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Puerto Rico Cruise Day 1 - April 27, 2007
After a minor delay, we set sail shortly after 10am and arrived on station around 4pm. We then proceeded according to plan, and so the crew and scientists prepared to launch three small boats for late afternoon dive operations.. Here we see scientist John Burke and Able-Bodied Seaman, Dave Leaphart, checking fuel levels in one of the small boats.
Two dive teams traveled around the southwest corner of Vieques to make a final test of our fish survey methodologies and to practice dive rescue skills.
The third dive team traveled to the northwest corner of Isla Vieques to revisit three of our seagrass blowout disturbance sites. The teams marked shoots of seagrass, by piercing the base of the leaf bundles with a hypodermic needle. Here we see scientist Giuseppe DiCarlo marking shoots of seagrass. As the leaves grow and elongate, the hole-punch will migrate up the blades, and a leaf growth rate can be calculated given the distance that the hole has moved. The shoots that were marked today will be harvested on the last day of the cruise.
Paula Whitfiedl and Brian Degan prepare scuba gear on deck |
 Giuseppe DiCarlo marking shoots of seagrass. As the leaves
grow and elongate, the hole-punch will migrate up the blades, and
a leaf growth rate can be calculated given the distance that the
hole has moved. |
Results To Date of Seagrass Blowout Study
We want to determine if in fact these sand “blowouts” are continuously migrating or if they represent a punctuated disturbance – a single disturbance event that is followed by a period of recovery. By learning about the dynamics of natural seagrass beds in space and time, we will be able to forecast responses to extreme events (e.g., hurricanes) and set realistic expectations for restoration of high energy seagrass habitats.
For this study, the twelve crescent-shaped blowout features examined are examples of Type I disturbances – that is, open, uncolonized areas within a larger, colonized substrate. The blowouts appear to have been created as a result of high energy waves and sediment transport during Hurricane Hugo (1989). In other Caribbean locations where similar crescent-shaped blowouts occur, it has been suggested that the blowouts continuously “migrate” across the seafloor. These blowouts are actually fairly shallow depressions in the seagrass bed – only about a foot deep. The leading edge of the blowout is a sharp drop-off where water motion has eroded away seagrass and sediment from the mature (undisturbed) portion of the bed, leaving behind a short “cliff” that slopes down into a bare area devoid of seagrass. Over time, algae and seagrass will begin to colonize the bare area. Given enough time, the once bare crescent will refill with seagrass. However, erosion continues at the leading edge migrating the entire feature towards the oncoming waves across the seafloor.
In 2004, we collected seagrass from three different zones at each blowout:
- Mature 1, on top of the escarpment at a distance of 1m from the leading edge
- Recovery 1, located in the trailing edge of the blowout where the first occurrence of seagrass was observed out to a point where visual observation indicated a transition to more dense vegetation
- Recovery 2, also located in the trailing edge of the blowout and extending a distance of 10 m from Recovery 1 .
We determined the age of each seagrass shoot collected by a commonly used metric in seagrass biology termed a plastochrone interval, or PI. The PI is the sum of all the green and white blades present on a shoot + the total number of leaf scars present on the stem of the shoot. A leaf scar is an actual scar left behind on the vertical stem of the shoot when a leaf is shed.
The PI represents the average time interval between the initiation of two successive leaves on a shoot - a measure of seagrass shoot “age”. Although the PI is subject to seasonal variability, it provides an accurate estimate of time at interannual time scales. It is a relative measure used for comparison.
We have found that the mean age for shoots in the Mature 1 zone is significantly higher than for either of the Recovery zones. We have also found that shoot ages in the two Recovery zones are not different from each other. This confirms our expectations. Mature 1 shoots are older because they have not yet been disturbed. Shoots in the Recovery zones are younger because they represent new shoot growth in response to having been disturbed.
By plotting shoot ages versus distance from the escarpment using a statistical method called regression, we can find out when the escarpment formed and we can examine the trend of the line that is formed in this plot. A straight line would suggest continuous migration while a line that is nonlinear would suggest otherwise. We are in the process of conducting these regressions.