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Dreissenid mussels (Dreissena polymorpha and D. bugensis) have re-engineered Great Lakes ecosystems since their introduction in the late 1980s. Dreissenids can have major indirect impacts on profundal habitats by redirecting nutrients and energy away from pelagic production (which supplies profundal production) and depositing nutrients and energy in the nearshore zones that they occupy. However, strong empirical evidence for the effects of this redirection of resources on fish populations is currently lacking. Here, we report significant shifts in isotopic signatures, depth distribution and diets of a coldwater profundal fish population that are all consistent with a greater reliance on nearshore resources after the establishment of dreissenid mussels in South Bay, Lake Huron. Isotopic signatures of scales collected from 5-year-old lake whitefish (Coregonus clupeaformis) demonstrated remarkable stability over the 50-year period prior to the establishment of dreissenids (1947-1997) and a sudden and significant change in isotopic signatures (3[per thousand] enrichment in δ¹³C and 1[per thousand] depletion in δ¹⁵N) after their establishment (2001-2005). These dramatic shifts in isotopic signatures were accompanied by a coincident shift in the mean depth of capture of lake whitefish towards the nearshore. A comparison of previously unpublished pre-invasion diets of lake whitefish from South Bay with contemporary diets collected between 2002 and 2005 also indicate a greater reliance on nearshore prey after the invasion of dreissenid mussels. This study is the first to report changes in the carbon source available to lake whitefish associated with restructured benthic communities after the appearance of dreissenid mussels. Further, this study contributes to a growing body of work that demonstrates the ecological insights that can be gained through isotopic analysis of archived fish bony tissues in ecosystems that have experienced significant levels of disturbance.

We hypothesized that native Leptodora kindtii would be shorter and have smaller feeding baskets in central Ontario lakes with greater abundances of small-bodied zooplankton prey, and that differences in zooplankton size among lakes could be attributed to the invasive cladoceran Bythotrephes longimanus . We evaluated these conjectures by comparing size metrics of Leptodora and the size of their preferred cladoceran prey in lakes invaded or not by Bythotrephes . Leptodora was less abundant in invaded lakes, but were smaller bodied with smaller feeding baskets only in lakes with long invasion histories. Small cladoceran abundance was greater in non-invaded lakes and was directly related to Leptodora abundance although not to Leptodora size. Mean Leptodora body size declined with increasing abundance of Bythotrephes . We evaluated three possible explanations for these patterns in Leptodora— (a) competition with Bythotrephes for zooplankton prey, (b) direct predation by Bythotrephes , and (c) size-selective predation by fish. While we were unable to unequivocally distinguish among these hypotheses, our observations are most consistent with predation by Bythotrephes changing zooplankton community composition and size structure in a manner that is detrimental to Leptodora . Our results indicate that Bythotrephes invasion may trigger more complex and subtle changes in food webs than previously thought.

We investigated the roles of local environmental conditions and dispersal limitation in zooplankton recovery from acidification in Swan Lake, Ontario, a historically acidified, metal-contaminated lake. We hypothesized that local environmental conditions (pH and the presence of resident, acid-tolerant zooplankton) would limit the establishment of several zooplankton colonist species. We tested this in a factorial mesocosm experiment that ran for 32 days during the mid summer. Ambient pH (5.6) reduced the abundance of two acid-sensitive cladoceran taxa, Daphnia spp. and Ceriodaphnia lacustris compared to elevated pH (6.5) but increased the abundance of cyclopoid copepod juveniles. The resident community suppressed Skistodiaptomus oregonensis and Diacyclops bicuspidatus thomasi, and to a lesser extent Mesocyclops edax, but slightly enhanced Daphnia spp. We also hypothesized that conditions in the sediments of acidified Swan Lake would limit zoo-plankton recruitment from diapausing eggs. We tested this by reciprocally transferring sediments containing eggs between Swan Lake and a nearby recovered lake, and incubating them for 15 weeks in 20-L emergence traps. Most zooplankton emerged from diapause in both lakes indicating that this mechanism contributes to the recolonization of acidified lakes once pH returns to normal. Some species, however, emerged in only one lake or the other, indicating that hatching cues such as light, temperature, oxygen, or appropriate pH may have been missing. Our experiments demonstrate that both local lake conditions and diapausing eggs can influence zooplankton recovery. Continued recovery may require additional management efforts to reduce and control regional acid emissions and active intervention in the form of food web manipulations.

The crustacean zooplankton community of Harp Lake, Ontario, Canada, has changed appreciably since the invasion by the spiny water flea, Bythotrephes. Crustacean species richness has declined, large-bodied Cladocera have replaced small-bodied ones, and there has been a downward trend in the total abundance of zooplankton because copepod abundance has remained stable while Cladoceran abundance has declined. Although the zooplankton community has now been stable for 4 years (1995–1998), the biology of the invader has changed dramatically. In particular, there have been 10-fold differences in the mean annual abundance of Bythotrephes in this 5-year period and substantial changes in the timing of population maxima. We attribute these changes to two factors: (i) transition from a summer to a fall switch from parthenogenesis to gametogenesis and (ii) interannual differences in the thickness of a warm, dark stratum in the lake. We hypothesize that this stratum provides a refuge for Bythotrephes from predation by lake herring, Coregonus artedii.

We explore the effects of the invasive predatory cladoceran Bythotrephes longimanus on the abundances and seasonal zooplankton consumption by the native predatory invertebrates Leptodora kindtii, Chaoborus spp., and Mysis relicta in inland lakes of Ontario. In lakes with Bythotrephes, the seasonal consumption by all invertebrate predators combined ranged from 2.39 to 13.50 g m⁻² and was 300% higher than in lakes without the invader. This was due to Bythotrephes because there was no invasion effect on Chaoborus or Mysis consumption, while it actually decreased Leptodora consumption. Leptodora and Chaoborus abundances were lower in invaded lakes, but only Leptodora abundance was negatively correlated with Bythotrephes abundance. There was no effect of Bythotrephes on Mysis abundance. Bythotrephes consume more zooplankton than most other predatory invertebrates, including copepods, and often consume more zooplankton than planktivorous fish. The large increase in predatory invertebrate abundance and consumption due to Bythotrephes means that substantial portions of zooplankton production are probably being diverted from other consumers, such as juvenile and planktivorous fish, and that the role of predatory invertebrates in the pelagia of inland lakes has been intensified by the arrival of Bythotrephes.

We compared changes in body condition (relative weight) and mercury concentrations ([Hg]) in two species of coregonid fish (lake herring Coregonus artedi, lake whitefish C. clupeaformis) among discrete populations in Ontario between 1967 and 2006. Temporal comparisons among populations were made to determine whether 1) the establishment of Bythotrephes longimanus had affected coregonid populations, or 2) if changes in body condition or [Hg] were related to regional differences in the degree of climate change during the time period investigated. Climate data from northwestern, northeastern and southern Ontario showed a general warming trend in all regions over the period of study. However, greater temporal changes in climate were observed in the northwest where growing degree days >5°C (GDD) increased and precipitation declined over the study period compared with relatively little change in southern or northeastern Ontario. Correspondingly, northwestern Ontario coregonid populations demonstrated significantly greater declines in body condition relative to those from northeastern or southern Ontario. Declines in [Hg] of both species were also greater among northwestern populations compared with those from northeastern or southern Ontario but only significantly so for lake herring. These declines were independent of the invasion of non-native Bythotrephes, and declines in [Hg] were opposite predictions based on the hypothesis that Bythotrephes invasion lengthened aquatic food chains. Based on our findings and further evidence from the literature, we propose that warming regional climates are capable of contributing to declines in both condition and [Hg] of fishes. Because fish condition affects both reproductive success and overwinter survival, observed condition declines of the magnitude reported here could have profound implications for the structure of future aquatic ecosystems in a warming climate.

Juvenile Daphnai pulex develop neck spines in response to a chemical agent released by predatory Chaoborus larvae. While these defensive structures reduce the vulnerability of Daphnia to the insect predator, they also entail a demographic cost. We investigated the nature and degree of this cost through an analysis of cohort life tables involving both the typical morph (TM), which lacks neck spines, and spined morph (SM) at 22°C in two different food regimes: \"natural\" food conditions (53—@m filtered pond water) and \"ideal\" food conditions (1 x 10⁵ cells/mL Chlamydomonas sp.). No consistent pattern of differences between TM and SM occurred with respect to survivorship, clutch sizes, body sizes, mean egg volume, or number of juvenile instars. Development rates of both juvenile and adult instars, however, were significantly slower in SM. The presence of neck spines increased the age maturity for D. pulex by 8.4—14.6%, and the duration of adult instars exposed to Chaoborus—factor, whose eggs will develop into SM, was 2.8% longer than for those not exposed. This caused delayed reproduction in SM and resulted in a population growth rate (°) that was °8—9% lower than in TM. This relatively large demographic cost of spine formation in D. pulex produces a strong selection pressure to forgo the formation of these spines when Chaoborus predators are absent.

The effects of pH, algal composition and algal biomass on abundance, size, reproduction and condition of Daphnia pulex and Bosmina longirostris were tested in a field experiment using water and natural phytoplankton assemblages from a circumneutral (pH 6.43) and a moderately acidic (pH 5.75) lake in south-central Ontario. Both species were affected by pH and phytoplankton composition, with decreased egg production, lipid reserves, body size or abundance in treatments containing algae and/or water from the more acidic lake compared to treatments containing water and phytoplankton from the circumneutral lake. This result was unexpected for Bosmina, which often increases in relative and/or absolute abundance in acidified lakes. The negative effect of acidic conditions on Bosmina suggests that the population increase observed in most acidified lakes is not due to a positive response to low pH or ambient phytoplankton, but to altered biotic interactions possibly involving reduced competition.[PUBLICATION ABSTRACT]

Bythotrephes cederströemi are a predatory cladoceran zooplankter that have invaded numerous inland lakes in North America, many of which are stratified and support offshore fishes like Cisco ( Coregonus artedi ). While changes in zooplankton community composition following Bythotrephes invasion predict an increase in Cisco mercury concentrations (Hg), this phenomenon was not detected from a survey evaluating temporal changes in Cisco Hg across a broad range of lakes varying in the presence or absence of Bythotrephes . Here, we compare temporal changes in Cisco bioaccumulation slopes (i.e., slopes of relationships between Cisco Hg and trophic position) from lakes experiencing Bythotrephes invasion over the study period to those already invaded (as a reference) over similar time periods. Our results show that bioaccumulation slopes after Bythotrephes invasion either changed direction entirely (from positive to negative relationships) or decreased in elevation relative to those prior to invasion. No such pattern was observed in previously invaded reference lakes. Reductions in Cisco bioaccumulation slopes and/or intercepts following Bythotrephes invasion suggest that conversion efficiency (and therefore growth) of Cisco increased after invasion (i.e., less Hg accumulates in fish at an equivalent trophic position after vs. before invasion). Back-calculated Cisco growth rates and size-at-age from a second complimentary study were greater in the presence of Bythotrephes than without, further supporting the hypothesis that changes in Hg bioaccumulation are likely due to increased conversion efficiency among invaded populations. These findings highlight the potential importance of foraging energetics over and above shifts in trophic position in modifying fish contaminant concentrations.

Young-of-the-year (YOY) smallmouth bass, Micropterus dolomieu, spend their first summer in littoral areas near their nests. Evidence indicates that nests of smallmouth bass in Lake Opeongo, Ontario, Canada are more abundant in downwind locations than in upwind areas. We hypothesize that wind-induced lower water temperatures and food availability in upwind nesting areas lead to lower growth rates of YOY bass in upwind than in downwind nesting areas. We show that water temperatures were 0.6-1.3 degree C higher in downwind than upwind littoral areas during the period from mid-June to mid-July, when the YOY bass were on or near their nests. Although quite variable, zooplankton concentrations were also higher at downwind sites. In addition, bioenergetic simulations based on time series of field-measured temperatures predicted higher growth rates of YOY bass in the downwind sites. Growth rates based on sequential sampling of bass fry from their nests did not, however, differ statistically between upwind and downwind sites, although fry consistently weighed more downwind than upwind in the basin with the longer fetch possibly due to earlier spring warming. Our hypothesis is thus only partially supported and we call for further research on effects of spatial gradients on smallmouth bass life history.