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DAY 7 – MAY 3
SEAGRASS RECOVERY FROM VESSEL GROUNDINGS
Four species of seagrass occur in southeastern Puerto Rico and Vieques Island: Thalassia testudinum (turtle grass), Halodule wrightii (shoal grass), Syringodium filiforme (manatee grass) and Halophila decipiens (paddle grass).
Thalassia is the dominant seagrass throughout this region, followed by Syringodium and lesser amounts of Halodule and Halophila. Sometimes locally high abundances of Syringodium and Halodule are found in physically disturbed environments such as high energy areas, deeper water, manatee grazing areas, and halos in the proximity of reefs subjected to herbivory and other bioturbation. One of the major objectives of our studies of disturbance and recovery dynamics is to calibrate our seagrass injury recovery model for conditions in the deeper tropical environment. The original model was formulated for conditions in the sub-tropical Florida Keys National Marine Sanctuary where there is much more seasonality in environmental conditions than in southeastern Puerto Rico.
One of our study sites is located where a Navy fuel barge grounded during Hurricane Georges in 1998. In about a 1000 m 2 impacted area, sediments were excavated and nearly all of the seagrass bed was removed. The dominant seagrass species in the area before the impact was Thalassia. An initial assessment was performed after the hurricane, and we have revisited this site numerous times between 1999 and 2006 to assess seagrass recovery by examining seagrass species composition, shoot density, and above and belowground biomass. We have compared the recovery process in the disturbed site to an adjacent undisturbed seagrass bed, in order to assess recovery.
Our data show how Thalassia above-ground biomass (leaves) has fully recovered, while belowground biomass (roots) has recovered about 60%. Syringodium, which originally represented a small component of the total seagrass biomass present in the site, but has showed no recovery. Thalassia biomass recovery has occurred nearly twice as fast in Puerto Rico as predicted by our recovery model developed for the Florida Keys, illustrating an adaptive response of this plant to overcome disturbance as well as the critical need to locally calibrate the model for fully tropical environments. In addition to the relatively rapid rate of Thalassia recovery at all of our “single event” disturbance sites, we have also noted that there is no succession of species. If a Thalassia dominated seagrass meadow is severely disturbed it recovers directly back to Thalassia instead of one of the faster growing opportunistic species, either Syringodium or Halodule. This is substantially different from succession models previously described in the scientific literature where the opportunistic species temporarily substituted for Thalassia during the earlier stages of recovery. The implications are that for tropical seagrass systems where disturbance occurs in a single event, the recovery models require some modificatiion.