Tuesday, August 3, 2010

Alex Beach Lovell's Island

Finally! We made it to an island! Sunday’s class consisted of a trip to Lovell’s Island, the outer most of the inner harbor islands. We arrived at the island at about 10am, just after low tide at 9:58. The extreme low tide was perfect for the tide pooling we would engage in shortly. Like at the Barking Crab, the weather was beautiful. A high of 75 degrees and very sunny but with a strong breeze off of the open ocean made it rather comfortable. The beaches around the dock at Lovell’s were sandy but as we approached the tide pools the beaches became composed mainly of cobbles. I have two hypotheses to explain this change in beach composition, and the true answer is probably a combination of the two. First, we learned from Bruce during lunch that Periwinkles are believed to turn sandy beaches and marshes into rocky beaches by eating the algae that hold the sediment together. Based on the countless number of Periwinkles at the site this hypothesis is not hard to accept. However, one would expect the Periwinkles to have done the same to the beaches near the dock. My second hypothesis is that the dock was on the sheltered side of the island and the tide pools were on the flank between the harbor side and the open ocean side. Increased exposure to weathering from the open ocean would remove more sand and loose sediment from the beaches, leaving the heavier cobbles behind. This hypothesis was further supported when we were left to explore the island looking for snails at the end of class. I ventured across the island to beaches directly exposed to the open ocean side. Here there was absolutely no sand left (unlike at the tide pools where there was some) and all of the small cobbles were gone as well. Only larger stones were left on these beaches. I didn’t notice any Periwinkles on this beach so the composition might be caused solely from the ocean mechanics but by this point high tide was coming in so perhaps the Periwinkles were submerged and out of view.

The path from the docks to the tide pools cut through the middle of the island like a spine. Lovell’s was highly vegetated. Even with my lack of botany knowledge I was able to recognize abundant blackberry bushes, milkweed, sumac, salt spray rose, and poplar lining the trail and some species of pine on higher ground farther inland. When the path opened onto the beach we were greeted by a sight that only I probably thought was really awesome and significant. An old World War II structure had crumbled down onto the beach and had taken a massive amount of the surrounding earth with it. This left an exposure of partially lithified (turned to rock) glacial till about 40 feet tall at the highest point and 50 feet across at the base. This exposure is by far the best example of the bedrock geology of the harbor islands I have ever seen (picture from http://epod.usra.edu/blog/2007/11/illinoian-till-in-boston-harbor.html). As we discussed in class, the islands are just a submerged drumlin field left behind during the Pleistocene glaciations. After the last glacial maximum around 20,000 years ago the glaciers started to recede back towards Hudson Bay in Canada. All of the sediment that they essentially bulldozed during their advance was simply left in place like a snowplow backing up and leaving the pile of snow at the edge of a parking lot. This process formed much of the geological features of Massachusetts Bay. Cape Cod is the terminal moraine of one lobe of the glacier, large boulders called glacial erratic were left scattered throughout the forests, kettle ponds such as Walden Pond and Jamaica Pond were formed, and the drumlin field now known as the Harbor Islands was created. This sediment left behind, called glacial till will eventually turn into a conglomerate rock called glacial tillite after millions of years. However, the till that forms the Harbor Islands is only 20,000 years old and has only just begun to lithify. This has profound consequences for the islands today. Since the sediment is comparatively soft and loose, it is easily eroded by natural and anthropogenic actions. A large storm surge would be enough to wash away large portions of islands or topple an old bunker like the one on Lovell’s. I was amazed at how easily I could erode the outcrop at Lovell’s with just my fingernail. If you want to see how the islands would look in a few million years after all the till lithified into tillite look no further than the abundant Roxbury Conglomerate formations all around Boston. These rocks started as the same type of material that now makes up the Harbor Islands. They were deposited by a glaciation somewhere around 600 million years ago. There has been sufficient time for those sediments to turn into a rock hard enough that it became an indispensable building material for settlers in this region for hundreds of years. Outcrops of this rock can be seen all over the area including at the Arnold Arboretum, Squantum Head in Quincy, and southern Hull near World’s End. Much more could be learned about the islands by studying and analyzing this outcrop but I am getting off topic.

The tide pools that we studies were located at the north-western end of the island. As mentioned above the beach was a combination of sand and cobbles. Four large tide pools were noticeable; each separated from one another and the ocean by barriers of large boulders, some of which appeared to be man made. The two tide pools closets to land were noticeably warmer than the two closest to the sea, since they had been isolated for a longer period of time and exposed to the sun without any heat loss to the open ocean. The water was very clear. On the arbitrary clarity scale used on Day 2 I would say the water was a 7 compared to the 5 at Long Wharf. The added clarity is most likely due to a combination of less pollution by being farther from the industrial inner harbor and less suspended material in the water column because of the relative calm of the tide pools.

We were able to spot many species directly and indirectly while wading in the tide pools over a two hour period. In order of incredibly subjective relative abundance:

Bay Barnacle
Common Periwinkle
Common Slipper Snail (alive and empty shells)
Blue Muscle (alive and empty shells)Atlantic Razor Clam (all empty shells)
Large Clawed Hermit Crab
Sea squirt (species?)

Orange Sheath Tunicate
Unidentified Bait Fish
Atlantic Rock Crab (various body parts)
Golden Star Tunicate
Red sand worm? (burrows)
European Green CrabSea Lettuce
Asian Shore Crab
Unidentified Urchin: possibly Green Sea Urchin (just the shell)
Northern Lobster
Unidentified Whelk: possibly Atlantic Oyster Drill, New England Dog Whelk

Bay Barnacles were on all the rocky surfaces but not on the bottom of the tide pools because they were mostly sandy. There were more barnacles on the rocks which were more exposed to the open ocean at low tide. These barnacles were also open compared to those in the tide pools, probably because they are always covered in water and can feed and do not have to worry about exposure to the air. The Common Periwinkles were everywhere as well. Some above low tide line on rocks and some below low tide line on any surface, but more commonly on hard surfaces rather than sandy bottom. Lady slippers where also very abundant. I would guess that I saw more empty shells than live specimens. Those that were alive were found on rocks and often layers 3 or 4 on top of each other. There were many empty Blue Muscle shells all around the tide pools. Live Blue Muscles were only on the rock barriers surrounding the tide pools and on the occasional isolated boulder oasis in the middle of a tide pool. The only evidence of Atlantic Razor Clams were their empty shells. The Audubon guide says that they live buried below low tide line so we would have to dig to try and fine any live organisms. Large Clawed Hermit Crabs were seen on the sandy bottoms of the tide pools almost exclusively in Periwinkle shells. The warmer tide pools had noticeably more dead hermit crabs floating around. On the rocky outcrops with the Blue Muscles were plenty of Sea Squirts. I am unable to identify the exact species, even numerous recent articles about their spread and abundance in the area simply refer to them as sea squirts. Perhaps this particular kind is an invasive species? Orange Sheath and Golden Star Tunicates were found exclusively on the rocky oases with the Blue Muscles and Sea Squirts, as well as the few examples of Sea Lettuce in the tide pools. More Sheath and less Golden Star than at the Barking Crab. There were some small schools of bait fish swimming around but I was unable to catch one in order to get a closer look. Plenty of Atlantic Rock Crab abdomens littered the tide pools, and their pincers were strewn just outside the tide pools into the open ocean. My hypothesis is that the crabs are taken here by shore birds where they can drop them on the exposed rocks to kill them and open their shells. However this does not explain why their abdomens are in the tide pool and pincers are outside of it. This cannot be coincidence. We saw quite a few burrows in the sand that looked like Play Doh that had been squeezed out of a tube. They varied from an inch to three inches in diameter. We tried to dig underneath the piles to see what created them but found nothing. Ranger Tim thought they might be Red Sand Worms so without further evidence I am going to trust him on this one. A few Asian Shore and European Green Crabs were spotted by the class. Both are invasive species, unlike the native Atlantic Rock Crab. There were less individuals of both these species seen than the Atlantic Rock Crab but only the Rock Crab’s body parts were found in the tide pool suggesting that the two invasive crabs are harder for predators to catch or are passed over for a more tasty Rock Crab. This is not a good sign. We found one urchin body near a rock wall. It was about an inch and a half in diameter and shaped like a donut that has been squished on one face. It was ribbed with little white bumps and had a small hole in the top. Based on pictures found on Google images and the description in the Audubon guide I am going to guess that it was a Green Sea Urchin. Ted found a juvenile Northern Lobster, perhaps 4 inches long, not quite a keeper yet. I also located one different kind of snail but released it before we had a chance to show Bruce. It was about 1 to 1.5 inches, white, spiral shaped, and linear to a point at the end. We could not get the snail to come out of its shell to see the body. Based on pictures in the Audubon guide the snail could have either been an Atlantic Oyster Drill or a New England Dog Whelk. It looked almost identical to the picture of the New England Dog Whelk but according to the guide they only grow to ¾ of an inch and prefer a sandy or muddy bottom. The individual we found was larger than that and was on a large rock. The mystery continues.

Parts of the tide pool looked remarkably similar to what we saw at the Barking Crab and other parts were completely different. Those few scattered rocks that were in the middle of the tide pools looked almost identical to the floats at the Barking Crab. They both had Blue Muscles, Sea Lettuce, Golden Star Tunicates, Sea Squirts, and barnacles. The arrangement of and relationship between the species looked the same in both locations. However, the rest of the tide pools were a completely different habitat. The sandy bottom was covered in hermit crabs and Periwinkles. I hypothesize that the differences occur because the sandy bottom does not provide any hard substrate for those species found at the Barking Crab and on the rocky oases to grow on. There was no visible bottom at the Barking Crab and it would have most likely been much different than the bottom of the tide pool because it was much deeper. Another contributing factor could have been the salinity of the water since the Barking Crab was brackish and the tide pools were sea water. I think this probably played a small roll but based on how similar some parts of the tide pools looked I suspect that the main determining factor is suitable hard substrate.

Now let’s talk about snails! First off, in regards to Bruce’s question about the holes in some periwinkle, lady slipper, and clam shells: They are formed by whelks, a large group of marine gastropods. The whelks (possibly the one we saw?) spend days drilling through the shell of their prey with a part of their mouth called a radula. They then stick their proboscis through the hole and inject digestive enzymes into the animal and then eat the liquefied soft tissue (http://www.edc.uri.edu/restoration/html/gallery/invert/new.htm). Yummy!

The Common Periwinkles we saw were about ¼ to ½ inch wide, varying patterns of brownish colors (probably better to camouflage with the rocks they live on), had brown bodies, lived in the intertidal and subtidal zones, and feed on algae. The Grove snails were larger (up to an inch); had various colors including brown, yellow, grey, and red, had brown and white swirl patterns on their shells, had a lighter brown/grey body, live on land, breathe air, and eats terrestrial vegetation. Both species of snails were introduced to North America from Europe. Considering that Periwinkles are a marine snail and Grove snails are terrestrial I do not think that there is much of a relationship between the two. They do not share a habitat or food source. My only hypothesis for an indirect interaction between them is that if the Periwinkles turn marshes and sandy beaches into cobble beaches over the years then the terrestrial vegetation available to the grove snails decreases slightly. However, most of the Grove snails we saw were a good ways inland and were primarily on trees, which would not be growing in a sandy beach or marsh even if Periwinkles weren’t there. So why were there five Grove snail shells on that rock down on the beach? I do not think that the snails went there themselves. All the other individuals we found were on vegetation, not exposed and a few meters across sand from the nearest tree. The fact that the shells were empty is also telling. Something or someone brought the shells to that rock. If it were a bird or other predator I would have suspected the shells to be broken or cracked from the animal trying to get at the meat. My hypothesis is that some visitor to the park collected the snail shells throughout their stay and placed them on that rock for some reason, perhaps while they went tide pooling for Periwinkles. I know Bruce said that this phenomenon was not caused by humans but it would not be the first time he misled us for the purpose of an exploratory blog post. I’m going with my gut here on this one.

-Alex Beach

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