Donn K. Branstrator

Range Expansion of Bythotrephes longimanus in North America: Evaluating Habitat Characteristics in the Spread of an Exotic Zooplankter

Sedimentary and water column evidence from 45 boat-accessible and eight backcountry lakes was used to investigate the distribution of Bythotrephes longimanus in northeast Minnesota, USA, and adjacent Ontario, Canada. The results expand the documented range of Bythotrephes in Minnesota from Lake Superior, Island Lake, and Saganaga Lake to Flour Lake, Greenwood Lake, McFarland Lake, Pine Lake, and Caribou Lake as well as to Saganagons Lake in Ontario. The latter three lakes are located in roadless landscapes without motorized boat access. Results confirm that Bythotrephes is no longer present in Boulder Lake or Fish Lake (St. Louis County, Minnesota), providing the first evidence of range compression (extinction following introduction) of this species in North America. Distributional expansion was confined to a corridor along the international border between northeast Minnesota and Ontario. Lakes along the invasion corridor were deeper, more transparent, and had lower chlorophyll concentration, on average, compared to other lakes studied. The pattern of range expansion provided an opportunity to test the predictions of a forecasting model for Bythotrephes occurrence (MacIsaac et al. 2000 Archiv für Hydrobiologie 149: 1–21) based on habitat characteristics. The model predicted 51% of the surveyed inland lakes to be susceptible to invasion, however, only 13% were actually invaded, implying strong dispersal constraints. Application of the forecasting model to a broader set of 179 Minnesota lakes predicted that 41% may be vulnerable to establishment by Bythotrephes based on habitat characteristics, offering an estimate of the state’s overall lake susceptibility (i.e., fundamental niche) to invasion. The results of this study provide evidence for the importance of a low-light refuge where Bythotrephes can minimize vulnerability to fish predation as a key habitat feature not considered by the forecasting model.

Ecological Interactions Between Bythotrephes cederstroemi and Leptodora kindtii and the Implications for Species Replacement in Lake Michigan

The zooplankton predator, Leptodora kindtii, declined in abundance at an offshore reference station in Lake Michigan in the mid-1980s following the invasion of another zooplankton predator, Bythotrephes cederstroemi. Both predators feed largely on daphnid prey and it was observed that densities of three Daphnia species declined in abundance at the reference station following the Bythotrephes invasion. Circumstantial evidence would suggest that the native predator, Leptodora, was competitively suppressed by Bythotrephes. However, results of laboratory experiments presented here show that Bythotrephes will readily attack and eat Leptodora when the predators are maintained under concentrated densities, even if alternative prey are available for the Bythotrephes. There was no evidence in these experiments that Leptodora attacked or ate Bythotrephes. These results imply that predation by Bythotrephes on Leptodora may alternatively account for the collapse of Leptodora in offshore Lake Michigan. In this note I discuss evidence in support of competition and predation as alternative hypotheses to explain the pattern of species replacement, Bythotrephes for Leptodora, observed in Lake Michigan. The existing data are not definitive and tempt further inquiry.

A Model for Growth, Development, and Diet Selection by the Invertebrate Predator Bythotrephes cederstroemi

Abstract A growth model is developed to predict C content, respiration, and rates of both embryonic and post-embryonic development by the invertebrate predator Bythotrephes . The model incorporates temperature dependent rates of embryonic and postembryonic development as well as respiration and variations in body mass. Estimated growth efficiencies indicate that this invertebrate predator converts about 25% of its prey biomass into predator mass, thus reducing potential resource availability to plank-tivorous fish. The physiological economics of Bythotrephes with respect to both P and C indicate that copepods can be only a minor component of the predators diet, owing to the high C:P composition of copepod tissues. Mass balance constraints dictate that field-collected populations of Bythotrephes in Lake Michigan consume a prey mixture with bulk tissue stoichiometries less than about 40:1 C:P by mass. The requisite stoichiometry correponds with the elemental composition of Cladocera, particularly Daphnia , which are the dominant Cladocera in offshore regions. Thus an analysis based on conservative elemental budgets of P and C points to Daphnia as the dominant prey of Bythotrephes in Lake Michigan.

Planktivory by bluegill (Lepomis macrochirus) on Leptodora kindti in a small North American lake

Leptodora kindti (Crustacea: Cladocera) is a large species of zooplankton (2–18 mm length) that is exceptionally transparent. This transparency is believed to be a means by which it successfully coexists in lakes with planktivorous fishes. We investigated the gut remains of bluegill (Lepomis macrochirus) that had been feeding on L. kindti and Daphnia (D. galeata and D. retrocurva) in the wild (Lake Zurich, Illinois) and found that bluegill readily preyed on L. kindti as small as 3–5 mm length, and strongly selected L. kindti over Daphnia galeata and Daphnia retrocurva. The large compound eye of L. kindti is one half to one complete order of magnitude larger than Daphnias eye, consistent with the hypothesis that eye area is an important visual cue for fishes. Moreover, the slope of the relationship between eye area and body length is an order of magnitude shallower in L. kindti than Daphnia, suggesting that eye area has been under stronger negative selection in L. kindti. Results suggest that L. kindtis large and dark eye compromises the transparent nature of its body.

Evidence for Predation by Young-of-the-year Alewife and Bloater Chub on Bythotrephes cederstroemi in Lake Michigan

Abstract The recent invasion of Bythotrephes cederstroemi into the Great Lakes has raised speculation as to its potential effects on the resident food webs. Its long tailspine has been implicated as a post-contact, antipredatory adaptation against small fishes but few field data exist on this subject. Here we present results of gut content analyses on 25 young-of-the-year alewife ( Alosa pseudoharengus ) and 84 young-of-the-year bloater chub ( Coregonus hoyi ) collected from Lake Michigan by mid-water trawl during August and September of 1988 and 1990. Alewife of 39.4–59.6 mm and bloater chub of 29.4–55.8 mm standard length contained remains of Bythotrephes . Although limited, these data lower the size range at which alewife and bloater chub in Lake Michigan have been found to eat Bythotrephes. Bythotrephes remains were counted in fish guts by numbers of tailspine kinks and mandible pairs. In bloater chub we found 1.2 times more tailspine kinks than mandible pairs but in alewife guts we found 9.4 times more tailspine kinks than mandible pairs suggesting that tailspine kinks were more strongly retained over mandibles in alewife compared to bloater chub.

A 2001 Survey of Crustacean Zooplankton in the Western Arm of Lake Superior

This study cataloged the current taxonomic composition and distribution of crustacean zooplankton at 12 locations in the western arm of Lake Superior during May and August 2001. Zooplankton were last surveyed to this taxonomic level in the western arm in 1973, prior to a number of biotic changes to the lake. Notable differences in the 2001 zooplankton assemblage compared to historical conditions include the establishment of the exotic Bythotrephes longimanus, declines in the density of Daphnia retrocurva and Bosmina longirostris, and an increase in density of Holopedium gibberum. These changes are consistent with an increase in invertebrate predation and similar to changes reported after B. longimanus invasions elsewhere in North America, but a cause and effect relationship is correlative. Densities and body lengths of Lake Superior B. longimanus were within the ranges reported for other Great Lakes.

Hydroacoustic Estimation of Zooplankton Biomass at Two Shoal Complexes in the Apostle Islands Region of Lake Superior

ABSTRACT Hydroacoustics can be used to assess zooplankton populations, however, backscatter must be scaled to be biologically meaningful. In this study, we used a general model to correlate site-specific hydroacoustic backscatter with zooplankton dry weight biomass estimated from net tows. The relationship between zooplankton dry weight and backscatter was significant (p < 0.001) and explained 76% of the variability in the dry weight data. We applied this regression to hydroacoustic data collected monthly in 2003 and 2004 at two shoals in the Apostle Island Region of Lake Superior. After applying the regression model to convert hydroacoustic backscatter to zooplankton dry weight biomass, we used geo-statistics to analyze the mean and variance, and ordinary kriging to create spatial zooplankton distribution maps. The mean zooplankton dry weight biomass estimates from plankton net tows and hydroa-coustics were not significantly different (p = 0.19) but the hydroacoustic data had a significantly lower coefficient of variation (p < 0.001). The maps of zooplankton distribution illustrated spatial trends in zooplankton dry weight biomass that were not discernable from the overall means.

Patterns in the abundance, phenology, and hatching of the resting egg stage of the invasive zooplankter Bythotrephes longimanus: implications for establishment.

To examine how dormancy contributes to the establishment and persistence of Bythotrephes longimanus, we investigated resting egg production and hatching in relation to the demography of the planktonic stage and environmental conditions in Island Lake Reservoir (USA). During a 3-year study, the largest contribution to the egg bank occurred in autumn and most eggs hatched in spring, but we also detected some resting egg production and hatching in summer. The difference between summer and late autumn densities of eggs in sediments averaged 47% (range 0–98%) for 18 sites throughout the reservoir, which was similar to experimental estimates of in situ hatching fraction of 67% for eggs in the spring and summer following their production. Based on emergence traps, neonates hatch in the field during May and June. We estimated mortality rates of 64% for resting eggs and embryos, and 59% for newly emerged neonates. Although hatching fraction saturated at the same level, eggs incubated offshore hatched later than those nearshore where water temperature was warmer and light was detectable at the sediment surface. Low dissolved oxygen concentration did not significantly reduce hatching fraction but resulted in some eggs that initiated development but failed to hatch. Collectively, our results demonstrate substantial annual turnover in the resting egg bank of B.longimanus and high mortality of resting eggs during recruitment from the egg to the first molt of the planktonic stage. These patterns suggest that propagule pressure in the form of resting eggs requires large numbers for establishment, and that considerable post-establishment resting egg production is necessary for inter-annual persistence.

Morphological changes in Daphnia mendotae in the chemical presence of Bythotrephes longimanus

Induction of helmet formation in Daphnia mendotae was demonstrated experimentally through chemical exposure to the predatory cladoceran Bythotrephes longimanus. The degree of helmet formation recorded in two of three clone lines of D. mendotae is comparable to other published studies on D. mendotae and Daphnia ambigua in the chemical presence of Chaoborus. The results are the first demonstration that B. longimanus, a recent invader of North America, elicits chemical induction of helmet formation in daphniids and, importantly, may help explain recent trends in the post-invasion success of D. mendotae in Lake Michigan and Harp Lake, Ontario, following species invasion by B. longimanus.

Seasonal dynamics in Bythotrephes diapausing egg emergence and production, and the role of dormancy in range expansion

The predatory zoop1ankton, Bythotrephes longimanus (Leydig 1860) (Crustacea: Cercopagidae), was first observed in the Laurentian Great Lakes in the ear1y 1980s (JOHANNSSON et al. 1991). Its introduction was 1ike1y the result o f accidental transport in ship ballast water carried from Eurasia (SPRULES et al. 1990). Bythotrephes has since spread to at 1east 40 smaller, in1and lakes across North America (MAclsAAC et al. 2000). These secondary invasions repeatedly demonstrate the ability of Bythotrephes to overcome 1and barriers and exploit a diverse group of lake ecosystems. Bythotrephes reproduce by cyclic parthenogenesis. Population development each spring begins with emergence of animals from diapausing eggs in the sediments. Diapausing eggs not only facilitate interannua1 persistence of Bythotrephes within invaded lakes, but they are thought to ai d colonization o f new ecosystems. Knowledge ofthe diapausing egg is thus imperative to understanding the eco1ogy of established populations as well as range expansion o f this invader. This contribution examines the dynamic relationship between demographics of the planktonic stage of Bythotrephes and the sediment diapausing egg bank over an annual cycle in Island Lake Reservoir, Minnesota, USA.