Watershed Riparian Management and Its Benefits to a Eutrophic Lake

Abstract

Sediment and nutrient loads from a 73 km 2 mixed pasture/forested watershed that drains to eutrophic Lake Rotorua (New Zealand) were measured before and after diffuse-source controls were implemented. In the pastoral areas, these controls were exclusion of grazing (“retirement”) from the banks of most perennial streams, erosion-prone hillslopes, and remnant pockets of native forest. After implementation, the loads changed by −85% for sediment, −27% for particulate P, −26% for soluble P, −40% for particulate N, and +26% for dissolved N. Calculated total phosphorus specific yields for the pasture portion of the watershed were 89 kg km −2 yr −1 (preimplementation) and 41 kg km −2 yr −1 (postimplementation). When these findings were applied to the whole Lake Rotorua watershed, we predicted that control measures had reduced TP loads by 20%. This reduction in phosphorus loads was predicted to reduce the chlorophyll a concentration by about 5 mg m −3 , and help shift the lake's trophic state from eutrophic to mesotrophic.