Coastal eutrophication driven by riverine nutrient loads continues to be one of the most prevalent water quality issues in estuarine and coastal waters. The process of anthropogenic eutrophication occurs when elevated nutrient loads result in increased phytoplankton biomass that may lead to harmful algal blooms, decreased water clarity, hypoxia, and coastal acidification. Recent studies have observed increased nutrient loading in watersheds draining to Mobile Bay. It is unknown if phytoplankton biomass in the sub-estuaries has increased in response to the increased loading. In this study, phytoplankton biomass was measured in Mobile Bay sub-estuaries approximately monthly starting in 2025 to obtain a baseline for future monitoring projects. Each sub-estuary has salinity gradients that are as low as 0 and can reach a maximum of 13 to 28. Phytoplankton biomass was measured as chlorophyll-a (chla), which was extracted from bay samples with methanol and analyzed on a fluorometer. The data from the sub-estuaries were compared to determine if there were any predominant trends in phytoplankton biomass in the sub-estuaries. The Weeks Bay sub-estuary has the highest chla, followed by Dog River, Mississippi Sound, and the Delta at 36.6, 20.1, 16.3, and 9.0 mg m-3, respectively.