Assessment of changes in algal biomass accrual and leaf decomposition along a gradient of increasing agricultural dis-turbance

Abstract: 1. Anthropogenic activities may impact stream ecosystems by distorting the energetic linkages between the aquatic and adjacent terrestrial habitats, or by modifying the aquatic environment directly. These impacts can affect biodiversity, ecosystem functioning, and stability in the stream ecosystem, all of which may compromise ecosystem services of importance to humans. Bio-monitoring schemes are required to assess such impacts. 2. Traditional biomonitoring focuses on the diversity and composition of biological groups, but there is increasing interest in monitoring ecosystem processes. I quantified two key ecosystem processes (litter decomposition and algal biomass accrual) to study the effect of different land use on stream ecosystem functioning. Ten streams were studied, representing a gradient of increasing agricultural land use in Östergötland County, Sweden, ranging from forested reference streams to heavily impacted agricultural sites. 3. Litter decomposition was assessed using the litter bag method, with coarse bags allowing access to invertebrates and fine mesh bags blocking invertebrate access. Thus decomposition rates in the fine mesh bags reflect microbial activity, while the coarse bags reflect microbial and detritivore activity combined. Algal biomass accrual rates was assessed by exposing tiles in the stream for a month, after which time the tiles were retrieved and algal biomass assessed. 4. Litter decomposition increased asymptotically along the land use gradient. This increase was associated with increased nutrient (N, P) flows from the fields to the streams. This increase was stronger in the coarse mesh bags than in the fine mesh bags, indicating stronger invertebrate than microbial responses to the gradient. 5. Little difference in algal biomass accrual was observed along the land use gradient despite the fact that there was an increase in nutrients, possibly reflecting elevated sediment loads to the most agricultural streams, which results in a poor light environment for algal growth. 6. My results showed that not a single assay is enough to give a complete picture of stream health, highlighting the value of multiple functional assays as a complement to traditional taxonomy-based monitoring, which can capture potentially contrasting impacts of human disturbances on different ecosystem processes.