Seagrasses around the world are threatened by human activities, especially those that result in degraded water quality and reduced light availability.. In this study, Forest Service scientists determined light thresholds for the four most common and abundant seagrass species along the Gulf coast of peninsular Florida. Among the seagrass species studied, light requirements ranged from around 6 percent of the light hitting the water's surface to around 26 percent. These variations are caused by morphological and physiological differences in the seagrasses, as well as adaptation to local light availability. Importantly, seagrasses were absent from locations with high nutrient concentrations and high abundances of phytoplankton. Coastal managers can use these targets for light penetration to protect seagrasses by calculating the maximum amount of phytoplankton that guarantees these targets are met, estimating nutrient concentrations that ensure phytoplankton populations stay in check, and reducing nutrient concentrations by managing wastewater disposal, fertilizer runoff and other human activities. Seagrasses are important to: help regulate global warming by taking up carbon dioxide; limit erosion by buffering shorelines from waves; and sustain fish, shellfish, birds and other wildlife by providing habitat and food. Thanks to this study, it is no longer a mystery about how much light these helpful organisms need to thrive.