A Slow Leak?
by Caroline Wiernicki -- June 16th, 2016
June 14, 2016 – 9:30 pm
Our boat has a hole in it. Not a particularly large one—the hole itself is probably a little over half an inch long and, at its broadest point, as wide as the tip of my finger. The boat itself is an old center-console, with a single outboard engine and a twin “v” hull that I believe—from the color of the fiberglass dust that has claimed one of my t-shirts—was once blue. When it is in the water, the boat is apparently a great asset to the station, who leases it out to researchers and other visiting groups. Propped up under an awning next to the toolshed, however, our 1980-something Twin Vee is as much an asset as an oversized fiberglass paperweight. With a slow leak.
According to my managers, the original plan was to get the then-listing boat out of the water, patched, painted, and back in to the water. Almost a month later, it doesn’t seem like the Twin Vee will be seaworthy anytime soon. Though not for a lack of trying. We’ve tried resins, epoxies, silicones, foams, liquid tapes, duct tapes; we’ve tried a folded square of t-shirt plastered down with some odd—yet highly effective—brown sealant whose name constantly escapes me. All wonderful, noxious-smelling combinations of chemicals. None quite effective at keeping the water out; or, in the case of our leak—in.
This afternoon, in a break from the boat-patching, I was able to get into the water and set up of the first of two studies, both of which are geared towards a larger project. The overall goal of this greater project is to better understand the nature and ecological characteristics of the invasive seagrass, Halophila stipulacea. The first study questions how well, if at all, the invasive displaces native grasses, such as Thalassia testudinium, or turtle grass. What is the nature of the interactions that are going on here in St. John? A little background: Halophila stipulacea is a tropical seagrass that is native to the Indian Ocean; it was first spotted in Grenada—not quite the Indian Ocean—in 2002 and has since spread to various islands throughout the Caribbean, including St. John. Thalassia testudinium is a flat, green-brown grass native to St. John that gets its name from one of its most loyal patrons—the island’s hawksbill and green turtles. And though there is much to be explored about how H. stipulacea operates—i.e., how it interacts with native species up and down the food web—as of now, these little green leaves are understood to be very good at what they do: grow.
Studies of previous invasions and successful expansions reveal that H. stipulacea grows at a relatively quick turnover rate and can tolerate a wide range of salinities and water depth. Yet, there are still questions to be addressed. Typically, the invasive species story is headlined by the villainous invader, and, more often than not, this is true. Think zebra mussels, Nile perch, or lionfish. The classic invasive species—invasive plants tend to be particularly guilty of this—is one that slips in quietly and whose presence isn’t quite picked up upon until the cascading effects are pouring through the ecosystem at full force…kind of like a certain hole in a certain boat. Its presence is small at first, when the effects are virtually unnoticeable, and the next thing you know, the seemingly stable structure around you has tilted, skewed and weighed down by something from the outside. Maybe it’s water. Maybe it’s a short, round blade of grass that looks more than anything else like a strip of green cellophane. The thing about H. stipulacea, however, is that one cannot quite stamp it as inherently “evil” yet—at least in the St. John ecosystem. So far, this particular leak doesn’t seem to have gotten out of control.
Is there any competition for space, and, if there is, does the rapidly growing H. stipulacea pose a threat to previously established colonies of turtle grass? And how does one go about measuring this displacement? PVC pipes. I packed thirty, one-foot lengths into my yellow duffel-bag from Maryland, (TSA was surprisingly okay with it) and, as of this afternoon, three sets of ten pipes each are currently lined up along Halophila-Thalassia borders in bays across St. John. With the invaluable help of one of my two phenomenal VIERS managers—shout-out to Asya for being the bomb with an underwater sledgehammer—we visited Francis, Waterlemon, and Maho Bays and drove PVC pipes into the sand so as to designate just where the native and invasive grasses collide head-on. In a few weeks, we’ll return to these sites—the pipes, hopefully, unscathed—and record any encroachment of either species into the other’s established turf. (Look for more details on the actual set-up process to come! Spoiler alert: tarpon were involved.)
Over the next few months, in combining this question with another study looking at foundational ecosystem function (read: habitat support, erosion prevention, and, more likely than not, a ton of invertebrates) I hope to gain a little more understanding of just what H. stipulacea is doing in St. John. Is this a serious leak on our hands? Countless battles against more blatantly sinister invasive species emphasize the need for early detection. To what extent can the damage—if damage is occurring—be anticipated? What type of solution, if needed, must be patched on, sooner rather than later?