Defining the Risk-Release Relationship of Daphnia magna, a Non-native Aquatic Species in the Environment of Duluth-Superior Harbor

Abstract

Understanding how inoculation characteristics (propagule pressure) affect establishment outcome of a non-native, invading species in the context of abiotic and biotic factors (ecological resistance) is a recent area of emphasis in ecology given an urgent need to prevent and manage invasive species globally. The overarching objective of this dissertation research is to quantify how a non-native species (Daphnia magna) establishes in response to different levels of propagule pressure in the context of a Laurentian Great Lakes port receiving system. Specifically, I described spatial and temporal patterns in the density and diversity of crustacean zooplankton in the context of abiotic factors of the Duluth-Superior Harbor and St. Louis River Estuary (DSHSLRE) and used those measures to develop establishment criteria for the propagule pressure experiments. Next, I evaluated the ability of a surrogate invader, D. magna to successfully colonize 200-L mesocosm aquaria by manipulating propagule pressure and several components of ecological resistance. Special attention was paid to the density of 10 individuals m-3 as this is the current standard suggested by the International Maritime Organization for density of permissible non-native species in ballast water discharge from oceangoing vessels. Lastly, I re-analyzed the results from the propagule pressure experiments to evaluate how different establishment criteria, which vary in rigor, influence the perceived outcome. Results demonstrate notable spatial and seasonal patterns in the abiotic conditions and zooplankton diversity and density measured at nine locations sampled biweekly during 2007 and 2008 in the DSHSLRE. Abiotic characteristics of upstream sites were indicative of littoral habitats and changed more dramatically within a year and from year to year. Overall 49 distinct taxa were identified, half at the genus level Copepoda and half Cladocera. Colonization of the mesocosm aquariums by D. magna populations ranged from 50-80% in the spring and fall experiments but never achieved success greater than 33% in the summer experiments. Only when the background assemblage of zooplankton was first removed from the tanks in the summer was D. magna able to establish suggesting that ecological resistance related to resource competition was prohibitive. Risk of establishment evaluated by different threshold criteria influenced the perceived outcome of the mesocosm experiments. This study demonstrates the multi-faceted nature of drivers that determine establishment success of a non-native species and the complexities of evaluating standards on permissible biological pollution using an experimental, mesocosm venue.