WRS Masters Defense


Nicki DeWeese
Masters Student, Water Resources Science

December 3, 2019


Long-term Population Growth and Food Web Impacts of Bythotrephes longimanus Revealed from Sediment Records

The spiny water flea (Bythotrephes longimanus) is an invasive predacious zooplankton species that has well-documented impacts on aquatic food webs. However, few studies have examined long-term population dynamics and food web impacts of the species. This study used Bythotrephes subfossils, as well as subfossils from bosminids, daphniids, Simocephalus, and Leptodora kindtii, in 210Pb dated sediment cores from Mille Lacs Lake and Kabetogama Lake in Minnesota, USA to estimate first invasion and population growth of Bythotrephes and corresponding impacts on the native zooplankton community. Bythotrephes evidence was found nearly 100 years prior to first detection in each lake, making these lakes the earliest invaded lakes in North America. Bythotrephes subfossils slowly accumulated until around 1990, when accumulation rates rapidly increased. Two-piece linear models provided a good fit for Bythotrephes annual accumulation rates (a proxy for population size), and modelled lag phases lasted approximately 70 years in each lake. Native zooplankton accumulation rates changed over time, but the major changes occurred prior to the transition of Bythotrephes populations from lag to growth phases. The only species that declined as Bythotrephes populations grew was Bosmina in Mille Lacs Lake. Bosmina antennules and mucros were measured to analyze if morphological changes occurred as Bythotrephes populations grew, but these feature lengths did not consistently change over time. This research suggests that Bythotrephes could be present in lakes decades before detection in zooplankton net samples, populations take several decades to overcome lag phases, and that other factors may be driving long-term food web changes in these lakes.