Trophic state index explained for source water reservoirs
If you've sat through a watershed advisory committee meeting, you've heard someone call a reservoir "eutrophic" like it settles the argument. It doesn't, by itself. The Trophic State Index is a number, usually 0 to 100, that Carlson built in the 1970s to put lakes on a common scale using three things you can measure: chlorophyll-a, total phosphorus, and Secchi depth. For a utility pulling raw water off a reservoir, it's a useful shorthand as long as you remember it's a shorthand, not a diagnosis.
The three inputs, and why they're not interchangeable
Each of the three TSI components is calculated from a log transform of the raw measurement, scaled so that roughly every 10-point jump means a doubling of algal biomass. Chlorophyll-a TSI tracks standing algal crop directly. Secchi depth TSI is really a proxy. It assumes water clarity is driven by algae rather than by clay, tannins, or glacial silt, which is a fair assumption on a lot of reservoirs and a bad one on others. Total phosphorus TSI estimates what the system could support if nutrients were the limiting factor, which in most freshwater reservoirs they are.
Carlson designed the index so all three should converge on roughly the same number when phosphorus is the limiting nutrient and algae dominate the light attenuation. On paper: 0-40 oligotrophic, 40-50 mesotrophic, 50-70 eutrophic, 70+ hypereutrophic. In practice, a reservoir that's clay-turbid from spring runoff will show a Secchi TSI well above its chlorophyll TSI even with a thin algal population. Checking the three scores against each other flags that pattern as a turbidity signal rather than a bloom signal, and you'll see it the moment you line the numbers up side by side.
When the three disagree, and what that disagreement tells you
The gap between TSI(chlorophyll) and TSI(Secchi) is where most of the actionable information sits. If chlorophyll TSI runs noticeably higher than Secchi TSI, you likely have large, efficiently light-harvesting algal cells, including cyanobacteria genera that can produce taste-and-odor compounds or toxins at biomass levels lower than you'd expect from clarity alone. If Secchi TSI runs higher than chlorophyll TSI, nonalgal turbidity is doing the work, which matters for coagulant dosing and filter runs but says less about cyanotoxin risk. TSI(phosphorus) running well above both of the others usually flags a system that's nutrient-loaded but not yet expressing it as biomass, worth watching for a lag effect after the next warm, still stretch.
None of this is new science. What's changed is how often a utility can run it. A single grab sample gives you one point on a curve that moves week to week with temperature, wind mixing, and tributary loading. Most source-water programs sample reservoirs monthly, sometimes less in the off-season, because a boat run with a Secchi disk and a lab draw for chlorophyll and total P isn't free, and it doesn't scale to every intake lake in a service area. That's the gap a weekly wide-swath pass closes: calibrated chlorophyll-a and turbidity readings across every reservoir on your list, delivered on a cadence tight enough to see a TSI climbing before it shows up as a complaint at the plant. Water Quality Retrieval builds that weekly read into a numbered memo per water body, so the trend line is sitting in your inbox instead of waiting on the next sampling trip.
A caveat worth keeping in mind
TSI was built for natural lakes, and reservoirs don't always play by those rules. Drawdown cycles, inflow-driven mixing, and variable retention time can decouple phosphorus from algal response in ways Carlson's original lakes never showed. Treat TSI as a screening tool that tells you where to look closer, not a number you report to the board without context on what's driving it that particular week.
If you're tracking trophic state across more than one reservoir and the sampling boat can't keep up, it might be worth seeing what a weekly reading on every one of them looks like.