How do we measure eutrophication?Trophic stateEutrophication is the progression of a waterbody towards a eutrophic state. Trophic state (or trophic status) is a way of classifying waterbodies based on the productivity of the ecosystem. Primary producers (e.g., phytoplankton, periphyton, macrophytes and macroalgae) convert inorganic nutrients and minerals to organic matter through photosynthesis, a process called primary production. Without this primary production, there would be no new biomass produced within the ecosystem to support other life forms.

Waterbodies range from nutrient-poor systems with low primary production (oligotrophic) to nutrient-rich systems with high primary production (eutrophic) or very high production (hypereutrophic). Those that fall somewhere in the middle are classified as mesotrophic. Waterbodies with extremely low nutrient status that are unable to support life are referred to as dystrophic.

Knowing a waterbodies trophic state provides basic information that can be used to measure biotic integrity, monitor human influence and guide restoration planning. However, there are no universal criteria for classifying trophic state and a combination of different indicators have been suggested depending on the water body type. Changes from oligotrophy to mesotrophy to eutrophy do not happen at a well-defined point nor do changes occur at the same rate across all indicators. Many different indicators have been used to describe a waterbodies trophic state.

Algal biomass

Algae are aquatic, plant-like organisms and can be found floating in the water column What is it?(phytoplankton) or growing attached surfaces (periphyton) or on the seafloor (macroalgae such as seaweed). Nutrient enrichment results in an increase in these primary producers.

How do we measure it? Chlorophyll a is a pigment used by algae and plants in photosynthesis. The concentration of chlorophyll a in the water provides an estimate of algal biomass, which is high in eutrophic waterbodies. Chlorophyll a can either be measured in the field using a fluorometer or by taking a sample back to the lab and analysing using a spectrophotometer and is reported as mg/m . -3

Macroalgae, due to its size, is typically measured in weight or % cover.  

Dissolved oxygen

What is it? Dissolved oxygen (DO) is the amount of oxygen dissolved in the water. Under eutrophic conditions, DO concentrations increase during the day but decline rapidly at night, leading to hypoxia (very low DO) or anoxia (no DO). The presence of sufficient DO in the water column is very important for aquatic life. Anoxic condition also result in the release of phosphorus from the sediment, causing further nutrient enrichment.

How do we measure it? DO can be measured using a titration or colorimetric method, or more commonly using a dissolved oxygen meter. DO sensors are also available for continuous monitoring. It is typically reported as mg/L.

Water clarity

What is it? The clarity of water is related to the depth that light will penetrate water (i.e., how clear the water is). The high density of algae in eutrophic waters reduces light penetration and so eutrophic waters will appear turbid (cloudy or opaque).

How do we measure it? Water clarity is typically measured using a Secchi disc (lakes and estuaries), or in rivers, a clarity tube or black disc set. The Secchi disc measures the depth that a person can see into the water (visual clarity) and is reported as a distance (in meters).

Nutrients

What is it? The amount of nitrogen and phosphorus and the ratio between the two can predict trophic state.

How do we measure it? Nutrients can be measured as water column totals (TN, TP) or only accounting for the bioavailable fraction (e.g., DIN, DRP). Water samples are typically sent back to the lab although there are sensors available for continuous measures of some nutrient constituents. Nutrient concentrations are reported in units of mg/l, g/m or ppm (parts per million).3

Benthic macroinvertebrates

What is it? These are small animals (but big enough to see with the naked eye) that live on the bottoms of lakes, rivers, and estuaries. They don’t have a backbone (invertebrates). The abundance, biomass, and types of invertebrates found can indicate eutrophic conditions.

 

Figure 3. Spot the Secchi disc? Water transparency is a key indicator of trophic status, with eutrophic water bodies appearing green and opaque rather than blue and clear.

 

Figure 4. Aquatic plants that grow to the surface of, but do not emerge from, shallow water are called submerged aquatic vegetation (SAV). Other aquatic plant lifeforms include emergent and free-floating. 

How do we measure it? Invertebrates samples are collected using a range of different sampling devices. The taxa are counted and identified and often a biotic index is calculated which can identify if the site is eutrophic or potentially eutrophic (e.g., MCI, NZ Hybrid AMBI). 

Submerged aquatic vegetation

What is it? Submerged aquatic vegetation (SAV) are aquatic plants that can grow up to the water’s surface in aquatic environments but not above it (excluding floating and emerged plants). SAV required adequate light penetration which can be reduced under eutrophic conditions due to phytoplankton blooms and epiphytic growth on leaves. Many long-lived species of SAV (e.g., seagrass in estuaries) are out-competed by the growth of filamentous short-lived macroalgae). The presence of SAV is associated with sufficient water clarity and good water quality. Find out more about freshwater SAV . here 

How do we measure it? SAV is typically measured as % cover.