Sargassum leachate initially inhibits then stimulates phytoplankton growth in coral reef waters

Massive Sargassum accumulations in the Atlantic form the Great Atlantic Sargassum Belt, which fuels recurrent coastal inundation events that disrupt ecosystems by reducing light availability for phytoplankton and enriching nearshore waters with nutrients released during decomposition. This study examined the short-term effects of decaying Sargassum leachate on phytoplankton productivity and growth in Guadeloupe's coral reef ecosystems using several photosynthesis measurements based on fluorescence and 13 C. A range of leachate concentrations were applied, under controlled conditions, to a natural phytoplankton community. The study revealed two major effects of Sargassum leachate on phytoplankton. First, leachate at concentrations ≥1 % initially inhibited photosynthesis and growth up to Day 2, likely due to allelopathic substances such as polyphenols. This was evidenced by near-zero Fv/Fm values and disrupted electron transport in PSII. By Day 3, these inhibitory effects diminished, suggesting degradation of labile inhibitory compounds or community modification. Second, from Day 3 onward, leachate addition stimulated phytoplankton growth, as seen in increased biomass, primary production (JVIImax), and carbon fixation. This nutrient-driven response was accompanied by reduced C/Chl a ratios and improved photosynthetic efficiency (lower Фe,C values), indicating alleviation of nutrient limitations, particularly nitrogen and phosphorus. These findings underscore the dual role of Sargassum leachate: an initial suppressive impact through allelopathy and a subsequent nutrient enrichment effect driving phytoplankton blooms. Such dynamics highlight the significant and complex influence of Sargassum strandings, combining ecological stress with nutrient-driven productivity changes. • Leachate ≥1 % inhibits phytoplankton photosynthesis up to Day 2. • Inhibitory effects lessen by Day 3. • From Day 3, leachate boosts growth via nutrient-driven stimulation. • Leachate has dual impacts: allelopathy first, enrichment later. • Complex influence of Sargassum strandings on Caribbean ecosystems.