Abstract

Blooms of green macroalgae have become a recurring phenomenon along many shores. Here, we conducted oceanographic surveys and field experiments to quantify daily growth rates of Ulva stenophylloides, the predominant species in the green tides that occur year-round at the southern end of the wave-protected open bay of Algarrobo, central Chile. We assessed the association between growth and nutrient availability, as well as other environmental variables, and compared growth rates between the south end of the bay and the north end (1.6 km away), where green tides are absent. Fronds of U. stenophylloides were suspended at similar to 3-4 m depth from moorings anchored at 10 m depth at both sites. Daily growth rates were high at both locations, ranging from a 9 %-121 % increase in wet biomass, but were significantly higher at the southern site. Nutrient concentrations in water samples collected once daily were not correlated with algal growth and showed no consistent differences between sites. However, high-frequency temperature records from moorings revealed rapid (minutes to hours) cold-water pulses at the southern end, which were nearly absent in the north. Given the strong inverse relationship between temperature and nitrate in the bay, we suggest that these cold-water intrusions-likely associated with internal waves-supply additional nutrients supporting higher growth rates at the southern site. Our findings indicate that once-daily water sampling may misrepresent the dynamic nutrient regimes experienced by fast-growing macroalgae, highlighting the need for high-frequency monitoring.