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 introductions in the Laurentian Great Lakes. The United States Coast Guard ballast discharge standard for organisms between 10 and 50 µm is 10 living cells/ml, implying ballast-borne introductions of live protists below this density are allowable. Data are lacking on protist risk-release relationships, i.e., the propagule pressure resulting in non-native species establishment. We investigated risk-release relationships for non-native protist taxa under varying water conditions using mesocosms, quantifying critical densities resulting in establishment and comparing these densities to the ballast water discharge criterion of “fewer than 10 cells/ml”. To mimic ballast discharge events, protists were inoculated into mesocosms and sampled over 4 weeks. Experiments evaluated seven surrogate “invaders”: Haematococcus pluvialis , Trachelomonas abrupta , Chrysosphaerella sp., Merismopedia elegans, Gymnodinium fuscum , Mallomonas splendens , and Melosira varians. Logistic models predicted the surrogate protist densities resulting in establishment ranged from 1 to 320 cells/ml. We noted how seasonality influences establishment success; a taxon’s establishment probability varied based on starting conditions, represented as ambient protist communities and water conditions. Our application of mesocosms to study non-native, risk-release relationships supports the use of this method in other locations and with additional taxa.