Oxic–anoxic conditions in the water column of a tropical freshwater reservoir (Peña-Larga dam, NW Venezuela)

A freshwater reservoir (area: 122 km2, volume: 2850 × 106 m3, maximum depth: 72 m) located at the outlet of a tropical Andean watershed (Rio Boconó, 1620 km2, 08°57′–09°31′N, 70°02′–70°34′W, NW Venezuela) was studied combining: (1) high resolution monitoring of the major environmental parameters using a CTD probe; (2) dissolved (major and trace) element concentration measurements; (3) water (δ[18]O, δ[2]H) isotope measurements and; (4) organic carbon (C) concentration – δ[13]C measurements on suspended matter and on sediments. Composed of two sub-reservoirs with contrasted water inputs, the Peña Larga dam provides the opportunity to better constrain C budgets and source – composition of suspended organic matter within two parts of a unique freshwater system with contrasted redox conditions. The redox status is reflected by drastic differences between the two parts of the reservoir, pictured by water residence time, water column ventilation, dissolved trace element concentration and suspended organic matter composition. Seasonal renewal of water near the main inlet supports higher surface productivity, oxic water column conditions, homogenous water chemical composition and, below the thermocline, the transport of suspended organic matter mainly derived from soil erosion on the watershed. Calculated sediment and organic C storages amount to 5.6 × 106 t a−1 and 21 ± 5 × 103 t C a−1, respectively. Despite a markedly lower suspended sediment load, outflow waters exhibit POC-δ[13]C values similar to those in inflow waters, due to a short water residence time and a rapid recycling of primary products. In the more isolated parts of the reservoir, where water discharge is thoroughly reduced, hypoxic to anoxic conditions are found immediately below the thermocline (ca. 16 m, [O2]<30 μmol L−1). They induce a correlative increase of trace element concentrations with water depth. Suspended organic matter display 13C-depleted compositions, contrasting sharply with that of photosynthetic organisms and land-derived detritus, that either reflect the contribution of microbial biomass (chemoautotrophic and/or methanotrophic bacteria) or the selective degradation of more labile organic compounds in the water column. The hypoxic to anoxic conditions displayed in the major part of the water column extend to the first cm of lake bottom sediments. A simplified budget based on the main redox processes active in the hypolimnion and on average water residence times, supports drastic differences in mineralization rate: 83–444 μmol C L−1 a−1 for the oxic reservoir and only 43 μmol C L−1 a−1 for the hypoxic to anoxic reservoir. This study shows that, if water renewal is not sufficient, tropical freshwater lakes may be subject to severe dissolved O2 depletion conditions at shallow depths, comparable to those observed in deep sections of the water column of temperate eutrophic lakes.