Microbial Ecology Influence of Hydrological Pulse on Bacterial Growth and DOC Uptake in a Clear-Water Amazonian Lake Vinicius F. Farjalla 1 , Debora A. Azevedo 2 , Francisco A. Esteves 1 , Reinaldo L. Bozelli 1 , Fabio Roland 3 and Alex Enrich-Prast 1 (1) Lab. Limnologia, Dept. de Ecologia, Inst. de Biologia, CCS-UFRJ, PO Box 68020, Rio de Janeiro, RJ 21941-590, Brazil (2) Dept. de Quı´mica Orga ˆnica, Inst. de Quı ´mica, CT-UFRJ, Rio de Janeiro, RJ 21949-900, Brazil (3) Dept. de Biologia Geral, UFJF, Juiz de Fora, MG 36036-330, Brazil Received: 22 February 2005 / Accepted: 7 March 2005 / Online publication: 12 May 2006 Abstract This study was conducted to evaluate: (1) the bacterial growth and the dissolved organic carbon (DOC) uptake in an Amazonian lake (Lake Batata) at high-water and low-water periods of the flood pulse; (2) the influence of nitrogen and phosphorus (NP) additions on bacterial growth and DOC uptake in Lake Batata at two flood pulse periods; and (3) the bioavailability of the main DOC sources in Lake Batata. Lake Batata is a typical clear-water Amazonian lake, located in the watershed of Trombetas River, Central Amazon, Brazil. Bacterial batch cultures were set up with 90% 0.2-mm filtered water and 10% inoculum from Lake Batata. N-NH 4 NO 3 and P- KH 2 PO 4 , with final concentrations of 50 and 5 mM, respectively, were added to the cultures, except for controls. Extra sources of DOC (e.g., algal lysate, plant leachates) were added to constitute six distinct treatments. Bacterial response was measured by maximum bacterial abundance and rates of bacterial production, respiration, DOC uptake, and bacterial growth efficiency (BGE). Bacterial growth and DOC uptake were higher in NP treatments than in controls, indicating a consistent nutrient limitation in Lake Batata. The composition of DOC also seems to be an important regulating factor of bacterial growth in Lake Batata. Seasonally, bacterial growth and DOC bioavailability were higher at low-water period, when the phytoplankton is a significant extra source of DOC, than at high-water period, when the forest is the main source of DOC. DOC bioavailability was better estimated based on the diversity and the diagenetic stage of carbon compounds than on single classes of labile compounds. Changes in BGE were better related to CNP stoichiometry in the water, and the Bexcess^ of organic substrates was oxidized in catabolism, despite the quality of these compounds for bacterial growth. Finally, we conclude that bacterial growth and DOC uptake vary throughout the flood pulse in clear-water Amazonian ecosystems as a result of changes in nutrient concentration and in DOC composition. Introduction Dissolved organic carbon (DOC) in aquatic ecosystems is one of the largest carbon pools in the biosphere. The role of DOC in aquatic ecosystems remained elusive for a long time, associated with metal binding and stability of aquatic ecosystems [23]. DOC importance as the main substrate and energy source for the growth of planktonic bacteria was enhanced after the introduction of the microbial loop concept [5]. Planktonic bacteria oxidize DOC into CO 2 , through respiration, or convert DOC into bacterial biomass, providing an important link to higher trophic levels through predation. Bacteria may be considered the basis of planktonic food chains in several aquatic ecosystems, and may also have an important role in CO 2 flux in the biosphere [13, 43]. Therefore, the possible interactions between the DOC pool and plank- tonic bacteria have a central role in carbon cycling or flux in aquatic ecosystems. DOC uptake by bacteria is mainly related to the composition of DOC pool, the degree of diagenesis, and the availability of other nutrients, such as N and P [3, 28, 30, 48]. For instance, DOC uptake by planktonic bacteria is directly linked to the C/N and C/O ratios, which indicate the degree of DOC diagenesis, and to the C/H ratio, which is related to the ratio of aliphatic and Correspondence to: Vinicius F. Farjalla; E-mail: farjalla@biologia.ufrj.br DOI: 10.1007/s00248-006-9021-4 & Volume 52, 334–344 (2006) & * Springer Science+Business Media, Inc. 2006 334