Nature Cruise and Mudflat

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Nature Cruise and Mudflat

Lab report by Shannon Modla

Introduction:
During the Devonian period (400-360 mya), the eastern coast of Maine collided with Europe to form the Acadian Mountain chain and the super-continent Pangea. Magma rose through the earth’s crust producing diorite and plutonic igneous rock, known as Cadillac granite, which now constitutes the majority of Mount Desert Island and the Porcupine Islands (APN, 2001). Approximately 100 million years later, the Atlantic Ocean opened, placing the coast of Maine on the edge of a new shoreline.

The last ice age further aided in the sculpting and formation of Mount Desert Island and the surrounding Porcupine Islands (APN, 2001). Glaciers, whose thickness ranged from 3,000 to 9,000 feet deep, carved parallel, U-shaped valleys between the mountain ranges (APN, 2001). Abrasion from the movement of glaciers and sediment caused the north-facing slopes to become smooth and rounded. The south-facing slopes experienced plucking, forming steep, jagged cliffs and sheer rock sides.

As temperatures increased, the ice receded and sea levels rose. The ocean filled low-lying land at a rate of two inches per one hundred years. Smaller mountains surrounding Mount Desert Island were separated from the mainland by the rising waterline, forming the Porcupine Islands and other small islands (APN, 2001). With the formation of Mount Desert Island and the surrounding islands, new marine habitats such as rocky intertidal zones, sandy beaches, and mudflats became available.

Mudflats possess several unique characteristics that separate them from other marine communities. First, they occur only in well-protected estuaries experiencing minimal wave action. In low energy areas, fine particles settle out of the water column, forming a soft, silt substrate. Second, mudflats are gradual sloping habitats possessing little grade. Third, they are often anoxic under the first few millimeters depth. Therefore, organisms distributed within the mud require access to the surface for oxygen. Bacteria greatly add to the process of decomposition while chemosynthetic bacteria break down hydrogen sulfide in the anaerobic layer. Fourth, mudflats are located subtidally and are almost always moist even when exposed. The fine particles that make up the substrate hold water in the interstitial spaces by capillary action. As a result, desiccation is minimized and organisms experience more moderate environmental conditions. Finally, unlike other tidal habitats, mudflats lack obvious vertical zonation (Bertness, 1999). Zonation in other habitats, such as the rocky intertidal zone, is primarily maintained by environmental tolerances, predation, and competition. In the mudflat, abiotic factors vary little across habitat width, and organisms in the mud are protected from fluctuations in water availability, salinity, and temperature. Competition for space is minimized because organisms are distributed at different depths within the layers of mud (Bertness, 1999).

Primary production on a mudflat is derived from microscopic and macroscopic algae. Diatoms and microscopic algae occupy the surface aerobic mud layer while macroscopic algae are found scattered along the surface, fastened to shells, tubeworm burrows, and other firm substrates. Most organisms feed on detritus, which is brought in with the tide and distributed throughout the water and sediment. Animals inhabiting the mudflat are mainly infauna deposit feeders that ingest and process sediment and digest organic material or filter feeders who obtain pelagic plankton from the water column (Castro, 2000). Deposit feeders usually outnumber filter feeders because the fine mud particulates often obstruct the filtering apparatuses of these organisms (Castro, 2000). Although not as abundant, epifauna are also found roaming on the surface of the mudflat (Castro, 2000).

The purpose of this study was twofold. First, a species list of animals inhabiting the islands surrounding Mount Desert Island, Maine was generated. Second, organisms from a local mudflat were identified and their location on the mudflat was recorded.

Materials and Methods:
Nature Cruise
A guided tour aboard the Acadian Nature Cruises was conducted. Species lists were generated with the aid of a professional naturalist and species identification books.

Mudflat
Teams used shovels to obtain organisms and species identification books to generate a list of animals found within or on a mudflat on Mount Desert Island, ME. Organisms were classified as algae, roamers, burrowers, benthic, or parasitic depending upon their location in the mudflat.

Results:
Nature Cruise
The nature cruise traveled out Frenchman’s Bay and around Bald Porcupine Island, Egg Rock Light House, Iron Bound Island, Jordon Island, Yellow Island, Long Porcupine Island, Burnt Porcupine Island, and Sheep Porcupine Island. Harbor Seals and Northern Gray Seals with pups and a variety of marine birds were observed off Egg Rock Light House (Table 1). Bald eagles or bald eagle nests were found on Iron Bound Island, Long Porcupine Island, and Sheep Porcupine Island. An active osprey nest was located on Yellow Island. Along Yellow Island, volcanic activity pushed an ancient sea floor composed of sedimentary rock to the surface, exposing it to erosion. Black guillemots utilized the cracks and fissures to lay pear shaped eggs designed to resist rolling off the cliffs. A total of twelve bird species were observed and identified, as demonstrated by Table 1.

Mudflat
The mudflat visited consisted of sediments with larger grain size, giving the substrate a sandy texture. As a result, most organisms identified, such as the common sand dollar, Northern rock barnacle, blue mussel, and sand shrimp were atypical of true mudflats (Table 2). Of the eighteen species identified, the majority of organisms were roamers on the surface of the mud (Table 2). Only one burrowing deposit feeder was identified, which was found in the low-lying areas of the mudflat. The remaining species were filter feeders, raptorial predators, herbivores, parasites, or scavengers, suggesting that detritus was not heavily relied upon as a food source. Filter feeders included the soft shell clam, blue mussel, and Northern rock barnacle. The most obvious source of primary production appeared to come from three species of macroalgae: spiral rockweed, rockweed, and sea lettuce (Table 2).

Discussion:
Nature Cruise
The rich diversity of species identified suggests that the steep cliffs and ledges on the islands surrounding Mount Desert Island provide numerous habitats and refuges to accommodate animals and their young. Ospreys, bald eagles, and black guillemots were observed nesting on many of the evergreen trees and plucked rock edges. The animals probably became adapted to or took advantage of the glacially carved landscape and used the geological features to enhance their fitness. By laying eggs among natural cracks and fissures, birds increase the survival rate of their young by keeping them out of reach from many predators. Seals also utilized the rocky landscape to rear pups and to rest.

Other methods to increase reproductive fitness were observed among male harbor seals, bald eagles and ospreys. Male harbor seals appeared to take on a reddish colored head during mating season, and they urinated on themselves to appear more attractive toward females. By attracting more females, the males would have enhanced parity, allowing them to reproduce more frequently and increase their chances of successful offspring. Next, bald eagles would lay two eggs in their nest. When the eggs hatched, sibling competition eliminated one chick, ensuring that only the strongest chick survived. Therefore, the offspring would be more likely to survive to adulthood. Similar to harbor seals, ospreys also participated in mating rituals to appear more attractive toward the opposite sex. Both partners would add sticks to their nest in a courtship ritual. The resulting nest may exceed over one thousand pounds.

Mudflat
The sandy texture of the substrate, the presence of atypical organisms and tidal riffles, and the lack of substantial burrowing deposit feeders in the substrate suggests that the mudflat was experiencing moderate levels of wave action. The wave action would prevent smaller particles from settling out in the water column. As a result, the substrate would consist of particles with larger grain size that would have larger interstitial spaces. Water and detritus would not be held by capillary action, and the detritus that did settle would be removed by the incoming tide. Therefore, burrowing deposit feeders would not have the food available to survive while suspension feeders could flourish because small, particulate silt would not block their filtering apparatuses. Herbivores of macroalgae, scavengers, and roaming predators would also do well in a sandier mudflat, as was observed from species lists (Table 2).

The sandy mudflat appeared to be undergoing a secondary successional process from a muddy, anoxic, low energy mudflat to a sandy, oxygenated, high energy sandy beach. The disturbance that initiated this temporal change in community structure may have been increased wave action due to increased grade or erosion. The change in structure may have also resulted from an autogenic facilitative process involving the burrowing deposit feeding organisms. According to Bertness (1999) the deposit feeders move sediments back into the water column and reintroduce them to the water by releasing castings on the surface. As a result, the suspended sediments are more likely to be washed away by abiotic factors. If this process continued uncheck over time, it is possible that the deposit feeders transformed the mudflat into a sandy environment. By modifying the mudflat, the burrowing deposit feeders would be replaced by organisms better adapted to sandy beach.

Additionally, if the mudflat continued to be transformed into a sandier habitat, vertical zonation would likely appear due to environmental factors and increasing competition. Since larger grain sizes hold less water, desiccation would become a problem for many organisms in areas far from shoreline. Finally, the increasing ratio of surface roamers to burrowers would elevate competition for space, further defining zones specific to particular species.

Bibliography:
American Park Network. “Acadia Geology”. 2001.
<http://www.americanparknetwork.com/parkinfo/ac/geology/>.

Berntness, M. D. The Ecology of Atlantic Shorelines. Sinauer Associates, Inc.: Sunderland, 1999.

Castro, H. and Huber, M. E., Marine Biology, 3rd ed. McGraw Hill: Boston, 2000.

Martinez, A. J. Marine Life of the North Atlantic. Down East Books: Camden, 1994.