Bythotrephes longimanus is an invasive zooplankton predator, negatively impacting zooplankton abundance and diversity in North American lakes. Previous studies have shown that Daphnia populations in lakes move to deeper waters during the day, in the presence of Bythotrephes, a visual predator occupying well-lit regions. However, Daphnia vertical position can be influenced by a variety of biotic and abiotic factors. We conducted a survey to determine (1) if Daphnia daytime vertical position differed between invaded and uninvaded lakes and (2) if Daphnia vertical position in invaded lakes was affected by water chemistry variables linked to water clarity, UV exposure, and phytoplankton production, and Bythotrephes density. Invaded lakes had a lower proportion of epilimnetic Daphnia as compared to uninvaded lakes. Daphnia vertical position was species-dependent, and with a lower proportion of epilimnetic Daphnia mendotae and greater proportion of epilimnetic D. longiremis observed in invaded lakes. D. mendotae were deeper in the water column in high Bythotrephes density lakes with low dissolved organic carbon levels. Our results show Daphnia vertical position response to Bythotrephes is species specific, dependent on Bythotrephes density and influenced by local abiotic conditions, with important implications for community structure and ecosystem function in invaded lakes.
To examine if abiotic variables influence the impact of Bythotrephes on vertical position of Daphnia species in south-central Ontario, we conducted a survey of 45 lakes from July to August 2013 (Table S1). Lakes were chosen along a gradient of DOC, TP, and Secchi based on samples collected by the Canadian Aquatic Invasive Species Network (CAISN) 2011 survey (N. Yan unpublished data, also see Cairns & Yan (2011) for details about sampling methods, values provided in Table S2). All lakes were thermally stratified. Because we were sampling across broad water chemistry gradients, we sampled Daphnia from each thermal layer of as many lakes as logistically possible during the months of July and August when lakes are or nearly completely thermally stratified in south-central Ontario. We were able to sample up to three times as many lakes as sampled in previous surveys that detected an impact of Bythotrephes on Daphnia density (Boudreau & Yan, 2003; Strecker et al., 2006b) and vertical position (Jokela et al., 2011).
Zooplankton sampling and enumeration
Zooplankton were collected during the day (between 10 am and 4 pm) at the deepest location of each lake using a 50-µm-mesh closing net with a 35 cm inner diameter. Temperature (°C) was measured using an YSI Model 550 Dissolved Oxygen and Temperature probe at 1-m depth intervals to determine the depth of the epi-, meta-, and hypolimnion. Zooplankton samples were collected from each thermal layer and preserved in 90% ethanol. Since the number of individuals sampled from each thermal layer would be affected by the volume of that layer, with more individuals sampled from larger layers as compared to smaller ones, all Daphnia in a sample volume representing 100 litres of water in each thermal layer were enumerated to standardize effort. Daphnia were identified to species using Ward & Whipple (1959), Witty (2004), and Haney et al. (2013). Since Daphnia pulex Leydig and Daphnia pulicaria can only be reliably distinguished using genomic methods (Černý & Hebert, 1999), we grouped them as D. pulex/pulicaria.
Bythotrephes densities in lake ecosystems vary throughout the year (Yan & Pawson, 1998), as well as between years (Pothoven et al., 2008). In south-central Ontario, Strecker & Arnott (2008) have shown that Bythotrephes densities vary significantly in invaded lakes throughout the year. Furthermore, a large degree of spatial heterogeneity in Bythotrephes density has been observed in invaded lakes in this region (Yan et al., 2002). Logistical constraints allowed us to sample Bythotrephes once in each lake during the day, which tends to under-represent densities and ignores seasonal variation in abundance. Therefore, we determined the invasion status of a lake based on whether Bythotrephes presence was detected in our study lakes by Jokela et al. (2011), who conducted an intensive nighttime macro-invertebrate survey of 80 lakes using five vertical tows per lake. All Bythotrephes individuals in our daytime samples were enumerated. Densities in our samples were up to an order of magnitude less than those in Jokela et al. (2011). This suggests that densities in our samples under-represented Bythotrephes densities in the lakes that we assessed. Additionally, Bythotrephes were not detected in our samples for many lakes where Bythotrephes presence had been observed by Jokela et al. (2011). We used densities in our daytime samples as a semi-quantitative measure of Bythotrephes abundance in our survey lakes rather than precise density values. Lakes with Bythotrephes presence detected by Jokela et al. (2011), but no Bythotrephes individuals detected in our samples were classified as low density lakes, while those with both Bythotrephes observed by Jokela et al. (2011) and Bythotrephes individuals present in our samples were classified as high-density lakes.