Part
2, Sec. 2B2b
Sanctuary
Resources - Natural Resources
Zooplankton
b.
Zooplankton
The
vast majority of zooplankton species are endemic in origin,
and reproduce with sufficient frequency to maintain a local
stock within the boundaries of the Gulf of Maine. The coastal
zone (shallower than 100 m) is generally more productive for
zooplankton (particularly for Calanus) than the central
basin area. Deeper waters of the Gulf are important reproduction
areas for Pseudocalanus and Oithona.
The
total amount of zooplankton biomass is usually greater in deeper
waters (greater than 100 m); though more concentrated in shallower
waters. Massachusetts Bay is highly productive for copepods
and pelagic fish eggs. Smaller copepods are found in greater
concentrations in shallow waters; larger species in deeper waters.
Zooplankton densities are greater in stratified western coastal
areas than along the turbulent eastern coast of the Gulf.
Although
the Gulf of Maine is rich in zooplankton species (more than
160 identified), fauna are dominated (over 80%) by only three
or four species. Fauna are dominated by crustaceans, primarily
copepods (and most prominently, Calanus finmarchicus).
An exception to this dominance occurs nearshore in the spring
when barnacle nauplii, or occasionally euphausids, ctenophores
and other zooplankters, may swarm locally (Fish and Johnson,
1937).
Other
less numerous species of the Calanus community include
the copepods, Pseudocalanus minutus and Metridia
lucens. Other, less abundant species include the chaetognath,
Sagitta elegans; the amphipod genus Euthemisto;
and euphausid genera Thysanoessa and Meganyctiphanes;
and the cetenophore, Pleurobrachia pileus (Cohen,
1975). The abundance of all zooplankton forms is greater in
the western coastal sector than in the eastern coastal sector.
The
two Calanus species, C. finmarchicus and
Pseudocalanus minutus account for more than 70%
of the zooplankton biomass in winter, spring and summer. Three
species -- Pseudocalanus minutus, Temora
longicornis, and Centropages typicus --
compose 85% of the zooplankton biomass during the autumn months
in the Gulf's coastal waters.
During
summer months, three major groups of zooplankton can be identified
based on their seasonal vertical distribution. The surface layer
contains small, young forms of copepod nauplii, copepodites,
fish eggs, fish larvae and small copepods. The second layer
contains the boreal Calanus community, which occurs generally
in mid-depths above 100-150 meters, but below the surface. The
third zooplankter group occurs in deeper waters of the Gulf,
and is characterized by the giant copepod, Euchaeta norvegica.
Also included, in lesser amounts, are the chaetognaths Eukrohnia
and Sagitta lyra; the decapod shrimp, Pasiphaea
and Meganyctiphanes norvegica.
Differences
among these three communities are most pronounced in the summer
when waters over the deep basins and in the western Gulf are
markedly stratified. Differences are least apparent in well-mixed
waters, i.e., shallow areas of heavy tidal mixing, and throughout
the Gulf during winter and spring. In general, copepod densities
are greatest at deeper levels in the nearshore areas of the
Gulf (Sherman, 1976).
Gulf
of Maine zooplankton generally may be divided into two fundamental
ecological subsets -- neritic and oceanic -- depending upon
their degree of dependence on shallow, food-rich coastal zone
waters. Water depth, in fact, is the single most important parameter
influencing the distribution of zooplankton in the Gulf of Maine
(Sherman, 1976). Typical neritic zooplankton are larval stages
of various benthic organisms, such as barnacles, worms, bivalve
and gastropod mollusks, decapod crustaceans, and echinoderms.
Also included in this group are pelagic eggs and larvae of all
fish species that spawn in shallow waters. Oceanic zooplankton
are pelagic throughout their life, and show no particular dependence
on coastal areas. Neritic organisms are rarely encountered outside
the 100-meter (328 ft.) isobath.
Zooplankton
do not pass through the seasonal pattern of succession as phytoplankton
species; rather, zooplankton stay qualitatively the same throughout
the year, while experiencing quantitative changes in total biomass.
Zooplankton begin spring increases along coastal waters of Massachusetts
Bay in waters north of Cape Ann sometime during March, evidenced
by copepod larvae. Copepod (primarily Calanus finmarchicus)
production expands seaward toward the mouth of the Bay during
late April, and continues over the southwestern Gulf of Maine
as spring season progresses. Peak zooplankton production occurs
by the end of May. Rapid decrease in zooplankton abundance is
evidenced in June. In the Gulf of Maine there is a gradual decline
in zooplankton biomass from spring (i.e., June) to fall.
Hydrographic
factors in the Gulf of Maine control the production, dispersal
and survival of zooplankton (Fish and Johnson, 1937). For instance,
water temperature dictates community type; and the horizontal
distribution of zooplankton is controlled by water circulation,
stability, and occasionally salinity. The dominant counter-clockwise
circulation pattern in the Gulf of Maine moves all plankton,
copepod eggs, and larvae in a southwestward direction (unless
they are situated in areas protected from these circulation
patterns).
Due
to the relatively "closed" nature of the Gulf of Maine, temporal
and spatial changes in zooplankton quantities are primarily
the result of reproduction, growth, and mortality of endemic
species, such as Calanus, Pseudocalanus, Oithona,
and Microstella.
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