What’s so special about Cashes Ledge? In this second of a planned series of dives on this New England biodiversity hotspot, Brian Skerry was joined by marine ecologist, Jon Witman, an expert on Cashes Ledge. Jon has been studying Cashes Ledge for 35 years, and has been watching how the diversity and abundance of sea life has been changing there, and how it has responded to its current limited-protection status. We talked to him and found out more about why Cashes Ledge is so important to the Gulf of Maine, and what we can do to keep it thriving.
Why have you spent so much time on Cashes Ledge?
Cashes Ledge is a fascinating and wild offshore place that helps us understand how marine ecosystems tick. It is also a unique storehouse of Atlantic marine biodiversity. Cashes Ledge provides an opportunity to understand why biodiversity matters in an ecological sense. Unfortunately, we are losing marine biodiversity in the world’s oceans faster than we can study it.
Currently, I’m trying to figure out how the whole benthic ecosystem out on Cashes Ledge – from the fish, to the kelp forests and the diverse invertebrates communities have changed over the past decades. I’m particularly interested in how resilient the system is to human disturbance and to climate-related changes in the oceanography.
When we studied Cashes Ledge intensively in the 1980’s, it was like a time machine providing a fleeting glimpse of what New England marine coastal communities might have been like hundreds of years ago, when lots of large predatory fish – especially cod, were commonplace close to shore. We videotaped over 100 cod an hour going by an area of bottom about the size of a large picnic table on Cashes Ledge, compared to no cod seen at the same depth at coastal sites in the Gulf of Maine.
I actually saw a whale cod as long as a diver and schools of Atlantic bluefin tuna while diving on Cashes Ledge then. There have been substantial reductions of predatory fish since then, which is something I’m studying, but Cashes Ledge is still a vitally rich ecosystem compared to coastal ones that have been more heavily impacted by humans.
What other kinds of interesting animals have you seen on Cashes Ledge?
There are layers of marine life on Cashes Ledge, including minke, right, humpback and pilot whales, blue sharks, basking sharks, atlantic white sided dolphins, big schools of bluefin tuna chasing herring, whale cod, red cod, pollock, wolffish, torpedo rays, squid, strange feather stars called crinoids, and unusual sponges and sea squirts typical of sub arctic areas of Scandinavia.
Can you talk about the internal waves and why they are important?
The top of the ridge on Cashes Ledge is an incredibly dynamic place – layers of plankton in warmer overlying waters are driven right down to the bottom as much as 20 times a day by these phenomena known as internal waves. This is a big deal because the downwelling plankton layers are pulses of concentrated food that sustain bottom dwelling organisms and, in effect, fuel the food web.
We stumbled across this phenomenon in the course of our scuba dives to the top of the ridge at 30 m. One dive team would go down and report that the water on the bottom was cold and beautifully clear but the next team an hour later found pea soup visibility in greenish warm water. This, of course, turned out to be the plankton layer pushed down onto the bottom like a yo-yo by internal waves.
The temperature increase was so large that we could feel the warm water through our drysuits. At that time, the prevailing view of the subtidal zone was that it was a stable place with nearly constant environmental conditions, compared to the rocky intertidal zone. But out on Cashes we were documenting as much as 5 degree centigrade temperature increases in 10 minutes right on the rocky sea floor at 30 m depth.
Internal waves are like a sine wave travelling along the boundary between the warm surface waters and the colder layer below. They can be huge – spanning 50 m vertically in some parts of the world and 30 m high on Cashes. I’ve seen these downwelling green water waves approaching the ridge on Cashes Ledge while scuba diving and sitting off the ridge in the Johnson Sea Link submersible – it’s one of the most spectacular things I’ve seen underwater.
What makes Cashes Ledge so unique?
There are at least three things make Cashes Ledge so unique. First of all, it is the largest continuous kelp forest in offshore waters on the entire east coast of the US. The kelp grow unusually deep there, beyond 30 m depth. The forest and the ledge itself provide many valuable goods and services to keep the offshore Gulf of Maine ecosystem healthy, vibrant, and productive. For example, it’s a nursery habitat for commercially valuable groundfish. It’s also an energy rich food source for marine life living in habitats both on the ledge and far away from it – in the form of detritus as the kelp breaks down.
Secondly, the Cashes Ledge ecosystem contains a wide range of different bottom types – it isn’t just all rocky ledge. Just like on a mountain slope in the Green or White Mountains in New England, there are cobble and boulder fields on the lower sides of rocky slopes on Cashes Ledge. Deeper down, the sea floor is covered in sand and gravel that grades into soft bottom areas of silt and mud in the basins. So what you have in the Cashes Ledge underwater landscape is a representative collection of most of the major types of bottom habitats found in the Gulf of Maine, but in an incredibly compact area, as ecosystems go.
Each of those different habitat types has its own community of species that do especially well in that particular habitat. For example, there are pink northern shrimp, clams, and tube worms living in the muddy basins at the edge of a boulder field, then communities of soccer ball-sized yellow sponges, bright red sea anemones, and little upright calcified candelabras called bryozoans that look like miniature coral reefs, attached to the boulder tops. Different habitats enhance biodiversity overall. If you sum up all the different species living in each of these different types of habitats from kelp forests to the muddy basins, you have some of the highest biodiversity levels in the Gulf of Maine right on Cashes Ledge.
Finally, as an abrupt topographic high in relatively clear, shallow, sunlit waters, Cashes Ledge is an especially productive offshore ecosystem in the Gulf of Maine. I mentioned the role of the kelp detritus exporting food to adjacent ecosystems, but the dynamic oceanography of the ledge itself also contributes to the productivity of the bottom community in the way that internal waves push concentrated layers of plankton to the top of the ridge.
I think both mechanisms help make Cashes Ledge such a productive area for many species – including groundfish and marine mammals. We’ve seen minke whales feeding in the slicks of internal waves on Cashes Ledge, presumably due to high concentrations of food there.
What kind of protection does Cashes Ledge need and why?
As special as it is, Cashes Ledge is a very vulnerable marine ecosystem. Right now Cashes Ledge has a small amount of protection from certain types of fishing activity as an Essential Fish Habitat and as a Habitat Area of Special Concern. This is laudable and a real achievement by fisheries managers in New England. However, this protection is only temporary and it could be eliminated at any moment. It could be opened to fishing practices that further deplete stocks of groundfish, damage biodiverse communities, and decrease the sustainability of the kelp forests.
Because it is such a unique, valuable, and diverse New England marine ecosystem, the rocky ridge, adjacent bottom habitats, and the overlying water column on Cashes Ledge need permanent protection from human impacts. It has been shown many times that marine protected areas help exploited stocks recover and can ensure the sustainability of biodiversity and other goods and services that keep our oceans healthy. We also know that really small protected areas don’t do these jobs very well, so it pays in the long run to preserve larger areas containing different types of habitats.
Globally, we aren’t doing a very good job of protecting the oceans as less than 2% of the worlds oceans are fully protected, despite all the scientific findings showing that marine ecosystems are under ever increasing levels of stress from all sorts of human impacts.
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