Sediment Distribution and Characteristics
Sediments at the bottom of ponds are a source or sink for nutrients in relation to the water column, contribute to turbidity during storm events, serve as the growth medium for aquatic plants and nesting sites for fish, and provide the necessary habitat needed to sustain a wide variety of invertebrates. Sediment characteristics described here for ponds in the Brandywine watershed are based primarily on a MS Thesis by West Chester University student Andrew Brainard.
Sediment characteristics were evaluated in 10 ponds within the Brandywine Creek watershed, in Central Chester County during summer 2009 (Fig. 1). The ponds varied in size from 0.04 ha to 0.56 ha, and from 0.7 m to 1.2 m in mean depth. All were constructed during the 20th century, and varied in age from 21 to 60 years.
Ten sediment cores were obtained at regularly spaced intervals from each pond using a hand-held corer (Fig. 2) deployed from the stern of a small pram. The sediments were analyzed for sediment volume, dry bulk density (dry mass/volume), particle size distribution (percent sand) and percent organic matter. Sediments in deeper portions of the ponds, further from shore, were characteristically finer-grained (reduced sand), had greater organic content, and lower dry bulk density (Fig. 3). In effect, heavier sand grains were typically deposited near their point of entry into the pond, whereas finer particles were carried by water currents into deeper water.
These patterns have important implications for ponds and their owners. First, organisms that either live in or on the sediments depend strongly on particle size and organic content, with sandier, near-shore sediments often providing better growing conditions for some plants and more organic and nutrient-rich sediments in deeper water providing more suitable habitat for others. Second, dry bulk density and particle size distribution can be used to determine the extent to which sediment inputs are retained within the pond, and thus the rate of in-filling.