Seagrass cover expansion off Caja de Muertos Island, Puerto Rico, as determined by long-term analysis of historical aerial and satellite images (1950–2014)

Abstract

Abstract Seagrass ecosystems affected by climate change and anthropogenic disturbances require baseline characterization of their cover, distribution, and dynamics for effective conservation strategies. Caja de Muertos Island (CMI) is an important nature reserve in Puerto Rico where long-term trends in seagrass cover are unknown but where availability of historical aerial photographs can provide elements for analyzing seagrass dynamics. The purpose of this study was to understand those dynamics in CMI, thereby providing a baseline for determining the severity of future disturbances and inform conservation strategies. We quantified changes in seagrass spatial extent using object-based image analysis in a 2014 WorldView-2 image and historical aerial photographs from 1950 to 2010. Contrasting with worldwide seagrass declining trends, seagrass extent in CMI showed a positive rate of expansion and increased 64%, which was mainly explained by the patchy seagrass cover category that was also the most persistent cover throughout the 64-year period analyzed. Findings suggest an increase in seagrass extent due to recolonization of seagrasses after impacts of tropical storm Baker in 1950, aided by other factors such as reduction in grazing pressure. Even though seagrass cover expanded, with other tropical storms and hurricanes, no negative effects were observed except between 1993 and 1999. The amount and frequency of changes in seagrass cover categories varied between zones and seemed to be linked to localized environmental conditions (e.g. wave exposure) and geomorphological characteristics. The present study represents a baseline of seagrass cover distribution and dynamics, offering valuable information for conservation strategies in Caja de Muertos Island Nature Reserve. In the absence of in situ long-term monitoring data, this study also shows how aerial photography and an object-based image analysis are effective for assessing trends and quantifying seagrass cover changes in clear oligotrophic waters.