Macrophytes and suspension-feeding invertebrates modify flows and fine sediments in the Frome and Piddle catchments, Dorset (UK)

This research investigated the ecosystem engineering by in-stream macrophytes, dominated by Ranunculus spp., and associated suspension-feeding blackfly larvae (Diptera: Simuliidae) for five reaches in the Frome and Piddle catchments, Dorset (UK) over one annual growth cycle (2003). This paper focuses on the modification of flow velocities and the trapping of fine sediment (particles < 2 mm in diameter) by in-stream macrophytes and the processing of dissolved organic matter (DOM), fine particulate organic matter (FPOM) and fine inorganic particles into faecal pellets by blackfly larvae attached to the leaves of Ranunculus plants. In-stream macrophyte growth was extensive, with maximum percentage cover of 80% recorded in September and October 2003. The macrophyte cover significantly altered flow patterns and flow velocities within and between the macrophyte stands. The reduced flow velocities within the plants promoted sediment trapping, reaching volumes of 0.085 m(3) of fine sediment trapped per metre square of vegetation at one site. The effective particle sizes of the sediments trapped within Ranunculus stands were dominated by the 250-500 mu m fraction from March to July 2003 whereas a higher proportion of smaller fractions occurred from October to December. Faecal pellets were highly abundant in the sediments trapped within Ranunculus stands (up to 2.2 x 10(8) faecal pellets per m(2)) and their dimensions (total size range 25-400 mu m) fall within the dominant size fraction of the trapped sediments. Our findings demonstrate the need to consider the biogenic component of the fine sediments in chalk streams in future studies of sediment and nutrient dynamics. (c) 2006 Elsevier B.V. All rights reserved.