Chlorophyll a
Photosynthetic pigment found in all green plants, and the main pigment in algae. The concentration of chlorophyll a is used to estimate the amount of algae (all phytoplankton in a given area) in surface water. Measured in micrograms per liter (µg/L).
While it is normal to find algae in streams and lakes, high levels of algae can lead to low levels of dissolved oxygen in a water body.
Consider Chlorophyll as a measure of the quantity of algae in the lake.

Total Phosphorus(TP)
A nutrient essential to the growth of organisms, and is commonly the limiting factor in the primary productivity of surface water bodies. Total phosphorus includes the amount of phosphorus in solution (reactive) and in particle form. Agricultural drainage, wastewater, and certain industrial discharges are typical sources of phosphorus, and can contribute to the eutrophication of surface water bodies. Measured in milligrams per liter (mg/L). See Phosphate.
Consider Phosphorus as a measure of the quantity of food available for the algae in the lake.

Secchi Disk
A device measuring the depth of light penetration in water.
Consider the Secchi depth as a measure of the clarity of the lake

Phosphate
An essential nutrient containing phosphorus and oxygen. Phosphate is often a critical nutrient in lake eutrophication management.

Source: Minnesota Pollution Control Agency (MPCA) Glossary, except for italicized comments. LINK