Organic matter losses from four watersheds in the humid tropics1

Organic matter ( OM ) losses in runoff water were examined for four watersheds in Guatemala, ranging in size between 170 and 5,247 km2. Total organic matter (TOM) concentrations for the largest river were highest (14.8-19.3 mg liter-l) at the beginning of the wet season, due to an abrupt increase in particulate fractions. This initial flushing effect was Followed by lower concentrations which had decreased to near dry season values (3.90-5.35 mg TOM liter-‘) 4 months later. Export of TOM was estimated to be botween 5.4 and 12 g mm2 yr-l for three of the four watersheds. The fourth, dominated by swamp forest, showed a higher maximum concentration (43.7 mg TOM liter-l) and greater export, the latter partially attributable to flushing by floodwaters that overflowed an adjacent watershed. These data and those from studies in higher latitudes suggest a direct relationship between the quantity of TOM exported and runoff volume per unit area of watershed. A plausible explanation is that dissolved OM is the major form of OM in most running waters and varies less in magnitude than hydrologic runoff from one watershed to another. Organic matter in stream or river waters has been studied as a source of energy for heterotrophic stream communities (Kaushik and Hynes 1968; Nelson and Scott 1962; Fisher and Likens 1973) and for downstream lakes, reservoirs, or estuaries (Barlow et al. 1963; Seki et al. 1968; Likens 1972). Regardless of the point of view taken, organic matter in runoff waters represents a one-way coupling of energy between upstream terrestrial ecosystems and downstream aquatic ecosystems. With the exception of the IIubbard Brook studies (Hobbie and Likens 1973; Bormann et al. 1974), few attempts have been made to compare the loss of organic matter from a watershed with such features as vegetation, rainfall, soils, topography, or latitude. These data from the humid American tropics may add to our understanding of the effects of organic matter runoff. In 1972 the organic matter budget of Lake Izabal in eastern Guatemala was estimated (Brinson 1973). As a measure of --_ __’ Part of a thesis submitted For the degree of Ph.D. at the University of Florida, This work was supportecl by the Foreign Area Fellowship Program administered by a Joint Committee of the American Council of Learned Societies and the Social Science Research Council under a gr,ant From the Ford Foundation. the lake’s allochthonous organic matter sources, the runoff waters from four of the surrounding watersheds were monitored for particulate and dissolved organic matter concentration. Here I report the results of these measurements and compare them with data from northern temperate and arctic regions. I thank G. E. Likens for critical review of the paper. Description of the watersheds The four watersheds examined are part of the Lake Izabal catchment basin (Fig. 1) which extends 205 km eastward from the interior of Guatemala to the Caribbean coast (88”41’W-90”34’W and 15”03’N-15” 52’N). The drainage pattern is from the highlands eastward to Lake Izabal only a few meters above sea level; and then, via the Rio Dulce, the water finally reaches Amatique Bay in the Gulf of Honduras. The major rivers emptying into Izabal are the Polochic and the Cahabbn, which convcrgc into a massive river that is actively building a multilobate delta across the west end of the lake. This river system originates some 2,100 m above sea level and passes a linear distance of 100 km before reaching the lake. Other rivers and small streams, about 40 in all, drain less extensive LIMNOLOGY AND OCEANOGRAPIIY 572 JULY 1976, V. 21(4)