Whole-lake Manipulation Research Articles

Do sympatric transgenic and non-transgenic common carps partition the trophic niche? A whole-lake manipulation study

Genetically modified organisms (GMO) are a potential driver of human-induced global change and interactions occurring among GMOs and non-GMOs have important implications for ecosystem structure and functioning. Genetically modified fish (transgenic fish) can improve the production and characteristics of conventional fish strains currently exploited in aquaculture, but their ecological impacts on local food web remain less studied. We settled “all-fish” growth hormone gene-transgenic and non-transgenic common carp (carpT and carpNT) sympatrically in an artificial lake, and used stable isotope analysis to test the hypothesis that carpT and carpNT alter their trophic niche to facilitate coexistence through local niche partitioning. A general paradigm for coexistence of carpNT and carpT in this study was that carpNT increased the use of low-quality food resources (such as submerged plants) to mitigate the competition for limited resources (i.e., zoobenthos) resulting from niche partitioning. This change resulted in an enlarged niche width of carpNT and increased the niche overlap with its sympatric competitor, that is, carpT, which led to an increase in the individual specialization within the population to enhance resource use efficiency. This study represents the first controlled empirical test of the trophic interactions between competitive coexisting transgenic and non-transgenic organisms at population and individual levels. Our results indicate that competitive niche partitioning operating between carpNT and carpT shifts through time, emphasizing the importance of changing interspecific interactions when considering the ecological risk of transgenic species release, and it can be inferred that it is difficult for the escaped carpT to achieve the form of an explosive and most successful population than the wild carp.

Changes in mixing depth reduce phytoplankton biomass in an Arctic lake: Results from a whole-lake experiment

ABSTRACTResearch has revealed contradictory responses of primary producers in Arctic lakes to increasing temperatures, making it unclear how future warming and climate change will alter lake productivity. We conducted a whole-lake manipulation to examine the effect of altered thermal structure on phytoplankton biomass in a lake in Greenland, one of the most rapidly warming regions of the world. Deepened lake mixing (from 4 to 8 m) in the experimental lake led to significant declines in phytoplankton biomass in the sediment traps despite warmer surface waters, indicating that changes in mixing depth may negate a positive, direct effect of warming on primary producer biomass. Light limitation (induced by deeper mixing) of phytoplankton played a greater role than temperature in structuring these Arctic lake phytoplankton communities. To put the manipulation in context, we surveyed twenty-four lakes across western Greenland to determine the strongest regional predictors of phytoplankton density. Across the landscape, lake chemistry and light attenuation in the epilimnion were the most important predictors of algal assemblages in the survey lakes. Though temperature can directly influence lake ecosystems, multiple factors will affect mixing depths of Arctic lakes, potentially leading to variable effects of warming on phytoplankton biomass and community structure.

Food Web Responses to Artificial Mixing in a Small Boreal Lake

In order to simulate food web responses of small boreal lakes to changes in thermal stratification due to global warming, a 4 year whole-lake manipulation experiment was performed. Within that time, period lake mixing was intensified artificially during two successive summers. Complementary data from a nearby lake of similar size and basic water chemistry were used as a reference. Phytoplankton biomass and chlorophyll a did not respond to the greater mixing depth but an increase was observed in the proportional abundance of diatoms, and the proportional abundance of cryptophytes also increased immediately after the onset of mixing. Obligate anoxic green sulphur bacteria vanished at the onset of mixing but gradually recovered after re-establishment of hypolimnetic anoxic conditions. No major effect on crustacean zooplankton was found, but their diversity increased in the metalimnion. During the mixing, the density of rotifers declined but protozoan density increased in the hypolimnion. Littoral benthic invertebrate density increased during the mixing due to Ephemeroptera, Asellus aquaticus and Chironomidae, whereas the density of Chaoborus larvae declined during mixing and lower densities were still recorded one year after the treatment. No structural changes in fish community were found although gillnet catches increased after the onset of the study. The early growth of perch (Perca fluviatilis) increased compared to the years before the mixing and in comparison to the reference lake, suggesting improved food availability in the experimental lake. Although several food web responses to the greater mixing depth were found, their persistence and ecological significance were strongly dependent on the extent of the disturbance. To better understand the impacts of wind stress on small lakes, long term whole-lake experiments are needed.

Experimental whole-lake increase of dissolved organic carbon concentration produces unexpected increase in crustacean zooplankton density.

The observed pattern of lake browning, or increased terrestrial dissolved organic carbon (DOC) concentration, across the northern hemisphere has amplified the importance of understanding how consumer productivity varies with DOC concentration. Results from comparative studies suggest these increased DOC concentrations may reduce crustacean zooplankton productivity due to reductions in resource quality and volume of suitable habitat. Although these spatial comparisons provide an expectation for the response of zooplankton productivity as DOC concentration increases, we still have an incomplete understanding of how zooplankton respond to temporal increases in DOC concentration within a single system. As such, we used a whole-lake manipulation, in which DOC concentration was increased from 8 to 11mgL(-1) in one basin of a manipulated lake, to test the hypothesis that crustacean zooplankton production should subsequently decrease. In contrast to the spatially derived expectation of sharp DOC-mediated decline, we observed a small increase in zooplankton densities in response to our experimental increase in DOC concentration of the treatment basin. This was due to significant increases in gross primary production and resource quality (lower seston carbon-to-phosphorus ratio; C:P). These results demonstrate that temporal changes in lake characteristics due to increased DOC may impact zooplankton in ways that differ from those observed in spatial surveys. We also identified significant interannual variability across our study region, which highlights potential difficulty in detecting temporal responses of organism abundances to gradual environmental change (e.g., browning).

Bacterial community response to changes in a tri-trophic cascade during a whole-lake fish manipulation.

Microbial communities play a key role in biogeochemical processes by degrading organic material and recycling nutrients, but can also be important food sources for upper trophic levels. Trophic cascades might modify microbial communities either directly via grazing or indirectly by inducing changes.in other biotic or in abiotic factors (e.g., nutrients). We studied the effects of a tri-trophic cascade on microbial communities during a whole-lake manipulation in which European perch (Perca fluviatilis) were added to a naturally fishless lake divided experimentally into two basins. We measured environmental parameters (oxygen, temperature, and nutrients) and zooplankton biomass and studied the changes in the bacterial community using next generation sequencing of 16S rRNA genes and cell counting. Introduction of fish reduced the biomass of zooplankton, mainly Daphnia, which partly altered the bacterial community composition and affected the bacterial cell abundances. However, the microbial community composition was mainly governed by stratification patterns and associated vertical oxygen concentration. Slowly growing green sulfur bacteria (Chlorobium) dominated the anoxic water layers together with bacteria of the candidate division ODI. We conclude that alterations in trophic interactions can affect microbial abundance, but that abiotic factors seem to be more significant controls of microbial community composition in sheltered boreal lakes.

Mercury Elimination Rates for Adult Northern Pike Esox lucius: Evidence for a Sex Effect

We examined the effect of sex on mercury elimination in fish by monitoring isotope-enriched mercury concentrations in the muscle tissue of three adult female and three adult male northern pike Esox lucius, which had accumulated the isotope-enriched mercury via a whole-lake manipulation and were subsequently moved to a clean lake. Mercury elimination rates for female and male northern pike were estimated to be 0.00034 and 0.00073 day(-1), respectively. Thus, males were capable of eliminating mercury at more than double the rate than that of females. To the best of our knowledge, our study represents the first documentation of mercury elimination rates varying between the sexes of fish. This sex difference in elimination rates should be taken into account when comparing mercury accumulation between the sexes of fish from the same population. Further, our findings should eventually lead to an improved understanding of mechanisms responsible for mercury elimination in vertebrates.

Changes in ecosystem resilience detected in automated measures of ecosystem metabolism during a whole-lake manipulation

Environmental sensor networks are developing rapidly to assess changes in ecosystems and their services. Some ecosystem changes involve thresholds, and theory suggests that statistical indicators of changing resilience can be detected near thresholds. We examined the capacity of environmental sensors to assess resilience during an experimentally induced transition in a whole-lake manipulation. A trophic cascade was induced in a planktivore-dominated lake by slowly adding piscivorous bass, whereas a nearby bass-dominated lake remained unmanipulated and served as a reference ecosystem during the 4-y experiment. In both the manipulated and reference lakes, automated sensors were used to measure variables related to ecosystem metabolism (dissolved oxygen, pH, and chlorophyll-a concentration) and to estimate gross primary production, respiration, and net ecosystem production. Thresholds were detected in some automated measurements more than a year before the completion of the transition to piscivore dominance. Directly measured variables (dissolved oxygen, pH, and chlorophyll-a concentration) related to ecosystem metabolism were better indicators of the approaching threshold than were the estimates of rates (gross primary production, respiration, and net ecosystem production); this difference was likely a result of the larger uncertainties in the derived rate estimates. Thus, relatively simple characteristics of ecosystems that were observed directly by the sensors were superior indicators of changing resilience. Models linked to thresholds in variables that are directly observed by sensor networks may provide unique opportunities for evaluating resilience in complex ecosystems.

An Unparalleled Scientific Resource Endangered

N half a century ago, a unique resource for freshwater research was created. The Experimental Lakes Area (ELA) consists of 58 lakes and their drainage basins in Ontario, Canada. The ELA was designed to allow whole-lake manipulation experiments; in addition, unperturbed ELA lakes have been monitored for the past 44 years. There is no other facility in the world that allows for the study of physical, chemical, and biological processes in whole-lake ecosystems at this order of magnitude. Now, this facility is under the threat of closure in 2013. The planned closure of the ELA has inspired outspoken support from the scientific community (see: http://saveela.org/). It has also highlighted the importance of two central features of the ELA: ecosystem-scale experiments and long-term monitoring. These features have enabled the ELA to generate policy-relevant science that has had major impact on political decision-making in the environmental arena for nearly four decades. Two classic examples of policy-relevant science at ELA are the studies of the roles of nitrogen and phosphorus in lake eutrophication 1 and of the impacts of lake acidification on fish. The former study produced one of the truly iconic images in ecology (see Figure 1) and motivated legislative and regulatory measures to control phosphate inputs to inland and coastal waters. Results of the latter study motivated controls on smokestack emissions of sulfur dioxide across all of North America. More recently, the complete collapse of a fish population in response to exposure to artificial estrogen has been observed in a new experiment. An ongoing experiment on the whole-lake effects of nanosilver will be compromised if the closure proceeds as planned. Whole-lake experiments produce more than just striking aerial photographs. They allow researchers to capture the effects of complex interactions across trophic levels and provide direct evidence of pollutant impacts on lakes without questionable extrapolation from small bottles to the ecosystem. This provides the robust results that are needed to justify significant expenditures of public funds for pollution control and mitigation. Less well-known are the long-term records of physical, biological and chemical parameters that have been compiled for unperturbed ELA lakes. These consistent, long-term records are invaluable for the identification of environmental trends in an era of global climate change. As early as 1990, a signature of

Physical and chemical consequences of artificially deepened thermocline in a small humic lake – a paired whole-lake climate change experiment

Abstract. Climate change with higher air temperatures and changes in cloud cover, radiation and wind speed alters the heat balance and stratification patterns of lakes. A paired whole-lake thermocline manipulation experiment of a small (0.047 km2) shallow dystrophic lake (Halsjärvi) was carried out in southern Finland. A thermodynamic model (MyLake) was used for both predicting the impacts of climate change scenarios and for determining the manipulation target of the experiment. The model simulations assuming several climate change scenarios indicated large increases in the whole-lake monthly mean temperature (+1.4–4.4 °C in April–October for the A2 scenario), and shortening of the length of the ice covered period by 56–89 days. The thermocline manipulation resulted in large changes in the thermodynamic properties of the lake, and those were rather well consistent with the simulated future increases in the heat content during the summer-autumn season. The manipulation also resulted in changes in the oxygen stratification, and the expansion of the oxic water layer increased the spatial extent of the sediment surface oxic-anoxic interfaces. In addition, the experiment affected several other chemical constituents; concentrations of organic carbon, TotN, and NH4 showed a statistically significant decrease, likely due to both changes in hydrological conditions during the experiment period and increased decomposition and sedimentation. In comparison with the results of a similar whole-lake manipulation experiment in a deep, oligotrophic, clear-watered lake in Norway, it is evident that shallow dystrophic lakes, common in the boreal region, are more sensitive to physical perturbations. This means that projected climate change may modify their physical and chemical conditions in the future.

Induced morphological defence: evidence from whole-lake manipulation experiments

Predator-induced defences are activated by cues associated with predators and confer some degree of resistance to subsequent attacks. Laboratory studies of many taxa have revealed such induced defences, and these data often conform to large-scale surveys of defence levels in habitats with and without predators. However, there have been no studies that make the direct connection between these laboratory studies and field surveys. We conducted a large-scale field manipulation of predators to provide this connection. Previous laboratory experiments on dragonfly (Leucorrhinia dubia) larvae have demonstrated that the presence of fish predators induces the development of elongated abdominal spines that serve to reduce mortality risk. In this study we determine the effect of whole-lake predator manipulation on this induced morphological defence of L. dubia. We monitored the spine lengths of final-instar larvae in two experimental lakes for 7 consecutive years. Fish were present during the first 2 years and then removed for the remaining 5 years. Results demonstrate that the spine lengths of L. dubia larvae decreased significantly in both lakes after the removal of fish. In contrast, there was no corresponding change in the spine lengths of larvae in reference lakes, and we found little change in food supply for larvae. Our results suggest that the plastic response in spine length is strong and attributable to the presence of predators.

Pelagic C:N:P Stoichiometry in a Eutrophied Lake: Responses to a Whole-Lake Food-Web Manipulation

Changes in the ecological stoichiometry of C, N, and P in the pelagic zone are reported from a whole-lake manipulation of the food web of Lake 227, an experimentally eutrophied lake at the Experimental Lakes Area, Canada. Addition of northern pike eliminated populations of planktivorous minnows by the third year (1995) after pike introduction, and in the fourth year after pike addition (1996), a massive increase in the abundance of the large-bodied cladoceran Daphnia pulicaria occurred. Accompanying this increase in Daphnia abundance, zooplankton community N:P declined, seston concentration and C:P ratio decreased, and dissolved N and P pools increased. During peak abundance, zooplankton biomass comprised a significant proportion of total epilimnetic phosphorus (greater than 30%). During the period of increased Daphnia abundance, concentrations of dissolved inorganic nitrogen (TIN) increased more strongly than dissolved phosphorus (TDP), and thus TIN:TDP ratios were elevated. Sedimentation data indicated that increased grazing led to greatly reduced residence times of C, N, and especially P in the water column during 1996. Finally, previously dominant N-fixing cyanobacteria were absent during 1996. Our results show that strong effects of food-web structure can occur in eutrophic lakes and that stoichiometric mechanisms play a potentially important role in generating these effects.

Acid-Induced Changes in DOC Quality in an Experimental Whole-Lake Manipulation

Fluorescence analyses of archived water samples were used to typify dissolved organic carbon (DOC) quality in experimentally acidified lakes and reference lakes at the Experimental Lakes Area, in northwestern Ontario. Carbon-specific DOC fluorescence (CSF) during peak acidification (pH 4.5) was 40−50% of that for a high-DOC reference lake and similar to a low-DOC reference lake. Reference lakes showed similar but smaller decreases in CSF during several years of prolonged drought in the late 1980s. During the 1990s, recovery from acidification resulted in increased CSF, whereas reference lakes remained unchanged during the same time period. In addition to causing decreased [DOC], acidification causes changes in fluorescence-peak geometry that indicate a switch in DOC quality from allochthonous to “autochthonous-like” during acidification. The acid-induced change in DOC quality was likely due to increased chemical oxidation or precipitation of the UV-absorbent aromatic portions of allochthonous DOC molecules, leaving more UV-transparent aliphatic chains. The change in the nature of DOC following acidification and drought may have an important role in physical, biological, and chemical processes within these lakes. With recovery from acidification, DOC quality has also recovered.

Stoichiometric Constraints on Food-Web Dynamics: A Whole-Lake Experiment on the Canadian Shield

A whole-lake manipulation of food-web structure (introduction of a top predator, northern pike, to a minnow-dominated lake) was performed in a Canadian Shield lake (L110) to examine the stoichiometric consequences of changes in planktonic community structure generated by altered food-web structure. Minnow abundance, zooplankton biomass and community composition, microconsumer abundance, and concentration and carbon–phosphorus (C:P) ratio of suspended particulate matter were monitored in L110 and unmanipulated L240 before (1992) and after (1993–95) pike introduction. Algal biomass in L110 determined from microscopic examination for postmanipulation and premanipulation periods was also compared with dynamics in a suite of unmanipulated reference lakes from long-term monitoring records. Pike were added in spring in 1993 and 1994 in sufficient quantity to raise pike biomass to levels of around 22 kg ha−1 by 1994. Minnow populations in L110 responded dramatically, decreasing to levels 30% (1993), 10% (1994), and less than 1% (1995) of premanipulation values. However, most components lower in the food web did not respond in a manner consistent with predictions of existing food-web theory, such as the idea of cascading trophic interactions (CTI). While Daphnia biomass increased in L110 in the first year following manipulation, consistent with CTI, this effect was temporary and Daphnia collapsed in 1995, the year of lowest minnow abundance. Total zooplankton biomass in both lakes declined during the study period and, contrary to CTI, this decline appeared somewhat stronger in L110 than in L240. Dominant microconsumers (heterotrophic microflagellates) did not differ among years in either lake and did not appear to respond to food-web manipulation. At the bottom of the food web, no changes in bacterial biomass occurred in either lake. However, total concentrations of particulate matter appeared to increase in L110 after manipulation (contrary to expectations based on the theory of CTI) while algal biomass did not change in the manipulated lake relative to reference systems. Finally, particulate C:P increased in both L110 and L240 during the study period. The lack of strong response of Daphnia, the lack of response of the microbial food web, decreases in zooplankton biomass and increases in particulate biomass following reduction of minnow populations after piscivore introduction are at odds with expectations from existing food-web theory, such as the idea of CTI as currently formulated. However, the extremely high C:P ratios in particulate matter at the base of the food webs in these lakes, the coincidence of zooplankton declines and increases in particulate C:P ratios, and the results of small-scale mesocosm food-quality experiments are consistent with a hypothesis of a stoichiometric constraint operating on food-web dynamics in this and similar ecosystems.

Changes in the Composition of Somatic and Gonadal Tissues of Yellow Perch following White Sucker Removal

We determined the response of the proximate composition of yellow perch Perca flavescens to removal of a food competitor, the white sucker Catostomus commersoni, in a whole-lake manipulation of Douglas Lake, Michigan. Following removal of 80% of the adult white sucker population, the composition of somatic tissue of adult yellow perch collected from mid-April to mid-July improved, with water content decreasing and fat and caloric contents increasing. In a nearby reference lake, yellow perch proximate composition showed either no change or became less favorable during the same time period. From mid-July to mid-October, somatic tissue composition showed no improvement in Douglas Lake; this is consistent with a previous study showing that feeding rate late in the summer was not significantly different after sucker removal. Gonadal development had a strong influence on the composition of somatic tissues. Immediately following spawning, fat and energy reserves were depleted and the water content of somatic tissues was high. During the summer (May through August), yellow perch stored fat and energy in somatic tissues, and their body water contents decreased in both lakes. Both sexes of yellow perch began gonad development during September and showed concurrent declines in fat and energy content in somatic tissues. Male yellow perch completed gonad development by mid-October, whereas gonadal development in females continued until spawning in late April. Male and female yellow perch showed similar rates of decline in fat and energy content during the winter, even though females were developing gonadal tissue during this time and males were not. Because female yellow perch required additional energy for gonadal development during the winter but showed a rate of somatic energy depletion similar to that of males, it appears that female yellow perch either rely on feeding or use energy-saving behaviors during the winter to supply the additional energy required for gonadal development. The improvement we observed in yellow perch body composition following white sucker removal indicates that fish respond to reductions in competition with qualitative changes in proximate composition as well as with quantitative changes in growth.

Introduction to the Northwest Ontario Lake Size Series (NOLSS)

The rationale, design, and limitations of the Northwest Ontario Lake Size Series (NOLSS) research program are described. The primary purpose of NOLSS is to discover how lake size per se influences limnological and fisheries phenomena, so that conclusions drawn from studies of particular lakes can be rigorously scaled and applied to lakes of other sizes. NOLSS consists of six lakes located in a remote wilderness region of Northwest Ontario. These lakes were chosen for their geological, hydrological, and morphological similarity (Canadian Shield geology; water renewal time> 5 yr; fully stratified in summer), but they form an exponential gradient in surface area (from 89 to 34 700 ha.) Associated with this gradient of lake size are gradients of physical properties (turbulent energy, mixing depth, thermal behaviour) to which biological communities must adapt. NOLSS fills the conspicuous gap in size that separates two well-studied groups of lakes in Northwest Ontario: the Experimental Lakes Area (ELA), where whole-lake manipulation experiments are performed, and the Laurentian Great Lakes (Nipigon, Superior), where these experiments find some of their most important applications.

Response of Yellow Perch and the Benthic Invertebrate Community to a Reduction in the Abundance of White Suckers

Abstract Competitive interactions between white suckers Catostomus commersoni and yellow perch Perca flavescens were evaluated in a whole-lake manipulation experiment with one treatment lake and one reference lake. After a 2-year pretreatment period, 80% of the adult white sucker population was removed from Douglas Lake, Michigan, with trap nets set during the spring spawning season of 1987. Following white sucker removal, the abundance of chironomid larvae and the mayfly Caenis sp. increased 13–18-fold in Douglas Lake, In Little Bear Lake, the control lake, Caenis sp. showed a 20% decline in abundance during the study period (1985–1989), and chironomid larvae showed a 2.2-fold increase over the same time period. Associated with the increase in benthic invertebrate abundance in Douglas Lake, the diet of adult (>age-0) yellow perch shifted from predominantly zooplankton to predominantly benthos. This shift was accompanied by an increase in mean stomach content weight and feeding rate, eventually becoming a...