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Plankton is a term used for any living thing that reside in the water column and are at the mercy of the currents and tides. There are many types of plankton. Phytoplankton includes the unicellular plants and zooplankton denotes the animal plankton of all sizes. Ichthyoplankton is the zooplankton component comprising fish eggs and larvae. Nanoplankton denote the very tiny microscopic organisms, including bacteria. |
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One of the most important species of zooplankton we see in the Gulf of Maine is Calanus finmarchicus, a tiny crustacean called a copepod. "Copepod" means "oar foot" in Latin which describes how these organisms move through the water. Copepods are the most abundant multicellular organisms on the planet and are in fresh as well as saltwater. The figures to the left are some examples of copepods common to the Gulf of Maine: top is a close-up (20x magnification) of a Calanus finmarchicus adult female with some other surrounding copepods and the bottom is a photo of a sample taken from our off-shore New Scantum station a 2x magnification. The largest animals are the Calanus finmarchicus, which represents how abundant they are in a sample from August. One of the distinctive behaviors of Calanus is the diapause phase of its life cycle (The dark blue curved arrow top left in the following diagram). Once the Calanus goes through its 6 naupliar stages and 5 copepodid stages it has stored up enough fatty lipids to allow it to sink down deep (a common place is Wilkinson Basin) and go into a resting state or hibernation. This hibernation period occurs during the colder months where food is a limiting factor. Then after some trigger in the spring, they rise up from the depths, molt to adult (Stage C6) and start reproducing.
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Dr. Jan Rines, of the University of Rhode Island, took this photo of a phytoplankton colony called Chaetocerous debilis. This is a beautiful representation of one species that can be found in temperate waters like the Gulf of Maine. Many of the phytoplankton cells/colonies that we identify are diatoms. Diatoms are single-celled plants that have a cell wall of a silica-like substance. These cells are like plants in that they need sunlight and nutrients to grow. They are most abundant in the spring and fall months duing periods called "blooms". They do really well during these months because of the increased nutrients in the water from the winter mixing up of deeper nutrient rich waters as well as the input from land runoff. Furthermore, they are kept up in the zone of sunlight (photic layer) by the thermal stratification that has developed from the increased warming and freshwater input. There has been exciting research showing that phytoplankton undergo
a succession of species during the spring bloom similar to the seasonal
succession of plants in pastures. For example, there are certain diatoms
that are the first to start developing once the light is strong enough,
then after they reach their nutrient limitation another specie will
take over in dominance and so on . We are collecting nutrient data from each station at 10 meter intervals
in order to examine the interactions between nutrient levels and phytoplankton
composition. |
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Ichthyoplankton is composed of fish eggs and larvae in
various stages of development. Pictures A-D on the left are examples:
A) American Plaice eggs within early stages of embryonic development;
B) Atlantic cod egg in a late stage of development just before hatch;
C) Atlantic cod larva in the act of hatching and D) newly hatched
Atlantic cod larva with yolk sac still attached.
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