What is Trophic State?
The term “trophic state” refers to the biological productivity of a body of water, describing its fertility. It measures the total amount of living biological material, or biomass, within the water. The amount of plant and algal life a lake can support is determined by nutrient availability, making it a fundamental characteristic of a lake’s ecological condition.
Just as soil fertility dictates the richness of a garden, the trophic state indicates how “green” a lake is. Lakes are classified by the quantity of available nutrients, primarily phosphorus and nitrogen, which fuel aquatic organism growth. A lake with low nutrient content has limited biological activity, while a nutrient-rich lake supports much more plant and algal growth.
A lake’s trophic state provides a snapshot of its biological condition. This state is not static and is influenced by factors like the watershed’s geology, climate, and the lake’s physical dimensions. Understanding a water body’s trophic state is an important step in assessing its health and water quality.
Key Parameters in Trophic Assessment
To determine the trophic state, scientists measure several interconnected indicators. The most common are total phosphorus, total nitrogen, chlorophyll-a, and water clarity.
Phosphorus is a nutrient for plant growth and is often the primary limiting factor for algae in freshwater ecosystems. This means its availability directly controls the potential for algal growth. Nitrogen is another nutrient that influences lake productivity, though it is often secondary to phosphorus in freshwater.
Chlorophyll-a is the main photosynthetic pigment in algae and cyanobacteria. Its concentration in a water sample is measured as a direct proxy for the amount of algal biomass. Higher chlorophyll-a levels indicate a more productive system.
Water clarity is assessed using a Secchi disk, a circular plate lowered into the water until it is no longer visible. This measurement, the Secchi depth, indicates water transparency. Lower water clarity is often a direct result of higher concentrations of suspended particles like algae and sediment.
Making Sense of the Index: Scores and Classifications
The measurements of nutrients, chlorophyll-a, and water clarity are converted into a standardized Trophic State Index (TSI) score. The most recognized system is Carlson’s Trophic State Index, which uses formulas to transform raw data from these parameters into a single score on a scale from 0 to 100.
In Carlson’s index, each 10-point increase on the scale represents a doubling of algal biomass. This logarithmic scale provides a continuous spectrum of trophic states. This system allows for comparisons between lakes and helps track changes in a single lake over time.
Based on TSI scores, lakes are grouped into four main classifications:
- Oligotrophic lakes (TSI 0-40) have low nutrient levels, deep clear water, and minimal plant or algae growth. They have high levels of dissolved oxygen and can support cold-water fish species.
- Mesotrophic lakes (TSI 40-60) have a moderate level of nutrients and productivity, with moderately clear water and potential for some algal blooms in late summer.
- Eutrophic lakes (TSI 60-100) are nutrient-rich with high biological productivity. These lakes are often murky and may suffer from oxygen depletion in deeper waters.
- Hypereutrophic lakes have extreme nutrient levels with frequent and severe algal blooms.
Why Trophic State Matters
Monitoring a water body’s trophic state is important for managing aquatic ecosystems and protecting human activities. Biological productivity directly impacts water quality, biodiversity, and the water’s suitability for recreation and as a drinking source. Changes in the Trophic State Index can signal a degradation in a lake’s health.
The process of a lake becoming enriched with nutrients is called eutrophication. While this is a natural aging process, it can be accelerated by human activities, a phenomenon called cultural eutrophication. Runoff from agriculture, wastewater discharge, and urban stormwater can introduce excessive phosphorus and nitrogen, rapidly shifting a lake to a higher trophic state.
High trophic states can lead to environmental problems. An overabundance of algae in eutrophic and hypereutrophic lakes can result in harmful algal blooms (HABs), which may produce toxins dangerous to wildlife and humans. When these blooms die and decompose, the process consumes vast amounts of dissolved oxygen, leading to hypoxic (low oxygen) or anoxic (no oxygen) conditions. These “dead zones” cannot support aquatic organisms.
A high trophic state also affects human use. Poor water clarity, algal scums, and potential toxins can make lakes unsuitable for swimming and other water sports. Treating water from a highly eutrophic source for drinking is also more complex and costly. Tracking a lake’s TSI helps water managers identify trends and implement strategies to reduce nutrient pollution.