A farmer has become aware of the problems that fertilizer use can cause in streams and lakes connected to farmlands, and would like to manage her farm to reduce its impact on those ecosystems. She can either reduce the amount of fertilizer added to her fields, or maintain a riparian "buffer" between her fields and the stream. The riparian buffer will promote nitrate retention via denitrification, but will have no effect on phosphate. In collaboration with a university ecologist, she conducts a short-term nutrient addition experiment with water and rocks from a nearby stream, adding nitrate, phosphate, nitrate + phosphate, or nothing (control) to the water. The ecologist analyzed the amount of algae (chlorophyll a) that grew on the rocks, and plotted the data (below, y-axis is algae amount). Based on the experimental results, what should the farmer do?