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

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

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

Niche Separation Increases With Genetic Distance Among Bloom-Forming Cyanobacteria

Bacterial communities are composed of distinct groups of potentially interacting lineages, each thought to occupy a distinct ecological niche. It remains unclear, however, how quickly niche preference evolves and whether more closely related lineages are more likely to share ecological niches. We addressed these questions by following the dynamics of two bloom-forming cyanobacterial genera over an 8-year time-course in Lake Champlain, Canada, using 16S amplicon sequencing and measurements of several environmental parameters. The two genera, Microcystis (M) and Dolichospermum (D), are frequently observed simultaneously during bloom events and thus have partially overlapping niches. However, the extent of their niche overlap is debated, and it is also unclear to what extent niche partitioning occurs among strains within each genus. To identify strains within each genus, we applied minimum entropy decomposition (MED) to 16S rRNA gene sequences. We confirmed that at a genus level, M and D have different preferences for nitrogen and phosphorus concentrations. Within each genus, we also identified strains differentially associated with temperature, precipitation, and concentrations of nutrients and toxins. In general, niche similarity between strains (as measured by co-occurrence over time) declined with genetic distance. This pattern is consistent with habitat filtering – in which closely related taxa are ecologically similar, and therefore tend to co-occur under similar environmental conditions. In contrast with this general pattern, similarity in certain niche dimensions (notably particulate nitrogen and phosphorus) did not decline linearly with genetic distance, and instead showed a complex polynomial relationship. This observation suggests the importance of processes other than habitat filtering – such as competition between closely related taxa, or convergent trait evolution in distantly related taxa – in shaping particular traits in microbial communities

Water column dynamics in Lake Nkuruba 1992–2010.

<p>a) annual means of depth to anoxia (squares) and transparency (diamonds), and inferred primary production (circles) expressed as sedimentary concentrations of β-carotene. b) 1992–2007 variations in the concentrations of four sedimentary pigments associated with diatoms (diamonds), cryptophytes (triangles), chlorophytes (squares *concentration values for sedimentary lutein were an order of magnitude higher than the others, and were divided by 10 to fit the graph) and cyanobacteria (circles).</p

Sedimentary pigment variations (1910–2008) and core chronology.

<p>a) Historical variations in selected pigments and important 20^th century human interventions in the Lake Nkuruba catchment. The dotted horizontal line indicates that the uppermost data points (shown as empty circles) were not considered in our analyses due to differences in digenesis between these and older samples <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086561#pone.0086561-Guillizzoni1" target="_blank">[29]</a>. b) Unsupported ²¹⁰Pb concentrations in 16 samples and constant rate of supply (CRS) model used to determine downcore ages (including error bars).</p