Pond Trophic State

        The trophic state of a pond, described qualitatively in Section E, may be quantified using Carlson’s (1977) Trophic State Index (TSI) for comparing lakes. The TSI is based on three separate calculations: 1) phytoplankton chlorophyll–a, 2) Secchi depth, and 3) total P in the water column during summer. Of these, TSIchl-a is usually deemed the most accurate. Oligotrophic water bodies are defined as having TSI < 40. Ponds with TSI values between 40 and 50 are classified as mesotrophic, ponds with TSI Fig. 31. Relationship of light depletion (LD) to phytoplankton chlorophyll–a. 42 between 50 and 70 are termed eutrophic and ponds with TSI > 70 are termed hypereutrophic (Carlson and Simpson 1996). Because Carlson‘s focus was on deeper lakes dominated by phytoplankton, TSI estimates are less sensitive to influences of metaphyton or aquatic plants, which are more abundant in most ponds than in lakes. Calculations of TSI are nonetheless useful as a first step in summarizing the susceptibility of ponds to nutrient–related management problems. TSI estimates for the 13 ponds based on phytoplankton chlorophyll–a, secchi depth, and total P recorded during July are shown in Figure 32. The 13 ponds were ranked in ascending order of TSIchl-a. Secchi depths were greater than expected given the phytoplankton biomass estimates (TSIsecchi estimates were typically less than TSIchl-a), and likely were influenced by additional light reflected from the shallow bottoms of the ponds.

Fig. 32  Carlson’s TSI for 13 ponds. Ponds were ranked on the X axis in order of ascending values of TSI for phytoplankton chlorophyll-a. Individual calculations for chlorophyll-a, Secchi depth, and total P are shown separately.