Optically determined sources of allochthonous organic matter and metabolic characterizations in a tropical oligotrophic river and associated lagoon

Abstract

The Cinaruco River, an oligotrophic ecosystem in the Venezuelan llanos (savanna), has strong seasonal hydrology and supports large populations of ecologically diverse fishes. The relative contributions of autochthonous and allochthonous production sources that support high stocks of secondary consumers are undetermined in this river. We used excitation-emission fluorescence spectroscopy and absorption spectra of dissolved organic matter to infer degradation of leaf material originating from the surrounding gallery forest. During the low-water period, a large fraction of fluorescent organic matter contained in leaves degraded quickly in river water and was an important allochthonous contribution of C to the system. However, the fluorescence signature of dissolved organic matter in lagoons was different from that of the main river channel during the falling-water period, suggesting that other sources of C were present. Allochthonous organic matter clearly fueled microbial respiration during the falling-water period, but our in-water experiments using light-dark bottle methods indicated that autochthonous production was an important supplementary C source in shallow nearshore waters. During the low-water and falling-water periods, water-column primary production in nearshore waters ranged from 150 to 500 mg C m À2 d À1 and was 23 greater than community respiration, i.e., the nearshore component of the water column was net autotrophic. Benthic primary production in nearshore areas where light reached the sediments ranged between 350 and 500 mg C m À2 d À1 and was about equal to community respiration. Primary production was probably limited by the availability of dissolved inorganic N, which sometimes was below detection limits of ;0.5 lM (mean ;0.25 lM). Our results support the idea that autotrophic production is an important C source in neotropical rivers.

Temporal and spatial variation in ecosystem metabolism and food web carbon transfer in a wet‐dry tropical river

“…, 2002). In the Cinaruco River, the average rates of GPP were similar for benthic and water column algae during the dry season (Roelke et al. , 2006), but the seasonal average concentration of suspended chlorophyll a ( c. 2.9 μg L −1 ) exceeded those of the Mitchell River (0.9–1.9 μg L −1 ).…”

A review of allochthonous organic matter dynamics and metabolism in streams

“…In some systems, readily labile DOM might support .50% of the community respiration near the point of input, whereas intermediately labile DOM might be an important subsidy to downstream systems (Wiegner et al 2005). Study of DOM was long limited by available methods, but techniques now exist to trace organic matter origins (e.g., fluorescence signature; Roelke et al 2006) and to assess its bioavailability (e.g., quantification of extracellular enzymatic activity; Harbot and Grace 2005). These and other new techniques should be used to quantify fully the role of DOM as a relevant energy resource to stream systems and to evaluate the common assumption that DOM simply flows through stream ecosystems with little instream processing.…”

Autochthonous production in shallow littoral zones of five floodplain rivers: effects of flow, turbidity and nutrients

“…The Cinaruco River is a moderate blackwater river with a high concentration of DOM and low pH. Previous studies measuring sources of DOM and net ecosystem production within the water column of the Cinaruco River have inferred that total DOM is maximised during the annual low-water period when net ecosystem production is highest, suggesting that DOM has little influence on light availability for algae Roelke et al, 2006).…”