Absolute estimates of the propagule pressure of viable dinoflagellates across Canadian coasts: the variable influence of ballast water exchange

Abstract

Estimates of propagule pressure in invasion biology are often unavailable due to widespread use of proxy variables, leading to uncertainty in absolute introduction effort of marine non-indigenous species (NIS). We present a first estimate of absolute propagule pressure (probability distributions of the total number of individuals introduced per release event and per year) for viable non-indigenous dinoflagellates transported via commercial vessels to major ports on the East and West coasts of Canada, thereby establishing a baseline metric against which to test future hypotheses about factors controlling invasion rates. To quantify the influence of ballast water management on absolute propagule pressure, three shipping routes were evaluated: (1) transoceanic with mandatory ballast water exchange (BWE), (2) coastal with BWE, and (3) coastal without BWE. Results provide insufficient evidence that BWE consistently reduces propagule pressure of dinoflagellates in ballast tanks. Moreover, controlled growth experiments confirmed that several dinoflagellate species were viable (e.g., capable to reproduce) following their release from arriving vessels, including those conducting exchange. We caution that the ecological significance of these findings is unclear without a better understanding of the factors that allow offshore dinoflagellates to establish in nearshore marine environments; however, several dinoflagellates NIS may have the potential to become established in Canadian coastal marine ecosystems following ballast water exchange.