Variation in germling growth in the green tide–forming alga Ulva intestinalis (Chlorophyta) in response to gradients in salinity, temperature, light, and nutrients

The frequency and magnitude of large ulvoid macroalgal blooms (green tides) have increased in recent years, causing concern for the collapse of benthic ecosystems. Ulva intestinalis is commonly considered a bloom-forming species and it is necessary to understand how variation in environmental conditions affects its ecophysiology in order to predict its bloom potential. To address this, we conducted a laboratory experiment to examine how variation in abiotic factors influences early development and growth in U. intestinalis. Specifically, germling growth characteristics were observed in primary thallus disks (PTD) and early filamentous stages (EFS) under exposure to gradients in salinity, temperature, and light and in concentrations of ammonium and phosphate over a period of 27 days. Our results show that growth in EFS occurred under all salinity and temperature conditions except for the highest salinity (S = 50) and temperature (25 °C) conditions examined. Growth in the EFS was also reduced under low light (< 30 μmol photons m−2 s−1) and under high nutrient (NH4+ and PO43−) concentrations. In contrast, light intensities of 90 ~ 120 μmol photons m−2 s−1 and a temperature of 15 °C stimulated the explosive growth, especially in EFS. Together, our results show that variation in abiotic conditions can both stimulate and limit germling growth, and it is expected that spore settlement and recruitment will become more active with the increased bloom potential of U. intestinalis under optimal light and temperature conditions.