Blooms of ulvoid macroalgae (mainly Enteromorpha and Ulva) have covered 80% of the intertidal seagrass bed at Wanlitung, southern Taiwan, effectively shading the seagrasss species Thalassia hemprichii resulting in a decrease in photosynthetic performance and low inorganic carbon (C i ) uptake. We looked for evidence of C i limitation and investigated the C i utilization characteristics of ulvoid-free and ulvoid-covered T. hemprichii. The rapid light curve (RLC) function of the Diving-PAM (Diving-PAM, Walz, Germany) was used to measure in situ photosynthetic performance (based on the effective quantum yield of PSII (Y) values) of intact seagrasses that were placed in small incubating chambers. Significantly, a lower RETR max (maximum relative electron transport rate) and E k (light intensity at the onset of saturation) were noted in the ulvoid-covered compared to the ulvoid-free T. hemprichii, suggesting that the former has acclimatized to the low light environment becoming a shade type plant. The ulvoid-covered T. hemprichii showed some evidence of C i limitation since a significant increase in RETR max (up to 46%; P < 0.05) was noted after an increase in the concentration of NaHCO 3 from 2.2 (normal seawater) to 6.2 mM. In terms of C i utilization characteristics, T. hemprichii could directly absorb HCO 3 - as the major C i source but partially depended on the extracellular carbonic anhydrase (CA) to convert HCO 3 - to CO 2 prior to uptake in the ulvoid-free, high light- adapted populations. A wastewater stream with a high nutrient load coming from the urbanized area may have caused the frequent blooms of ulvoid macroalgae.