Rolf D. Vinebrooke

Past ultraviolet radiation environments in lakes derived from fossil pigments

Natural levels of ultraviolet (UV) radiation can harm organisms inshallow aquatic ecosystems in which concentrations of photo-protective dissolved organic carbon are low. These compounds can be removed as a result of acidic precipitation and climate changes, an effect which may have recently been manifested in up to 200,000 boreal lakes. Unfortunately, meteorological and biological monitoring studies are usually too brief to record the magnitudes of past changes in UV radiation fluxes and their effects. Here we demonstrate that certain fossil pigments in lake sediments can be used to document historical changes in the UV radiation environment of lakes. These pigments are produced by benthic algae when exposed to UV radiation and show sedimentary concentrations that are correlated to the depth of penetration of UV radiation within lakes. Analysis of fossil profiles from the sediments of two mountain lakes suggests that past UV radiation penetration has sometimes been—at least in these mid-latitude lakes—greater than during the period of anthropogenic stratospheric ozone depletion.

Net effects of multiple stressors in freshwater ecosystems: a meta-analysis.

The accelerating rate of global change has focused attention on the cumulative impacts of novel and extreme environmental changes (i.e. stressors), especially in marine ecosystems. As integrators of local catchment and regional processes, freshwater ecosystems are also ranked highly sensitive to the net effects of multiple stressors, yet there has not been a large-scale quantitative synthesis. We analysed data from 88 papers including 286 responses of freshwater ecosystems to paired stressors and discovered that overall, their cumulative mean effect size was less than the sum of their single effects (i.e. an antagonistic interaction). Net effects of dual stressors on diversity and functional performance response metrics were additive and antagonistic, respectively. Across individual studies, a simple vote-counting method revealed that the net effects of stressor pairs were frequently more antagonistic (41%) than synergistic (28%), additive (16%) or reversed (15%). Here, we define a reversal as occurring when the net impact of two stressors is in the opposite direction (negative or positive) from that of the sum of their single effects. While warming paired with nutrification resulted in additive net effects, the overall mean net effect of warming combined with a second stressor was antagonistic. Most importantly, the mean net effects across all stressor pairs and response metrics were consistently antagonistic or additive, contrasting the greater prevalence of reported synergies in marine systems. Here, a possible explanation for more antagonistic responses by freshwater biota to stressors is that the inherent greater environmental variability of smaller aquatic ecosystems fosters greater potential for acclimation and co-adaptation to multiple stressors.

DIFFERENTIAL RESPONSES OF LITTORAL COMMUNITIES TO ULTRAVIOLET RADIATION IN AN ALPINE LAKE

Differential sensitivities of benthic and planktonic communities to UV ra- diation may involve differences in habitat conditions (e.g., availability of physical refuge), taxonomic composition, UV-A (320-400 nm) and DNA-damaging UV-B (280-320 nm) irradiances, and potential indirect effects via food-web processes. These hypotheses were tested using 18 enclosures (corrals) within an alpine lake. The factorial design consisted of three UV treatments (+UV, -UV-B, -UV) and two macroinvertebrate densities (ambient, 3 X). High performance liquid chromatography was used to quantify changes in periphyton and phytoplankton abundance and composition in response to UV radiation and macroin- vertebrates over a period of 1 mo. Algal and invertebrate responses to UV radiation were habitat- and taxon-specific. Epilithic standing crop was significantly suppressed by UV radiation, primarily due to UV- B radiation inhibiting diatoms by 40%. In contrast, standing crop of epipelic (sediment- dwelling) organisms was significantly enhanced by UV-A radiation, which increased the abundance of cyanobacteria by 50%. UV radiation also significantly altered the taxonomic composition of both epilithon and epipelon. In comparison, picocyanobacterial phytoplank- ton were unaffected by UV radiation. Zoobenthos (Gammarus lacustris, Chironomidae) and zooplankton (Hesperodiaptomus arcticus, Rotifera) did not significantly alter periph- yton or phytoplankton biomass or taxonomic composition. Although total zoobenthos and zooplankton biomass were unaffected by UV radiation, UV-B significantly suppressed the final density of rotifers but not that of heavily pigmented calanoid copepods. These results show that UV radiation affects shallow-water communities in cold and unproductive systems mainly through direct effects, rather than by indirect effects mediated by food-web processes. Access to physical refuges was evidently a key factor determining habitat-specific responses to UV radiation. UV radiation did not adversely affect motile epipelon and zoobenthos that could seek refuge in sediments, but it did suppress attached epilithic taxa. In habitats devoid of physical refuge, UV tolerance was associated with photoprotective pigmentation (i.e., H. arcticus), and possibly a capacity for DNA repair (i.e., epilithic filamentous cyanobacteria and planktonic picocyanobacteria). Our findings suggest that UV exposure can affect abiotic regulation of littoral food webs in extreme environments, such as alpine, polar, and anthropogenically acidified ponds and shallow lakes.

Recent climate extremes alter alpine lake ecosystems

Here, we show that alpine lake ecosystems are responsive to interannual variation in climate, based on long-term limnological and meteorological data from the Canadian Rockies. In the 2000s, in years with colder winter temperatures, higher winter snowfall, later snowmelt, shorter ice-free seasons, and dryer summers, relative to the 1990s, alpine lakes became clearer, warmer, and mixed to deeper depths. Further, lakes became more dilute and nutrient-poor, the latter leading to significant declines in total phytoplankton biomass. However, increased concentrations of dissolved organic carbon in lake water stimulated the appearance of small mixotrophic algal species, partially offsetting the decline in autotrophic phytoplankton biomass and increasing algal species richness. The climate regime in the 2000s altered the physical, chemical, and biological character and the function of high-elevation aquatic ecosystems. Forecasts of increased climatic variability in the future pose serious ramifications for both the biodiversity and ecosystem function of high-elevation lakes.

Acceleration of cyanobacterial dominance in north temperate‐subarctic lakes during the Anthropocene

Increases in atmospheric temperature and nutrients from land are thought to be promoting the expansion of harmful cyanobacteria in lakes worldwide, yet to date there has been no quantitative synthesis of long-term trends. To test whether cyanobacteria have increased in abundance over the past ~ 200 years and evaluate the relative influence of potential causal mechanisms, we synthesised 108 highly resolved sedimentary time series and 18 decadal-scale monitoring records from north temperate-subarctic lakes. We demonstrate that: (1) cyanobacteria have increased significantly since c. 1800 ce, (2) they have increased disproportionately relative to other phytoplankton, and (3) cyanobacteria increased more rapidly post c. 1945 ce. Variation among lakes in the rates of increase was explained best by nutrient concentration (phosphorus and nitrogen), and temperature was of secondary importance. Although cyanobacterial biomass has declined in some managed lakes with reduced nutrient influx, the larger spatio-temporal scale of sedimentary records show continued increases in cyanobacteria throughout the north temperate-subarctic regions.

Trophic Dependence of Ecosystem Resistance and Species Compensation in Experimentally Acidified Lake 302S (Canada)

AbstractEcosystem resistance to the impacts of diverse human insults depends on the replacement of sensitive species by ones more tolerant of the stressor. Here we present evidence from a whole-lake acidification experiment (Lake 302S, Experimental Lakes Area, Canada) that resistance and species compensation decline with increasing trophic level. Diverse and fast-growing algal and rotifer assemblages with high dispersal potentials showed significant compensatory species dynamics, resulting in the maintenance of total biomass despite 30%–80% declines in species richness. Canonical correspondence analysis showed that significant compensatory algal and rotifer dynamics were best explained by differential species tolerances of acidified chemical conditions coupled with release from resource limitation and predation. However, less diverse cladoceran, copepod, and fish assemblages showed significant declines in total biomass and weak species compensation with loss of species during acidification. In comparison, algal and zooplankton species dynamics remained relatively synchronized in a nearby unperturbed reference lake (Lake 239) during the experiment. As a result, Lake 302S showed limited ecosystem resistance to anthropogenic acidification. Therefore, we hypothesize that lost species will increase the susceptibility of acidified lakes to the adverse impacts of other environmental stressors (for example, climate warming, stratospheric ozone depletion, invasive species). Consequently, the ecosystem stability of boreal lakes is expected to decline as global change proceeds.

Phytobenthos and Phytoplankton as Potential Indicators of Climate Change in Mountain Lakes and Ponds: A HPLC-Based Pigment Approach

Shallow mountain lakes and ponds may function as reference systems for monitoring the effects of global climate change. A survey of phytobenthos and phytoplankton communities was conducted along an altitudinal gradient of Canadian Rocky Mountain lakes and ponds to relate patterns in algal abundance and community composition to catchment and climate-related variables. Algal abundance and community composition were quantified using pigments as analyzed by high performance liquid chromatography (HPLC). Regression analyses revealed that the abundance of rock-attached algae (epilithon) was negatively correlated (r2 = 0.54, p < 0.001) to lake elevation, and positively correlated to conductivity and dissolved organic carbon (DOC) content (r2 = 0.52, pcond. < 0.03, pDOC < 0.01). Redundancy analysis (RDA) showed that elevation, conductivity, and DOC were also significant predictors of epilithon community composition. Epilithic diatoms (diatoxanthin, diadinoxanthin, fucoxanthin) declined disproportionately with increasing water transparency and decreasing chemical concentrations. In contrast, patterns in sediment-dwelling algal (epipelon) abundance and community composition were not well-explained by the suite of measured environmental variables. Phytoplankton community composition, but not abundance, was best predicted by zooplankton biomass and elevation as cryptophytes (alloxanthin) were favored in low-elevation, montane lakes and ponds containing abundant zooplankton. Also, elevated conductivity and low DOC content were associated with a compositional shift away from planktonic cryptophytes and green algae (lutein, violaxanthin) towards siliceous algae (fucoxanthin, chlorophyll c) and colonial cyanobacteria (myxoxanthophyll). These comparative results corroborate experimental findings that suggest epilithon is regulated by DOC, inorganic nutrients, and exposure to ultraviolet radiation (UV) in alpine littoral habitats. Thus, epilithon appears better suited than either phytoplankton or epipelon as a bioindicator of climatically induced variations in the abiotic environments of shallow mountain lakes and ponds.

Truncated foodweb effects of omnivorous minnows in a recovering acidified lake

Cyprinids (Margariscus margarita, Phoxinus spp., Pimephales promelas) have resumed reproduction in a boreal headwater lake (Lake 302S, Experimental Lakes Area, northwestern Ontario) that is recovering from experimental acidification. Concomitant changes to the littoral food web suggested that these omnivorous minnows suppressed the development of green algal mats, termed metaphyton. We tested this hypothesis by conducting an experiment using minnow enclosures, minnow exclosures, and open control plots in the shallow littoral zone of Lake 302S. Minnows significantly suppressed zooplankton biomass, and altered community composition by disproportionally reducing large daphnids and chydorids. Epiphytic chironomids were also significantly less abundant in the presence of minnows. Minnows had a significant time-dependent, negative effect on benthic invertebrate biomass and community composition because chironomids and anisopterans were suppressed during the second half of the 6-wk experiment. However, minnows did not reduce the abundance of the dominant primary producer, namely metaphyton. Stable isotope analyses revealed that minnows did not suppress metaphyton because these algae were not the primary C source for the food web. Instead, our findings suggest that the littoral food web depended mainly on sedimentary C, which resulted in the foodweb effect of minnows being truncated at the level of invertebrates. Therefore, metaphyton appears to be regulated primarily by abiotic factors (e.g., availability of dissolved inorganic C) and not herbivory in recovering acidified lakes.

Abiotic and biotic regulation of periphyton in recovering acidified lakes

Anthropogenic acidification of lakes alters periphyton growth and taxonomic composition. I tested whether these changes were regulated by abiotic (i.e., resources, acidity) and biotic (i.e., herbivory, competition) conditions, or both. Periphyton was reciprocally transplanted across lakes of low acidity (pH∼6.3), moderate acidity (pH∼5.6), and high acidity (pH < 5) undergoing recovery from anthropogenic acidification. Transplants were also placed in exclosures that excluded either macrograzers or all grazers and potential algal competitors. Data from 4-wk incubations suggested that periphyton growth and taxonomic composition were primarily regulated by abiotic conditions. Total biomass was significantly lower for periphyton that originated from, or was transferred into, the most acidic lake. Canonical correspondence analysis showed abiotic regulation of periphyton in acidified lakes was best explained by dissolved inorganic carbon availability. Regulation by biotic factors was detectable only in the least acidic lake where macrograzers suppressed loosely attached taxa, but not total biomass, owing to compensatory species replacements. When periphyton was transferred into less acidic lakes, only taxa that were already native to the host lake persisted through the 4-wk incubation, suggesting that algal recolonization during recovery is not impeded by dispersal barriers. My findings suggest that abiotic regulation of epilithic periphyton is superseded by biotic control as acid-sensitive macrograzers recolonize recovering acidified lakes.

Increased mercury loadings to western Canadian alpine lakes over the past 150 years.

We reconstructed historical trends in mercury (Hg) accumulation over the past ∼ 150 years in nine western Canadian alpine lakes. Recent Hg accumulation rates (fluxes) ranged between ∼ 7 and 75 μg m(-2) yr(-1), which were an average of 1.8 times higher than preindustrial (i.e., pre-1850) fluxes. Increased Hg fluxes in these lakes were less than at lower elevation sites, showing that despite the potential for increased deposition, alpine lakes are no more susceptible to Hg accumulation. Unlike other studies, we found that geographic setting, changes in chlorophyll-inferred algal production, and climate were not significant predictors of [Hg] or Hg flux in lakes. Instead, our findings highlight how a combination of atmospheric deposition and site-specific processes, including organic matter supply and catchment weathering, better explain sequestration of Hg in alpine lakes.