Neotropical River Ecology – Cinaruco River, Venezuela

Collaborators – Kirk Winemiller, James Cotner, Steve Davis, Carlos Del Castillo

Graduate Students – Jose Montoya

Seasonal variation of productivity and respiration in a tropical blackwater river: The role of allochthonous organic matter and inorganic nutrients

The Cinaruco River is a blackwater ecosystem in the Venezuelan llanos (savannah). The river has strong seasonal hydrology and supports large populations of ecologically diverse fishes. Undetermined are the relative contributions of autochthonous (aquatic) and allochthonous (terrestrial) production sources supporting high stocks of secondary consumers. Using excitation-emission fluorescence spectroscopy and absorption spectra of colored dissolved organic matter we were able to infer degradation of leaf material originating from the surrounding gallery forest. Our results suggest that during the low-water period a large fraction of fluorescent organic matter contained in leaves was degraded quickly in river water. This represented a prevalent allochthonous contribution of carbon into the system at this time. During the falling water period, however, the contribution of organic matter from trees was much less. Furthermore, the fluorescence signature of dissolved organic matter in lagoons was different from that of the main river channel, which suggests that organic matter originated from the savannah at this time. Despite this clear allochthonous organic matter source fueling microbial respiration, our in-water experiments using light and dark incubator technology indicated that autochthonous productivity was an important supplementary carbon source, perhaps fueling higher trophic levels. During the low-water and falling-water periods, water column primary productivity in the euphotic zone ranged from 150 to 500 mg-C m-2 d-1, 2-fold greater than respiration. Similarly, along the river edges, where light penetration reached the sediments, benthic primary productivity was on the same order as respiration, i.e., ranging between 350 and 500 mg-C m-2 d-1. Primary production was likely limited by the availability of dissolved inorganic nitrogen, which ranged from below detection limits to 0.5 µM, with averages typically of ~0.25 µM.

Hydrological seasonality and spatio-temporal dynamics of physico-chemical variables of a tropical floodplain river.

Rio Cinaruco is a tropical floodplain river in the Orinoco River basin (Venezuela), characterized by relatively high transparency, and low conductivity, pH, and suspended sediment load. The purpose of this study was to evaluate spatio-temporal dynamics of physico-chemical variables of the main channel and floodplain lakes of Rio Cinaruco. We hypothesized higher similarity among all sites during periods with high lateral hydrological connectivity (LHC) and lower similarity during low-water phases. Also, we predicted divergence among floodplain lake sites during the low-water period. Samplings occurred monthly on twelve occasions between 2002 and 2003 at 10 sites in the main channel and lakes. We measured water level, temperature, DO, pH, conductivity, water transparency, and water flow in situ. Water samples were taken superficially for nitrates, nitrites, ammonia, urea, orthophosphates, silica, chlorophyll-a, and phaeophytin, and analyzed using standard methods. Correspondence analyses showed that over 80% of the variation among sites and sampling dates is explained by two axes. During the high-water period, river sites and lakes were similar with low values for conductivity, flow, silica and chl-a concentrations. Strong shifts for these four variables were detected for most sites during transitional periods between high and low LHC. Nevertheless, after these shifts that tended to differentiate sites, the dynamics of physico-chemical variables became similar among lakes. Repeated spatio-temporal trends in the dynamics of physico-chemical variables of Rio Cinaruco and its lakes can be directly attributable to a strong effect of dilution and transitions between lentic and lotic conditions in the floodplain, driven by the annual flood-pulse.

Experimental demonstrations of seasonal fish effects on benthic ecology of a Neotropical floodplain river

The Cinaruco River, a lowland floodplain river in the Venezuelan llanos, has seasonal hydrology, low nutrient availability, and high fish diversity and abundance. Algae- and detritus-feeding fishes are abundant and may influence benthic ecology. During the dry season (2002), large-mesh exclosures in the channel accrued significantly more sediment, organic material, and chlorophyll than control cages. Grazing scars revealed that the sapuara, Semaprochilodus kneri, was a major consumer of organic-rich sediments. Further experiments were conducted to test the hypothesis that the relative strength of bottom-up (nutrient availability) and top-down (grazer) control of organic matter in sediments varies according to species and hydrologic period. Sapuara densities are highest (0.28 m-2) during the low-water season. At flooding onset (May), sapuaras migrate to the Orinoco River to reproduce and feed, and their densities are extremely low in the Cinaruco (0.02 m-2) during the interval when nutrient inputs from newly flooded plains are greatest. Experiments conducted during March 2002 (low water) in the channel and floodplain lakes revealed significant treatment effects (large fish exclosure; total fish exclosure; sapuara enclosure; control) for sediment mass, organic material mass, and chlorophyll a concentration on hard substrates, but no treatment effects for sand. Chlorophyll a concentrations were significantly affected by habitat. Mean mass of sediments and organic material matched our prediction of grazer control during the low-water season, but due to high within-treatment variance, the only statistically significant mean differences were sapuara enclosure < control and total fish exclosure. Subsequent experiments of daily sediment accrual revealed that sapuaras infrequently enter 3-sided control cages, but quickly remove sediments from 1 or 2-sided control cages. Experiments during the early rising-water phase, yielded no significant habitat or treatment effects.