In most aquatic ecosystems, the productivity of microscopic marine plants (phytoplankton) is limited by the availability Nitrogen, Phosphorus or sunlight.  Sunlight only limits “primary productivity” during winter months in non – tropical ecosystems.  Nitrogen typically limits productivity in saltwater ecosystems and Phosphorus is typically limiting in lakes and non-tidal rivers.  Research at Horn Point Lab has also helped to show that in many tidal rivers, nutrient limitation has a seasonal component with Phosphorus limitation in the spring and Nitrogen Limitation in the summer and fall.  Today, most coastal bodies of water adjacent to population centers are impacted by nutrient enrichment via agricultural fertilization, sewage disposal, and the combustion of fossil fuels.  All three sources are significant to the Chesapeake Bay and cause levels of primary productivity to be substantially elevated above pre settlement times.  Today, due to high sediment loads and the self-shading of algal blooms, primary productivity is constrained to the upper portions of the water column in enriched systems (the “photic zone” of many Chesapeake Bay tributaries is often less then 1 meter).  When blooms of phytoplankton die off, their cells rain down into deeper waters and become food for ambient bacteria.  This “respiration” process consumes dissolved oxygen, and if the deep water is blocked from atmospheric oxygen by a pycnocline (see Temperature) as is much of the main stem and large tributaries of Chesapeake Bay, Oxygen levels will no longer be sufficient to support marine animals including crabs, oysters and fish.  The reduction of nutrient inputs is considered the first and most important step to improving water quality in Chesapeake Bay and numerous ecosystems worldwide.

How we measure nutrients:

Our in-situ sensors (manufactured by EnviroTech LLC) automate excepted traditional wet chemical, colorimetric methods.  The basics of the analysis include the following:

1. Ambient water is collected with a syringe pump and dosed with small amounts of chemical reagents (different chemicals for different nutrients).
2. The resultant chemical reaction is allowed to develop for periods of seconds to several minutes and in the case of Ammonium (NH₄⁺), Phosphate (PO₄^3-) and Urea ((NH₂)₂CO) the color development is accelerated with heating. 
3. The colored reaction solution is inserted into a flow through cell and a beam of light is transmitted through the cell.  The more nutrients in the water, the more colored the solution, and the less light transmits through the cell.  The nutrient concentration is obtained by feeding sample and standard transmission values into the Beer-Lambert equation.

AIMS nutrient monitors run a standard nutrient solution before each sample to improve accuracy.  The raw data is telemetered to a master computer at HPL where it is processed and error checked before display on the AIMS web site.