In the Gulf of California, marine organisms can be exposed to great fluctuations of environmental parameters that regulate their metabolism and survival. We evaluated over a two-year period the photosynthetic characteristics, growth rates and nitrate uptake rates as a function of irradiance and temperature in two rhodophytes, Chondracanthus squarrulosus and Eucheuma uncinatum, endemic to the Gulf of California. In the field, plants were exposed to an annual temperature and irradiance variation of more than 14 °C and 30 mol quanta m−2 d−1, respectively. Maximum cover of C. squarrulosus in the field occurred during the winter-spring while maximum values for E. uncinatum occurred during the summer-fall. Maximum photosynthesis (Pmax) values were approximately two- to three-fold greater in C. squarrulosus than in E. uncinatum, and were out of phase in the two species by approximately three months. The greater Pmax values found for C. squarrulosus than E. uncinatum are consistent with its two-fold greater cover and growth rates in the field. Photosynthetic efficiency of C. squarrulosus decreased as temperature increased, while it remained relatively constant in E. uncinatum throughout a wide range of assayed temperature suggesting a photosynthetic advantage for C. squarrulosus over E. uncinatum during the colder/less illuminated months of the year.
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