Stan,

I am not sure of your level of understanding phosphorus cycling and availability. It is a complex process. It is clear from the sentence from Gwen that she does not understand it. I was always very proud of the County Council members in their grasp of the process. One of my suggestions regarding the issues is that every two years after elections of council members a one hour presentation be made to the councils on the subject. It is not simple!

That said here is a one shot stab at the scenario of a landslide into a stream in the Lake Whatcom watershed.

If a landslide sends sediment into a stream there will be an increase of sediment in the stream. Within the sediment a certain proportion of the sediment will contain phosphorus. Some of the phosphorus will be dissolved or readily dissolved. This phosphorus will of course be transported down stream and into the lake as fast as the water. Most of the dissolved or readily available phosphorus will be in the organic material and top soil layer portion of the slide material. Hence the amount of phosphorus in the water available for biological activity will be increased beyond natural conditions.

Much of the phosphorus in the landslide mass of soil will be mineralized - that is it may be in phosphorus bearing minerals such as apatite or it will be bound by other elements within the soil. This phosphorus is not readily available for biological activity. Plant roots have to extract phosphorus from the soil via biochemical means in the root systems. Some plants are better at it there others.

Here is the problem though. The sediment with phosphorus will be transported to the lake and the sediment will be deposited on the lake bed. Initially this will not cause a phosphorus problem as long as oxygen levels in the lake are high. However, during the spring the lake stratifies. The water is warmed by the sun and air. The warm water stays on the surface and the bottom of the lake stays cold and insulated. As biological activity in the cold water at the bottom of the lake continues the bottom of the starts to become low in oxygen. The shallower the lake and the more biological activity, the faster the lower portion of the lake will run low on oxygen.

Once the lower levels of the lake run out of oxygen, the chemical conditions radically change. Without oxygen many chemical reactions reverse such that elements bound by oxygen begin to be released. At this point otherwise unavailable phosphorus begins to be released into the water. As phosphorus is often a limiting nutrient for biological activity the sudden release of phosphorus from the new loose landslide sediments will cause a large increase in biological activity in the lake. And we are then right back to the last sentence in the above paragraph. More biological activity creates faster no oxygen conditions releases more phosphorus.

So what? You may ask.

Three problems: 1) One low oxygen conditions if they become too widespread and impact more of the water column will kill every living thing in the lake that needs oxygen. The added loose sediment hence speeds up the low oxygen generating conditions. 2) besides phosphors being released from lake sediments, other elements and chemicals will be released that normally would stay put. Mercury is one such example. 3) The other limiting factor for biological activity is nitrogen. As excessive biological activity takes place feeding on the available phosphorus the readily available nitrogen in the water column is used up. This creates a condition where only algae that can extract nitrogen from N2 will survive. The ecological balance of the lake collapses and the blue green algae take over. Some of these algae are very toxic and the lake will begin to stink. This is the condition that all lakes ultimately reach. We have been accelerating Lake Whatcom towards that path.

So back to Gwen's question or statement.

It makes no difference where the sediment loading comes from. In the case of landslides into creek the volume can be very large and may actually contain very large volumes of readily available phosphorus as all the top soil and plant matter will have significant available phosphorus.

In the case of development the increase sediment may come from bear land cleared during development. Other impacts of development will be increased stream flow because of impervious surfaces, lack of trees and native soils to hold water. The increase stream flows may be erosive and cause additional sediment. Impervious surfaces such as streets, driveways and roofs will allow organic material containing phosphorus such as pine needle and grass clippings and leaves as well as atmospheric phosphorus deposition (dust) to be readily washed downstream into the lake. Add phosphorus fertilizers, gravel driveways and dirt driveways and other poor land management such as handling of animal waste or over grazing by livestock as well as a variety of other trace sources and the consequences of development though not as spectacular as debris flows will result in water quality degradation.

During my time on the County Council we passed down zones, fertilizer bans, boat regulations, multiple development standards and sues the state to implement better forest management of trust lands. Every single time people would say we ought to do something besides what we were doing. The boaters pointed fingers at development and clear cuts. The builders pointed at already damaged streams and areas with no storm water controls. The timber people pointed at boats. Residences point at loggers. Builders point to cows and horses. Horse owners point to motorcycle riders. On and on. It is one of our national mottoes "Don't tread on me". All of these activities are treading on the water quality of the lake.

The fact is all these activities cause water quality problems from phosphorus loading.

Hope this thrown together answer helps. If literature or references are needed I can put some things together.

Regards

Dan McShane