<![CDATA[Lake Toba, the largest lake in Indonesia and Southeast Asia, is a multifunctional ecosystem supporting fisheries, tourism, transportation, domestic water supply, hydropower generation, agriculture, and surrounding human settlements. This study comprehensively evaluated the spatial and temporal variations in water quality and trophic status using integrated water qualityindices and trophic state indicators. Surface water samples were collected from 21 stations during the dry and rainy seasons of 2024. Water quality was assessed using the Pollution Index (PI) and the Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI), and trophic status was determined using the Trophic State Index (TSI) and TSIlamp. PI results classified Lake Toba as lightly polluted under Class I criteria (3.53 ± 0.82 dry season; 3.44 ± 0.81 rainy season) and good to lightly polluted under Class II standards (1.72 ± 0.59 dry; 1.74 ± 0.78 rainy season). Trophic conditions ranged from oligotrophic to eutrophic across the stations, withan overall mesotrophic status. The mean trophic index increased from 42.37 ± 3.43 in the dry season to 47.03 ± 3.98 in the rainy season, indicating a gradual increase in ecosystem productivity. Although the overall water quality remains suitable for Class II uses, the upward trophic trend signals increasing nutrient pressure. These findings underscore the need for proactive management, particularly the regulation of floating net-cage aquaculture expansion, to mitigate organic loading and prevent progressive eutrophication in Lake Toba.]]>
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