Hourly photosynthetic rates were determined in situ for Bostrychia binderi and Gracilaria verrucosa at a salt marsh (WWR) and a mangrove swamp (CRB) on the west coast of Florida over a 24-h period using the Winkler L/D bottle technique. Each population had a peak
in photosynthesis in mid-morning, dropping to low levels by the afternoon. A conversion between L/D bottles and manometric determinations under identical conditions in the laboratory was determined, 0.0115 mg-at O2/μl O2. Diel fluctuations in photosynthesis
were found when plants were run 24 h under constant conditions in the laboratory. Labelled carbon (C14) was used to obtain photosynthetic quotients for Bostrychia (WWR: 1.4; CRB: 1.2) and Gracilaria (WWR: 1.3; CRB: 1.2). Net production (g C/m2/day) was determined
for Bostrychia (WWR: 0,179; CRB: 0.141) and Gracilaria (WWR: 0.216; CRB: 0.160) and was found to be lower than that reported for other macroalgae, probably due to the lack of diel data on photosynthetic activity in previous studies.
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