COS 255-3 - CANCELLED - The Impacts of Salinity and Temperature on Early Stages of the Invasive Wakame (Undaria pinnatifida) at its Northern Range Limit in California

Abstract: Climate change represents a threat to many ecosystems via changes in temperature and precipitation. Many coastal ecosystems, including those dominated by kelps, are experiencing variable effects on community structure and function due to increasing mean sea-surface temperatures (SST), marine heatwaves, and variation in salinity due to changing precipitation patterns. The effects of these stresses have been well documented in mature kelps, but effects on the microscopic kelp life stage (gametophyte), which are often the most vulnerable to the effects of environmental change, are much less understood. Wakame (Undaria pinnatifida) is a globally invasive kelp first introduced to the West Coast of North America in 2000. Along the West Coast, Wakame can be found primarily in ports and harbors from Ensenada in the south to San Francisco in the north. We sought to answer the question: how do increased temperatures and low salinities affect the reproduction of Wakame individuals at its northern range? In August 2021, we collected reproductively mature individuals of Wakame from the San Francisco Small Boat Harbor and brought them back to Bodega Marine Laboratory for spore release and cultivation. Using a full factorial design, we exposed Wakame gametophytes to five salinity conditions ranging from low to ambient salinity (10, 15, 20, 25, and 33 psu) and two temperatures representing ambient and increased sea surface temperature (15°C and 20°C). These temperatures and salinities were based off of documented averages near the mouth of the San Francisco Bay and projected extremes from increasingly intense and frequent marine heatwaves or atmospheric rivers under climate change. Each week, we photographed three random points within each petri dish and counted the number of male and female gametophytes and the number of eggs and juveniles produced. After six weeks our results show that both increased temperature and lowered salinity had significant impacts on kelp reproduction. We found that microscopic stages of Wakame were unable to survive at 10 and 15 psu, but exhibited relatively equal survival and juvenile production rates at 20, 25, and 33 psu. Additionally, we saw significantly higher survival and reproduction of Wakame rates under warmer temperatures (20°C) than ambient temperature (15°C). Our results indicate that under predicted higher temperatures and reduced annual rainfall and lower salinity in drought years, future northward expansion of Wakame populations in California may be likely.