This study aimed to evaluate the influence of irradiance on photosynthetic rates of Bostrychia calliptera, Bostrychia pinnata, Bostrychia radicans and Bostrychia montagnei. These algae were collected in mangrove trees on mesohaline area of Babitonga Bay (SC) and were kept in laboratory under continuous aeration, 20oC, photoperiod of 16:8h, irradiance »40µmol photons m-2s-1. The production versus irradiance curves (PI Curves) were measured for all 4 species in salinity 20. Additional PI curves were measured for B.calliptera in salinities 10 and 30. Photosynthetic and respiratory rates were estimated using variations on dissolved oxygen concentration (YSI53 oximeter) in BOD flasks with 0.015 to 0.030g dry weigh of the algae and about 250 mL of filtered seawater, diluted to desired salinity. Incubations expended 2 hour and irradiances ranged from 0 to 1000 µmol photons m-2 s-1 and temperature about 23oC. The curve parameters (maximum production=Pmax; Respiration=resp; photosynthetic efficiency=a) were estimated using three usual models. The model which adjusted to observed data with minimum summed square of residues was choose for all estimations. Saturating irradiance (Ik) was estimated by: Ik=Pmax/a. Compensating irradiance (Ic) was estimated by algebraic manipulation of adjusted equation. The higher values of Pmax were observed for para B.calliptera in salinity 30 and for B.pinnata and B.radicans in salinity 20. Higher values of photosynthetic efficiency were observed for B.calliptera in salinity 30 and B.pinnata in salinity 20. Respiration values were higher for B.radicans and B.montagnei in salinity 20 and B.calliptera in salinity 30. Increasing salinity increased photosynthetic rates for B.calliptera and B.montagnei. In the same salinity, maximum production was observed for B.radicans, followed by B.pinnata, B.calliptera. Values of saturating irradiance and compensating irradiance were very low and values of photosynthetic efficiency were very high for all seaweed in this work, characterizing them as shadow plants. Though the seaweeds did not show photoinhibition, what suggests great tolerance to irradiances variations, as normally observed in mangroves.