Meghan E. Brown

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.

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.

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.

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.

Historic nutrient loading and recent species invasions caused shifts in water quality and zooplankton demography in two Finger Lakes (New York, USA)

We investigated the paleolimnology of Owasco and Seneca Lakes (New York, USA) and compiled water-quality monitoring data to describe environmental change during the past two centuries. Trophic shifts were detected in the oligotrophic to mesotrophic range and were likely driven by nutrient loading and species invasion. Based on box core reconstructions, primary production increased in both lakes during the last century, which is evidenced by the amount, type and isotopic composition of material preserved in the sediment. Organic matter accumulation and its stable carbon isotopic composition, as well as carbonate abundance, began to increase during the 1960s in Owasco Lake and the 1850s in Seneca Lake. Further, the abundance of phytoplanktivorous cladocera subfossils increased beginning in the 1910s in Seneca Lake and in the 1960s in Owasco Lake. The different timing and magnitude of the trophic shifts likely resulted from contrasts in lake residence time and species assemblages between the two lakes. The increases in primary and secondary production paralleled, and are interpreted to reflect, increased allochthonous nutrient loading. However, nutrient loading was not detected in the water-quality data, perhaps because of strong uptake of phosphates by phytoplankton or due to limited data collection. Rapid changes in cladoceran subfossils and water quality were also detected during the last decade. Concurrent with the establishment of non-native Cercopagis pengoi (fishhook waterflea), phytoplanktivorous cladocera remains declined, and the mucrone length of Bosmina increased, consistent with predictions of increased invertebrate predation. Additionally, the post-1990 decline in sediment carbonate, increased Secchi depth and decreased chlorophyll a concentrations followed the establishment of filter-feeding Dreissena spp. (zebra and quagga mussels). Collectively, paleolimnological data and water-quality monitoring provided a more complete and consistent record of shifts in the productivity of Owasco and Seneca Lakes, which were useful to understand environmental changes over different time scales. Physical, geochemical and biological changes were temporally consistent among three cores collected from different locations in each lake, but differed in magnitude for several variables (e.g., grain size and cladoceran subfossils), which could reflect near-shore to offshore gradients.