The Effect of Bythotrephes longimanus on Copepod Populations in Lake Mendota Henry Schmit with Jake Walsh and Dr. Jake Vander Zanden UW Hasler Center for Limnology Background Copepods are a type of native microscopic crustacean that play an intrinsic role in the ecosystem of many lakes. The invasive predatory zooplankter, Bythotrephes longimanus, was first detected in Lake Mendota in 2009 (Vander Zanden, personal communication), but its effects on native copepod communities are poorly understood (Lathrop et al., 2002). In Lake Mendota, cyclopoids consist of two main species Diacyclops thomasi and Mesocyclops edax. Research question Have copepod populations in Lake Mendota changed since the spiny water flea invasion, and if so why? Data/Methods In order to count zooplankton a mesh net is dropped down and then drawn up; the contents of the net are then washed with ethanol into a jar. At the lab, the zooplankton were suspended in water and from that a subsample was analyzed. Near the center of Lake Mendota, data were gathered by letting a plankton net drift down 15 meters and then pulling it up at a constant rate of three seconds per meter. The mesh size of the net is 83um. The net was always dropped at a point marked by a buoy that measures temperature and other parameters. The plankton at the bottom of the net then were sprayed with ethanol, washing them into a jar. A subsample of the data were analysed in the lab under a microscope by counting the number and kind of plankton. This procedure consisted of: ●putting the sample with the ethanol through a 35 or 80 um mesh ●rinsing it into a beaker with water until it was filed to the 140 ml mark ●a 1 ml subsample was drawn using Hensen-Stempel pipette ●the sub-sample was put onto a counting wheel for examining under the microscope. Some types of plankton were identified down to the species, while others were not. Summary It was discovered that M. edax population density was reduced by an order of magnitude after the B. longimanus invasion. However, D. thomasi and Calanoids increased after the invasion, which may have occurred because of reduced competition from M. edax and Daphnia pulicaria, a dominant grazing zooplankton in Lake Mendota that has been negatively impacted by B. longimanus predation. However in 2013 the number of D. thomasi sharply dropped off, and we are not sure why. The number of Copepodites increased in 2011 most likely because they did not reach full size and so could not be identified down to the species. Analysis References Confer, John L. "Intrazooplankton Predation by Mesocyclops Edax at Natural Prey Densities." Limnology and Oceanography 16.4 (1971): JSTOR. Web. 19 Aug Lathrop, R. C., Johnson, B. M., Johnson, T. B., Vogelsang, M. T., Carpenter, S. R., Hrabik, T. R., Kitchell, J. F., Magnuson, J. J., Rudstam, L. G. and Stewart, R. S. (2002), Stocking piscivores to improve fishing and water clarity: a synthesis of the Lake Mendota biomanipulation project. Freshwater Biology, 47: 2410–2424. doi: /j x Rollwagen-Bollens, Gretchen, Stephen M. Bollens, Alejandro Gonzalez, Julie Zimmerman, Tammy Lee, and Josh Emerson. "Feeding Dynamics of the Copepod Diacyclops Thomasi Before, during and following Filamentous Cyanobacteria Blooms in a Large, Shallow Temperate Lake - Springer." Feeding Dynamics of the Copepod Diacyclops Thomasi Before, during and following Filamentous Cyanobacteria Blooms in a Large, Shallow Temperate Lake - Springer. Hydrobiologia, 01 Mar Web. 17 Aug Diacyclops thomasi, Cyclopoid copepod density over several years ( ). The y-axis is a logarithmic plot. The vertical dashed line indicates the approximate date of the invasion of the spiny water flea. Mesocyclops edax, Cyclopoid copepod density over several years ( ). The y- axis is a logarithmic plot. The vertical dashed line indicates the approximate date of the invasion of the spiny water flea. Nauplii density over several years ( ). The y-axis is a logarithmic plot. The vertical dashed line indicates the approximate date of the invasion of the spiny water flea. Calanoid copepod density over several years ( ). The y-axis is a logarithmic plot. The vertical dashed line indicates the approximate date of the invasion of the spiny water flea. Diacyclops thomasi,(left panel), Mesocyclops edax, (right panel), USGS Limited research has been done on these animals and how they affect aquatic food webs. They eat plankton earlier in their development, then turn carnivorous, eating more zooplankton, daphnia, and younger copepods as they grow older (Confer, 1971). Copepods are eaten by planktivorous fish. If their numbers are decreasing it may be from direct spiny water flea predation or spiny water fleas eating their prey such as Daphnia. If their numbers are increasing it may be from spiny water fleas eating their competition (Rollwagen-Bollens et al., 2013). Acknowledgements Thanks to the Long Term Ecological Research (LTER) network, and the Science Research Internship program. Another thanks to the UW students who worked in the lab: Bridget, Jared, and Sam. Years M. edaxD. thomasiCalanoids Copepodites B. longimanus Before invasion (n=210) After invasion (n=74) Logarithmic average of Mesocyclops edax, Diacyclops thomasi, Calanoids, Copepodites, and Bythotrephes longimanus before and after the spiny water flea invasion. Ward counting wheel Mesh cupHensen-Stempel Pipette Spiny Water Flea (Bythotrephes Longimanus) Photo by Jake Walsh Links Copepodite copepod density over several years ( ). The y-axis is a logarithmic plot. The vertical dashed line indicates the approximate date of the invasion of the spiny water flea.