Phosphorus: The Key to Freshwater Productivity
In many freshwater lakes, phosphorus limits algal growth, though nitrogen limitation or co-limitation occurs in some systems. Reducing phosphorus input is often an effective strategy for controlling algal blooms and improving water clarity where phosphorus limits growth.
Phosphorus enters lakes from point sources (wastewater treatment plant discharges, industrial effluent) and non-point sources (agricultural runoff, stormwater, septic system leachate, atmospheric deposition). In some lakes, internal loading from reducing bottom sediments is a significant additional source. Quantifying these inputs requires both concentration measurements in the water column and streamflow-weighted loading calculations from tributaries.
Forms of Phosphorus
Not all phosphorus in a water sample is equally available to algae. Total phosphorus (TP) includes everything: dissolved, particulate, organic, and inorganic forms. Soluble reactive phosphorus (SRP), also called orthophosphate, is the dissolved inorganic form that algae can take up directly. The difference between TP and SRP represents phosphorus locked in organic particles, bound to sediment, or in dissolved organic compounds.
For most lake management purposes, total phosphorus is the primary metric because it captures all sources and forms. SRP is useful for understanding immediate algal uptake potential. In a productive lake during summer, SRP may be near zero in the epilimnion because algae are consuming it as fast as it becomes available, even while TP remains elevated due to the phosphorus contained within the algal cells themselves.
- Total phosphorus (TP): the standard metric for lake trophic state and nutrient budgets
- Soluble reactive phosphorus (SRP/orthophosphate): the form immediately available for algal uptake
- Particulate phosphorus: bound to sediment particles or contained within algal cells
- Dissolved organic phosphorus: in dissolved organic compounds, released through decomposition
Nitrogen Forms and Their Roles
Nitrogen is present in lakes in several forms: nitrate (NO3), nitrite (NO2), ammonia (NH3/NH4+), and organic nitrogen bound in living and dead biological material. Total nitrogen (TN) captures all forms. Total Kjeldahl nitrogen (TKN) measures ammonia plus organic nitrogen but excludes nitrate and nitrite.
While phosphorus often limits algal growth in many lakes, nitrogen can become co-limiting or primary limiting in certain conditions, particularly where nitrogen-to-phosphorus ratios are low on a specified mass or molar basis. Some cyanobacteria (blue-green algae) can fix atmospheric nitrogen, giving them a competitive advantage in nitrogen-limited conditions and potentially promoting harmful algal blooms. Understanding the nitrogen-to-phosphorus balance, with the ratio basis clearly stated, helps predict which algal species may dominate.
Sample Collection Best Practices
Nutrient sample quality depends on collection, handling, and preservation matched to the laboratory method and analytical objective. Bottle type, filtration, preservatives, holding times, and detection limits are specified by the lab and the parameter being measured; follow those requirements exactly.
For total phosphorus and total nitrogen, collect samples in the epilimnion (typically at 1 meter depth or at half the Secchi depth) using a grab sampler or Van Dorn bottle. Transport and holding requirements depend on the lab method; many programs use ice storage and delivery within the lab's specified holding window.
Filtered samples for dissolved nutrient forms (SRP, dissolved nitrogen species) must be filtered in the field through a membrane pore size specified by the lab (commonly 0.45 micrometers) using clean equipment, because biological activity in an unfiltered sample can alter dissolved nutrient concentrations during transport.
For nutrient loading calculations, pair water chemistry samples with streamflow measurements at tributary inflow points. A concentration of 50 micrograms per liter of phosphorus at a streamflow of 10 cubic feet per second represents a very different loading than the same concentration at 100 cubic feet per second.
- Use lab-supplied containers and follow the lab's bottle, filtration, preservation, holding time, and detection-limit requirements for each analyte.
- Keep samples on ice or as specified by the laboratory method from collection through delivery.
- Filter dissolved nutrient samples in the field using the pore size and equipment specified by the lab.
- Record sample depth, time, location, and weather conditions with every sample
- Pair tributary nutrient samples with simultaneous streamflow measurements for loading calculations
Interpreting Nutrient Data
Total phosphorus is widely used to describe lake trophic state. Descriptor ranges (oligotrophic through hypereutrophic) vary by lake type, ecoregion, and classification system; they are not universal treatment or regulatory triggers. State water quality standards typically set site-specific criteria based on ecoregion and lake class.
A single sample provides limited information because nutrient concentrations fluctuate with season, weather, and biological uptake. Multiple samples spread across the growing season, with additional sampling after major storm events, help characterize conditions. Multi-year records support trend detection. When comparing your data to published standards or reference conditions, match the same metric (growing-season mean, annual median, epilimnetic grab sample, or depth-integrated composite) that the standard specifies.