The North American invasion of a non-native mysid, Hemimysis anomala, has been expanding since 2006, with the first inland lake invasions detected for Oneida Lake in 2009 and Seneca Lake in 2010. Although we know that Hemimysis primarily consumes zooplankton, our ability to predict the community-level impact of an invasion is hampered by a lack of information on taxon-specific effects. To investigate the effects of Hemimysis on native zooplankton, we conducted two mesocosm experiments that compared composition between communities with and without Hemimysis and studied how the effects of this predator on zooplankton species composition varied across a natural gradient of low to high invader densities (0.01–0.1 individuals·L−1). Our first experiment found that Hemimysis primarily affected cladocerans, and particularly Daphnia, shifting communities towards dominance by copepods. Our second experiment showed that Hemimysisinvasions may do little to suppress Daphnia abundances until between 0.067–0.11 individuals·L−1or higher. Cladocerans are important links in freshwater trophic transfer and the nutrient cycle, and disruption of these linkages following Hemimysis invasion could result in both bottom-up and top-down impacts in nearshore food webs. However, Hemimysis can also fill a similar trophic role as the zooplankton they consume, and longer-term experiments are required to better assess their eventual impacts on native communities.
The first experiment (EPA) compared zooplankton composition between communities with and without Hemimysis. Twelve 378 L cattle tanks (134 cm x 78 cm x 63 cm) were filled on 11 June with 350 L of Lake Opinicon water (see online supplementary material Table S1 (1) for details of all lakes used in the experiments) filtered through 80 [micro]m mesh to remove large zooplankton but allowing most phytoplankton and some small zooplankton to pass through. Shards of two 14 cm diameter, unglazed ceramic pots were added to each mesocosm to provide a daytime refuge for the Hemimysis. Tanks were also covered with 1 mm mesh to provide additional shade and to protect from insects and debris. Mesocosms were allowed to acclimate for 1 week prior to the addition of zooplankton. To create a diverse, native zooplankton community, we filtered zooplankton from 1050 L of water from the pelagic and littoral regions of four nearby, uninvaded lakes (totaling 4200 L) on 18 June using an 80 µm net. All zooplankton were condensed into cooled containers, transported to the mesocosm site, and added to the tanks on the same day they were collected. All tanks were inoculated with zooplankton from the equivalent of 350 L of lake water from a condensed and constantly mixed pool, with this inoculum divided evenly over two randomly added aliquots. Tanks were then acclimated for 1 week prior to Hemimysis addition. Hemimysis was collected after sunset from the St. Lawrence River at Montreal, Quebec, Canada (45.499804°N, 73.551433°W) on 25 June using vertical plankton net tows. All individuals were then transported that same night in cool, dark containers to a laboratory near the field site. Six tanks were haphazardly assigned to the "present" treatment, and 35 Hemimysis (0.1 individuals·L-1) were introduced to each of these tanks (n = 6) on the same night they were captured. We chose a volumetric treatment density of 0.1 individuals·L-1 to approximate the higher Hemimysis densities observed in Lake Ontario between the spring and fall of 2009 by Taraborelli et al. (2012). Body size was standardized across replicates by using the same number of large (18 individuals between 9 and 11 mm) and small (17 individuals between 6 and 8 mm) Hemimysis in each replicate. Additionally, to standardize behaviour across replicates, only adult male Hemimysis were used in the experiment. Different migratory behaviours have been reported between male and female Hemimysis (Ketelaars et al. 1999; Kipp and Ricciardi 2007), which could also be influenced by local abiotic and biotic conditions, since different populations sampled around the same period often have widely different sex ratios (e.g., Taraborelli et al. 2012). The diets of adult male and female Hemimysis are the same (Marty et al. 2010), so our use of only male Hemimysis would not have influenced the types of prey affected by our treatments.
The second experiment (EGrad) built upon the first by incorporating a gradient design to investigate how the community effects of Hemimysis invasion may differ across a range of low to high densities. Twenty 220 L cylindrical tanks (84 cm diameter x 53 cm high) were filled on 28 July with 180 L of Lake Opinicon water filtered through 50 µm mesh, and ceramic shards were added to each tank as in EPA. Tanks were covered with 1 mm mesh and allowed to acclimate for 1 week prior to the addition of zooplankton. On 4 August, zooplankton were filtered from 900 L of water from each of the same four lakes as in EPA (totalling 3600 L) using a 50 µm net. All tanks were inoculated with zooplankton from the equivalent of 180 L of lake water following the same methodology as in EPA and left to acclimate until 15 August. Hemimysis were collected on 15 August, then transported and introduced as in EPA. Mesocosms were haphazardly assigned to Hemimysis treatment densities, with two replicates of each treatment of 2, 4, 6, 8,10,12, 14,16,18, and 20 Hemimysis (0.011, 0.022, 0.033, 0.044, 0.055, 0.067, 0.078, 0.089, 0.1, 0.11 Hemimysis·L-1, respectively), corresponding to a range of low to high natural Hemimysis densities (0.010.1 individuals·L-1; Taraborelli et al. 2012). Body size was again standardized by adding 50% large and 50% small Hemimysis to each treatment, and only adult male Hemimysis were used.