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Sediment Nutrients


Sediment Nutrients

        The sediments contain inorganic particles, organic “detritus” (the remains of algae, zooplankton, etc.), live benthic algae, a host of bacteria, very small invertebrates termed the meiobenthos, and larger macroinvertebrates. The sediments also contain much higher quantities of nutrients than are found in the water column; some of these are bound in solid phase organic molecules, while a portion is present in inorganic form in the interstitial water.
        Contrary to expectations, reducing external phosphorus inputs from the watersheds of P-limited shallow ponds frequently has little immediate impact on pond water quality (Perrow et al., 1994; Moss et al., 1996; Nixdorf and Deneke, 1997). This occurs because of the large reserves of phosphorus remaining in the sediments. Phosphorus in the sediments can be resupplied to the water column both by upward diffusion of dissolved PO43– under anoxic conditions and by resuspension of particulate phosphorus by storms or human activity. Increases in total P in the water column during July in most of the study ponds likely occurred because of increased PO43– release from the sediments as the bottom waters became more anoxic. Even if external sources of P are reduced, recycling of P from the sediments (unless dredged) may thus maintain high levels of phosphorus in the water column for many years until sediment concentrations are depleted.
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Fig. 4. Sediment P content vs. total phosphorus in the water column during July. The sediments may serve as a “sink” for phosphorus as organic particles settle, but also release phosphorus to the overlying water, especially when oxygen concentrations are very low.

        Results presented here are based on data collected for 13 Chester County ponds in 2002. Cores of the top 0.5 cm of the sediments were obtained from each pond during visits in July. The relationship between particulate phosphorus in the surface sediments to total phosphorus in the water column is shown in Figure 4. Sediment P was typically slightly greater in ponds with higher concentrations of P in the water column. A positive correlation between sediment P and water column P is expected because not only does sediment-associated phosphorus reenter the water column (directly via resuspension and indirectly via remineralization and diffusion), but particulate phosphorus within the water column (e.g., as phytoplankton) sinks to the sediments.
        In effect, nutrients in particles that settle to the sediments are in fact frequently exchanged with the water column, are the chief nutrient source for rooted plants, and are a reservoir of nutrients for plants and algae in the water column. Homeowners concerned with pond management must thus consider the large supply of nutrients residing in the sediments as well as nutrients entering from the watershed.