Inland fisheries contribute to Louisiana’s economy yet are threatened by eutrophication, hypoxia, and harmful algal blooms driven by agricultural expansion, urbanization, and hydrological alterations. One such system is Lac des Allemands in the Upper Barataria Basin. Often referred to as the “Catfish Capital of the World,” the lake is impaired by low dissolved oxygen and widely reported as hypereutrophic. Despite its economic importance, baseline comprehensive studies of the lake’s ecosystem and sources of impairment are limited. This research integrates long-term data to evaluate nutrient sources, nutrient retention services, and lake ecosystem responses in terms of cyanobacteria abundance, floating vegetation extent, dissolved oxygen (DO), turbidity, and fish community assemblages. Results indicate urban expansion, with developed land increasing by 2,933 ha and mostly replacing cultivated cropland, which nevertheless remained the second most dominant land-use type after woody wetlands. Forest cover remained relatively stable (–0.076 ha yr⁻¹) compared to earlier decades. Nutrient-export hotspots are concentrated in urban and agricultural areas along higher elevations. While wetland vegetation is dominant at the lower elevations, heavily canalized levee margins and canalization disrupt natural hydrological connectivity and limit wetland nutrient-retention services for water purification. Lake observations revealed persistently hypereutrophic conditions, with elevated TN (1.39–1.55 mg L⁻¹) and TP (0.13–0.28 mg L⁻¹), and cyanobacteria concentrations consistent with harmful algal bloom conditions (>100,000 cells mL⁻¹), with peak levels in spring 2025. Floating vegetation cover remained low (<1%) throughout the study period. Dissolved oxygen exhibited strong seasonal variability, with spring conditions within stressful to hypoxic ranges, while fall conditions were supportive of aquatic life. Fish communities showed seasonal and interannual variability, with total abundance higher in fall than spring, peaking in 2022 and declining thereafter, alongside post-2022 reductions in dominant species such as Largemouth Bass and Black Crappie. Turbidity was highest during spring 2020 and declined over time. Together, these results provide a recent baseline for Lac des Allemands and highlight the need for watershed nutrient management and restored hydrological connectivity, while ongoing long-term analyses will further clarify decadal water-quality trends.