Water Temperature
Lakes go through a seasonal process known as thermal stratification. This process occurs as the water column separates into layers due to differences in temperature and density. During late fall and much of winter after the water has cooled, lake temperature, and therefore water density is fairly uniform top to bottom. (This is represented as a fairly straight line for each of the parameters illustrated in the profile chart). As spring approaches, the upper water layer (the epilimnion) warms and becomes less dense, eventually becoming thermally isolated from and virtually floating on top of the cooler, denser, deeper water layer (the hypolimnion). The layers are separated by a middle transition layer know as the metalimnion. (During periods of stratification, the metalimnion is graphically represented as the area of the bend in the temperature profile chart). As the temperature difference between the water at the top of the lake and the bottom increases, the density differences increase and thermal stratification becomes more pronounced through the summer. The upper and lower layers of water no longer mix and the water chemistry becomes different in each layer.

pH
The pH is a measure of the concentration of hydrogen ions. Photosynthesis uses up carbon from the water and reduces the concentrations of hydrogen ion, which increases pH readings. For this reason, pH may be higher at the surface during daylight hours in the summer. Diffusion of carbon dioxide from the atmosphere, respiration and decomposition processes result in lower pH. Organic matter that settles onto the bottom of the lake and decomposes contributes to differences in pH readings with depth in the lake.

Dissolved Oxygen
Dissolved oxygen concentrations may change with dramatically with depth, particularly as thermal stratification persists. Oxygen is added to the water via diffusion through wind mixing and produced in the top portion of the lake during photosynthesis. Respiration also consumes dissolved oxygen. Oxygen consumption is greatest near the bottom of the lake where settled organic matter decomposes. The consumption of oxygen is greatest in the hypolimnion, which is seperated from the epilimnion and sources of oxygen by thermal stratification.

Water temperature influences the amount of gas that water can hold. As water becomes warmer, it becomes saturated more easily with oxygen, meaning it can hold less of the dissolved gas. This is graphically represented in the percent saturation profile.

Chlorophyll
Chlorophyll is the green pigment in plants that allows them to create energy from light -- to photosynthesize. Chlorophyll a is an indirect measure of the amount of algae actively respiring and photosynthesizing at the time of sampling. The amount of algae present can influence the dissolved oxygen concentration and the pH as well as the amount of nutrients and Secchi depth readings. Algae produce oxygen during daylight hours but use up oxygen during the night in respiration and when they die, sink, and decay. These same processes cause changes in lake pH.

Conductivity
Conductivity is a measure of waters' capacity to convey an electric current. The concentration of dissolved ions in water largely determines its conductivity. Because of the local geology and mellennia of rainfall, water in the Puget Sound region generally has low levels of dissolved minerals and relatively low conductivity. Urbanization tends to increase conductivity, and increases indicate the presence of dissolved ions potentially from a pollutant source (e.g., nitrite-nitrate from fertilizers).