The North American invasive zooplanktivore Bythotrephes longimanus is less hypoxia-tolerant than the native Leptodora kindtii

The predatory cladoceran Bythotrephes longimanus (spiny water flea) has been invading lakes and damaging food webs across the central part of North America since the early 1980s. To understand its niche and that of the taxonomically related and native predatory cladoceran Leptodora kindtii, we investigated species survival after 12 h exposures to low dissolved oxygen (DO) concentrations in the laboratory. Bythotrephes longimanus (n = 690) exhibited a hypoxia tolerance limit (LC50) of 1.65 mg·L−1 DO (95% confidence interval: 1.59, 1.72 mg·L−1) and was significantly less tolerant of hypoxia than L. kindtii (n = 380), which exhibited an LC50 of 0.58 mg·L−1 DO (0.51, 0.65 mg·L−1). These lab-based physiological results are consistent with landscape-scale observations that B. longimanus successfully invades primarily mesotrophic and oligotrophic lakes, while L. kindtii inhabits a wider range of lakes that includes eutrophic ones. Climate change throughout the 21st century may increase the occurrence and severity of hypoxia in the hypolimnia of lakes and may provide a growing barrier to B. longimanus invasion.

Leptodora kindtii Population Dynamics in the Island Region of Western Lake Erie before and after the Invasion of the Predacious Cladoceran Bythotrephes longimanus

Competition among native and non-native species can cause decreases in population size and production of both species. The native predaceous crustacean zooplankter Leptodora kindtii shares a similar niche with the invasive Bythotrephes longimanus in Lake Erie. This niche overlap may contribute to the decline in abundance and production of Leptodora in the western basin of Lake Erie. Historical (1946) and recent (2006) data were used to determine if the decline in Leptodora abundance and production was associated with the effects of Bythotrephes, which invaded Lake Erie in the mid-1980’s. Pre-invasion abundances and lengths of L. kindtii were compared with current data (2006). A change in prey community abundance, composition and dynamics were observed, relative to pre-invasion, with a marked decline in.abundance and size of L. kindtii after the invasion of Bythotrephes. Competition for food and direct predation are two explanations, among others, for the declines observed in L. kindtii size, abundance and production that have occurred since B. longimanus invasion.

Hypoxia modifies planktivore–zooplankton interactions in Lake Erie

We evaluated vertical distributions of fish and zooplankton, planktivore consumption, and prey production in Lake Erie during 2005 to determine how hypolimnetic hypoxia alters fish (i.e., rainbow smelt (Osmerus mordax) and emerald shiners (Notropis atherinoides)) and invertebrate planktivore (i.e., Bythotrephes longimanus and Leptodora kindtii) relationships with their mesozooplankton prey. Hypoxia concentrated 45%–76% of fish into a narrow (<2 m) metalimnetic layer, but only 3%–13% of zooplankton production was in this layer. The epilimnion may have served to some degree as a refuge for mesozooplankton because high temperatures may have excluded rainbow smelt. High concentrations of fish above the hypolimnion likely resulted in increased competition for large prey (i.e., predatory claodcerans). Although hypoxia did not result in overall high predation demands by planktivores relative to total zooplankton production, planktivore consumption rates within the metalimnion exceeded zooplankton production in that layer.

Abundance dynamics and functional role of predaceous Leptodora kindtii in the Curonian Lagoon

The abundance and distribution of predatory cladoceran Leptodora kindtii was investigated in the estuarine lagoon (Curonian Lagoon, SE Baltic Sea). Three hydrodynamically different parts of the lagoon were selected, representing transitory oligohaline, intermediate freshwater and stagnant freshwater sites. L. kindtii was least abundant at the oligohaline site, never occurring at salinities greater than 4 psu. At the two freshwater sites, the abundance of L. kindtii varied from a low of <0.1 up to 2.2 indv. L−1 during peak abundance. Two peaks of L. kindtii abundance were observed with timing differences between stations: at the stagnant site the population of L. kindtii peaked two weeks earlier relative to the more hydrodynamically active sites, likely due to a 2°C higher May temperature. The small body size of L. kindtii in the lagoon (seasonal mean 2.68±0.6 mm) shows high fish predation pressure and predicts small cladocerans, juvenile copepods and rotifers being in the preferred prey size range. The calculated L. kindtii daily consumption during the population peak was as high as 100% of the daily zooplankton production, which implies high potential of this predator to shape the grazing zooplankton community in the lagoon.

Seasonal variations in the length of zooplankton related to certain physicochemical variables in two freshwater reservoirs

Seasonal variations in the body length of zooplankton were studied in relation to water temperature, nitrate (NO3), soluble reactive phosphorus (SRP), total chlorophyll, Secchi disk depth, pH, conductivity, and oxidation-reduction potential (ORP) in a mesotrophic (Ikizcetepeler) and a eutrophic (Çaygören) reservoir from February 2007 to March 2008. During the study, the body lengths of a total of 7590 zooplankton specimens (1110 rotifers, 3270 cladocerans, and 3210 copepods) were measured. The length of the majority of the species was significantly smaller in summer than in winter, fall, and spring, including that of the most dominant species, Asplanchna priodonta, Daphnia galeata, Daphnia longispina, Diaphanosoma brachyurum, Bosmina longirostris, Leptodora kindtii, Ceriodaphnia pulchella, Cyclops vicinus, Metacyclops gracilis, and Acanthocyclops robustus (F > 5, ). Correspondence analysis (CA) showed that the body length of the zooplankton studied was inversely related to water temperature, whereas it was positively related to ORP and pH. The results of our study suggest that, although nutrients (NO3 and SRP) apparently have an effect on zooplankton body length only in the mesotrophic reservoir, temperature influences the body length in both the mesotrophic and the eutrophic reservoir.