Retention of particulate matter by macrophytes in a first-order stream

Abstract

Retention of coarse particulate organic matter (CPOM) in streams is affected by channel complexity, especially three-dimensional structures in stream channels. Much attention has focused on woody debris as retention structures, but macrophytes should function similarly. Retention and flow characteristics were measured in 5 m long reaches at four-replicate sites in the Breitenbach (Hessen, Germany). CPOM retention was measured by releasing chips of paper (6 mm diameter) as analog CPOM. Triplicate releases (200 chips each) were done before and after submerged, emergent and overhanging macrophytes were removed. Discharge ranged between 3 and 4.5 l s −1 during the experimental releases. Macrophyte biomass removed was 3.047, 3.012, 0.164, and 1.392 kg m −2 (fresh weight) respectively from sites 1 to 4. Instantaneous retention rates, calculated with an exponential decay function, were significantly higher ( P<0.001) with macrophytes than without macrophytes (0.8±0.2 versus 0.2±0.1). Velocity also was significantly reduced by macrophyte presence ( P<0.001). Macrophyte biomass was a significant predictor of retention ( P<0.001), whereas discharge was not ( P=0.3). Hence, retention appears to be positively related to macrophyte biomass. Macrophytes in the Breitenbach increase retention by forming effective sieve-like structures in the stream physically trapping CPOM and by reducing water velocity.