The reproductive phenology of Asparagopsis armata in New Zealand – Potential shifts 35 years later

Abstract

The reproductive phenology (the timing of key reproductive events) of seaweeds is largely controlled by environmental factors related to seasonal change, particularly temperature. Therefore, it can be expected that climate change-induced increases in sea temperature can cause shifts in the reproductive phenology of seaweeds, although this has been difficult to detect due to the lack of baseline data for reproduction. We investigated the seasonal occurrence and reproductive phenology of the emerging aquaculture-target species Asparagopsis armata on the North Island of New Zealand from 2021 to 2022, which was previously assessed 35 years ago (1987) when temperatures were on average 1.5 °C cooler. Seasonality in reproduction was assessed via the presence of tetrasporangia, tetraspore release and germination rate, presence of cystocarps, cystocarp density and cystocarp opening. Tetrasporophytes occurred year-round with tetrasporangia present from March – September (austral autumn – spring), with populations most fertile from May – July (austral autumn – winter). Tetraspore release peaked in June 2021 and September 2022, but was otherwise low during our sampling. Gametophytes were present from August – November (austral winter –spring) with cystocarps present from September (austral spring), which was delayed in comparison to the 1987 assessment. Viable carpospores were also only released under controlled conditions during September, as opposed to during all reproductive months in the 1987 assessment. Discolouration was a common occurrence in natural populations during 2022 that translated into poor reproductive output and high mortality under controlled conditions, which could be explained by prolonged heat stress throughout 2022. This study presents the first temporally longitudinal analysis of potential climate driven shifts in reproductive phenology of a seaweed and provides a quantitative baseline for detecting changes in reproductive phenology in the future.