Long-Term Population Growth And Food Web Impacts Of The Spiny Water Flea (Bythotrephes Longimanus) Revealed From Sediment Records

Abstract

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 potential prey and competitor taxa (bosminids, daphniids, Simocephalus, and Leptodora kindtii) and pigment concentrations 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 lower food web. Bythotrephes evidence was found nearly 100 years prior to first detection in each lake, potentially 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. Of the native zooplankton species, Bosmina was the only species group that declined in correspondence with Bythotrephes population growth. 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. Sediment pigment concentrations did not increase with Bythotrephes population growth as expected in either lake. This research suggests that 1) Bythotrephes could be present in lakes decades before detection in zooplankton net samples, 2) populations take several decades to overcome lag phases, and 3) that other ecosystem factors may be more important than Bythotrephes in driving long-term food web changes in these lakes.