Bythotrephes Research Articles

Bythotrephes longimanus (spiny waterflea)

This datasheet on Bythotrephes longimanus covers Identity, Overview, Distribution, Dispersal, Diagnosis, Biology & Ecology, Environmental Requirements, Natural Enemies, Impacts, Prevention/Control, Further Information.

Application of species-specific primers to estimate the in situ diet of Bythotrephes [Cladocera, Onychopoda] in its native European range via molecular gut content analysis.

The study of invasive species often focuses on regions of recent introduction rather than native habitats. Understanding an invasive species in its natural environment, however, can provide important insights regarding the long-term outcome of invasions. In this study we investigated the diet of the invasive spiny water flea, Bythotrephes longimanus, in two Austrian perialpine lakes, where it is native. The gut contents of wild-caught Bythotrephes individuals were estimated by quantitative polymerase chain reaction, targeting species-specific fragments of the barcoding region of the cytochrome c oxidase I gene of potential prey. The observed prey spectrum of Bythotrephes in the study lakes consisted primarily of Eudiaptomus gracilis and Diaphanosoma brachyurum. The Daphnia longispina complex, Leptodora kindtii and Mesocyclops leuckarti also contributed to the diet. Results indicate that Bythotrephes is a generalist feeder with a preference for epilimnetic prey.

Early presence of Bythotrephes cederstr\xf6mii (Cladocera: Cercopagidae) in lake sediments in North America: evidence or artifact?

The spiny water flea (Bythotrephes cederströmii), a freshwater crustacean considered to be the world’s best-studied invasive zooplankter, was first recorded in North America in the Laurentian Great Lakes during the 1980s. Its arrival is widely considered to be the result of ocean-going cargo ships that translocated contaminated ballast water from Eurasia to the Great Lakes during the 1970–1980s. The subsequent first discovery of the species in inland lakes is consistent with the hypothesis that propagules dispersed initially from established Great Lakes populations. Here we present evidence of exoskeletal remains, including mandibles, tail spines, and resting eggs, in 210Pb-dated lake sediment cores, which suggests that B. cederströmii was already resident in four inland North American lakes (two in Minnesota, USA; two in Ontario, Canada) by at least the early 1900s. Densities of exoskeletal remains were low and relatively steady from first appearance until about 1990, after which time they increased in all cores. The earliest evidence that we found was a mandible at 33-cm depth (pre-1650) in the sediments of Three Mile Lake, Ontario, Canada. These unexpected findings challenge the current paradigm of B. cederströmii invasion, renew uncertainty about the timing and sequence of its colonization of North American lakes, and potentially question our ability to detect invasive species with traditional sampling methods. We attempted to eliminate errors in the dated stratigraphies of the exoskeletal remains that might have been introduced either methodologically (e.g., core-wall smearing) or naturally (e.g., bioturbation). Nonetheless, given the very low numbers of subfossils encountered, questions remain about the possible artifactual nature of our observations and therefore we regard our results as ‘preliminary findings’ at this time.

Lines snag spines! A field test of recreational angling gear ensnarement of Bythotrephes

Branstrator DK, Dumke JD, Brady VJ, Wellard Kelly HA. 2021. Lines snag spines! A field test of recreational angling gear ensnarement of Bythotrephes. Lake Reserv Manage. 37:391–405. Recreational angling gear is a high-risk pathway of dispersal for the invasive spiny water flea (Bythotrephes cederströmii). We measured the number of Bythotrephes individuals ensnared on trolled shallow angling lines (3 line materials), a trolled downrigger angling line, a trolled downrigger steel cable, a trolled simulated livewell, a trolled bait bucket, and stationary anchor ropes (3 rope materials) in 2 Minnesota (United States) lakes. The shallow angling lines and the downrigger angling line had the greatest mean ensnarement rates (number of Bythotrephes individuals ensnared/transect transit), followed by the downrigger cable and the livewell, followed by the bait bucket and the anchor ropes. Added together, the shallow angling lines (as a mean of the 3 line materials) and the downrigger angling line accounted for 87–88% of the mean total ensnarement rate. Among the shallow angling lines, monofilament and fluorocarbon lines had greater mean ensnarement rates than braided line but the distinction was only statistically significant in one of the 2 lakes. The ensnarement rate of all angling gear combined was positively related to the density of Bythotrephes in the water column at the time of study (ambient density). On the downrigger angling line (monofilament), instar-3 Bythotrephes were ensnared at a relative frequency disproportionately greater than ambient density would predict, while instar-1 Bythotrephes were ensnared at a relative frequency disproportionately less than ambient density would predict. Our results suggest that education and outreach messaging should include instructions on removing Bythotrephes from angling lines in addition to the reminder to drain all water.

Lake thermal structure drives interannual variability in summer anoxia dynamics in a eutrophic lake over 37 years

Abstract. The concentration of oxygen is fundamental to lake water quality and ecosystem functioning through its control over habitat availability for organisms, redox reactions, and recycling of organic material. In many eutrophic lakes, oxygen depletion in the bottom layer (hypolimnion) occurs annually during summer stratification. The temporal and spatial extent of summer hypolimnetic anoxia is determined by interactions between the lake and its external drivers (e.g., catchment characteristics, nutrient loads, meteorology) as well as internal feedback mechanisms (e.g., organic matter recycling, phytoplankton blooms). How these drivers interact to control the evolution of lake anoxia over decadal timescales will determine, in part, the future lake water quality. In this study, we used a vertical one-dimensional hydrodynamic–ecological model (GLM-AED2) coupled with a calibrated hydrological catchment model (PIHM-Lake) to simulate the thermal and water quality dynamics of the eutrophic Lake Mendota (USA) over a 37 year period. The calibration and validation of the lake model consisted of a global sensitivity evaluation as well as the application of an optimization algorithm to improve the fit between observed and simulated data. We calculated stability indices (Schmidt stability, Birgean work, stored internal heat), identified spring mixing and summer stratification periods, and quantified the energy required for stratification and mixing. To qualify which external and internal factors were most important in driving the interannual variation in summer anoxia, we applied a random-forest classifier and multiple linear regressions to modeled ecosystem variables (e.g., stratification onset and offset, ice duration, gross primary production). Lake Mendota exhibited prolonged hypolimnetic anoxia each summer, lasting between 50–60 d. The summer heat budget, the timing of thermal stratification, and the gross primary production in the epilimnion prior to summer stratification were the most important predictors of the spatial and temporal extent of summer anoxia periods in Lake Mendota. Interannual variability in anoxia was largely driven by physical factors: earlier onset of thermal stratification in combination with a higher vertical stability strongly affected the duration and spatial extent of summer anoxia. A measured step change upward in summer anoxia in 2010 was unexplained by the GLM-AED2 model. Although the cause remains unknown, possible factors include invasion by the predacious zooplankton Bythotrephes longimanus. As the heat budget depended primarily on external meteorological conditions, the spatial and temporal extent of summer anoxia in Lake Mendota is likely to increase in the near future as a result of projected climate change in the region.

Diversity and Structure of Pelagic Zooplankton (Crustacea, Rotifera) in NE Poland

This study presents the diversity and structure of pelagic zooplankton in north-eastern Poland. The research was conducted in 47 lakes with different trophic conditions in the middle of summer. Samples were collected close to the deepest part of the lakes to avoid the diverse benthic and littoral zones. We found 119 zooplankton species of which 32 were Cladocera, 16 were Cyclopoida, 4 were Calanoida, and 67 were Rotifera. We determined which species occurred most frequently in the region, as well as the species that were characteristic of different trophic conditions. We also recorded the presence of eight cold-adapted species which some of them are considered as glacial relicts (e.g., Eurytemora lacustris, Heterocope appendiculata, Cyclops lacustris). Our research revealed potential glacial refugia for planktonic species in 14 lakes of NE Poland. Our study suggests that the presence of stenotherm species may be an excellent indicator of the ecological status of deep lakes and could be considered in lake monitoring programs. Furthermore, we did not find Bythotrephes longimanus which has been reported from Poland. Instead, we found that B. brevimanus was the most common representative of the genus in the study area.

Ancestral genetic variation in phenotypic plasticity underlies rapid evolutionary changes in resurrected populations of waterfleas

The role of phenotypic plasticity in adaptive evolution has been debated for decades. This is because the strength of natural selection is dependent on the direction and magnitude of phenotypic responses to environmental signals. Therefore, the connection between plasticity and adaptation will depend on the patterns of plasticity harbored by ancestral populations before a change in the environment. Yet few studies have directly assessed ancestral variation in plasticity and tracked phenotypic changes over time. Here we resurrected historic propagules of Daphnia spanning multiple species and lakes in Wisconsin following the invasion and proliferation of a novel predator (spiny waterflea, Bythotrephes longimanus). This approach revealed extensive genetic variation in predator-induced plasticity in ancestral populations of Daphnia It is unlikely that the standing patterns of plasticity shielded Daphnia from selection to permit long-term coexistence with a novel predator. Instead, this variation in plasticity provided the raw materials for Bythotrephes-mediated selection to drive rapid shifts in Daphnia behavior and life history. Surprisingly, there was little evidence for the evolution of trait plasticity as genetic variation in plasticity was maintained in the face of a novel predator. Such results provide insight into the link between plasticity and adaptation and highlight the importance of quantifying genetic variation in plasticity when evaluating the drivers of evolutionary change in the wild.

Climate warming restructures an aquatic food web over 28years.

Climate warming can restructure lake food webs if trophic levels differ in their thermal responses, but evidence for these changes and their underlying mechanisms remain scarce in nature. Here we document how warming lake temperatures by up to 2°C, rather than changes in trophic state or fishing effort, have restructured the pelagic food web of a large European lake (Lake Maggiore, Italy). Our approach exploited abundance and biomass data collected weekly to yearly across five trophic levels from 1981 to 2008. Temperature generally had stronger effects on taxa than changes in fish predation or trophic state mediated through primary productivity. Consequently, we found that, as the lake warmed, the food web shifted in numerical abundance towards predators occupying middle trophic positions. Of these taxa, the spiny water flea (Bythotrephes longimanus) most prospered. Bythotrephes strongly limited abundances of the keystone grazer Daphnia, strengthening top-down structuring of the food web. Warmer temperatures partly restructured the food web by advancing peak Bythotrephes densities by approximately 60days and extending periods of positive population growth by three times. Nonetheless, our results suggested that advances in the timing and size of peak Bythotrephes densities could not outpace changes in the timing and size of peak densities in their Daphnia prey. Our results provide rare evidence from nature as to how long-term warming can favour higher trophic levels, with the potential to strengthen top-down control of food webs.

Changes in water quality related to permafrost thaw may significantly impact zooplankton in small Arctic lakes.

Rising temperatures are leading to permafrost thaw over vast areas of the northern hemisphere. In the Canadian Arctic, permafrost degradation is causing significant changes in surface water quality due to the release of solutes that can alter conductivity, water clarity, and nutrient levels. For this study, we examined how changes in water quality associated with permafrost thaw might impact zooplankton, a group of organisms that play an important role in the food web of Arctic lakes. We conducted a biological and water quality survey of 37 lakes in the Mackenzie Delta region of Canada's Northwest Territories. We then used this data set to develop models linking variation in the abundance, diversity, and evenness of zooplankton communities to physicochemical, biological, and spatial variables. Subsequently, we used these models to predict how zooplankton communities might respond as water quality is altered by permafrost thaw. Our models explained 47%, 68%, and 69% of the variation in zooplankton abundance, diversity, and evenness, respectively. Importantly, the most parsimonious models always included variables affected by permafrost thaw, such as calcium and conductivity. Predictions based on our models suggest significant increases in zooplankton abundance (1.6-3.6 fold) and decreases in diversity (1.2-1.7 fold) and evenness (1.1-1.4 fold) in response to water quality changes associated with permafrost thaw. These changes are in line with those described for significant perturbations such as eutrophication, acidification, and the introduction of exotic species such as the spiny water flea (Bythotrephes). Given their important role in aquatic food webs, we expect these changes in zooplankton communities will have ramifications for organisms at higher (fish) and lower (phytoplankton) trophic positions in Arctic lakes.

Metabarcoding of native and invasive species in stomach contents of Great Lakes fishes.

As aquatic invasive species (AIS) proliferate worldwide, a better understanding of their roles in invaded habitats is needed to inform management and introduction prevention strategies and priorities. Metabarcoding of stomach content DNA (scDNA) shows considerable promise in such regard. We thus metabarcoded scDNA from two non-native fish species (alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax)), and three native ones (bloater (Coregonus hoyi), ninespine stickleback (Pungitius pungitius), and slimy sculpin (Cottus cognatus)). Fishes (N = 376) were sampled in spring 2009 and 2010 from 73–128 m depths at three Lake Michigan sites. Four mitochondrial cytochrome oxidase 1 (CO1) primer sets designed to target five potential AIS prey, and a universal aquatic invertebrate CO1 primer set targeting both native and AIS prey were used. Quality controlled prey amplicons were matched to three AIS prey: Bythotrephes longimanus (mean percent frequency occurrence, all samples = 7%), Cercopagis pengoi (5%), and Dreissena rostriformis bugensis (11%). Neither invasive prey Dreissena polymorpha nor Hemimysis anomala were detected. Native prey Leptodiaptomus sicilis, Limnocalanus macrurus, and Mysis diluviana were relatively common in scDNA (respective mean percent occurrences, all samples: 48%, 25%, 42%). Analysis of variation in prey occurrences for sample site, predator species, sample year, sample depth, and predator total length (TL) indicated site and predator species were most important. However, B. longimanus occurrence in scDNA depended upon predator TL, perhaps indicative of its unique defensive spine limiting susceptibility to predation until fishes exceed species-specific gape-based limitations. Our analysis of native and invasive prey species indicated possible indirect AIS impacts such as native predators switching their diet due to AIS-driven losses of preferred native prey. Metabarcoding demonstrated that AIS are integrated components of the offshore Lake Michigan food web, with both native and non-native predators, and both invasive and native prey are affecting species interactions across multiple trophic levels.

Piscivory in recovering Lake Michigan cisco (Coregonus artedi): The role of invasive species

Contemporary conditions in Lake Michigan where cisco (Coregonus artedi) populations are expanding are vastly different from those encountered by the historic fish community. Invasive species introductions have substantially altered the Lake Michigan ecosystem in the last half century. Successful management efforts for cisco in Lake Michigan hinge on our ability to understand their contemporary ecology, especially diet. We collected 725 cisco stomachs opportunistically from commercial fisheries (2%) and in agency surveys (98%) over six years (2014–2019). The majority (70%) of stomachs were from East Grand Traverse Bay and 96% of these were collected at Elk Rapids. Additional samples were collected from Charlevoix (8%), Little Traverse Bay (11%), other sites in northern Lake Michigan (4%), Central Lake Michigan (6%), and Green Bay (1%). Our results indicated a high degree of piscivory, in contrast to historical and contemporary accounts of planktivory for cisco in the other Laurentian Great Lakes. The top three prey items by mass were not native to the Great Lakes and these accounted for 87% of all observed prey mass consumed: round goby (Neogobius melanostomus) (58%), Bythotrephes longimanus (15%), and alewife (Alosa pseudoharengus) (14%). Round goby dominated the prey in the spring and summer, while B. longimanus and alewife occurred more in summer and fall diets. The contemporary population of cisco in Lake Michigan has been able to uniquely capitalize on abundant invasive prey resources, which may be less limiting and more energy-rich than a more typical planktivorous cisco diet.

Diet and bathymetric distribution of juvenile Lake Trout Salvelinus namaycush in Lake Huron

Rehabilitation efforts for Lake Trout Salvelinus namaycush in Lake Huron have resulted in increased capture of young wild Lake Trout in annual bottom trawl surveys conducted by the U.S. Geological Survey. To better understand the ecology of juvenile (<400 mm) Lake Trout, we summarized the spatial distribution of their capture in bottom trawls at six ports in Lake Huron during October/November 2008–2017 and analyzed diets of wild (n = 306 of 337 total) and hatchery-origin (n = 18 of 30 total) fish captured. Lake Trout ranged in size from 27 to 399 mm, representing at least three age-classes, and 92% were wild origin. Most wild juvenile Lake Trout (83%) were captured at 46–64 m depths at the two northernmost ports, typically below the thermocline. Mysis diluviana was the most prevalent prey type, found in 75% of wild fish with non-empty stomachs, followed by two non-native species: Spiny Water Flea Bythotrephes longimanus (31%) and Round Goby Neogobius melanostomus (12%). Small Lake Trout (<185 mm) consumed invertebrates but transitioned to mostly fish-based diets by >185 mm (∼age 2). The variety of taxa consumed by young Lake Trout increased with length. Further declines in Mysis populations due to increased predation pressure after the loss of Diporeia from the system may hinder the recovery of wild Lake Trout, and although they have been able to utilize invasive species as prey, impacts to Lake Trout growth remain unknown. Additional research on the habitat use and diets of wild juvenile Lake Trout may provide insight into the reasons behind the recent successful natural reproduction and recruitment of Lake Trout in Lake Huron.

Comparing models using air and water temperature to forecast an aquatic invasive species response to climate change

AbstractUnderstanding invasive species spread and projecting how distributions will respond to climate change is a central task for ecologists. Typically, current and projected air temperatures are used to forecast future distributions of invasive species based on climate matching in an ecological niche modeling approach. While this approach was originally developed for terrestrial species, it has also been widely applied to aquatic species even though aquatic species do not experience air temperatures directly. In the case of lakes, species respond to lake thermal regimes, which reflect the interaction of climate and lake attributes such as depth, size, and clarity. The result is that adjacent waterbodies can differ notably in thermal regime. Given these obvious limitations of modeling aquatic species distributions using climate data, we take advantage of recent advances in simulating lake thermal regimes to model the distributions of invasive spiny water flea (Bythotrephes cederströmii) for current and projected future climates in the upper Midwest of the USA. We compared predictions and future projections from models based on modeled air temperatures with models based on modeled water temperature. All models predicted that the number of suitable lakes in the region will decrease with climate change. Models based on air and water temperature differed dramatically in the extent of this decrease. The air temperature model predicted 89% of study lakes to be suitable, with suitability declining dramatically in the late century with climate warming to just a single suitable lake. Lake suitability predictions from the water temperature model declined to a much lesser degree with warming (42% of lakes were predicted to be suitable, declining to 19% in the late century) and were more spatially independent. Our results expose the limitations of using air temperatures to model habitat suitability for aquatic species, and our study further highlights the importance of understanding lake‐specific responses to climate when assessing aquatic species responses to climate change. While we project a contraction in the potential range of Bythotrephes with warming in the study region, we anticipate that Bythotrephes will likely continue to expand into new lakes that will remain suitable in the following decades.

Immunochemical methods for identification of prey in freshwater zooplankton

AbstractLarge, predatory, non‐indigenous species such as the cladoceran, Bythotrephes longimanus, can have a profound effect on aquatic food webs, but determining the prey consumed by such species is problematic because they shred prey externally and consume only soft body tissues. We assessed the usefulness of an immunochemical approach, in which putative zooplankton prey were homogenized and polyclonal antibodies against total proteins were raised in rabbits and purified against predator proteins. Using either dot‐blot or enzyme‐linked immunosorbent assay‐plate formats, assays easily detected prey proteins in single B. longimanus. Issues encountered included some degree of cross‐reaction with predator proteins and among prey species for some antisera and questions concerning determination of a correct blank value for assays, however, there are feasible approaches to resolving these issues. Immunochemical assays were readily applied in to a total of 185 individual B. longimanus in the Lake Michigan pelagic ecosystem in summer of 2008 and 2009 and indicated ingestion of cladoceran and copepod species, typically described in the literature, but also dreissenid veliger larvae and the large copepod Limnocalanus macrurus, not usually considered prey for Bythotrephes. Assays results showed some degree of correlation with numerical abundances of zooplankton prey in the water column, suggesting generalist feeding. Protein immunochemical assays offer a quick, sensitive and practical approach to examining planktonic food‐web questions that are compatible with, but have certain advantages over, alternatives such as nucleic acid techniques.

Walleye growth declines following zebra mussel and Bythotrephes invasion

Invasive species represent a threat to aquatic ecosystems globally; however, impacts can be heterogenous across systems. Documented impacts of invasive zebra mussels (Dreissena polymorpha) and spiny water fleas (Bythotrephes cederströmii; hereafter Bythotrephes) on native fishes are variable and context dependent across locations and time periods. Here, we use a hierarchical Bayesian analysis of a 35-year dataset on two fish species from 9 lakes to demonstrate that early life growth of ecologically important fishes are influenced by these aquatic invasive species. Walleye (Sander vitreus) in their first year of life grew more slowly in the presence of either invader after correcting for temperature (measured by degree days), and were on average 12 or 14% smaller at the end of their first summer following invasion by Bythotrephes or zebra mussels, respectively. Yellow perch (Perca flavescens) growth was less affected by invasion. Yellow perch on average grew more slowly in their first year of life following invasion by zebra mussels, although this effect was not statistically distinguishable from zero. Early life growth of both walleye and yellow perch was less tightly coupled to degree days in invaded systems, as demonstrated by increased variance surrounding the degree day-length relationship. Smaller first-year size is related to walleye survival and recruitment to later life stages and has important implications for lake food webs and fisheries management. Future research quantifying effects of zebra mussels and Bythotrephes on other population-level processes and across a wider gradient of lake types is needed to understand the mechanisms driving observed changes in walleye growth.

Pumpkinseed sunfish (Lepomis gibbosus) counter spiny cladoceran (Bythotrephes longimanus) defenses

How do native species react to the presence of an invader? Consumption by resident fish is important in determining the yearly mortality of the invasive cladoceran Bythotrephes. In aquarium experiments, we discovered that small pumpkinseed sunfish (Lepomis gibbosus) were efficient at removing spines from Bythotrephes longimanus and consuming the adults. Experiments quantified how frequently pumpkinseeds showed successful removal of spines, and compared predatory performance relative to other species of fish. We also checked if Bythotrephes' resting eggs pass through the guts of pumpkinseeds in viable condition. The experiments revealed that pumpkinseed sunfish (45–70 mm total length, TL) from two geographic regions removed spines 87% of the time. Spine-handling techniques allowed pumpkinseeds to consume Bythotrephes faster than other species of small (<70 mm, TL) fish [yellow perch (Perca flavescens) and smallmouth bass (Micropterus dolomieu)]. Forty three percent of Bythotrephes’ resting eggs fed to pumpkinseeds passed through digestive systems intact, a property shared with other fish species. Our findings suggest that a native fish that possesses specialized morphology and behavior for hard-shelled prey manipulation (mollusk consumption) is pre-adapted to counter Bythotrephes’ spine defense. The studies raise two questions: 1) to what degree does pumpkinseed predation influence the geographic distribution of Bythotrephes, and 2) could pumpkinseeds become an effective biological control agent?

Concurrent improvement and deterioration of epilimnetic water quality in an oligotrophic lake over 37 years

AbstractSingle‐lake studies offer an opportunity for understanding, predicting, and mitigating local or regional threats to lake ecosystems. Our goal was to understand how concurrent environmental stressors such as climate change, eutrophication, and salinization affect long‐term lake water quality. We report epilimnetic changes in 18 water‐quality parameters collected at seven sites from 1980 to 2016 in Lake George, a large oligotrophic lake in the Adirondack Park, New York, USA. Improvements and deteriorations in water quality occurred over 37 years. We observed a 32% increase in chlorophyll a associated with an increase in orthophosphate, but not total phosphorus or a warming epilimnion (0.05°C/year). Salinization from road deicing salts contributed to the largest deterioration in water quality. However, chloride concentrations and the current rate of increase are low enough that few ecological impacts are likely to occur over the next few decades. Increasing calcium concentrations were not high enough to facilitate the persistence of invasive species in the lake such as zebra mussels (Dreissena polymorpha) but are sufficient for Asian clams (Corbicula fluminea) and the spiny water flea (Bythotrephes longimanus). Similar to other lakes, environmental legislation has supported recovery from acidification, indicated by reduced sulfate and nitrate, and increased alkalinity and pH. Declines in water quality were minor relative to other lakes, suggesting that decades of tourism and development occurred without major deterioration in water quality, but management efforts are needed to curb salinization in the Lake George watershed, particularly as it relates to sodium concentrations to prevent a loss of drinking water quality.

Long‐term survey data reveal large predator and temperature effects on population growth of multiple zooplankton species

AbstractPredators can strongly affect prey communities, but their influence may be difficult to distinguish from bottom‐up and other environmental effects. The problem of assessing predator impact is especially difficult in large systems that do not allow for comparisons across multiple units (e.g., small lakes) that have varying predator density. For instance, the invasion of the predatory zooplankter, Bythotrephes longimanus, into the Laurentian Great Lakes contributed to the nearly complete disappearance of several zooplankton species, but current effects on extant zooplankton are not well understood. We used generalized additive models (GAMs) applied to long‐term data time series (1994–2012) to examine B. longimanus effects on zooplankton species in Lake Michigan. Because B. longimanus abundance varied over time, our approach allowed assessment of predator effects from field data while accounting for other factors, including food resources, temperature, and seasonality. Results suggest that B. longimanus substantially reduces some zooplankton population growth rates, with the largest effects on species that B. longimanus affected more strongly in experiments. For example, at maximum B. longimanus abundance, Daphnia mendotae, Bosmina longirostris, and Diacyclops thomasi population growth rates were estimated to be reduced by 17%, 30%, and 21%, respectively, compared to no effect on calanoid copepods. Results further indicated positive temperature effects on population growth that differed by species. Our study thus provides field‐based evidence for ongoing impacts of invasive species and temperature on zooplankton production and composition, with potential consequences for planktivorous fish, and exemplifies how GAMs can be used to determine predator effects from time series data.

Evaluation of a method that uses one cubic meter mesocosms to elucidate a relationship between inoculation density and establishment probability for the nonindigenous, invasive zooplankter, Bythotrephes longimanus

The introduction and spread of aquatic invasive species by ships is a global concern, and development and implementation of effective prevention measures are an urgent priority. Two questions plague science and policy. First, what level of reduction in live organism discharges from ships yields a given desired level of reduction in species establishment probability? Second, how can this question be empirically tested in a practicable and replicable manner? We evaulated the extent to which experiments using 1 m3 mesocosms could help elucidate a relationship between inoculation density and reproductive output (a proxy for establishment probability) of a surrogate invader, Bythotrephes longimanus (Crustacea: Cladocera). The inoculation densities were 1, 5, 10, 20, or 40 individuals m−3. All individuals in each inoculum were added as a single event at the start of each of four, two-week-long experiments. B. longimanus were inoculated into ambient water pumped from the Duluth-Superior Harbor at the start of each experiment between June and August, 2015. The mesocosm volume and range of inoculation densities has relevancy to the International Maritime Organization’s Ballast Water Management Convention D2 standard which allows a density of 50 μm minimum dimension. Reproductive output was detected among 78 of 80 mesocosms after 2 weeks. Among 13 abiotic and biotic variables, inoculation density was overwhelmingly the best predictor, and water temperature was the second best predictor, of B. longimanus reproductive output. Net reproductive value of the inoculants was ≥ 1 among 35 of 80 mesocosms after 2 weeks. There was evidence for density-dependent reduction in population growth rate in the higher inoculation density mesocosms. We examined various parts of our method for robustness. Translation of the results into meaningful estimates of establishment probability in the field remains a vivid challenge. We introduce the idea of using annual hatching rates of natural banks of dormant eggs in lake sediments as a method to characterize relationships between propagule pressure and establishment probability at the scale of an ecosystem for seasonally transient species such as B. longimanus.

Phototactic behavior of native Daphnia in the presence of chemical cues from a non-native predator Bythotrephes.

Chemical cues are used by many taxa to communicate within and among species. Behavioral defenses induced by predator cues are a mechanism by which prey species resist or avoid predator attack. This study examined the egg bank of native Daphnia species in a lake that has been invaded by Bythotrephes longimanus, an invertebrate zooplanktivore native to northern-central Europe and Asia (initial invasion 1994, population boom in 2009). Daphnia resting eggs from both pre- and post-B. longimanus invasion lake sediments were hatched and established as isofemale clonal lines. Phototactic behavior (a proxy for vertical migration behavior) was assessed in the presence and absence of B. longimanus cue. This was done to evaluate the hypothesis that the heavy predation imposed by B. longimanus would have been selected for Daphnia clones that are more negatively phototactic in the presence of B. longimanus cue, because B. longimanus is a visual predator. The behavior of the clones derived from pre-B. longimanus era resting eggs was not significantly different from the behavior of the clones from the post-B. longimanus era and exposure to predator cue did not affect the phototactic response of the clones. There was a significant difference in the phototactic behavior of the three Daphnia species tested (Daphnia ambigua, Daphnia mendotae, and Daphnia pulicaria). These results suggest that predation by B. longimanus is not the main factor that is influencing the phototactic behavior of Daphnia in the lake. Other factors such as fish predation may be playing a more significant role in this system.