Trophic state index analysis of six wetlands in Lake Tana, Ethiopia: a comprehensive assessment for sustainable water resources management.

This study assessed the trophic status of six wetlands (Wonjeta, Zewdie Girar, Gumara River Mouth, Megech River Mouth, Avaj, and Ras Abbay) in Lake Tana, Ethiopia, using Carlson's trophic state index (TSI) based on Secchi disk transparency, chlorophyll a, total phosphorus, and total nitrogen. Water samples were collected across four seasons (dry, early rainy, rainy, and late rainy) and analyzed using standard methods. Results revealed significant spatial and seasonal variations in trophic conditions (p < 0.05). The total trophic state index ranged from 43.74 to 86.81, classifying wetlands as mesotrophic to hypereutrophic. Megech River Mouth exhibited hypertrophic conditions (TSITOT = 81.4 ± 4.9), while Wonjeta showed eutrophic status (TSITOT = 55.2 ± 6.4), representing a 47% difference in TSI values between the most and least impacted wetlands. Significant interactions between wetland type and season affected all TSI parameters (p < 0.05), indicating that trophic assessment must consider both spatial and temporal dimensions. TSITN showed the highest values (67.7 ± 5.2), indicating significant nitrogen enrichment. However, TSITN:TSITP ratios (2.2-2.9) revealed consistent phosphorus limitation, suggesting that phosphorus control strategies may be most effective for algal bloom mitigation, while nitrogen reductions are also needed given the widespread enrichment. These findings demonstrate progressive eutrophication in Lake Tana's wetlands compared to historical data, attributed to agricultural runoff, urban wastewater discharge, and catchment degradation. This research provides essential baseline data for informing sustainable management strategies to mitigate eutrophication and preserve the ecological integrity of Lake Tana's wetland ecosystems.