A Risk-Release Assessment Of Aquatic Invasive Species Using Protist Surrogates In Mesocosm Studies

Abstract

Untreated ballast water discharge is considered responsible for nearly thirty percent of aquatic invasive species (AIS) introductions in the Laurentian Great Lakes. Currently, the ballast discharge standard for organisms between 10 and 50 µm in size is 10 cells/mL, implying introductions of live protists below this density in released ballast water are allowable. However, data are lacking on protist risk-release relationships; i.e., the level of propagule pressure that results in non-native species establishment. This research investigates the risk-release relationships for various non-native protist taxa under varying water conditions using mesocosms. To mimic ballast discharge events, protist densities of 0 - 100 cells/mL were inoculated into mesocosms filled with Duluth-Superior Harbor (western Lake Superior) water and sampled over 4 weeks. Experiments evaluated four surrogate “invaders”: Haematococcus pluvialis, Trachelomonas abrupta, Chrysosphaerella sp., and Melosira varians. Logistic models predicted the minimum surrogate protist densities resulting in establishment ranged from 11 to 127 cells/mL for non-native species, thereby providing information on risk-release relationships relative to the discharge standard. We also noted how seasonality (e.g., varying ambient water conditions) influences establishment success. Though there are limitations of the mesocosm method in invasive species studies, our successful application of mesocosms to study non-native, risk-release relationships supports the potential use of this method in other locations (e.g., other harbors, ports) and with additional taxa.