms may be disproportionately stressed and mixotrophs advantaged due to changing nutrient stoichiometry and forms of nutrients, temperature, stratification and oceanic pH. Modeling is advancing, but there is much yet to be understood, in terms of physiology, biogeochemistry and trophodynamics and how both harmful and nonharmful taxa may change in an uncertain future driven by anthropogenic activities.

Spatial and Temporal Variability of Nutrient Dynamics and Ecosystem Metabolism in a Hyper-eutrophic Reservoir Differ Between a Wet and Dry Year

Authors

Tanner J. Williamson, Michael J. Vanni & William H. Renwick

Climate change alters hydrologic regimes, including their variability. Effects will be pronounced in aquatic ecosystems, where resource subsidies (e.g., nutrients, carbon) drive key ecosystem processes. However, we know little about how changing hydrologic regimes will modulate the spatiotemporal dynamics of lake biogeochemistry and ecosystem metabolism. To address this, we quantified ecosystem metabolism and nutrient dynamicsat high spatial resolution in Acton Lake, a hyper-eutrophic reservoir in the Midwestern US. We captured two consecutive growing seasons with markedly different watershed discharge and nutrient loading. Temporal variability often exceeded spatial variability in both wet and dry years. However, relative spatial variability was higher in the dry year, suggesting that internal processes are more important in structuring spatial dynamics in dry years. Strikingly, marked differences in watershed discharge and nutrient loading between years produced relatively small differences in many lake metrics, suggesting resilience to hydrologic variability. We found little difference in gross primary productivity between wet and dry years, but ecosystem respiration was higher in the wet year, shifting net ecosystem production below zero. Discrete storm events produced strong, yet ephemeral and spatially explicit effects, reflective of the balance of stream input and discharge over the dam. Increases in limiting nutrients were restricted to near stream inlets and returned to pre-storm baseline within days. Ecosystem metabolism was suppressed during storm events, likely due to biomass flushing. Understanding how changing hydrologic regimes will mediate spatiotemporal dynamics of ecosystem metrics is paramount to preserving the ecological integrity and ecosystem services of lakes under future climates.

Climate warming and heat waves alter harmful cyanobacterial blooms along the benthic–pelagic interface

Authors

Pablo Urrutia‐Cordero Huan Zhang Fernando Chaguaceda Hong Geng Lars‐Anders Hansson

n addition to a rise in mean air and water temperatures, more frequent and intense extreme climate events (such as heat waves) have been recorded around the globe during the past decades. These environmental changes are projected to intensify further in the future, and we still know little about how they will affect ecological processes driving harmful cyanobacterial bloom formation. Therefore, we conducted a long‐term experiment in 400‐L shallow freshwater mesocosms, where we evaluated the effects of a constant +4°C increase in mean water temperatures and compared it with a fluctuating warming scenario ranging from 0 to +8°C (i.e., including heat waves) but with the same +4°C long‐term elevation in mean water temperatures. We focused on investigating not only warming effects on cyanobacterial pelagic dynamics (phenology and biomass levels), but also on their recruitment from sediments—which are a fundamental part of their life history for which the response to warming remains largely unexplored. Our results demonstrate that (1) a warmer environment not only induces a seasonal advancement and boosts biomass levels of specific cyanobacterial species in the pelagic environment, but also increases their recruitment rates from the sediments, and (2) these species‐specific benthic and pelagic processes respond differently depending on whether climate warming is expressed only as an increase in mean water temperatures or, in addition, through an increased warming variability (including heat waves). These results are important because they show, for the first time, that climate warming can affect cyanobacterial dynamics at different life‐history stages, all the way from benthic recruitment up to their establishment in the pelagic community. Furthermore, it also highlights that both cyanobacterial benthic recruitment and pelagic biomass dynamics may be different as a result of changes in the variability of warming conditions. We argue that these findings are a critical first step to further our understanding of the relative importance of increased recruitment rates for harmful cyanobacterial bloom formation under different climate change scenarios.