MEASUREMENTS OF PRIMARY PRODUCTION IN COASTAL SEA WATER USING A LARGE‐VOLUME PLASTIC SPHERE

Abstract

A free‐floating 20‐ft diameter thin transparent sphere, with its center 5.5 m below the sea surface, was filled with nutrient rich water filtered free from plants and animals and inoculated with a natural population of coastal phytoplankters. The resulting phytoplankton “bloom” was then studied in this well‐mixed water mass under near‐natural conditions of temperature and illumination with the minimum of complications arising from grazing, sinking or lateral transport of the plant cells.Daily measurements were made of temperature, light and photosynthesis and the water was analyzed for oxygen, carbon dioxide and all common micronutrients. The production of plant material was followed by cell counts and pigment values and by analyses for particulate carbon, nitrogen, phosphorus, carbohydrate, protein and fat.The results have a bearing on nearly all aspects of marine primary productivity and are discussed in detail. In particular, the growth kinetics and chemical composition of a natural mixed crop have been related to the environment with the minimum of ambiguity. Growth was finally limited by nitrogen deficiency, although the plant biomass nearly doubled after nitrate became depleted, with a shift of metabolism to carbohydrate and fat synthesis. The composition of the plankton at various stages in a bloom is summarized by ratios involving carbon and chlorophyll a.Differences between the results for photosynthesis obtained by the oxygen and C14 method have emphasized the importance of knowing photosynthetic quotient values and possible unlabeled sources of plant carbon. During an initial period of low illumination there was strong evidence for heterotrophic growth.The principal species found in the “bloom” were Skeletonema costatum, Thalassiosira nordenskiöldii, T. rotula, Gyrodinium fulvum, Glenodinium danicum, Nitzschia delicatessima and Asterionella japonica. The last two grew very slowly and the two dinoflagellates only appeared after the nitrate was depleted.