Effects of Small Ponds on Stream Water Chemistry

In many regions, small constructed ponds greatly exceed natural lakes in number and aggregate area. Many of these ponds are impoundments of small streams. Their effect in modifying stream water chemistry, however, remains poorly understood. Here we compare 19 physicochemical variables upstream vs. downstream of 11 ponds, sampled in March, May and July. The ponds greatly reduced inflow concentrations of SiO2 (by 71%), NO3 − (by 82%) and PO4 3− (by 46%), while exporting water of higher pH, alkalinity and dissolved oxygen content, and much higher quantities of particulate and dissolved organic C, N and P than were present upstream. Higher % removals of NO3 − and SiO2 were observed in ponds with longer hydraulic residence times. Based on ambient N:P ratios, algal periphyton below the ponds were likely P limited, but differential transformations of the components of total N vs. total P within the ponds greatly reduced N:P ratios downstream.