FINE SEDIMENT RETENTION AS AFFECTED BY ANNUAL SHOOT COLLAPSE: SPARGANIUM ERECTUM AS AN ECOSYSTEM ENGINEER IN A LOWLAND STREAM

To examine the effect of different growth forms of Sparganium erectum (Sparganiaceae) on its sediment trapping and retention characteristics, the plant phenology and morphological attributes based on stands along the edge of a river channel, river flow velocity distribution and sedimentation rates in-and outside the stands of a downstream site were examined over a 3-year period. A decomposition experiment was carried out to determine the respective fragmentation rates of S. erectum shoots and rhizomes. The preliminary monitoring revealed that S. erectum shoots attained distinct phonological stages; submerged in winter, subsequently emerging in late-spring, while the same cycle was followed by successive secondary cohorts. Our results highlighted that the growth form substantially affected the flow condition in-and outside the stands, thus affecting associated sedimentation rates. Although the sedimentation was high if the shoots were submerged in early-spring, the accumulated fine sediment layer was unleashed following shoot emergence, despite their large biomass and the resulting low flow velocity. The collapse of S. erectum shoots accelerated sedimentation again by increasing constriction to flow, producing a more preferable habitat for its soft roots and rhizomes. The collapse of emergent shoots, therefore, appeared to be a vital part of the inherent phonological cycle of S. erectum. Further, due to the high-decomposition rate, the collapsed shoots disappeared within 40-60 days, whereas the decomposed materials occupied a large fraction of the floating organic matter. The collapse of shoots increased flow resistance by similar to 50%, though the stand area occupied a mere one-fifth of the channel. The seasonal observations provided further insight into the modification of flows due to the growth and changing growth form affecting associated fine sediment trapping and retention characteristics within the stands, deriving important management implications and highlighting the role of S. erectum as an ecosystem engineer in lowland streams. Copyright (C) 2009 John Wiley & Sons, Ltd.