Organic Matter Retention by Macrophyte Beds in 2 Southeastern USA, Low-Gradient, Headwater Streams

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We examined particulate organic matter (POM) storage and retention in aquatic macrophyte beds (Sparganium americanum Nutt.) from 2, first-order, blackwater tributaries of Tinker Creek, South Carolina, USA. Measurements were made during the autumn (high litter input) and summer (low litter input), in 100-m study reaches, 1 with low macrophyte density (T-l) and 1 with high macrophyte density (T-2). Three experiments assessed: 1) POM retention by individual plant beds, 2) short-term retention of leaf analogs in plant beds and by other retention structures, and 3) benthic and transported POM at the reach scale. Upon completion, all above ground macrophyte biomass was removed and the experiments were repeated. At the scale of 1 bed, low plant density effectively trapped and stored POM in summer and autumn. High-density beds stored POM in the summer but during autumn POM output exceeded input. At the reach scale, both tributaries effectively retained coarse POM in summer, but only high-density beds did so in autumn. Likewise, only high-density beds trapped and retained fine POM. Following plant removal, both coarse and fine POM were transported with minimal retention. When plants were present, beds played a major role in leaf analog retention and increased the retentive capabilities of other channel structures (debris dams, overhanging vegetation). When beds were removed, most analogs were retained by debris dams or settled to the substrate. Macrophyte beds play a major role in POM retention and short-term storage in low-gradient blackwater streams at several spatial scales and are linked to retention by other channel structures. The importance of macrophyte beds varies depending on bed density, total surface coverage, and timing of leaf fall.