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

Abstract

Natural algal populations in 4-, lo-, 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 pg Chl a.liter-l and epilimnetic NH,+, N03-, and POa3-concentrations were usually ~0.3 HM. 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 -=z 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 pg Chl a-liter-', turbidity increased, and epilimnetic inorganic N and P were usually ~0.3 PM. 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.Evaluation of the worldwide importance of nutrient supply to the productivity and abundance of phytoplankton is hindered by a paucity of information from the tropics (Schindler 1978). Tropical floodplain lakes are especially important because immense floodplains border most large tropical rivers; in South America and Africa about 320,000 and 250,000 km2 are inundated at peak flood (Welcomme 1979). Bordering the middle reaches of the Amazon River is a 60,000-km2 floodplain with thousands of lakes. As the Amazon River and its tributaries rise and fall each year, the floodplain lakes are filled by the rivers and local rain and then drain back to the river. Fisher and Parsley (1979) estimated that 5-l 0% of the Amazon's discharge may be stored in the lakes during rising water and suggested that the riverine inputs of nitrate and phosphate are of primary importance in the productivity of the lakes. We report here experiments to determine the relative importance to phytoplankton of nitrogen and phosphorus supply in Amazonian floodplain lakes, using two complementary approaches over the full range of conditions during a year. We enriched natural populations in enclosures and followed the responses for about 5 days. The added inorganic N and P simulated concentrations in Amazon river water. Chang...

Consequences of riverine flooding for seston and theperiphyton of floating meadows in an Amazon floodplain lake

“…Although SRP was not drawn down in river water after passage through the Apure floodplain, DIN decreased strongly, suggesting higher demand for dissolved N by floodplain biota. Evidence of N deficiency has been obtained in lakes of the mainstem Amazon River (Devol et al 1984;Forsberg 1984;Setaro and Melack 1984), but those studies did not reveal the nutrient status of phytoplankton when the external supply of labile N and P was high and dominated by inflows of river water.…”

“…Phytoplankton biomass generally ranges between 10 and 25 pg liter-l in the lake (Fisher et al 1991;Smith-Morrill 1987;Setaro and Melack 1984). However, in 19 89, chlorophyll concentrations declined steadily from 20-30 yg liter-l in April to well below 3 pg liter-l during the throughflow phase, rising again only on the falling limb of the hydrograph.…”

Factors controlling nutrient concentrations in Amazon floodplain lakes1

“…There are also many leguminous macrophytes on the Amazon floodplain that actively fix N (Salati et al 1982;Victoria unpubl. data), and there are reports of N-fixing, blue-green bacteria in some Amazon floodplain lakes (Schmidt 1973a;Setaro and Melack 1984;B. Forsberg unpubl.…”

“…These lakes can be highly productive (Schmidt 19733;Fisher and Parsley 1979;Devol et al 1984b) Araujo-Lima et al 1986). As in temperate systems, nitrogen and phosphorus appear to be the principal nutrients limiting primary producers in these floodplain lakes (Zaret et al 198 1;Forsberg 1984;Devol et al 1984a; Setaro and Melack 1984;Pinheiro 1985) and are likely to play key roles in maintaining lacustrine productivity. However, the factors controlling the concentrations of N and P in these lakes are poorly understood.…”

Flood pulse influence and anthropic impact on the chemical composition and energy content of Oryza glumaepatula in an Amazonian lake

“…This characteristic is even more pronounced in ecosystems with low nutrient concentrations, such as the clear and black water Amazonian ecosystems (Sioli, 1984;Setaro & Melack, 1984;Esteves et al, 1990). The participation of aquatic macrophytes in nutrient cycling has been verified by many authors (e.g., Granéli & Solander, 1988;Nogueira et al, 1996), who have demonstrated the role of this community in the absorption of nutrients from the sediment as well as their liberation in the water column.…”