origins of oceanic plankton in the middle atlantic bight
TRANSCRIPT
Estuarine and Coastal Marine Science (1979) 9, sag-527
Origins of Oceanic Plankton in the Middle Atlantic Bight”
James Cox Marine Science Institute, University of California at Santa Barbara, Santa Barbara, CA. 93106, U.S.A.
and Peter H. Wiebe Woods Hole Oceanographic Institution, Woods Hole, MA. 02543, U.S.A.
Received 9 June I978
Keywords: zooplankton; Gulf Stream; meandering; eddies; continental shelves; continental slopes; species composition; U.S.A. east coast
Expatriated species of zooplankton found in the mid-Atlantic Bight include Arctic-Boreal species derived from shelf waters northeast of Cape Cod, transition zone species from the adjoining Slope Water and tropical- subtropical species that commonly reside in the Gulf Stream, and Sargasso Sea. Introduction of expatriates is largely associated with the pattern of advective movements of water onto the shelf: Arctic-Boreal species are brought in from the northeast largely by over-shelf transport; transition zone species by Slope Water penetration at the surface when horizontal density gradients are minimal and at mid-depth in response to physical processes such as estuarine-type circulation, wind-driven upwelling, cold shelf water ‘bubble’ formation and movement out into the Slope Water or to shelf-Slope Water interactions associated with warm core rings; warm water species by injection of warm core ring surface water in over the shelf. There is little evidence that Carolinian species are introduced into the mid-Atlantic Bight directly around Cape Hatteras. In general, the occurrence of expatriate warm water species is more important in terms of species numbers and total biomass when compared to the occurrence of expatriate cold water species. The Bight region can be divided into three regions with regard to oceanic influences: (I) the band of low salinity water along the coast south of the mouth of the Hudson River, extending to the mouth of the Chesapeake; (2) the Continental Shelf edge extending from about 37 “30’N to 40 “N and extending shoreward towards the eastern half of the Long Island and Block Island Sound, but not including the region south- east of Cape Cod and Nantucket: (3) the southern sector, including the shelf edge south of 37 “N and extending landward south of Chesapeake Bay. Each of these regions is characterized by types of expatriate species and by hydrographic features.
A mechanism is postulated whereby warm water species which cannot withstand harsh winter conditions in the mid-Atlantic Bight can ‘overwinter’ by the movement offshore of adults or larvae in shelf water entrained at Cape Hatteras in late summer or early fall, by transit alongside or within the Gulf Stream, by incorporation into a warm core ring and by return to shelf waters in the spring when the ring impinges on the shelf margin. “Contribution no. 4164 from the Woods Hole Oceanographic Institution. This work was supported by the Office of Naval Research under contract Nooor4-74-Coz62, NRoQ-oo4.
509 0302~352&9/110509+19 $02.00/O 0 1979 Academic Press Inc. (London) Ltd.
510 J. Cox M P. H. Wiebe
Introduction
The Middle Atlantic Bight area is a region of complex water movements where diverse water types converge and mix over the shelf. The endemic shelf zooplankton fauna is com- prised of many holoplanktonic organisms normally considered to be coastal forms and many species such as hydroid medusae, scyphomedusae, and crustacean larvae which are mero- planktonic. This characteristically coastal plankton, however, is augmented by mixing with oceanic plankters carried in over the shelf where the latter may persist for varying lengths of time. These sporadic incursions of non-resident species are likely to have an appreciable impact on the structure and function of planktonic shelf communities, especially because of the large volumes of source waters transported into the region. Bigelow & Sears (1939) conservatively estimated that from 8% to 16% of total zooplankton displacement volume in Middle Atlantic Bight waters were due to expatriate species from offshore. Other published records have documented numerous intrusions of non-resident zooplankton (Moore, rgo3 ; Bigelow, 19x5, 1917, 1926; Wilson, 1932; Deevey, 1952, 1960; Colton & Temple, 1961: Colton et al., 1962; Grice & Hart, 1962; Van Engel & Eng-Chow Tan, 1965; Sherman & Shaner, 1968; Grant, 1977).
These intrusions have been generally attributed to a shoreward movement of oceanic waters, and the seasonal westerly drift over the Georges Bank region. Zoogeographers have traditionally considered the Gulf Stream to represent a discrete fauna1 boundary (Briggs, 1974). Ekman (1953, p. 136) concluded that the Gulf Stream influences coastal water ‘only to an inconsiderable extent’, although he stated that coastal water temperatures are ‘determined by a mixture of the marginal ramifications of the Gulf Stream and the cold coastal water from the north.’ Sherman & Shaner (1968) describe an oceanic group of pontellid copepods which were ‘carried over the southern margin of Georges Bank in the northward extension of the main axis of the Gulf Stream’.
Recent evidence suggests that the frequent occurrence of isolated warm core rings formed by shoreward meanders of the Gulf Stream (Saunders, 1971; Gotthardt, 1973) may provide a major mechanism whereby warm water oceanic species of zooplankton are carried across the Gulf Stream into Slope Water and ultimately mix into shelf waters. Rings may also promote exchange of shelf and Slope Water and thereby further affect zooplankton com- position in the shelf region. The existence of warm core rings has also been confirmed in the Kuroshio region off Japan (Sugiura, rg55a, b; Masuzawa, rg55a, b; Ichiye, 1955; Kawai, 1955; Kuroda, 1968). Warm core rings in both the Kuroshio and Gulf Stream areas possess certain similarities: they show similar physical structure and appear to form primarily between 38 ’ and 40 “N latitude.
Both the Northwestern Atlantic and Northwestern Pacific have Slope Water masses which are composed of a complex mixture of colder waters of northern origin in addition to the water derived from warm core rings. This Slope Water and the warm core water of the rings represent a consistently recurring source of zooplankton of diverse origins for the adjacent shelf waters. The shelf environments in these regions are, in effect, extreme examples of ‘wide open’ ecotones (McGowan, 1974) and are a challenging area of study for plankton biogeographers and ecologists.
The purpose of this paper is to review documented occurrences of oceanic and other non- indigenous zooplankton species in the Middle Atlantic Bight region and to use recent knowledge of hydrographic features, especially warm core rings, to explain the origins of the expatriated species. To aid in these interpretations we include data on zooplankton samples taken in warm core rings and their surroundings.
Origins of oceanic plankton 511
Defining expatriate and endemic species
Most plankton communities are subject to advective movements which can mix large numbers of individuals into a totally different water mass. In the new surroundings, the interplay of physical and biotic factors will work to eliminate these individuals, or permit their extended survival. Since these factors have a different effect on each species, there is probably great variability in survival capabilities. This is especially true for the Middle Atlantic Bight, where advective introduction of species from other areas is a common occur- rence. For the purposes of this discussion, we recognize five degrees of survival capability:
(I) Exotic species, those that die out immediately upon arrival in shelf waters. While these may have some value as indicators of water mass movement, they have little impact on zooplankton shelf communities.
(2) Species which persist in slowly diminishing numbers, but fail to breed and undergo gradual physiological degradation until elimination. These may mildly interact with shelf communities.
(3) Species which achieve individual and population growth during part of the year, but which fail to complete their annual cycle because of countervailing factors (e.g., unfavorable seasonal temperatures, unusual predation) which cause their elimination. These species are likely to interact significantly in community processes.
(4) Species which breed successfully and maintain healthy populations, but whose long term stability depends upon continued inocula from that portion of their home range where they are endemic. These species will be eliminated during occasional years of extreme conditions (e.g., unusually low salinities or high temperatures), but can be re-established when normal conditions are restored and new inocula are introduced.
(5) Endemic shelf fauna for which the shelf is part of the home range. These species never experience conditions extreme enough to cause total elimination during an annual cycle, although they may show variations in their range of occurrence along the shelf.
Exotic species (Category I) occur at irregular intervals along the margin of the continental shelf and show little or no penetration over the shelf. Species of class 2 show more regular occurrence and penetration, but lack signs of reproductive success. Species of class 3 may be identified by their complete absence during some part of the year, despite signs of in situ population growth before disappearance. We define the term expatriate species as any species belonging to the first three classes. Expatriate species in the Middle Atlantic Bight include Arctic-Boreal species derived from shelf waters northeast of Cape Cod, transition zone species from the Western Slope Water of the North Atlantic, and warm water species that commonly reside in the Gulf Stream and Sargasso Sea.
Physical mechanisms of transport
The shelf water of the Middle Atlantic Bight is a mixture of freshwater from the continent and more saline water from offshore. Generally, the salinities in the region are 35x, or less, defining the seaward limits of surface shelf water at the shelf/Slope Water boundary (Wright &Parker, 1976). The annual range of surface temperatures is approximately I 5 “C (Schroeder, 1966). A strong horizontal temperature gradient exists near the shelf break during the colder months of the year, so that the seaward limits of the shelf water can be defined by the position of surface temperature isotherms. During the warmer months, especially in late summer and early fall, the shelf water frontal position is difficult to detect from surface isotherms because of surface warming of the shelf water to Slope Water temperatures. Determining the
512 J. Cox @ P. H. Wiebe
position of the front at the surface during this period requires salinity measurement (Wright, 1976; Wright & Parker, 1976).
The shelf water boundary at the surface shows a constantly changing and convoluted profile with ‘bubbles’ of thin (80 to IOO m) lenses of shelf water being released into Slope Water (Bigelow, 1933; Cresswell, 1967; Moore, 1972; Ingham, 1974; Wright, 1976). Variations in the position of the surface boundary suggest that surface Slope Water (upper 200 m) can penetrate over the shelf, especially during the warm season when horizontal surface temperature gradients are negligible. The boundary at the sea floor closely follows the IOO m contour and apparently does not vary in position nearly as much as the surface boundary, which usually extends 40 to 80 km seaward as a shelf water bulge (Wright, 1976). Water in the shelf water bulge designated ‘shelf edge water’ (Wright & Parker, 1976) is
characterized by a higher salinity range (33*6-35x0) than the less saline (<33.6x,) shoreward ‘coastal water’ component and comprises nearly half of the total shelf water volume. There is evidence of a slow shoreward flux at mid-depths under the bulge and along the bottom further in from the edge of the shelf (Bumpus, 1973; Beardsley et al., 1976) with upward mixing into the fresher water outflow. Various processes on the outer zone of the continental shelf may contribute to this upward mixing of water, including estuarine circulation, wind- driven upwelling, and mixing due to internal waves impinging at the shelf break (Riley, 1975)
Documentation of shelf water mass transport is incomplete, although estimates of along- shore flow within the IOO m contour indicate that such flow is substantial, on the order of 8000 km3 year-l (Beardsley et al., 1976). Most of this flow can apparently be accounted for by transport from the northeast via the Gulf of Maine and Georges Bank regions. This flow is difficult to measure except by long term averaging, since current variance is high due to the strong influence of semidiurnal and diurnal tides, internal waves, storms, and interactions on the outer shelf with mesoscale processes occurring in Slope Water (Beardsley et al., 1976).
The extent to which shelf water and Slope Water exchange occurs is unknown. Calcula- tions by Wright (1977), based in part in salt balance, indicate that Slope Water input across the shelf margin may exceed the export of cold water ‘bubbles’ at the shelf margin (Table I). Beardsley et al. (1976) conclude that data are insufficient to estimate volume exchange, but that there is probably relatively little net flow in either direction, since they account for virtually all of the flow over the shelf on the basis of over-shelf input from the northeast. This
TABLE I. Estimated volume transport over Middle Atlantic Bight Shelf region.” The total volume of shelf water is 23146 km3 as defined by Wright and Parker (1976).
Transport process Volume transport (km” year-‘)
Off shelf On shelf
Entrainment of shelf water at Cape Hatterasb Export of cold water bubbles at Shelf margin’ Freshwater runoff’ Slope Water input across shelf margin
(salinity balance)” Overshelf from northea& transport
Totals
8 ooo 2 000-4 000
Iji
4 250 j 000 000 -7
10000-12000 9 4077-I2 407
“Analysis based on Wright (1977). bBeardsley, Boicourt & Hansen (1976). ‘Wright (I 976). “Bue (1970).
Origins of oceanic plankton 513
input of shelf water from the northeast apparently consists of Scotian Shelf water plus at least an equal volume of Slope Water with which it mixes in the Gulf of Maine (Brown & Beardsley, 1978).
A more important influence on the composition of Slope Water than shelf water ‘bubbles’ are warm core Gulf Stream rings. They achieve considerable size (ca. 18 ooo km2) and occur so frequently (5 to 8 annually) that they are a major part of the Slope Water lying off the shelf area of the Middle Atlantic Bight (Fuglister, 1972). At times, they may occupy more area of the Slope than the Slope Water itself, as judged by satellite imagery (Figure I).
rRROWS INDICATE ‘ERMANENT CURRENTS
SARGASSO
Figure I. Distribution of warm core rings in the Slope Water region during the week of I I May, 1977 based on the interpretation by personnel at the U.S. Naval Oceanographic Office of satellite photographs. Spring of 1977 was a period when a particularly large number of rings were formed and were present in the Slope Water. There can be periods when few or no rings are observable north of the Gulf Stream between 55 “W and 75 “W.
TABL
E 2.
Eup
haus
iids
iden
tifie
d fro
m
zoop
lank
ton
tow
s to
80
0 m
in
the
wes
tern
Sl
ope
Wat
er
and
Gul
f St
ream
; 0/
0 co
mpo
sitio
n by
sp
ecie
s of
tot
al
euph
ausi
id
coun
t
Stat
ions
Sl
ope
wat
er
Gul
f st
ream
R
ings
M
OC
-20
SL-6
M
OC
-39
GS-
4 G
S-3
WFO
-4,
5 w
so-I
SHEL
F”
Euph
ausi
a am
eric
ana
Euph
ausi
a br
evis
Eu
phau
sia
gibb
oide
s Eu
phau
sia
hem
igib
ba
Euph
ausi
a kr
ohni
i Eu
phau
sia
mu&
a Eu
phau
sia
tene
ra
Nem
atob
ranc
hion
bo
opis
N
emat
obra
nchi
on
sexs
pino
sus
Nem
atos
celis
at
lant
ica
Nem
atos
celis
m
egal
ops
Nem
atos
celis
m
icro
ps
Nem
atos
celis
te
nella
Styl
oche
iron
abbr
evia
tum
St
yloc
heiro
n af
ine
Styl
oche
iron
carin
atum
St
yloc
heiro
n el
onga
tum
St
yloc
heiro
n lo
ngic
orne
St
yloc
heiro
n su
hmii
Thys
anoe
ssa
greg
aria
Th
ysan
aess
a lo
ngic
auda
ta
Thys
anoe
ssa
parv
a Th
ysan
oess
a in
erm
is
Bent
heup
haus
ia
amby
lopi
0 0'02
0'02
0 1'2 0.03
c 0 0 670.
2 0 0 0.
03
0 0 0 0 0 29’9
0 0.
6 0 0
0'2 0 0 1-s
27’3
0.
8 27
.2
0 0
4’5
2.6
0.3
2'4 0 I.1
7.6
0 0
1.8
7’7
5.6
0 0
18.7
6.
9 20
.4
6.8
0
0'4
0 0
1’7
0
12.8
3'
1 14
.8
8.5
0
0 0'
02
0 0
25.1
0
2'1
1’9
1’7
0
21'2
23
.8
14.8
28
.8
0.8
2'0
0
0'1
0
0.8
2.1
0.4
0'7
26.2
0’
1 0
0
0.6
4'5
3'1
3'0
0’1
1'4
2.4
O’j
0 0'2 0.3
0.4
0 0 0’4
$6’
0 0
I.8
"3
"3
4'0
16.6
3'
6
9'5
1.6
37.8
5'7
12.5
4'
2 0
0.8
0
2.6
2'9
3'1
2'1
34.2
9.6 0
0’2
0 0 0
0
c
0 I'9
0 0
11'1
3'7
0 1.9
0 9.3
0 0
13'C
0 0 0 0
0 0
0 0
5’1
0
0 0.
6
3’4
0
0 0
1’7
13.6
i.5
1-7
I.7
3'4
I.7
5" 0
0
15.0
2.7
0
14.8
0
Thys
anop
odu
acut
ifron
s O
’j 0.
16
0‘2
0 0
0 0
0 Th
ysan
opod
a ae
qual
is
0 0.
35
0.6
0.9
1‘4
0 0
0 Th
ysan
opod
a nm
wca
ntha
0
0 2.
6 0
0 0
0 0
Thys
anop
oda
obtu
sifro
ns
0 0
0.2
0" 3
0
0 0
0 Th
ysan
opod
a t&
u&da
ta
0 0’
1 0
0 0.
6 0
0 0
Thys
nnop
oda
orie
nt&s
0
0 0
0 0
1’9
Cl
0
Meg
anyc
tipha
nes
norv
egic
a 0.
5 I.0
0
84
0 0
6:
4t ‘
0 In
divi
dual
s/ro
oo
m3
893
154
I12
352
I02
-
“Per
ce
nt
of
tota
l eu
phau
siid
ca
tch
(adu
lts)
aver
aged
fro
m
sixt
een
Shel
f st
atio
n (G
rice
& H
art,
1962
).
TABL
E 3.
Pe
r ce
nt
sim
ilarit
y an
alys
is”
of
30 s
peci
es
of
Euph
ausi
ids
from
80
0 m
zoo
plan
kton
to
ws
in t
he
wes
tern
Sl
ope
Wat
er
and
Gul
f St
ream
b,
incl
udin
g re
pres
enta
tive
Shel
f sa
mpl
es
for
com
paris
onC
MO
C-2
0 SL
-6
Moe
-39
WFO
4,
s
Com
posi
teC
(S
lope
) (S
lope
) (S
lope
) G
S-4
GS-
3 (W
arm
C
ore)
Sh
elf
Stat
ion
Wat
er
Wat
er
Wat
er
(Gul
f St
ream
) (G
ulf
Stre
am)
Rin
g
5’9
13.8
0.
82
0.80
5’
9 30
.8
17’3
4.6
13
0.28
0’
07
3’4
29
39
19
29
25
26
45
34
45
36
46
65
67
5r
42
49
WS
O-I
(War
m
Cor
e R
ing)
W
FO-4
, 5
GS-
3 G
S-4
MO
C-3
9 SL
-6
MO
C-2
0
“Whi
ttake
r &
Fairb
anks
(1
958)
. bM
OC
-20
was
tak
en
at a
pos
ition
w
ell
rem
oved
fro
m
the
Gul
f St
ream
an
d w
arm
co
re
rings
, as
judg
ed
by a
naly
sis
of
conc
urre
nt
sate
llite
imag
ery,
D
esig
natio
n of
wat
er
type
s at
eac
h st
atio
n w
as b
ased
on
tem
pera
ture
an
d sa
linity
pr
ofile
s.
‘Sta
tions
A,
B,
C
, an
d D
of
G
rice
& H
art
(196
2)
for
Mar
ch,
Dec
embe
r, Ju
ly,
and
Sept
embe
r w
ere
aver
aged
to
ob
tain
ov
eral
l o/
o co
mpo
sitio
n of
ad
ult
euph
ausi
ids
over
th
e Sh
elf
regi
on.
A to
tal
of
16 t
ows
are
incl
uded
in
the
co
mpo
site
of
th
e fo
ur
stat
ion
loca
tions
; pe
rcen
tage
s ap
pear
in
Ta
ble
2.
J. Cox &f P. H. Wiebe
Warm core rings appear to be formed primarily in the region of large Gulf Stream meanders east of 66 “W, and contain a core of Sargasso Sea water surrounded by an anti- cyclonic rotating ring of Gulf Stream water. They occupy the relatively narrow corridor between the Gulf Stream and the shelf/Slope Water boundary. The rings usually follow a westward path that brings them in contact with the edge of the shelf at about 40 “N, where they continue a southwesterly course along the shelf edge (Bisagni, 1976). Salinity and temperature sections of the rings and their environs (Saunders, 1971), and satellite imagery show that rings can entrain shelf water and transport it out over Slope Water (Figure I). Although direct measurement of shelfward transport are lacking, there is evidence that Slope Water and surface water from the ring can be injected in over the outer shelf, as judged from temperature sections (Morgan & Bishop, 1977) despite the lack of a surface manifesta- tion visible by satellite infrared imagery. Warm core rings in the Kuroshio clearly show warm water export at the surface in the form of ‘. . . thin and wide patches . . . transported dis- continuously from the boundary layers of the warm eddy to surrounding regions’ (Hata, 1975). This spreading is evident in the irregular outlines of warm core rings seen in satellite images of Gulf Stream warm core rings (Figure I), despite the much greater regularity of the underlying temperature and salinity structure (Bisagni, 1976).
Many warm core rings are absorbed by the Gulf Stream as they pass down the Slope Water corridor, often resulting in complex circulation patterns which can result in entrain- ment of Gulf Stream water close to the shelf/Slope Water boundary. This is potentially an additional mechanism for injection of warm oceanic waters over the shelf.
Sources of slope water and composition of slope water fauna
The western Slope Water, adjoining the Middle Atlantic Bight, is a mixture of Gulf Stream water, Sargasso Sea water, shelf water, and Labrador Coastal water. Surface western Slope Water composition is affected partially by the contributions of shelf water bubbles and warm core rings. These sources account for the shelf water and part of the Gulf Stream and Sargasso Sea water contributions to the western Slope Water. Measurements of the surface westerly drift at site D (39 ‘IO’N, 70 “W) (Webster, 1969) indicate that roughly half of western Slope Water originates to the east. Eastern Slope Water, according to McClelland (1957) is comprised of half Gulf Stream water and half Labrador Coastal water. Thus, well over one-quarter, and possibly as much as one-half or more of surface western Slope Water is of Gulf Stream or Sargasso Sea origin (Wright, 1977). The remainder originates from the Middle Atlantic shelf and Labrador coastal water. Deeper western Slope Water is comprised almost entirely of eastern Slope Water and Gulf Stream water (Wright, 1977).
This picture of the composition of surface western Slope Water leads to certain predictions regarding its zooplankton composition. Western Slope Water would be expected to show strong fauna1 affinities with eastern Slope Water, especially at greater depths, and at least some affinity with both Gulf Stream and coastal water faunas, especially at lesser depths.
Analysis of euphausiid species composition of warm core rings, of Slope Water in the vicinity of warm core rings, of Slope Water away from warm core rings, of Gulf Stream water, and of shelf water, indicates that Slope Water can have variable zooplankton com- position showing some affinity with each contributing water type (Tables 2 and 3 ; Figure 2). Greater similarity of the Gulf Stream fauna and that of warm core rings is shown in sample MOC-39, which was taken in Slope Water situated between two warm core rings. Other Slope Water samples, taken at positions outside the immediate influence of warm core rings show much less warm water similarity, indicating that Gulf Stream and warm core ring
Origins of oceanic plankton 5’7
I I I I I I I I I I I 750 700 65’
Figure 2. Positions and sampling dates of tows in Tables z and 3. Tows designated MOC employed a multiple opening and closing net system described by Wiebe et al. (1976). Other tows were with meter nets. All tows were oblique and made with nets of 333 nm mesh, and represent integrated vertical sampling of roughly equivalent water volumes from a depth of 800 m to the surface. Shallow tows by Grice & Hart (1962) are indicated A, B, C, D. These were taken from near the bottom to the surface with a meter net using 240 nm mesh nets. Depths were within the upper zoo m, except when bottom depths were shoaler.
euphausiids have a limited span of existence when mixed into Slope Water, or that warm core rings had not yet mixed with the Slope Water from which these samples were taken. The latter possibility is less likely in consideration of the extent and frequency of warm core rings in western Slope Water.
Slope Water, in some locations, apparently can have a zooplankton fauna1 composition with almost no overlap with Gulf Stream fauna. Shelfward transport of Slope Water can introduce Gulf Stream species, but only if such Slope Water has recently been influenced by the formation of, or passage of, a warm core ring. This implies that transport of warm water species is more likely to occur as a direct result of warm core ring impingement on the shelf margin rather than eriu Slope Water.
The low percent similarity of shelf water euphausiid assemblages with those of the Gulf Stream (Table 3) d emonstrate that warm water euphausiids do not survive long in shelf water, although occasional strays of Stylocheiron and the warm water representatives of Euphawia and Thysanopodu have been recorded along the shelf margin, never closer to more than 60 m isobath. The shelfward penetration of some of these euphausiids may in part be limited by their vertical migratory behavior, since numerous non-migratory, warm water zooplankton species penetrate and survive well in shelf waters.
Slope water and arctic-boreal species in the shelf region
Expatriate Arctic-Boreal species in the Middle Atlantic Bight are few in number. This is due largely to the fact that the Bight represents the southern limits of many Arctic-Boreal species which are counted as endemics in the region. Arctic-Boreal expatriates are in fact
518 J. Cox & P. Ii. Wiebe --
TABLE 4. Arctic-Boreal species of occasional occurrence in the Middle Atlantic Bight region - ~~ ~~ ~~ ~~~
Species Seasonal occurrence Location Citation -
Annelida Tomopteris helgolandica” December-July South of Long Island
at Woods Hole I, ‘2 Euphausiids South of Long Island
at Woods Hole f Thysanoessa gregaria ’ A as southern limit” 1, 3 Thysanoessa longicaudata February-.-June Within distribution D 1, 3
Siphonophorae Nanomia cara March, July South of Long Island 1, 3
Hyperiidea (Amphipods) Vibilia armata June South of Long Island I Vibilia borealis June South of Long Island 1 Hyperoche medzuarum March South of Long Island i
I, Grice & Hart (1962). 2, Fish (1925). 3, Bigelow & Sears. “Synonymous with T. catherina ? %ee Figure 3 ‘This species is actually a broadly eurythermic boreal species with unusual high temperature tolerance
species which occur seasonally beyond the southern fringes of their normal endemic range, apparently arriving the region via circulation over the shelf from the Northeast rather than from Slope Water. Arctic-Boreal species show shelfwide distributions (Figure 3, distributions types A and D), indicating their shoal water affinities and tolerance of lower salinities < 3 I x0
close to shore. Arctic-Boreal species carried in over the shelf from the northeast show a high year to yea1
variability (BigeIow & Sears, 1939), possibly reflecting irregularities in shelf water flow or unfavorably high temperatures. These species include those with shelf-wide distributions depicted in Figure 3, and additional less abundant species listed in Table 4.
Slope Water species enter the shelf region over the shelf margin, and show distributions with boundaries indicating their seaward origins (Figure 3, distribution types B and C). These species are apparently intolerant of the conditions in the shoreward and southern portions of the region. Euphausiids found over the shelf (Table z and Figure 3) show this Slope Water distribution type except for Thysanoessa inermis and probably T. longicaudata, although data are scant), which are Arctic-Boreal species. The most abundant shelf euphausiid species, Meganyctiphanes norvegica, shows an advancement of its southern limits in late winter through early summer, retreating to the North in late summer (Figure 3). This euphausiid, along with the Slope Water euphausiid Nematoscelis megalops and the large Slope Water copcpod Pareuchaeta norvegica persist through late summer and early fall, despite very high surface temperatures. Their persistence is undoubtedly linked to the cold water pool which lies south of Long Island, centered at a depth of 50 m (Ketchum & Corwin, 1964), Seasonal warming of the water at this depth is delayed several months, providing a temporary refugium for these species from the elevated temperatures at the surface. Shelfward penetration of these species beyond the IOO m contour may in fact, be dependent on the pool’s existence. Pareuchaeta norvegica disappears by October, presumably due to its normal preferred temperature being exceeded in the cold pool by that month. Both euphausiids disappear from the plankton by November, when the warming trend in the
Origins of oceanic plankton 519 -
76-W 740 72’ 70’
AISEASONAL SOUTHERN LIMIT)
s 2 B~SEASON~L N-S Lft.wsI
B(LIMITS FEE-JUNE! c bOUTHERN LIMIT JULY-OCTI
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s
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Figure 3. Seasonal and geographical occurrence of more abundant Arctic Boreal and transition zone species in the Middle Atlantic Bight. All data were derived from Bigelow (1916), Bigelow & Sears (1939), and Grice and Hart (1962). An asterisk on the seasonal abundance graphs indicates the species was totally absent from all hauls during that month. Eukrohniu hamata is a common Slope Water chaetognath; Erythrops erythrophthalma is a cold water shelf mysid. Oikopleura labradorensis (Larvacea), Fritillaria borealis (Larvacea), and Calanus hyperboreus are Arctic Boreal species. The regions A, B, C, D are based on data of Bigelow & Sears (1939). Note that B lines represent both a North and South limit.
cold pool pushes temperatures past IO “C, the apparent preferred temperature of these species (Brinton, 1962; Dunbar, 1964; Wiebe & Boyd, 1978).
Warm water species in the shelf region
The direct association of warm water oceanic species occurrences over the outer shelf with
warm core rings is indicated by the increase in the frequency of these species occurring near the zoo m isobath along the impingement path [Figure 4(a)], Introduction of warm water species occurs with roughly equal frequency throughout the year [Figure 4(b)], corresponding to warm core ring production, which shows little or no seasonality (Bisagni, 1976). Somewhat higher numbers of warm water species are encountered in August and October possibly due to the lesser restraints on lateral water exchange resulting from the isopycnal nature of shelf
J. Cox &f P. H. Wiebe
No lat etude
NO of offshore stations 0 23162324 8
4 (b) 2 /
, /
3- /*x / ,
.---.y N.-O
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I, I I I I I II ” JFMAMJJASOND
Months
Figure 4. Occurrence of warm water oceanic species near the shelf margin in the Middle Atlantic Bight (species data from Bigelow & Sears, 1939, p. 251). Note that in (a), the greater occurrence of species corresponds to impingement of warm core rings at the shelf margin (data on ring trajectories from Bisagni (1976). In the lower graph, the numbers at the top correspond to the total number of offshore stations used in the analysis. For example, in February, from 20 stations sampled over several years, an average of 2.5 of these stations yielded samples containing warm water oceanic species, as defined and listed by Bigelow & Sears (1939).
and warm core ring water during the late summer and early fall (Bisagni, 1976) with survival and population growth being enhanced by the higher surface temperatures characteristic of those months.
Shelfward penetration is likely to be limited for warm water vertically migratory spccics whose migrations in shoaler water may cause their aggregation near the bottom where they may be susceptible to bottom feeding predators. In their normal environment, their spacing in the water column and behavioral responses minimize the effects of predation. These protective mechanisms are inoperative for expatriate vertical migrators in shoal waters. It is not surprising, then, that most successful oceanic invaders of the shelf water enviromnent are non-migratory, upper water column dwellers.
In general, the occurrence of expatriate warm water species is more important in terms of species numbers and total biomass when compared to the occurrence of expatriate cold water species. Warm water expatriates can comprise up to 160,b of the total zooplankton displacement volume for the entire shelf region during part of the year, often exceeding io’:, at individual stations (Bigelow & Sears, 1939). Cold water expatriates, at their maximum, account for no more than 3% of total displacement volume, although they may at times be important in subregions of the Bight. Among the many factors contributing to the pre- dominance of a warm water expatriate fauna is the lesser importance of low temperature stress (Le., below the optimum for the species) as compared to high temperature stress in the survival of a species (Ekman, 1953). Cold water species have upper lethal temperature limits which are much closer to their optimum temperature, greatly restricting their penetration into waters where water temperatures are seasonally above optimum. The cold tolerance of
Origins of oceanic plankton 521
TABLE 5. a, Seasonal occurrence of warm water cosmopolitan and tropical sub- tropical species in the Middle Atlantic Bight region by number of stations where displacement volume exceed I y0 of the totalY
Species
Sampling months Distribution
February April May June July October type *
Chordates Salps 2 4 * * * * i\ Oikoplcurn dioica 0 0 I 4 * * A Doliolum sp. 0 0 2 3 * * C
Copepods Pltxwomamma gracilis 5 2 5 I * I A Mecynocera clausi 0 0 0 I * 4 A Euchirella rostrata 0 2 I 0 c 0 A
Centropages violaceus 0 0 0 0 * I B Temora stylifera 0 2 I 0 I 2 c Oncaea sp. 0 0 0 0 * I C
Malacostraca Euphausia, Thysanopoda,
Stylocheiron 2 0 0 0 * 0 B Palinurid larvae 0 0 0 0 * 0 C Lucifer typus 0 I 0 0 * 6 c
*Present at > 50% of all stations and constituting > I ‘/” of total plankton displace- ment volume; all stations combined, actual values not available. “All data from Bigelow & Sears (1939). bl~istribution types identified in Figure 6.
b, Warm water cosmopolitan and tropical-subtropical species of lesser abundance listed by distribution type
Species Distribution type’
Malacostraca Sromatopod larvae Pkronima
Copepods Eucalanus attenuatus Scolecithrix danae
Chaetognatha Sagitta en&la
Siphonophores Stephanomia cara
Agalma elegans
“Distribution types identified in Figure 6.
warm water expatriates, on the other hand, allows their persistence into the colder months in shelf waters and apparently permits the overwinter survival of some species (Table sa, b).
Overwinter survival, in another sense, can be achieved by transit of larvae or adults of warm water forms leaving the shelf in late summer via entrainment of shelf water at Cape Hatteras. These plankters could remain in transit alongside or within the Gulf Stream, become incorporated into a warm core ring, and ultimately be returned to shelf waters when the warm core ring impinges on the shelf margin. An ideal cycle would start with south- westerly transport in October at the end of the period of maximum population growth (Table sa). Using published data on current velocities along the outer continental shelf during
522 J. Cox @ P. H. Wiebe ---
‘/ ‘,” w 70” 65'
Figure 5. Theoretical path of warm-water oceanic plankter originating in October at 40 “N and 72 “W. Figures for southwesterly drift over the outer shelf region are from Bumpus (1973). Plankter arrives at Cape Hatteras during period of maximum entrainment of shelf water (Parker, 1976). Transit time in or on left edge of the Gulf stream is relatively short. Incorporation into a warm core ring occurs at 40 “N, 64 “W, at the center of the zone of warm core ring formation and travels back to its origin at a mean warm core ring velocity (Bisagni, 1976). This may be ‘short- circuited’ at any time by a variety of mechanisms, including early departure of shelf water into the Slope Water, and earlier warm core ring formation.
this month, mean Gulf Stream velocity, and mean warm core ring translational velocity away from the major area of formation east of 66 “W, it can be calculated that transit time would, on the average, be about 7 months (see Figure 5 for a sample calculation). This returns the plankter in April or May, at which time surface warming has begun again and production is high. During transit, it is unlikely that the plankter would ever encounter temperatures less than IO “C, judging from surface temperature data for the shelf waters (Schroeder, 1966). The extent to which this mechanism operates to maintain seasonal expatriate populations can only be guessed at present, but the timing of shelf currents and the correspondence of observed seasonal population changes with the expectations of this model mechanism support the possibility of its importance.
This recycle mechanism may also operate to permit the northward propagation of mero- planktonic larvae over the shelf region, despite the southwesterly drift of shelf currents. The longevity of such larvae in oceanic currents has been documented by Scheltema (1971);
survival times of many species apparently would permit a 8 month transit time. An interesting observation in Bigelow & Sears (1939) charts of plankton distributions is
the occurrence of what might be termed warm water ‘overlap species’. These are not expatriate species as we have defined them, but are distinguished by a range which includes Slope Water and overlaps the outer continental shelf. An excellent example is &g&a en.uta, whose extent over the shelf is restricted to a fairly narrow band along the shelf break (i.e., distribution type A, Figure 6). Other examples are listed in Table sa, b. These species apparently depend upon continued reintroduction into shelf water by the mixing processes previously discussed. The regularity of occurrence of such species indicates that their endemic status is dependent upon these mixing processes.
Origins of oceanic plankton 523
Figure 6. General distribution types of warm water cosmopolitan and tropical- subtropical oceanic species occurring in the Middle Atlantic Bight, derived from data of Bigelow & Sears (1939). Boundary C represents the northern limit from species which achieve shelf-wide distribution in the southern sector. See Tables ga and b for species which fit these distributions.
Quantitative estimates of the abundance of tests of planktonic foraminifera in cores from shelf sediments (Parker, 1948) reveal distribution patterns which closely parallel the distribution of living warm water plankton. A peak in abundance occurs at the 200 m isobath, indicating increased mortality where these normally oceanic organisms are likely to encounter colder and less saline water from the shelf as warm core rings pass along the shelf margin, Virtually no planktonic foraminifera are found shoreward of the 50 m isobath, and all of the species found in sediments show distributions which indicate that they have a limited span of survival in shelf waters (see Figure 6, distribution type B).
Carolinian shelf species
The incursion of Carolinian shelf species into Middle Atlantic Bight shelf waters is likely to be a rare occurrence because of the barrier formed by Cape Hatteras between the high salinity water of the Carolinian shelf (>35xO) and the low salinity waters of the Middle Atlantic Bight (<33x,,). Southward water movements have been reported (Bumpus 8i Lanzier, 1965; Stefansson, Atkinson & Bumpus, 1971). Bigelow & Sears (1939) noted the transport of palinurid phyllosome larvae and stomatopod larvae into Middle Atlantic Bight waters in June, but this transport is probably linked to Gulf Stream flow rather than northward flow over the shelf, The occurrence of callinectid crabs in coastal areas of the Bight (Norse, 1977) may be linked to a similar Gulf Stream transport of larvae accompanied by shelfward dispersal by warm core rings.
524 J. Cox & P. H. Wiebe
Discussion
From the standpoint of the shore, the Middle Atlantic Bight region can be viewed as the extension of a system of semi-enclosed estuaries onto a broad continental shelf where the circulation dynamics remain essentially estuarine. Viewed from the ocean, the Bight is a marginal, shallow reservoir of relatively Iow salinity water which is subject to significant interactions with oceanic circulation. The ability of the various expatriate species assemblages to penetrate shelf waters and interact with endemic components of shelf communities attests to the significant influence of oceanic circulation, as does the equally important loss of shelf water plankton and effects of fish larvae in shelf waters (Colton, 1959). Input of oceanic water due to warm core rings and the ingression and upward mixing of Slope Water into the surface shelf waters undoubtedly have a significant effect on productivity and trophic structure at all levels of the shelf community. Thus, an overall conceptual framework for plankton dynamics of the Middle Atlantic Bight requires consideration of both oceanic and estuarine circulation systems.
The high probability of advective movements of planktonic communities of the Bight poses several problems for future studies of their trophodynamics and functional structure. For example, assumptions regarding in situ growth rates of constituent species may not yield reliable predictions of future population size because of the probability of large scale advective losses from the population, or the advective entry of predators or competing species which may cause unanticipated population losses. Expatriate species (e.g., salps, predatory. copepods) may have a major trophic impact on shelf communities and then disappear. Essential data for these species such as mortality, recruitment and growth are impossible to gather without following populations through their dispersal movements. Some knowledge of dispersal routes and the mechanisms which control their seasonal arrival in the shelf region is essential for predictive modelling. Further study may show that biological processes are coupled with physical processes which control the timing and extent of water exchanges at the shelf/Slope Water interface, and arrival of colder shelf water from the northeast.
An important implication of certain of the plankton distributions discussed here is the possibility that some expatriate species exploit the potential for the seasonally-timed return circulation inherent in warm core rings as an overwintering mechanism. These species, which derive energy from shelf water productivity, may reside for a good portion of their existence in oceanic waters, representing an export of energy from the shelf system.
The Bight region can be divided into three regions with regard to oceanic influences: (I) The band of low salinity water along the coast south of the mouth of the Hudson River,
extending to the mouth of the Chesapeake. (2) The continental shelf edge, extending from about 37 “30’N to 40 “N and extending
shoreward towards the eastern half of the Long Island and Block Island Sound, but not including the region southeast of Cape Cod and Nantucket.
(3) The southern sector, including the shelf edge south of 37 “N, and extending landward south of Chesapeake Bay.
Cluster analysis of seasonal transect data off New Jersey by Grant (1977) verifies the fauna1 distinctness of regions I and 2. Each of these regions is characterized by types of expatriate species and by hydrographic features. For example, region 2 is occupied by Slope Water species such as Nematoscelis megalops, Pareuchaeta norvegica, Meganyctiphanes norvegica, and Calanus hyperboreus. This region roughly defines the limits of the ‘cold water pool’ described by Ketchum & Corwin (1964) and Cresswell (1967), and shows strong seaward flow of surface waters in May and June (Bumpus, 1973). Region I shows little oceanic
Origins of oceanic plankton 525
influence in terms of expatriate species, perhaps due to salinities which are consistently lower than 31%~ in summer. The southern sector is characterized by higher surface temperatures in summer and fall than the rest of the Bight (> 25 “C in July) and the presence of certain warm water oceanic species such as Temora stylifera, Doliolum sp. and Oncaea sp. The existence of these different regions should be considered in future studies of planktonic ecosystems of the Middle Atlantic Bight.
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