Stacey M. Williams; Jorge Sabater; Yasmin Detres; Ricardo Colon-Rivera; William J. Hernandez; Dorimar Ortiz

Assessing the impacts of the invasive seagrass, Halophila stipulacea, and two major hurricanes on the distribution of native seagrass communities of Culebra Island, Puerto Rico

2019

Coastal Survey Solutions LLC

"This study was funded by the Coral Reef Conservation Program of the National Oceanic and Atmospheric Administration to better understand the distribution of the invasive seagrass, Halophila stipulacea, in Culebra Island, Puerto Rico. A total of 200 sites were randomly surveyed for the presence of seagrass. At each site, biological factors such as seagrass composition, cover (Braun Blanquet) and condition were recorded and estimated. Also, environmental parameters like conductivity, temperature and dissolved oxygen were recorded. Out of the 200 sites sampled, only 40 sites were absent of seagrass. There were three species of seagrass identified at the sites surveyed, and these were Thalassia testudinum, Syringodium filiforme, and H. stipulacea. The community structure of seagrass species significantly varied between locations and depth ranges. Spatially, seagrass communities were overall significantly different between locations. The only areas with similar seagrass assemblages were within protected areas, like bays, Ensenada Honda, and Bahia de Almodovar. T. testudinum was the dominant seagrass species in these bays, contributing between 50 to 70% of the overall seagrass communities. Out of the 160 sites with seagrass present, H. stipulacea was present at 97 sites. The overall mean cover (plus or minus SE) of H. stipulacea in Culebra was 1.59 plus or minus 0.08% based on the Braun Blanquet (BB) method, which corresponds to any number of individuals covering 5-25% of the area. There was a significant negative relationship between the cover of H. stipulacea and other native seagrasses, especially with T. testudinum. Other factors affecting the cover of H. stipulacea were depth, dissolved oxygen and conductivity. Depth was the most influential factor, contributing 45% of the models variation. In addition to developing geographical maps of the occurrence and cover of H. stipulacea, a risk assessment map was also produced. Given the results of the mapping and statistical analyses, the factors affecting H. stipulacea invasion were identified as boat activity, H. stipulacea presence, continuous cover of native seagrass, optimal depth for H. stipulacea, habitat availability, and within a Marine Protected Area. Through the use of satellite imagery, we were able to identify benthic habitats, especially seagrass areas affected by the passing of two major hurricanes, Irma and Maria, in Culebra Island. From the supervised classification of the two Sentinel 2 images, we estimated that the cover of seagrass decreased by 232.02 ha."

seagrass, invasive species, Halophila stipulacea, Culebra, Puerto Rico

NOS

OCM

FY2019 CRCP Project ID 31209; Project Title: FY18 - Domestic Coral Reef Conservation Grants; Principal Investigator: Liz Fairey; Grant NA18NOS4820107.

NA18NOS4820107