Invasion dynamics of quagga mussels within a Southern California reservoir and its spatially intermittent watershed

Since its discovery in Lake Mead, Nevada in 2007, the invasive
quagga mussel ( Dreissena rostriformis bugensis )
spread throughout the lower Colorado River drainage and into
connected Southern California water systems. In December 2013,
quagga mussels were found in Lake Piru, California, a reservoir with
no connection to the Colorado River drainage. An initial “boom”
period occurred in the first year after colonization. High densities
and settlement rates continued for three years while lake water
levels were low and relatively stable, despite periodic removals of
mussels from lake infrastructure. Mussels were initially restricted
to hard substrates but were regularly found on soft sediments within
two years of colonization. Storms in 2017 dramatically increased the
lake level and deposited substantial sediment, which eliminated
mussels on soft sediments and reduced the overall mussel population.
Reproduction and juvenile settlement rebounded within 6 months,
despite the low population of adult mussels in the lake.
Environmental conditions, particularly fill status and water
temperature, rather than adult density, appear to be the primary
driver of veliger abundance in this system, while recruitment was
primarily explained by veliger abundance. Elevated water releases
from the reservoir increased the flux of veligers downstream and led
to mussel recruitment >15 km downstream. Sustained establishment
of quagga mussels downstream has not occurred in the Santa Clara
River and seems unlikely due to the unstable habitat conditions.
However, periodic downstream colonization increases the likelihood
for the infestation to spread and impact agricultural and municipal
water systems that receive water from the river.