Plant characteristics of the non-native seagrass Halophila stipulacea in recently colonized beds of Puerto Rico

Abstract

The seagrass Halophila stipulacea is native to the Red Sea and Indian Ocean, and since entering the Caribbean in 2002 has spread rapidly throughout the eastern islands including Puerto Rico, and as far as Venezuela and Florida, USA. Despite the research attention to potential ecosystem disruption from H. stipulacea there is little data on the morphology and plant characteristics (e.g. biomass and density) of these new populations. In 2022 we surveyed shallow seagrass communities at 10 sites in 3 bays and characterized plant traits. H. stipulacea was present at 50 % of the sites. We measured leaf morphology on individual shoots, and collected samples for biomass, density, and leaf C:N:P. Density estimates ranged from 3000 to over 11,000 shoots m⁻² and biomass varied from 65 and 266 gdw m⁻². Mean leaf C:N:P varied between 393:18:1 and 772:27:1 with no strong indications of nutrient limitation. Leaf lengths ranged between 3.7 and 5.4 cm, width varied between 6.7 and 8 mm, with resulting leaf areas ranging between 2 and 3.5 cm². H. stipulacea leaf area index ranged between 0.6 and 3.9 m² leaf m² sediment, with higher LAI values represent a mechanism of self-shading in shallow, clear waters. The range expansion and H. stipulacea beds observed in this study provide supporting evidence that the Caribbean seagrass landscape is changing. Establishing long-term seagrass mapping and monitoring programs would help to assess the impact of this changing seagrass landscape.