Watershed riparian management and its benefits to a eutrophic lake

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.