Cladophora rupestris (L.) Kütz. can encounter either abrupt or more gentle declines of salinity down to full fresh water during combined low tide and rainfall. With the rising tide salinity increases immediately. Such salinity regimes were simulated in the laboratory (“abrupt regime” and “gentle regime”) and respiration and photosynthesis were measured during exposure to them. Respiration changed little but was partly enhanced after sudden salinity changes. Photosynthesis was stimulated at the beginning of gentle regimes but was later somewhat inhibited as hypoosmotic stress became greater. In the abrupt regime photosynthesis was inhibited strongly during the low salinity period and showed only partial recovery after re-immersion in full strength sea water indicating severe damage to the photosynthetic apparatus. This damage occurs only when low salinity exposure is accompanied by high illumination (480 μE · m−2 · s−1). Although Cladophora can grow in the field in full sunlight it does not encounter these combined conditions because intense self-shading and epiphytic growth on the alga prevents exposure to high illumination. As a result photosynthesis is not inhibited after re-immersion in full sea water.