Effects of Short Exposure to Temperature Stress on Gametophyte Survival and Development of Three New Zealand Kelps

Kelp life stages are highly sensitive to temperature shifts, making early development particularly vulnerable to both acute and chronic heat stress. This study investigated the effect of short‐term exposure to heat stress on the development of gametophytes (survival, fertilisation and embryonic sporophyte formation) of three New Zealand kelp species: Lessonia variegata , Ecklonia radiata and Macrocystis pyrifera . Gametophytes of the three species were exposed for 24 h to a temperature challenge, ranging from 10 to 25 °C, before returning to control conditions (15 °C). Survival of gametophytes was analysed immediately after the temperature challenge, fertilisation after 48 h and embryonic sporophyte formation after 7 days post‐challenge. Sex was found to be an important predictor of gametophyte survival. Higher mortality was observed in male gametophytes for all species at temperatures >23 °C compared to cooler temperatures, whereas the survival of female gametophytes was not impacted and remained high (>75%) across all temperatures. Fertilisation and the formation of embryonic sporophytes had a narrower thermal range than survival for E. radiata and L. variegata , with a higher thermal tolerance for M. pyrifera . Overall, L. variegata gametophytes were more vulnerable to high temperatures, with fertilisation occurring at less than half the rate of M. pyrifera at 25 °C, and with significantly lower rates of embryonic sporophyte formation compared to E. radiata and M. pyrifera . The sex‐ and species‐specific responses observed in this study provide preliminary but critical insight into the thermal sensitivities of kelp species after a short heat exposure, offering a foundation for predicting how these key species will respond to ongoing ocean warming, with important implications for the ecosystems they support.