Temperature and Phytoplankton Growth in the Sea

The variation in growth rate with temperature of unicellular algae suggests that an equation can be written to describe the maximum expected growth rate for temperatures less than 40°C. Measured rates of phytoplankton growth in the sea and in lakes are reviewed and compared with maximum expected rates. The assimilation number (i.e., rate of photosynthetic carbon assimilation per weight of chlorophyll a) for phytoplankton photosynthesis is related to the growth rate and the carbon/chlorophyll a ratio in the phytoplankton. Since maximum expected growth rate can be estimated from temperature, the maximum expected assimilation number can also be estimated if the carbon/ chlorophyll a ratio in the phytoplankton crop is known. Many investigations of phytoplankton photosynthesis in the ocean have included measures of the assimilation number, while fewer data are available on growth rate. Assimilation numbers for Antarctic seas are low as would be expected from the low ambient temperatures. Tropical seas and temperate waters in summer often show low assimilation numbers as a result of low ambient nutrient concentrations. However, coastal estuaries with rapid nutrient regeneration processes show seasonal variations in the assimilation number with temperature which agree well with expectation. The variation in maximum expected growth rate with temperature seems a logical starting point for modeling phytoplankton growth and photosynthesis in the sea. Temperature does not seem to be very important in the production of phytoplankton in the sea. For example, Steemann Nielsen (1960) has written, "Recent investigations have shown, however, that the direct influence of temperature on organic production in the sea is fairly insignificant." Other reviewers of photosynthesis in the sea likewise give little or no consideration to a role of temperature and Steemann Nielsen's statements find widespread endorsement in the published data on geographic and seasonal variation in marine phytoplankton photosynthesis. In response to this, the reader may ask at least two questions: (1) Why is temperature of so little importance and (2) why would anybody write a review on temperature and phytoplankton growth in the oceans? Several answers to the first question have appeared in the literature and some of these will be discussed , Institute of Marine Resources, University of California, San Diego, P.O. Box 109, La Jolla, CA 92037. Manuscript accepted May 1972. FISHERY BULLETIN: VOL. 70. NO.4. 1972. in this account. I have two answers for the second question. The purpose of this review is (1) to suggest maximum growth and photosynthetic rates which might be reasonably expected for natural marine phytoplankton and (2) to point out the interrelationship among growth rate, photosynthetic assimilation number (Le., rate/chlorophyll), and carbon/chlorophyll a ratios in the phytoplankton. What follows is an attempt to show that temperature sets an upper limit on phytoplankton growth rate and on the rate of photosynthesis per weight of chlorophyll, and that this upper limit can be predicted from a knowledge of temperature and the carbon and chlorophyll content of the plants. It can perhaps be inferred, from above, that phytoplankton growth in the oceans seldom approaches the upper limits imposed by the temperature of the water and that temperature does not figure prominently in simulation models for primary production in the marine environment. Other factors effect reduced rates of growth and