Responses of phytoplankton to experimental nutrient enrichment in an Amazon floodplain lake1

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Abstract
Natural algal populations in 4‐, 10‐, and 800‐liter enclosures were fertilized to ascertain to what extent ammonium, nitrate, or phosphate enrichment changed phytoplankton abundance and physiology in an Amazon floodplain lake (Lake Calado). When water was rising and was between 8 and 11 m (March–May 1982), algal abundance ranged from 13–24 µg Chl a liter−1 and epilimnetic NH4+, NO3−, and PO43− concentrations were usually µM. Chlorophyll increased over the control by 60–265% in the P treatment, 5–40% in the N treatments, and 75–270% in the N+P treatments after about 5 days. In three of the four experiments, increases in chlorophyll after addition of phosphate alone were significantly greater (P < 0.05) than after additions of nitrate or ammonium alone. Increases in net photosynthesis and in particulate C and N concentrations corroborated the chlorophyll data. Deficiency indices (ammonium‐enhanced dark 14C fixation, N and P uptake rates, and sestonic N:P ratios) implied that the phytoplankton were impoverished with respect to nitrogen and phosphorus; nutrient enrichment relieved this condition but deficiency often returned after a few days. When water depth was falling and was between 7 and 2 m (September–November 1982) algal abundance ranged from 4 to 15 µg Chl a·liter−1, turbidity increased, and epilimnetic inorganic N and P were usually µM. During September both N and P additions increased algal standing crop, whereas in early October enrichment with N increased abundance substantially (P < 0.05) above that after P addition. The physiological evidence indicated that the lake was N‐deficient during September; in October neither N nor P deficiency was discernible except for slight alkaline phosphatase activity.