The Metabolism of the Río Mameyes, Puerto Rico: Carbon Fluxes in a Tropical Rain Forest River

Department of Environmental, Population, and Organismic Biology

Abstract

Metabolism has seldom been studied in tropical rivers. Some potential differences between tropical and temperate rivers were tested in a study of the Río Mameyes, a tropical river that drains the Luquillo Experimental Forest in eastern Puerto Rico. It was hypothesized that in the Río Mameyes, the high water temperatures and the constant supply of litterfall will promote higher rates of community respiration (CR) and of primary production (PP) than in temperate rivers. Another aspect that was investigated was the prediction of the River Continuum Concept (RCC) regarding changes in metabolism along a river.

Stream metabolism was measured with the open-channel technique at eight different reaches located throughout the Río Mameyes. Diel changes in PAR, dissolved oxygen concentration, and water temperature were measured at the reaches from February 1995 to June 1996. In the Río Mameyes, PP increased downstream as a function of increased irradiance from 0.19 g C/m²/d near the headwaters to 0.63 g C/m²/d near the mouth. CR averaged 2.5 g C/m²/d throughout the river. The ratio of PP to CR (P/R ratio) was used as an index of metabolism. The P/R ratio increased downstream but was always < 1, suggesting that the entire river is heterotrophic.

A carbon budget was developed for the upper Río Mameyes (upstream of USGS gaging station 50065500). During 1995, organic carbon sources totaled 257,254 kg, of which 40% entered as DOC from groundwater, 37% entered via hillslope erosion, 18% entered as direct litterfall, and 5% as aquatic PP. Carbon left the river mostly via respiration (49%), followed by export of dissolved (39%) and particulate organic carbon (13%).

Data on channel morphology, canopy cover, and stream metabolism supported a basin-wide model of stream metabolism. This model predicts changes in dissolved oxygen concentration from changes in discharge, which is expected to change in the near future due to water diversion.

Compared to temperate streams of similar size, Río Mameyes has similar rates of PP and higher rates of CR. The Río Mameyes does not conform to the RCC because it is heterotrophic along its entire length. In tropical rivers, the role of respiration in carbon cycling may be more important than previously thought.

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