Effects of Ponds on Stream Nutrients


        Most ponds in Chester County are connected to stream networks, usually providing the source for headwater streams or occurring as impoundments of headwater streams. The impoundment of small streams may strongly impact their water chemistry. Water flow is slowed, and a much larger portion of the water surface becomes exposed to direct solar radiation. These changes have the effect of warming the impounded water, and stimulating photosynthesis by algae and vascular plants. The growth of these primary producers in turn increases nutrient uptake.
        All but one of the target ponds had both inflows and outflows, although drought conditions prevented measurement of water chemistry during some visits. By comparing nutrient concentrations and particulate matter entering vs. leaving the ponds, it was possible to estimate the probable impact of the ponds on the streams with which they were associated. Changes in nitrogen, phosphorus and silica and suspended particles between the inflowing waters and outfalls of the 13 ponds are shown in Table 3.

Table 3. Mean concentrations (averaged across all ponds) of PO43–P), dissolved organic P (DOP), particulate P (PP), NH4+–N, NO2+3, dissolved organic N (DON), particulate N (PN), silica (SiO2), TN and TP, all expressed in µg/L, and particle concentrations (TSS = total suspended solids, in mg/L) in the inflow vs. outflow from the 13 study ponds during March, May and July 2002.


Table 3
        As expected, the ponds sequestered much of the incoming nutrients. Retention of orthophosphate (PO43–P) increased later in the season, presumably because of uptake by primary producers. Similar patterns of net uptake were observed for the inorganic nutrients nitrate (NO2+3–N) and silica (SiO2). By contrast, ammonium (NH4+ –N), which results primarily from the decomposition of organically bound N, consistently showed net export downstream.
        Ponds produced by impounding streams have the well-deserved reputation of trapping particles suspended in the inflow during rain events. At other times (when streams are at normal flow), however, ponds are likely to be net exporters of particles. Basically, the ponds may be viewed as reaction chambers, converting dissolved nutrients into phytoplankton tissue, bacteria and other organic particles, exporting a portion of these particles downstream (the remaining fraction may settle to the sediments or be returned to dissolved inorganic forms by decomposition). Particulate forms of nitrogen (PN) and phosphorus (PP) both showed net export from the ponds. A further indication of the net export of particles is indicated by the slightly higher concentrations of total suspended solids in the outflows than in the inflows of the ponds.