Experiment 06LimnologyTrophic Profiling
Ecosystem Physicochemical Profiling: Water Transparency (Secchi Disc Depth & Carlson’s TSI), pH, Conductivity & TDS
Duration: 4 Hours • Field Limnology Survey
🎯 Experimental Aim & Learning Objectives
To measure aquatic light penetration and extinction depth using a standard 20 cm Secchi disc, calculate the euphotic zone depth ($Z_{eu}$) and vertical attenuation coefficient ($k$), determine the lake’s trophic status using Carlson’s Trophic State Index (TSI), and correlate optical transparency with field pH, electrical conductivity (EC), and Total Dissolved Solids (TDS).
🔬 Theoretical Foundation & Optical Equations
Water transparency integrates suspended mineral particulates, organic detritus, and phytoplankton biomass:
- Secchi Depth ($Z_{sd}$): The vanishing depth of a 20 cm circular plate divided into alternating black and white quadrants. Reflects the diffuse attenuation of visible solar radiation.
- Euphotic Zone Depth ($Z_{eu}$): The photic depth where downwelling irradiance drops to 1% of surface values. Empirically related to Secchi depth by:
$$Z_{eu} approx 2.7 times Z_{sd} quad (text{or } 2.5 – 3.0 times Z_{sd})$$ - Poole-Atkins Extinction Coefficient ($k$):
$$k = frac{1.7}{Z_{sd}}$$ - Carlson’s Trophic State Index (TSI): Carlson (1977) formulated a continuous numerical scale (0 to 100) based on algal biomass:
$$text{TSI(SD)} = 60 – 14.41 ln(Z_{sd} text{ in meters})$$
• $text{TSI} < 40$: Oligotrophic (Clear water, deep photic zone, low productivity).
• $text{TSI } 40 – 50$: Mesotrophic (Moderately clear, intermediate nutrient load).
• $text{TSI } 50 – 70$: Eutrophic (Frequent cyanobacterial blooms, shallow Secchi depth).
• $text{TSI} > 70$: Hypereutrophic (Severe scum, nocturnal anoxia, low biodiversity).
Figure 1.6: Secchi disc deployment geometry, euphotic depth estimation ($Z_{eu} pprox 2.7 cdot Z_{sd}$), and Carlson’s Trophic State Index (TSI) multi-parameter limnological classification.
🧪 Field Instruments & Calibration Standards
- Optical Tools: Standard 20 cm alternating black/white Secchi disc with non-stretch calibrated nylon lanyard marked at 0.1 m intervals and brass sounding weight; viewing tube (aquascope) to eliminate surface glare.
- Electronic Probes: Portable multi-parameter digital meter calibrated with pH 4.01, 7.00, and 9.21 buffers, and conductivity standards ($1413 mutext{S/cm}$ KCl).
📋 Step-by-Step Field Measurement Protocol
- Secchi Disc Deployment: Lower the Secchi disc vertically on the shaded (leeward) side of the boat between 10:00 AM and 2:00 PM without wearing polarized sunglasses.
- Vanishing & Reappearance Depths: Note the exact depth $d_1$ at which the disc just disappears from view. Lower an additional 0.5 m, then slowly raise it until the white quadrants just reappear ($d_2$). The true Secchi depth is: $Z_{sd} = frac{d_1 + d_2}{2}$.
- Calculations: Compute euphotic zone depth $Z_{eu}$, extinction coefficient $k$, and Carlson’s $text{TSI(SD)}$.
- Physicochemical Profiling: Immerse calibrated probe sensors at $0.5text{ m}$ depth. Record temperature, pH, electrical conductivity ($mutext{S/cm}$), and Total Dissolved Solids ($text{mg/L}$).
📊 Representative Field Survey Data & Classification
| Parameter | Oligotrophic Lake Site | Urban Eutrophic Pond Site |
|---|---|---|
| Secchi Depth ($Z_{sd}$) | $4.50text{ m}$ | $0.65text{ m}$ |
| Euphotic Zone Depth ($Z_{eu}$) | $12.15text{ m}$ | $1.75text{ m}$ |
| Attenuation Coeff ($k$) | $0.378text{ m}^{-1}$ | $2.615text{ m}^{-1}$ |
| Carlson’s $text{TSI(SD)}$ | $mathbf{38.3}text{ (Oligotrophic)}$ | $mathbf{66.2}text{ (Eutrophic)}$ |
| Field pH | 7.42 | 8.85 (High photosynthetic $CO_2$ drawdown) |
| Conductivity / TDS | $115 mutext{S/cm}$ ($75text{ mg/L}$) | $680 mutext{S/cm}$ ($442text{ mg/L}$) |
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