Land-Use Legacies Are Important Determinants of Lake Eutrophication in the Anthropocene

Background: A hallmark of the latter half of the 20 th century is the widespread, rapid intensification of a variety of anthropogenically-driven environmental changes—a ‘‘Great Acceleration.’ ’ While there is evidence of a Great Acceleration in a variety of factors known to be linked to water quality degradation, such as conversion of land to agriculture and intensification of fertilizer use, it is not known whether there has been a similar acceleration of freshwater eutrophication. Methodology/Principal Findings: Using quantitative reconstructions of diatom-inferred total phosphorus (DI-TP) as a proxy for lake trophic state, we synthesized results from 67 paleolimnological studies from across Europe and North America to evaluate whether most lakes showed a pattern of eutrophication with time and whether this trend was accelerated after 1945 CE, indicative of a Great Acceleration. We found that European lakes have experienced widespread increases in DI-TP over the 20 th century and that 33 % of these lakes show patterns consistent with a post-1945 CE Great Acceleration. In North America, the proportion of lakes that increased in DI-TP over time is much lower and only 9 % exhibited a Great Acceleration of eutrophication. Conclusions/Significance: The longer and more widespread history of anthropogenic influence in Europe, the leading cause for the relatively pervasive freshwater eutrophication, provides an important cautionary tale; our current path of intensive agriculture around the world may lead to an acceleration of eutrophication in downstream lakes that could tak

Can we detect ecosystem critical transitions and signals of changing resilience from paleo-ecological records?

Nonlinear responses to changing external pressures are increasingly studied in real-world ecosystems. However, as many of the changes observed by ecologists extend beyond the monitoring record, the occurrence of critical transitions, where the system is pushed from one equilibrium state to another, remains difficult to detect. Paleo-ecological records thus represent a unique opportunity to expand our temporal perspective to consider regime shifts and critical transitions, and whether such events are the exception rather than the rule. Yet, sediment core records can be affected by their own biases, such as sediment mixing or compression, with unknown consequences for the statistics commonly used to assess regime shifts, resilience, or critical transitions. To address this shortcoming, we developed a protocol to simulate paleolimnological records undergoing regime shifts or critical transitions to alternate states and tested, using both simulated and real core records, how mixing and compression affected our ability to detect past abrupt shifts. The smoothing that is built into paleolimnological data sets apparently interfered with the signal of rolling window indicators, especially autocorrelation. We thus turned to time-varying autoregressions (online dynamic linear models, DLMs; and time-varying autoregressive state-space models, TVARSS) to evaluate the possibility of detecting regime shifts and critical transitions in simulated and real core records. For the real cores, we examined both varved (annually laminated sediments) and non-varved cores, as the former have limited mixing issues. Our results show that state-space models can be used to detect regime shifts and critical transitions in some paleolimnological data, especially when the signal-to-noise ratio is strong. However, if the records are noisy, the online DLM and TVARSS have limitations for detecting critical transitions in sediment records

Prioritizing taxa for genetic reference database development to advance inland water conservation

Biodiversity loss has accelerated over the past century and freshwater species overall are among those experiencing greatest declines. Genetic resources have the potential to help evaluate the full magnitude of this loss and represent a key tool to effectively allocate conservation resources and monitor the success of restoration efforts. The full power of genetic resources will be realized when the daunting task of referencing all DNA sequences of freshwater organisms is complete. Here, we quantified the availability and distribution of barcode and genome data for freshwater macroscopic organisms in Canada, a country rich in inland water resources and thus particularly vulnerable to aquatic species losses. Impressively, most inland water species (86 %) were represented by barcodes recorded in the BOLD Systems database, while very few had full genomes available (<4 %) in the NCBI database. We identified barcode data deficiencies in northern regions and for taxa assessed as most at risk or without sufficient information for conservation status classification. As expected, the speciose insect group had a lower-than-average number of records per species and a high proportion of data deficient species without adequate barcode coverage. This study highlights where future sequencing resources should be prioritized within initiatives such as the Canada BioGenome Project and BIOSCAN Canada and provides a workflow that could be applied internationally to inform conservation management plans and to mitigate biodiversity loss

Historical and contemporary drivers of cyanobacterial dynamics: regional and global perspectives

Numerous authors are reporting that anthropogenic changes to the environment (i.e., cultural eutrophication and climate warming) have provided opportune conditions for a global dominance of phytoplankton communities by cyanobacteria. Yet, to date there has been no quantitative synthesis to support this claim and we have very little empirical evidence of synergistic relationships among the environmental drivers that could be responsible for an expansion of cyanobacteria through time. Multi-year lake monitoring data coupled with paleolimnological reconstructions proved to be an effective strategy to address these questions. In my doctoral thesis, I quantified temporal (decadal; 1985-2011 CE to centennial; 1800-2011 CE) cyanobacterial trends, both at the regional (Alberta, Canada) and global scale. Furthermore, to explain cyanobacterial trends observed in certain sites or time points, I evaluated additive and synergistic effects among drivers. For example, using phytoplankton time series data from five Albertan lakes, I show that the increase in cyanobacterial biomass is greatest under a combination of warmer water temperatures, increased intensity of thermal stratification, and elevated nutrient concentrations. I further show that space for time substitutions are valid for predicting cyanobacterial biomass by comparing empirical models developed from lake surveys to decadal-scale time series. To test whether the response of cyanobacteria in the face of environmental development has changed since the start of Anthropocene (ca. 1850 CE), I then compared models based on different time scales (decadal to centennial scales). In extending the timeframe, I found as much as a 70% loss in variance explained with the centennial models. At this regional scale, I showed that the anthropogenic changes that have taken place since ca. 1945 CE (eutrophication and warming of surface waters) have facilitated and explained the expansion of cyanobacteria. Finally, I quantified the increases in cyanobacterial abundance (rate and magnitude) over the past ~200 years by conducting a synthesis of more than 100 paleolimnological records and ~20 long-term monitoring records. With this dataset, I have shown that in the Northern Hemisphere, the increase in cyanobacteria is related to nutrient concentrations, lake elevation and lake morphometry. In general, my thesis has advanced our knowledge of the spatio-temporal dynamics of cyanobacteria in temperate lakes and the factors that can explain these dynamics. My thesis also helped isolate some of the causes of the accelerated expansion of cyanobacteria when environmental thresholds are crossed and when interactions amongst these drivers develop.De nombreux auteurs suggèrent que les changements environnementaux d'origine anthropique (c.-à-d., eutrophisation et réchauffement climatique) ont créé des conditions propices pour une dominance globale des cyanobactéries dans les communautés phytoplanctoniques. Cependant, à ce jour, il n'existe pas de synthèse quantitative pour appuyer cette présomption et nous avons très peu d'évidences empiriques des relations synergétiques entre les facteurs environnementaux qui peuvent être responsables d'une expansion des cyanobactéries à travers le temps. Un échantillonnage pluriannuel des lacs, couplé avec des reconstitutions paléolimnologiques s'est avéré une stratégie efficace pour répondre à ces questions. Dans ma thèse de doctorat, j'ai quantifié les tendances temporelles (bi-décennale; 1985-2011 CE à centenaire; 1800-2011 CE) de cyanobactéries, aussi bien au niveau régional (Alberta, Canada) qu'au niveau mondial. De plus, afin d'expliquer les tendances de cyanobactéries observées dans certains sites ou points temporels, j'ai également évalué les effets additifs et synergétiques entre les facteurs environnementaux. Par exemple, en utilisant des suivis de données de phytoplancton à long terme, provenant de cinq lacs albertains, j'ai démontré que l'augmentation de la biomasse des cyanobactéries est due à l'effet additif de la hausse des températures d'eau, de l'intensification de la stratification thermique, et des concentrations de nutriments élevés. En comparant des modèles empiriques basés sur des données spatiales à des données temporelles, j'ai démontré que l'utilisation de données spatiales est une méthode valide pour prédire la biomasse des cyanobactéries à l'échelle décennale. J'ai ensuite comparé des modèles basés sur différentes échelles temporelles (bi-décennale à centenaire) pour vérifier si la réponse des cyanobactéries face aux développements environnementaux a changée depuis le début de l'Anthropocène (ca. 1850 CE). En prolongeant l'échelle temporelle, j'ai noté une perte de variance expliquée de plus de 70% chez les modèles centenaires. Au niveau régional, les changements anthropiques qui sont survenus depuis les années ca. 1945 CE (eutrophisation et augmentation de la température de l'eau) ont facilité et expliquent l'expansion des cyanobactéries. Finalement, j'ai quantifié l'augmentation de l'abondance des cyanobactéries (accélération et ampleur) au cours des dernières ~200 ans en menant une synthèse de plus de 100 archives paléolimnologiques et ~20 suivis de phytoplancton à long terme. Avec ces données, j'ai démontré que dans l'hémisphère nord, l'augmentation des cyanobactéries est reliée à l'augmentation de l'apport en nutriments, l'élévation et la morphométrie des lacs. En termes généraux, ma thèse a amélioré nos connaissances de la dynamique spatio-temporelle des cyanobactéries dans les lacs tempérés et des facteurs expliquant cette dynamique. Ma thèse a également permis d'isoler certaines causes de l'expansion accélérée des cyanobactéries lorsque des seuils environnementaux sont franchis et lorsque des interactions entre ces facteurs se développent

Tracking changes in water quality due to catchment land-use and lake morphometry across spatial and temporal scales

Past studies have shown that diffuse nutrient loading from agricultural activities is an important cause of lake eutrophication. The degree to which this relationship can be scaled-up (e.g. at an inter-regional scale) has not, however, been widely addressed. My thesis objectives were therefore to define the generality and the impact of agriculture land use and lake morphometry on lake water quality. Analyses along spatial and temporal scales were conducted to evaluate the significance of these effects. In the first manuscript, we tested whether agricultural activities explain a significant proportion of the variation in lake water quality at a broad inter-regional scale. The degree to which lake mean depth modulates this response was also assessed. From our meta-analyses of 358 lakes, we noted a significant correlation between total phosphorus concentration and the extent of agricultural catchment development. This relationship was further strengthened by including lake mean depth as a second predictor. We also observed among-study variability in the relationship between these three variables. Thus, although there is a general relationship between total phosphorus concentrations and our two predictors, agriculture catchment development and lake mean depth, regional baseline nutrient differences modify this relationship. To address the issue of lake morphometry more closely, we adopted a spatio-temporal approach to investigate whether the effect of agricultural catchment development on water quality differed between dimictic and polymictic Albertan lakes. We found that the correlation between surface water total phosphorus concentration and the percent of agriculture in a lake's catchment was strongly modified by lake mixis pattLes études antécédentes ont démontré que les chargements de nutriment diffus parvenant d'activités agricoles sont une cause importante de l'eutrophisation des lacs. Par contre, le degré auquel cette relation peut être extrapolé à l'échelle interrégionale n'a pas été largement étudié. Ma thèse a pour but de définir la généralité et l'impact de l'usage des terres agricoles et la morphométrie des lacs sur la qualité d'eau. Des analyses selon des échelles spatiale et temporelle ont évalué la significativité de ces effets. Dans ce premier manuscrit, nous avons examiné si les activités agricoles pouvaient expliquer une proportion significative de la variation de la qualité de l'eau sur une vaste échelle interrégionale. Notre métaanalyse, basée sur des données parvenant de 358 lacs, a démontré une corrélation significative entre la concentration de phosphores totaux et le pourcentage de terres agricoles des basins versants. Cette corrélation a été améliorée en incluant la profondeur moyenne comme seconde variable prédictive. Nous avons aussi observé une variabilité parmi les études pour la relation entre ces trois variables. Donc, étant donné qu'il y a une relation générale entre la concentration de phosphores totaux et deux indices, le développement de l'agriculture des bassins versants et la profondeur moyenne des lacs, des différences de niveau de référence régionales modifie cette relation. Pour déterminer l'importance de la morphométrie des lacs plus en détaille, nous avons adopté une approche spatio-temporelle pour vérifier si les effets du développement de l'agriculture des bassins versants sur la qualité de l'eau sont différents entre les systèmes dimictiques et

Using the Diversity, Taxonomic and Functional Attributes of a Zooplankton Community to Determine Lake Environmental Typology in the Natural Southern Boreal Lakes (Québec, Canada)

Herein, we used zooplankton as a study model for determining how biodiversity components as well as taxonomic and functional attributes reflect lake typology in the natural southern boreal lakes. We estimated the regional and local variation in zooplankton diversity and the community structure across a set of fourteen lakes within a national park. Regional diversity (γ diversity) accounted for 40 species including 20 rotifers, 10 cladocerans and 8 copepods. Local diversity (α diversity) averaged 15 species per lake. Spatial variation in β diversity was inversely related to spatial variation in α diversity. Inter-lake variation in zooplankton communities based on taxonomy, functional traits and biotic indices was explained by two major limnological gradients: namely lake trophic status and fish community. The community structure reflected a gradient of rotifer to calanoid copepod dominance in response to trophic status. Several key species of rotifers (Kellicottia longispina and Conochilus unicornis) and of small (Bosmina and Diaphanosoma birgei) or large (Daphnia catawba and Holopedium gibberum cf glacialis) cladocerans were good indicators of lake zooplankton typology, as in other boreal lakes. We distinguished two main groups of lakes: (1) oligotrophic lakes inhabited by brook trout and dominated by the calanoid copepods and (2) mesotrophic lakes inhabited by northern pike and dominated by rotifers. Overall, our study can help managers better define monitoring and conservation strategies for lake ecosystems in natural parks

Meteorological and Nutrient Conditions Influence Microcystin Congeners in Freshwaters

Cyanobacterial blooms increasingly impair inland waters, with the potential for a concurrent increase in cyanotoxins that have been linked to animal and human mortalities. Microcystins (MCs) are among the most commonly detected cyanotoxins, but little is known about the distribution of different MC congeners despite large differences in their biomagnification, persistence, and toxicity. Using raw-water intake data from sites around the Great Lakes basin, we applied multivariate canonical analyses and regression tree analyses to identify how different congeners (MC-LA, -LR, -RR, and -YR) varied with changes in meteorological and nutrient conditions over time (10 years) and space (longitude range: 77°2′60 to 94°29′23 W). We found that MC-LR was associated with strong winds, warm temperatures, and nutrient-rich conditions, whereas the equally toxic yet less commonly studied MC-LA tended to dominate under intermediate winds, wetter, and nutrient-poor conditions. A global synthesis of lake data in the peer-reviewed literature showed that the composition of MC congeners differs among regions, with MC-LA more commonly reported in North America than Europe. Global patterns of MC congeners tended to vary with lake nutrient conditions and lake morphometry. Ultimately, knowledge of the environmental factors leading to the formation of different MC congeners in freshwaters is necessary to assess the duration and degree of toxin exposure under future global change

Quantifying microcystin concentrations, their composition and drivers across over 400 north-temperate and boreal Canadian lakes

Microcystins (MCs) are the most commonly measured of the cyanotoxins. Roughly 250 MC congeners have been identified to date with noted differences in their toxicity and persistence. Regional and national scale models for different parts of the world have been instrumental in identifying the drivers of total MC concentrations including toxin-producing cyanobacteria biomass, nutrients (total nitrogen and phosphorus), agricultural development in the watershed, temperature and light. Most of the MC work in Canadian lakes has focused on total concentrations and congener-specific data are lacking. Using the first Canada-wide lake set, with standardized sampling generated through the NSERC LakePulse Network, we quantified the importance of biotic and abiotic predictors of total MCs as well as several congeners from up to 440 lakes. Overall, MCs were detected in 30% of lakes using ELISA, mostly in central Canada within the Prairies and Boreal Plains ecoregions. Total concentrations were generally low, with just 10% of lakes exceeding the WHO drinking water guideline. While considering a broad suite of variables – physiography, water quality, land use, zooplankton abundance, climate, and cyanobacteria enumerations - MCs were most associated with high nutrients, low hypolimnetic oxygen, high chlorophyll-a and the biomass of known MC producers, particularly Microcystis

Extrinsic vs. Intrinsic Regimes Shifts in Shallow Lakes: Long-Term Response of Cyanobacterial Blooms to Historical Catchment Phosphorus Loading and Climate Warming

To evaluate the relative influence of intrinsic and extrinsic factors on ecosystem dynamics and regime shifts, we examined the algal response to historical catchment phosphorus loading from two shallow lakes located in Quebec, Canada. Roxton Pond is a eutrophic shallow lake with submerged macrophytes, and Lake Petit Saint-François (PSF) is a hypereutrophic shallow lake with no submerged macrophytes. Specifically, we inferred past cyanobacteria dynamics using pigment analyses, and tested whether the most parsimonious response model for cyanobacteria dynamics was congruent with the response model for phosphorus loading to the catchment. For both lakes, we found that an abrupt increase in cyanobacteria concentration lagged behind the initial increases in agricultural phosphorus use in the catchment as well as climate warming by over a decade. The delayed cyanobacterial response to these external drivers, observed in both lakes, suggests that intrinsic factors more than likely played important roles in ecosystem dynamics. These results show that cyanobacteria dominance in shallow lakes can be brought on by intrinsic responses to catchment phosphorus loading, climate warming, or both, but the timing depends on the antecedent conditions and the magnitude of the external forcing

Small changes in climate can profoundly alter the dynamics and ecosystem services of tropical crater lakes.

African tropical lakes provide vital ecosystem services including food and water to some of the fastest growing human populations, yet they are among the most understudied ecosystems in the world. The consequences of climate change and other stressors on the tropical lakes of Africa have been informed by long-term analyses, but these studies have largely focused on the massive Great Rift Valley lakes. Our objective was to evaluate how recent climate change has altered the functioning and services of smaller tropical lakes, which are far more abundant on the landscape. Based on a paired analysis of 20 years of high-resolution water column data and a paleolimnological record from a small crater lake in western Uganda, we present evidence that even a modest warming of the air (∼0.9°C increase over 20 years) and changes in the timing and intensity of rainfall can have significant consequences on the dynamics of this common tropical lake type. For example, we observed a significant nonlinear increase (R(2) adj  = 0.23, e.d.f. = 7, p<0.0001) in thermal stability over the past 20 years. This resulted in the expansion of anoxic waters and consequent deterioration of fish habitat and appears to have abated primary production; processes that may impair ecosystem services for a vulnerable human population. This study on a system representative of small tropical crater lakes highlights the far-reaching effects of global climatic change on tropical waters. Increased research efforts into tropical aquatic ecosystem health and the development of sound management practices are necessary in order to strengthen adaptive capabilities in tropical regions