As I mentioned earlier, the phosphorus model also estimates the amount of phosphorus released by specific land use types and other sources of phosphorus.
This chart shows the percent contribution of phosphorus from specific external sources.
Of all external sources, agricultural uses, forest, and shoreline development and septic systems contributed the greatest amounts of phosphorus.
According to our best estimate, agricultural land accounted for 23% of the total phosphorus from external sources (point out maroon).  This is comprised of both cropland and pasture.  Although cropland represents only 0.02% of the watershed, relatively high amounts of phosphorus can flow into the lake from fertilizers applied to the land.  Pasture, on the other hand, exports much less phosphorus per area, but represents a much larger area within the watershed.  These characteristics cause agricultural land to contribute relatively high amounts of phosphorus to the lake.
Forest also contributed a large amount of phosphorus to the lake (purple), however, this does not mean that mature forest is bad!  In fact, forested land contributes very little phosphorus per area, because the full canopy helps to reduce the velocity of rain, and the roots help to hold soil in place and absorb nutrients.  Forested land contributes so much phosphorus because it accounts for 62% of the total watershed area.  Remember that the total amount of phosphorus contributed by a particular land type is equal to the export coefficient multiplied by the area, so even with a very low export coefficient, a large area will cause the total amount of phosphorus contributed to be large.
Together, shoreline development and shoreline septic systems contributed 13% of total phosphorus from external sources.  Although shoreline lots account for a small proportion of the total area of the watershed, this small amount of land can have a large impact on water quality.  Water running off lawns, roofs, and other surfaces can carry phosphorus directly into the lake if buffer strips are not adequate, and septic systems built too close to the shore or in unsuitable soil types can lead to the movement of nutrients from the system into the lake.
So far, IÕve talked about three different slices of the pie chart that represent relatively large contributors of phosphorus to the lake, but if you around the chart at all of the percentages listed,  youÕll see that there are many other sources that contribute similar amounts (For example, 9% atmospheric (periwinkle), 7% commercial development (lt. green), 7% from roads(pink and blue)).  With the exception of atmospheric deposition, which is phosphorus that enters the lake directly through rainfall, the amount of phosphorus from the majority of these sources could be reduced by improving buffer strips, maintaining septic systems, and  maintaining roads.
Remember, these are only external sources of phosphorusÉ