Seagrass distribution, areal cover, and changes (1990–2021) in coastal waters off West-Central Florida, USA
Luis Lizcano-Sandoval, C. J. Anastasiou, Enrique Montes, Gary Raulerson, Edward T. Sherwood, Frank Müller‐Karger
Abstract
Seagrass meadows of West-Central Florida (USA), between 27 3 N -28 12 N; 82 24 W -82 50 W, are closely monitored by federal, state, and local groups for benthic composition, density, and areal cover (extent). Biennial aerial mapping, annual in situ surveys, and monthly water quality measurements inform management and conservation actions. Here we leverage a complete archive of multiple satellite imagery products (i.e., Landsat-5, Landsat-7, Landsat-8, and Sentinel-2 imaging sensor data) to reconstruct seagrass areal cover estimates in four regions of West-Central Florida: St. Joseph Sound, Clearwater Harbor, Tampa Bay, and Sarasota Bay. Satellite imagery hosted in Google Earth Engine (GEE) was screened for low cloud cover (<40%), and clear atmosphere and water conditions using a semi-automated process followed by visual inspection. Imagery meeting these conditions for seagrass mapping was available for 16 years: 1990, 1992, 1996, 1999, 2000, 2004-2006, 2010, and 2015-2021. Space-based time series of seagrass areal cover for each region showed a positive correlation with estimates from aerial mapping (r > 0.61). Over the period 1990-2021, satellite-derived estimates show seagrass areal cover increased by 24.3 km 2 (34.4%) in Tampa Bay, 18.0 km 2 (74.1%) in St. Joseph Sound, 6.3 km 2 (182.6%) in Clearwater, and 1.3 km 2 (5.3%) in Sarasota. Overall, the combined seagrass areal cover for the entire region increased by 49.9 km 2 (40.6%), or ~1.4% yr -1 , from 1990 through 2021. Increases in seagrass areal cover before 2020 coincide with improvements in water quality throughout the region (i.e., a general decrease in chlorophyll-a, phosphorous, nitrogen concentration, and turbidity). The publiclyavailable satellite datasets in GEE provide resource managers with complementary and unique tools for synoptic and repeated seagrass areal cover assessments. This is an important approach to monitor the seagrass cover Essential Ocean Variable (EOV) of the Global Ocean Observing System (GOOS).