Marine Biology 44, 85-96 (1977) MARINE BIOLOGY 9 by Springer-Verlag 1977 An Annual Carbon Budget for the Kelp Laminaria Iongicruris B.G. Hatcher, A.R.O. Chapman and K.H. Mann Department of Biology, Dalhousie University; Halifax, Nova Scotia, Canada Abstract Oxygen evolution and uptake by whole thalli of the large marine alga Laminaria longi- cruris de la Pylaie were measured for 24 h, once every 2 weeks for a year, using large chambers to incubate the plants on the sea bed. Diel rates of photosynthesis and respiration were calculated from these measurements and continuous light mea- surements were used to extrapolate the data between observation dates. The resulting estimates were combined with measurements of growth and carbon content to give an annual carbon budget for a typical mature plant. Annual net assimilation was 6.8 mgC per cm 2 of frond surface (71 cal cm-2). Approximately 45% of this appeared in the production of new frond tissue, and a further 12% was accounted for by storage of carbon in mature frond tissue. About 8% was needed for stipe growth, and the re- maining 35% was assumed to be lost as dissolved organic carbon. Diel net photosyn- thetic rates reached a maximum in June and July and were negative only in November, indicating an ability to produce a photosynthetic surplus throughout winter. In early winter the plants drew on stored reserves to supplement photosynthesis in providing carbon for growth, but from January onwards photosynthesis provided more than enough carbon for growth. Introduction Large marine algae in temperate waters often exhibit periods of rapid growth in winter and early spring, when both light and temperature are at low levels. Para- doxically, they grow less rapidly in summer. The phenomenon has been docu- mented in Desmarestia aculeata (Chapman and Burrows, 1971), sublittoral fucoids (Suto, 1951) as well as in the lami- narians studied by Black (1950), L~ning (1971), Mann (1972), and L~ning et al. (1973). L~ning and his co-workers showed for Laminaria hyperborea that carbon was stored in summer and was translocated to the meristematic region to provide mate- rials and energy for late winter growth. There was little or no photosynthetic surplus in winter. This study was designed to find out whether Laminaria ion~cruris has a photo- synthetic surplus in winter, and to re- late annual patterns of photosynthesis and growth to environmental conditions. Materials and Methods Field Measurements on Plants For reasons given in Hatcher (1977), the principal measurements of photosynthesis and respiration were made on whole plants, using specially designed large incubation chambers on the sea floor. The apparatus (Hatcher, 1977) consisted of a transparent and an opaque chamber, each 30 cm diameter x 110 cm high, and fitted with mechanisms for (i) passing the water through 0.45 ~m filters to re- Laminaria lon~cruris in Nova Scotia shows move phytoplankton and most bacteria, uninterrupted growth throughout the win- (ii) removing bubbles, (iii) stirring to ter, and tissue present in summer is break down diffusion gradients at the pushed to the tip of the blade and plant surface, (iv) reducing the oxygen eroded during the winter. Hence, the op- tension prior to incubation, to prevent portunities for storage and translocation supersaturation and (v) removing water seem more limited, samples during incubation.