Quantitative assessment of planktonic AIS eDNA signal duration in Great Lakes harbor water microcosms

U.S. Laker ships move large quantities of untreated ballast water from the lower Great Lakes to Lake Superior, making early detection of AIS in harbors of ballast uptake critical to spread prevention. Environmental DNA (eDNA) techniques using qPCR methods targeting AIS of concern can facilitate early detection surveillance by signaling presence of the target species. However, such methods detect genetic material of an organism regardless of the organism’s vital status, making the question of how long eDNA persists in the environment after removal or death— a duration known to vary across species and conditions—highly relevant to interpretation of eDNA screening outcomes, with implications for invasion management and action. While eDNA decay rates have been measured in closed systems under varying conditions for organisms easily removed from the system, data are lacking for zooplankton since removal experiments are not practical. We developed a method that utilizes microcosms to measure eDNA signal decay in zooplankton after the death of the organisms and applied the method to persistence of target-specific, qPCR signal in eDNA extracted from Lake Erie water microcosms inoculated with the invasive mysid, Hemimysis anomala (GO Sars, 1907). At temperatures between 19 and 23 °C, qPCR signal rapidly decayed in the first 48 hours. During the exponential decay phase, T90 and T99 values were 14.8 hours and 29.5 hours respectively, the eDNA half-life was 4.5 hours and the decay rate constant was 0.154 h^-1. The limit of detection was reached in seven days and there was no detectable signal for the remainder of the 29-day experiment. Similar to what others have shown for larger organisms, these results indicate that eDNA from this species of zooplankton is rapidly metabolized and support the use of screening for eDNA as a valuable tool to enable early detection of new populations in Great Lakes harbors.