Effects of water chemistry and leaf species on leaf breakdown in neotropical headwater streams

by (Lindsay Ann), 1977- Stallcup

Stream ecosystem function is dependent on organic matter inputs from surrounding forests. The rate of leaf breakdown is determined by stream water quality, such as prevailing nutrient concentrations, as well as characteristics of organic matter. We tested the effects of phosphorus (P) in driving leaf breakdown processes in headwater streams draining La Selva Biological Station, Costa Rica. We also tested the hypothesis that at high P concentrations, dissolved inorganic nitrogen (N) becomes secondarily limiting. Finally, we predicted that breakdown of leaves with relatively higher %N (Trema integerrima) would occur more rapidly than breakdown of leaves with relatively lower %N (Ficus insipida). Leaves were incubated in litter bags in two streams: a whole-stream P-enrichment experiment ( > 200 µg SRP L-1), and a reference stream with naturally low P concentrations (~10 µg SRP L-1). To achieve localized N-enrichment, fertilizer was added upstream of half of the litter bags. Results suggest P effects on leaf breakdown, fungal biomass, microbial respiration, leaf chemistry, and insect abundance. Our short-term N addition had no effect on any of the variables measured, indicating that N is probably not limiting in high-P streams draining La Selva. Leaves with higher %N exhibited faster breakdown, higher microbial respiration, and supported a higher abundance of invertebrates, suggesting that differences in leaf N content may be important in determining colonization by microbes and invertebrates, and subsequent leaf breakdown.