Breakdown of tussock grass in streams along a gradient of agricultural development in New Zealand

Abstract

1. We measured the breakdown rate of tussock grass in 12 New Zealand streams in catchments that provided a gradient of agricultural development. We also examined the microbial and invertebrate communities associated with decomposing tussock litter. 2. Pristine streams in the study had low concentrations of dissolved inorganic nitrogen (<10 μg L-1) and dissolved reactive phosphate (<3 μg L-1), whereas streams in the most developed catchments had high concentrations of nitrate (>2500 μg L-1) and phosphate (35 μg L-1), as well as greater amounts of suspended sediment and fine sediment covering the streambed. 3. Breakdown rate and microbial respiration were significantly related across the sites, and both were positively related to concentrations of nitrate and phosphate. Fungal biomass, measured as ergosterol, was positively related to microbial respiration and was also higher at sites with higher concentrations of nutrients. Total and shredding invertebrates were most abundant at the sites with high nutrient concentrations, but abundance of shredding invertebrates was not significantly related to breakdown rate. Amphipods were the most common shredding invertebrate at most sites, but probably did not contribute greatly to high rates of breakdown in streams in agricultural catchments. 4. With the exception of one site, nutrients from agricultural development appeared to have larger positive effects on litter breakdown than negative effects from sedimentation. Litter breakdown can serve as a functional measure of ecosystem health in streams, but caution should be exercised when a stress, such as land use, can have both positive (nutrients) and negative (sedimentation) effects.