Whole-lake Experiment Research Articles

Adding Oxygen to a Lake to Explore Methane Emissions

A rare whole-lake experiment suggests that in some cases, low-oxygen conditions may have a smaller impact on methane release to the atmosphere than previously thought.

Density- and size-dependent mechanisms modulate the outcome of stocking in a naturally recruiting freshwater piscivore (northern pike, Esox lucius): A replicated whole-lake experiment

Despite the widespread use of stock enhancements to improve fisheries across the world, there is a lack of robust knowledge under which conditions fish stocking provides additive effects rather than merely replacing a fraction of natural recruitment. Fully controlled and replicated studies at the level of entire ecosystems are needed to provide answers. Properly monitored experimental releases also allow testing mechanisms of density- and size-dependent population regulation. In this study, the population-level outcome of stocking juvenile pike (Esox lucius L.) in naturally reproducing lentic stocks was investigated. We used a replicated before-after-control-impact design in 15 experimental lakes involving two stocking densities and unstocked controls. Releasing age-0 pike failed to generate additive effects at the age-2 cohort. As expected from theory, we observed density-dependent mortality and differential survival of wild and stocked pike. Stocked and wild fishes showed contrasting responses in terms of growth to variation in predator density, competitor density and the forage base, suggesting both subpopulations differed in their response to stocking-induced changes in population traits (e.g., density). Despite the lack of additive effects caused by stocking, a fraction of the stocked individuals established in the stock-enhanced cohort, indicating that replacement of wild recruits by stocked conspecifics had occurred. Depending on the origin of the stocking material, pike stocking thus has the potential for genetic hybridization, while not necessarily benefiting fisheries catch. We conclude that whenever a natural pike population exists, stocking juvenile pike will not produce additive effects in lakes and that enhanced pike populations will be strongly regulated by size- and density-dependent juvenile mortality and less by density-dependent growth.

Reprint of: The ecology of the zooplankton in Mexican inland waters: What we know so far

The varied physiography and climate of the Mexican territory have produced a large mosaic of inland aquatic ecosystems. However, zooplankton ecology has been studied mainly from the water bodies of Central (Trans Mexican Volcanic Belt), eastern (the Yucatán Peninsula), and southern Mexico. In contrast, the zooplankton from the Mesa Central water bodies has been insufficiently investigated. After a long history of zooplankton taxonomic studies, the first contributions to the zooplankton ecology dated to the 1990 s, rapidly increasing afterward. Most ecological information arises from field investigations and experiments under laboratory conditions, lacking mesocosms and whole-lake experiments. In the same way, the studies have been carried out in lentic, natural, and artificial water bodies, primarily eutrophic. Differently, the zooplankton ecology of lotic ecosystems is largely unknown. Here we summarized the progress made in the study of the zooplankton ecology of Mexican inland waters, revealing the most important research topics developed so far.

Ecosystem-based management outperforms species-focused stocking for enhancing fish populations.

Ecosystem-based management is costly. Therefore, without rigorously showing that it can outperform traditional species-focused alternatives, its broad-scale adoption in conservation is unlikely. We present a large-scale replicated and controlled set of whole-lake experiments in fish conservation (20 lakes monitored over 6 years with more than 150,000 fish sampled) to examine the outcomes of ecosystem-based habitat enhancement (coarse woody habitat addition and shallow littoral zone creation) versus a widespread, species-focused alternative that has long dominated fisheries management practice (i.e., fish stocking). Adding coarse woody habitats alone did not, on average, enhance fish abundance, but creating shallow water habitat consistently did, especially for juvenile fish. Species-focused fish stocking completely failed. We provide strong evidence questioning the performance of species-focused conservation actions in aquatic ecosystems and instead recommend ecosystem-based management of key habitats.

Nontraditional biomanipulation: A powerful ecotechnology to combat cyanobacterial blooms in eutrophic freshwaters

<p>Cyanobacterial blooms, occurring frequently in eutrophic freshwaters worldwide, are considered as potential hazards to ecosystems and human health, and it is often difficult and expensive to control their outbreaks in large lakes through reduction of nutrient loadings. Biomanipulation, launched as an ecology-based solution in the 1970s, was once believed to be an effective way to counteract cyanobacterial blooms. It is divided into traditional biomanipulation (TB) and nontraditional biomanipulation (NTB) that use filter-feeding <i>Daphnia</i> and filter-feeding fish, respectively. There have been numerous reviews on the former, yet few on the latter. Here, we first revisit the debate on the digestibility of cyanobacteria in silver and bighead carp. Then, we review 42 experiments that clearly mention cyanobacterial changes and reveal substantial reductions in cyanobacterial abundance by filter-feeding carp in 88% of the cases. In particular, in a whole-lake experiment in Lake Donghu, increased stock of silver and bighead carp effectively decreased <i>Microcystis</i> blooms from a coverage of 87% in 2021 to 0% in 2022. Finally, we discuss possible factors related to NTB��s effectiveness that depends not only on standing stock, niche divergence and shape preference of the fish but also on trophic status of the waterbodies. Particularly, silver and bighead carp feed more effectively on colony-forming <i>Microcystis</i> than on filamentous cyanobacteria, but are capable of increasing small-sized algae. NTB can be used to prevent or diminish cyanobacterial blooms that are poorly grazed by <i>Daphnia</i>, providing an effective and sustainable in-lake ecotechnology to combat heavy cyanobacterial blooms in eutrophic waterbodies.</p>

The ecology of the zooplankton in Mexican inland waters: What we know so far

The varied physiography and climate of the Mexican territory have produced a large mosaic of inland aquatic ecosystems. However, zooplankton ecology has been studied mainly from the water bodies of Central (Trans Mexican Volcanic Belt), eastern (the Yucatán Peninsula), and southern Mexico. In contrast, the zooplankton from the Mesa Central water bodies has been insufficiently investigated. After a long history of zooplankton taxonomic studies, the first contributions to the zooplankton ecology dated to the 1990 s, rapidly increasing afterward. Most ecological information arises from field investigations and experiments under laboratory conditions, lacking mesocosms and whole-lake experiments. In the same way, the studies have been carried out in lentic, natural, and artificial water bodies, primarily eutrophic. Differently, the zooplankton ecology of lotic ecosystems is largely unknown. Here we summarized the progress made in the study of the zooplankton ecology of Mexican inland waters, revealing the most important research topics developed so far.

Freshwater Colonization, Adaptation, and Genomic Divergence in Threespine Stickleback.

The Threespine Stickleback is ancestrally a marine fish, but many marine populations breed in fresh water (i.e., are anadromous), facilitating their colonization of isolated freshwater habitats a few years after they form. Repeated adaptation to fresh water during at least 10 My and continuing today has led to Threespine Stickleback becoming a premier system to study rapid adaptation. Anadromous and freshwater stickleback breed in sympatry and may hybridize, resulting in introgression of freshwater-adaptive alleles into anadromous populations, where they are maintained at low frequencies as ancient standing genetic variation. Anadromous stickleback have accumulated hundreds of freshwater-adaptive alleles that are disbursed as few loci per marine individual and provide the basis for adaptation when they colonize fresh water. Recent whole-lake experiments in lakes around Cook Inlet, Alaska have revealed how astonishingly rapid and repeatable this process is, with the frequency of 40% of the identified freshwater-adaptive alleles increasing from negligible (∼1%) in the marine founder to ≥50% within ten generations in fresh water, and freshwater phenotypes evolving accordingly. These high rates of genomic and phenotypic evolution imply very intense directional selection on phenotypes of heterozygotes. Sexual recombination rapidly assembles freshwater-adaptive alleles that originated in different founders into multilocus freshwater haplotypes, and regions important for adaptation to freshwater have suppressed recombination that keeps advantageous alleles linked within large haploblocks. These large haploblocks are also older and appear to have accumulated linked advantageous mutations. The contemporary evolution of Threespine Stickleback has provided broadly applicable insights into the mechanisms that facilitate rapid adaptation.

Can top-down effects of planktivorous fish removal be used to mitigate cyanobacterial blooms in large subtropical highland lakes?

Removal of planktivorous fish is used extensively in northern temperate lakes to reduce phytoplankton abundance via enhanced zooplankton grazing. However, whether this method would work also in large subtropical highland lakes to alleviate cyanobacterial blooms is unknown. We conducted a one-year pilot in situ experiment where we removed a substantial biomass of fish in a fenced-in area, followed by a 3-year whole-lake experiment where the dominant fish species (Japanese smelt) was removed in Lake Erhai in southwest China. The fencing experiments showed that between July and November, when the biomass of the removed stock reached 4 g/m2, the zooplankton biomass inside the fence increased significantly compared to a control fence. In the full-lake experiment, we found that sustained removal of Japanese smelt led to an increase in the biomass of cladocerans (Daphnia spp. but especially of Bosmina spp.) and a significant decrease in the biomass of Cyanobacteria and Chlorophyta. Additionally, a marked increase in the ratio of zooplankton to phytoplankton biomass, as well as an increase in the body size of cladocerans, emphasising the importance of enhanced top-down control for mitigating cyanobacterial blooms following extensive fish removal. Our results reveal that removal of small fish (here Japanese smelt) can lead to a reduction of the phytoplankton and cyanobacteria biomass through a trophic cascade in highland deep subtropical lakes. Thus fish removal may be a feasible additional restoration tool to external nutrient loading reduction in such lakes.

Effects of Vertical Spatial Overlap on Phytoplankton Diversity under Experimentally Altered Lake Stratification Regimes.

In phytoplankton communities, competitive exclusion might occur when functionally similar species are impeded from regulating their positions along light and nutrient gradients to reduce niche overlap. Greater spatial overlap (SO) between species due to water column mixing could thus promote competitive exclusion, reducing community taxonomic diversity. However, greater SO could also promote coexistence of functionally different taxa. Using data from a whole-lake experiment, we investigated the effects of SO and other relevant environmental factors on phytoplankton diversity across the water columns of lake basins with different thermocline manipulations. We estimated SO using an in situ fluorometer, and overall community diversity microscopically. Using structured equation models, we estimated directional relationships between phytoplankton diversity, SO, the lake physical structure and the zooplankton community. No significant effect of SO on phytoplankton taxonomic diversity was observed, but higher SO was associated with greater functional diversity. Change in lake physical structure and in the zooplankton community also affected diversity, with a negative response to increased top-down interactions. Overall, despite the fact that the alteration of water column stratification structure and top-down interactions were stronger drivers of phytoplankton diversity in our system, some effect of spatial overlap on the outcome of inferred competitive interactions were observable.

David William Schindler (1940-2021).

Trailblazing aquatic researcher, advocate, and whole-ecosystem experimentalist

David W. Schindler—Icon and Iconoclast

David W. Schindler—Icon and Iconoclast

Behavioural and fitness effects of translocation to a novel environment: Whole-lake experiments in two aquatic top predators.

Translocation into a novel environment through common fisheries management practices, such as fish stocking, provides opportunities to study behavioural and fitness impacts of translocations at realistic ecological scales. The process of stocking, as well as the unfamiliarity with novel ecological conditions and the interactions with resident fish may affect translocated individuals, leading to alterations of behaviours and causing fitness impacts. Our objectives were to investigate how aquatic top predators behaviourally establish themselves and compete with resident individuals following introduction in a novel lake environment and to investigate the resulting fitness consequences. Using high-resolution acoustic telemetry, we conducted whole-lake experiments and compared the activity, activity-space size and fate of translocated and resident individuals in two model top predators, the northern pike Esox lucius (n=160) and European catfish Silurus glanis (n=33). Additionally, we compared the reproductive success of translocated and resident northern pike. The experiment was conducted with large (adult) individuals of different origins, resilient to predation, but subject to agonistic interactions and competition with resident fish. Over a period of several months, the translocated catfish exhibited consistently larger activity-space sizes than resident catfish, but did not differ from residents in activity and survival. The pike from one of the two translocated origins we tested also showed elevated space-use, and both translocated origins revealed higher mortality rates than their resident conspecifics, indicating maladjustment to their novel environment. When non-resident pike reproduced, they overwhelmingly produced hybrid offspring with resident fish, indicating that introductions fostered gene flow of non-native genes. Our study indicates that fish introductions result in behavioural and fitness impacts even in large-bodied top predators that experience low levels of natural predation risk.

The relationship between eDNA particle concentration and organism abundance in nature is strengthened by allometric scaling.

Organism abundance is a critical parameter in ecology, but its estimation is often challenging. Approaches utilizing eDNA to indirectly estimate abundance have recently generated substantial interest. However, preliminary correlations observed between eDNA concentration and abundance in nature are typically moderate in strength with significant unexplained variation. Here, we apply a novel approach to integrate allometric scaling coefficients into models of eDNA concentration and organism abundance. We hypothesize that eDNA particle production scales nonlinearly with mass, with scaling coefficients <1. Wild populations often exhibit substantial variation in individual body size distributions; we therefore predict that the distribution of mass across individuals within a population will influence population-level eDNA production rates. To test our hypothesis, we collected standardized body size distribution and mark-recapture abundance data using whole-lake experiments involving nine populations of brook trout. We correlated eDNA concentration with three metrics of abundance: density (individuals/ha), biomass (kg/ha) and allometrically scaled mass (ASM) (∑(individual mass0.73 )/ha). Density and biomass were both significantly positively correlated with eDNA concentration (adj. r2 =0.59 and 0.63, respectively), but ASM exhibited improved model fit (adj. r2 =0.78). We also demonstrate how estimates of ASM derived from eDNA samples in "unknown" systems can be converted to biomass or density estimates with additional size-structure data. Future experiments should empirically validate allometric scaling coefficients for eDNA production, particularly where substantial intraspecific size distribution variation exists. Incorporating allometric scaling may improve predictive models to the extent that eDNA concentration may become a reliable indicator of abundance in nature.

Effects of Whole-Lake Additions of Ethynylestradiol on Leech Populations.

Leeches are widespread, found in many freshwater habitats, and have diverse dietary habits. Despite their close phylogenetic relationships to Mollusca, a phylum with species affected by exogenous estrogens, it is unclear whether Hirudinea may also be impacted. A whole-lake experiment was done at the Experimental Lakes Area in Ontario, Canada, to assess whether 17α-ethynylestradiol (EE2) affected fishes and other species. Herein, we examined whether EE2 impacted leech community composition, species abundance, growth rates, gonad size, and cocoon production, when compared with reference lakes using a before-after-control-impact design. Each month baited leech traps were set overnight in the littoral zone at 10 sites around experimental Lake 260 and 2 reference lakes, and individuals were identified, weighed, and measured. Male and female organs of common species Haemopis marmorata were measured. Across all lakes, 9 species representing 3 families were collected. There were no apparent effects of EE2 on numbers, species richness, or community composition; however, condition in 1 of 5 species increased significantly after EE2 exposures. Total gonadosomatic index (GSI) and the GSI for all male or all female organs combined for H. marmorata were not affected by EE2 additions. However, some individual reproductive organs including relative sperm sac length (+), relative epididymis weight (-), relative vaginal bulb length (+), and relative ovisac + albumen gland length (+) changed after EE2 additions. Finally, whereas overall cocoon production was similar, it occurred earlier in the EE2-amended lake. In summary, few individual through community measures of leeches responded to low ng/L concentrations of EE2, suggesting that Hirudinea are less sensitive to this endocrine disruptor than other invertebrates and vertebrates. Environ Toxicol Chem 2020;39:1608-1619. © 2020 SETAC.

A small native predator reduces reproductive success of a large invasive fish as revealed by whole-lake experiments.

The extent to which native fish communities might control the success of invasive fish has been of interest to ecologists, but it has been rarely addressed using experiments. We conducted an experiment in six small lakes in the Upper Mississippi Region to test the effects of a small native predator, bluegill sunfish (Lepomis macrochirus) on the recruitment of a large, invasive fish, the common carp (Cyprinus carpio). Bluegills are predominant throughout the region and were previously shown to consume carp eggs and larvae. We stocked both lakes at each of our 3 sites with adult carp (spawners) and one lake at each site with bluegills. We repeated the experiment at two of the three sites for two consecutive years. In each lake we assessed the abundance of post-larval carp one month after spawning (backpack electrofishing surveys) and at the end of the season (mark-recapture). For each site/year combination, catch rate of post-larval carp was typically an order of magnitude higher in control than bluegill lakes, but it often declined quickly over time. The abundance of end-of-seasonal juveniles was significantly higher (no 95% CI overlap) in control lakes than in bluegill lakes, except for one pair of lakes during one year when both the control and bluegill lake had similar, low abundance of end-of-season carp. Overall, our results support the hypothesis that common carp recruitment is substantially reduced in habitats dominated by bluegills. We also suggest our results may be applicable to other species, and that managers should explore how predation on early life stages may control other invasive species.

Endocrine disruption: From a whole-lake experiment to a calibrated ecosystem model

The unique Canadian whole-ecosystem study on the impact of 17α-ethinylestradiol (EE2) on a freshwater food web (Karen et al., 2014) provides evidence of the value of whole-ecosystem experiments for understanding indirect effects of endocrine disrupting compounds (EDCs) and other aquatic stressors. To further explain the indirect effects of EE2 observed in the experimental lake, an ecosystem model based on AQUATOX equations was successfully developed and calibrated. Discussions with the scientists who gathered the experimental data were necessary to ensure the consistency of the parameters used in the model and the realism of the biomass dynamics observed in such ecosystems. The prediction results helped further explain how the other fishes were impacted by the fathead minnow collapse. This study also suggests that a mix of reduced gamete production, increased gamete mortality and fish mortality is a potential mechanism for the collapse of the fathead minnow population.

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.

Biological Nitrogen Fixation Prevents the Response of a Eutrophic Lake to Reduced Loading of Nitrogen: Evidence from a 46-Year Whole-Lake Experiment

A whole-ecosystem experiment in Lake 227 (L227) at the Experimental Lakes Area, ongoing since 1969, examined the roles of carbon (C), nitrogen (N), and phosphorus (P) in controlling eutrophication. During 2011, we conducted a series of sub-experiments and more intensive monitoring to improve estimates of N fixation and its ability to meet algal growth demands in the decades following the cessation of artificial N loading, while maintaining long-term high artificial P loading. Stoichiometric nutrient ratios indicated both moderate N and P limitation of the phytoplankton during spring, preceding a shift in phytoplankton community structure toward dominance by N fixing cyanobacteria. During bloom development, and for the remainder of the stratified period, stoichiometric nutrient ratios indicated moderate to strong P limitation. N fixation rates, corrected using 15N2 methods, increased 2× after 1990, when N loading ceased. Ambient dissolved inorganic nitrogen prior to the bloom represented less than 3% of N demands of the phytoplankton. N fixation accounted for between 69–86% of total N loading to the epilimnion during the period of rapid bloom development, and 72–86% of total N loading during the May–October period. Phytoplankton biomass did not decline in L227 during the 40 years since artificial N loading was reduced, or the nearly 25 years since artificial N loads ceased entirely (1990–2013), and remained approximately 20× higher than four nearby reference lakes. These results suggest that despite constraints on biological N fixation, it retains a large capacity to offset potential N loading reductions in freshwaters.

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.

It Takes Two to Tango: When and Where Dual Nutrient (N & P) Reductions Are Needed to Protect Lakes and Downstream Ecosystems.

Preventing harmful algal blooms (HABs) is needed to protect lakes and downstream ecosystems. Traditionally, reducing phosphorus (P) inputs was the prescribed solution for lakes, based on the assumption that P universally limits HAB formation. Reduction of P inputs has decreased HABs in many lakes, but was not successful in others. Thus, the "P-only" paradigm is overgeneralized. Whole-lake experiments indicate that HABs are often stimulated more by combined P and nitrogen (N) enrichment rather than N or P alone, indicating that the dynamics of both nutrients are important for HAB control. The changing paradigm from P-only to consideration of dual nutrient control is supported by studies indicating that (1) biological N fixation cannot always meet lake ecosystem N needs, and (2) that anthropogenic N and P loading has increased dramatically in recent decades. Sediment P accumulation supports long-term internal loading, while N may escape via denitrification, leading to perpetual N deficits. Hence, controlling both N and P inputs will help control HABs in some lakes and also reduce N export to downstream N-sensitive ecosystems. Managers should consider whether balanced control of N and P will most effectively reduce HABs along the freshwater-marine continuum.