Abstract
Land use has an important role influencing stream ecosystem processes, such as leaf litter breakdown. Here, we assessed rates of leaf litter breakdown in low-order tropical streams draining forest, agriculture, and urban land uses in Puerto Rico. To measure leaf breakdown rates, we placed litter bags made of coarse mesh in nine streams, three for each land use type. At each stream, we measured changes in leaf mass over time, leaf breakdown rates, macroinvertebrate assemblages, and stream physicochemistry. Streams differed in their water physicochemistry, with urban streams showing high values for most variables. Stream physical habitat was evaluated using a visual assessment protocol, which indicated that agricultural and urban streams were more degraded than forested streams. Leaf breakdown rates were fast in all streams (k values ranging 0.006-0.024). Breakdown rates were significantly related to the physical conditions of the stream channel (e.g., visual protocol scores), with fastest rates in forested streams. Invertebrates colonizing leaves were mainly mayflies (Leptophlebiidae, Baetidae, and Caenidae), dipterans (Chironomidae), caddisflies (Polycentropodidae), and beetles (Elmidae and Gyrinidae). Our streams lacked large decapod populations, contrasting with other Puerto Rican streams. We found little evidence for an insect effect on leaf breakdown. Results suggest that land use is an important factor affecting leaf litter processing in streams. In contrast to studies in temperate regions, we found little evidence for a positive nutrient related effect of agricultural land use on decomposition rates. Changes in the physical characteristics of streams appear to be the main drivers behind observed decomposition patterns.
Highlights
The importance of the connection between streams and their watersheds via the movement of water and materials is well established in stream ecology (Hynes, 1975; Allan, 2004)
Our main goal is to assess the relation between land use and leaf litter breakdown rates in low order tropical streams that drain a range of land uses from forest, agricultural, to urban
Understanding how stream ecosystems are affected by land use on the watershed is critical to assess their vulnerability to change
Summary
The importance of the connection between streams and their watersheds via the movement of water and materials is well established in stream ecology (Hynes, 1975; Allan, 2004). In warm-water tropical streams, microbial activity potentially plays a larger role in making leaf material a palatable resource to shredders, and to omnivorous consumers, in particular insect collectors, omnivorous decapods (e.g., crabs and shrimps), and fishes (Crowl, McDowell, Covich & Johnson, 2001; Encalada, Calles, Ferreira, Canhoto & Graça, 2010; March, Benstead, Pringle & Scatena, 2001; Rosemond, Pringle, Ramírez & Paul, 2001; Bobeldyk & Ramírez, 2007) In both tropical and temperate streams a great number of aquatic organisms can be potentially connected to leaf detritus entering streams (Wallace et al, 1997; Benstead et al, 2003). Our study has two main objectives, to: (1) assess the overall effect of land use on leaf breakdown rates, and (2) assess the relative importance of biotic and abiotic factors in controlling leaf breakdown
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