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Introduction
I. Beaches, Waves, and Longshore Drift
Most people would define a beach as a ribbon of sand between the land (where beach sand is no longer visible) and the current limits of the ocean (usually high-tide, but some will include low tide). Most don't consider a rocky shore to be a beach, instead referring to this as, well, a rocky shore.
A beach is the interface between the land and ocean, and can include sand, coarse sand, gravel, etc. It is a dynamic zone than can change overnight from a single storm, seasonally with a net loss of beach material during winter and a net gain during summer, and does change over many years. FIGURE 1 (below) shows a typical beach profile. A beach (shore) is divided into the foreshore and backshore, with the foreshore extending from the Mean High Water (MHW) mark to the Mean Low Water (MLW) mark. The backshore is that part of a sandy beach that is only washed by waves during severe storms. This is that part of the beach where umbrellas and beach towels are safe from high-tide (usually).
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FIGURE 1: Typical Beach Profile
From: USACE Glossary Of Coastal Terminology
The nearshore includes the surf zone and extends beyond the area where waves will begin to break, usually to a depth of ~20 meters. The remaining part of "the beach" people are familiar with are the dunes located "behind" the beach, usually with what is called "beach grass". This area (see FIGURE 1 above) begins at the dune scarp, the nearly vertical rise caused by wave erosion, and extends landward for some distance. This area can be extensive, dotted with "salt water ponds", or it may not exist at all depending on geography or human development. A mix of dunes, bogs, and marshes exist in many locations along the New England coast and are often traversed by an elevated "board walk" connecting a home or public access point to the backshore.
There are two other beach profiles, variations of that in FIGURE 1, found on the Cape and Islands:
Waves seldom approach a beach head-on. Instead, waves typically strike the beach at some oblique angle. The wave's energy is absorbed by the beach along the length of the wave, which means that one part of the wave's energy is absorbed before the next part, and so on down the beach. This results in the wave being refracted (bent) towards, but never quite parallel to, the shoreline. As a consequence, there is a net movement of water (and suspended particulates) parallel to the shoreline in the direction of the wave - the longshore current. As the wave swashes up the beach, it does so at its original angle to the shoreline, but as the water looses its energy to the beach and gravity takes over, the backwash flows down nearly perpundicular to the shoreline. As in the case of the longshore current, this too results in a net movement of sand parallel to the shoreline in the direction of the wave.
IMAGE 2 (below) shows the result of this beach-wave interaction: a net movement of both sand and water along the beach in the direction of the wave. It is important to note the idea of the overall system of inputs to the beach being in balance with losses to down-current locations and to deeper water.
FIGURE 2: Longshore Drift
From: UCSB Web Site
FIGURE 3: Longshore Drift Landforms
From: NC State University Web Site
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Many coastal landforms are the result of the processes depicted in FIGURE 2 above. FIGURE 3 (left) shows the natural coastal landforms resulting from longshore drift.
Our efforts to inhibit consequences of this natural process often results in an intensification of the original problem being solved, or in an equally undesirable outcome. Thus, we are presented with a new problem to solve.
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In order to stop the unwanted effects of this phenomena, for example the closing of a bay or channel from a baymouth bar reaching the opposite headland (see FIGURE 3 above), structures are put in place to interrupt the longshore current. IMAGE 1 (below) is an aerial view of the jetty constructed at the East opening of the Cape Cod Canal in Cape Cod Bay.
IMAGE 1: Cape Cod Canal East Entrance
IMAGE 1A: East Entrance Profile
Stephen C. Daukas, Google Maps
Stephen C. Daukas, Google Maps
The longshore current runs from the top of IMAGE 1 to the bottom (NW to SE). The effects of the jetty are easily seen. As the longshore current approaches the jetty, it slows as it works its way around the obstacle. A slower moving current is said to be less competent, meaning it can't carry the same suspended load of sediment as it once could, so it drops some of it on the proximal (up current) side of the jetty. Similarly, beach drift encounters the barrier and the drift accumulates. This is also referrred to as beach nourishment.
The down-current jetty is showing the opposite effect - beach starvation. The current will take up whatever its competency will allow and carry this material farther down current. IMAGE 1A (above right) highlights how the shoreline has been modified by the jetty. The portion of the shoreline at this location is the rather inaccurately named Sandcatcher Recreation Area.
II. Marine Communities
Plankton, Nekton, & Benthic
Nektonic communities are the free-swimming organisms including fish and squid. Benthic communities are located at or near the bottom. and include such bottom dwellers as clams and lobsters.
Plankton are are microscopic plants (photoplankton) and animals (called zooplankton) that live in the uppermost layer of the water column and are carried by currents. Plankton is from the Greek "planktos" meaning "to drift" or "drifting". While some plankton have limited abilities for movement, these drifters are unable to move against a current or tide.
Photoplankton are plant-like photosynthetic organisms that use CO2, H2O, and sunlight to photosynthesize, that is, to produce carbohydrate (sugars). These plankton must remain in the photic zone of the water column in order to capture the sunlight necessary for photosynthesis. Photoplankton are producers forming the foundation of the food chain supplying energy to the consumers that eat them.
Zooplankton are animal-like organisms that include copepods (crustaceans: lobsters, crabs, shrimps, barnacles - phylum Arthropoda inclusive of spiders and insects) and the larval form of other marine organisms. Zooplankton feed on the photoplankton. Many zooplankton have small hair-like cells (cilia) that beat in a wave-like motion to allow the animal to move while others move using a tail (flagella) that whips side-to-side. Still others move by alternately contracting and relaxing their bowl or bell-shaped body (medusae), and the copepods use their appendages to swim through the water column.
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Paralytic Shellfish Poison (PSP) or Red Tide, a common occurance that wreaks havock with the local shellfish industry, is a zooplankton (dinoflagellate) population explosion or a bloom.
The zooplankton bloom results from a corresponding bloom in the zooplankton's food supply, the photoplankton, when the water conditions (temperature and nutrients) are optimal for reproduction. The nutrients are often provided as a byproduct of Human activity where phosphorus from lawn fertilizers or soap finds its way into the coastal waters. Some Dinoflagellate contain a toxin that is harmless to humans unless consumed in large amounts. Benthic organisms, for example clams, are filter feeders and ingest the Dinoflagellate in large amounts thereby accumulating and concentrating the toxin in their bodies (bioamplification) to levels dangerous to Humans if consumed.
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IMAGE 2 - PSP Closures
MA Disivion of Marine Fisheries Web Site
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When samples taken from shallow marine water (photic zone) are examined under a hand lense or larger magnifying glass, it is sometime possible to see planktonic organisms. Several examples of planktonic organisms are presented below:
Photoplankton - Diatoms
IMAGES 2 - 7 are examples of Diatoms, small algae that have a silica / quartz skeleton. Diatoms can be found in the fossil record going back to the cretaceous with some rocks (e.g., the White Cliffs of Dover on the south coast of England) composed almost entirely of these skeletal remains. These are the most abundant photoplankton in the middle and high latitude oceans.
IMAGE 3 -
Diatoms
SCD
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IMAGE 4 -
Cosinodiscus
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IMAGE 5 -
Triceratium
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IMAGE 6 -
Bascillaria
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IMAGE 7 -
Thalassionema
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IMAGE 8 -
Chaetoceros
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Photoplankton - Dinoflagellates
IMAGES 9 - 14 are examples of Dinoflagellates, small single-celll organisms often referred to as algae. About half of the species have an "armored" shell, other produce toxins, and some are parasites.
IMAGE 9 -
Peridinium
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IMAGE 10 -
Gymnodinium
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IMAGE 11 -
Noctiluca
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IMAGE 12 -
Dinophysis
SCD
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IMAGE 13 -
Ceratium
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IMAGE 14 -
Gonyaulax
SCD
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Photoplankton - Radiolaria &smp; Foraminifera
IMAGES 15 - 18 are examples of Radiolaria & Foraminifers. Foraminifera are small organisms, with silicone-based skeletons and spines, that the are decomposers. They make up much of the sea-floor material when they sink to the ocean bottom. Foraminiferans are made of calcium carbonate or silica and can be an indictaor of oil-bearing rocks.
IMAGE 15 -
Acanthonia
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IMAGE 16 -
Radiolarians
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IMAGE 17 -
Globigerina
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IMAGE 18 -
Tintinnid
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Zooplankton - Holoplankton & Meroplankton
IMAGES 19 - 29 are examples of Zooplankton. IMAGES 19 - 24 are Holoplankton, marine organisms that remain as plankton for their entire lifecycle. IMAGES 25- 29 are Meroplankton, organsims that are planktonic for the early stage of their lifecycle, but mature into benthic or nektonic organisms.
IMAGE 19 -
Amphipod
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IMAGE 20 -
Copepod
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IMAGE 21 -
Shrimp
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IMAGE 22 -
Physophora
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IMAGE 23 -
Man-o-War
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IMAGE 24 -
Rotifer
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IMAGE 25 -
Crab
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IMAGE 26 -
Fish Larva
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IMAGE 27 -
Sea Star
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IMAGE 28 -
Barnacle
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IMAGE 29 -
Sea Urchin
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III. Intertidal Zonation
There are five different layers, or zones, of life on a typical beach. These zones become noticeable upon examination of the different organisms found at different elevations relative to the water. At low tide, certian organisms can be seen exposed to air and sunlight, but others are not able to survive in this environment and so are only seen in the water.
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The Supralittoral Zone is where salt-water spray provides the only moisture above the waterline. The easiest way to identify this zone is to look for a tiny black snail - the periwinkle. The periwinkle will consume the algae growing on the moist surface of rocks and can attach to rocks as a way of sealing-in moisture during low tides when spray is not available.
The Supralittoral Fringe is where organisms have to be adapted for life both in and out of the water. This narrow zone lies between the normal high-tide mark and the spring high-tide mark, is submerged and exposed based on the tides, and is where wave action is present. The easiest way to identify this zone is to look for barnacles (a crustacean) affixed to rocks or other solid surface with one of the strongest adhesives known.
The Midlittoral Zone lies between the normal high-tide mark and the normal low-tide mark. The easiest way to identify this zone is to look for mussels (a bivalve mollusk) affixed to rocks or other solid surface with a fibrous "root". The mussels and barnacles compete for territory with the barnacles occupying the higher elevation as they are better adapted to exposure to air and sun.
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FIGURE 4 - Intertidal Zonation
Modified From: University of Haifa Web Site
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The Infralittoral Fringe lies between the normal low-tide mark and the spring low-tide mark. This area is almost always submerged, so there is an abundance of life. The easiest way to identify this zone is to look for many different types of species, including crabs and algae.
The Infralittoral Zone lies below water at all times and where you will find fish and invertebrates.
IV. Tides & Canal Currents
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FIGURE 5
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Tide Height
Very Low <---------> Very High
FIGURE 5A
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Tides are caused by the gravitational attractions of the Earth-Moon-Sun system. FIGURE 5 & FIGURE 5A (left) show the effect of the relative positions of the Earth. Moon, and Sun in their orbits.
Frame #1 (FIGURE 5 top) shows a Spring Tide caused by the combined gravitational attraction of the Moon and Sun. In this configuration, the ocean is being pulled toward the Sun and toward the Moon causing the ocean to bulge. Locations on the Earth in-line with the Earth-Moon-Sun axis will experience very high tides while location at 90° to that axis will experience very low tides.
Frame #2 shows a Neap Tide. In this configuration, the ocean is being pulled toward a point between the Sun and the Moon (45°) reducing the bulge resulting in lower than normal high tides, and higher than normal low tides.
Frame #3 shows an Astronomical Spring Tide. In this configuration, the ocean is being pulled more strongly toward the Sun and the Moon resulting in the highest-high tides, and the lowest-low tides.
Frame #4 shows a Neap Tide similar to that of Frame 2, and Frame #5 begins the cycle again (same as Frame #1).
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Tides have an especially interesting effect on the Cape Cod Canal. Because there is a 3-hour difference between the arrival of high and low tides in the two bays, a rising tide takes place in Buzzards Bay during a falling tide in Cape Cod Bay and a rising tide in Cape Cod takes place during a falling tide in Buzzards Bay. As a result, the current through the canal alternates direction, moving from high-water in one bay to low-water in the otherabout every six hours.
[Click Image to Enlarge]
FIGURE 6 - Current Navigation Chart
From: IRBS Web Site
Summary
The background information presented here covers the basics that will help you get more out of the field trip. We covered beach profiles, longshore current and drift. We next reviewed the zonation of the intertidal zone, and touched on the effect of tides on the currents of the Cape Cod Canal.
What Next?
During the field trip, additional information detailing the East Entrance to the Cape Cod Canal will be provided.
Continue on to the
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