Sources of Nitrogen and Phosphorus
Nutrients affect nearly all aspects of pond water quality. Understanding and controlling nutrients is therefore one of the most essential components of pond management. Plants and algae require a "balanced diet" consisting of many kinds of nutrients. Only two nutrients, however, are likely to directly control the abundance of most species. Nitrogen and phosphorus inputs are often the biggest threat confronting pond management, as high concentrations in hypereutrophic ponds typically lead to excessive algal or aquatic plant growth. Nitrogen and phosphorus originate from a variety of land uses within the watershed. The N:P:K formula in lawn and agricultural fertilizers, for example, refers to the relative amounts of nitrogen, phosphorus and potassium being added. Domestic sewage is typically high in both nitrogen and phosphorus, and septic drain fields release substantial quantities of both to groundwater which may ultimately enter the pond. Rainfall is actually a major source of nitrogen (but contains little phosphorus) in southeast Pennsylvania. Of the two nutrients, phosphorus is often the nutrient found in "least supply" relative to demand for it by plants and algae, and can also be better managed. Most of this section is therefore devoted to controlling phosphorus inputs from the watershed, and reducing the concentrations and impacts of phosphorus already found within ponds.
As shown in the diagram below, phosphorus (P) enters a pond either in dissolved or particulate form. If a stream inflow is present, sediment particles transported into the pond especially during storm events often have lots of P incorporated into organic molecules or adsorbed to their surfaces. Surface runoff and groundwater inflow may similarly add P in both dissolved and particulate form.
Because P is such a scarce and desired commodity, dissolved phosphorus is usually taken up rapidly by algal phytoplankton (see Section III below) within the water column, and converted to cellular constituents. The phytoplankton cells typically settle slowly to the sediments. Sediment particles from the watershed may similarly settle to the bottom. Although ponds also export P downstream in both dissolved and particulate form, much remains trapped within the pond sediments.
As a result pond sediments are usually very rich in P. During storms, when the sediments are disturbed, and also when oxygen levels are depleted near the bottom, much of this P is returned to the water column; this is termed "internal fertilization". In effect, efforts to control the growth of algae and aquatic plants by reducing inputs of P from the watershed are often doomed to failure unless the P-rich sediment is removed as well.
Because large portions of P inputs are particulate, controlling sediment inputs (above) also reduces phosphorus entry into the pond; owners should consider settling basins and bank erosion control upstream of the pond itself to contain the sediments as one example of joint sediment and phosphorus control. Reducing the phosphorus content of fertilizers can reduce dissolved P inputs, usually without harming lawns and gardens (low P fertilizers are advertised on-line). Finally, if a pond is becoming progressively "greener" and dominated by phytoplankton or free-floating plants year after year, consider as one possibility the enrichment of groundwater entering the pond from a malfunctioning septic tank/drain field.