Memoirs MCZ 5404

/Demoirs of tbe /IDuseum of Comparative Zoolocis

AT HARVARD COLLEGE
Vol. LIV.

No. 4

STUDIES OF TPIE WATERS OF THE
CONTINENTAL SHELF, CAPE COD
TO CHESAPEAKE BAY
III

A VOLUMETRIC STUDY OF THE ZOOPLANKTON

BY

Henry

B.

Bigelow and Mary Sears

MUS. COMP. ZOOL
LIBRARY

NOV 2 1964
HARVARD
UNIYERSCDC

CAMBRIDGE,

U.S.A.

IprtnteO for tbe /IDuseum
1939


>>


STUDIES OF THE WATERS OF THE CONTINENTAL SHELF, CAPE COD
TO CHESAPEAKE BAY. III.' A VOLUMETRIC STUDY
OF THE ZOOPLANKTON
By Henry

B.

Bigelow and Mary Sears

Contribution No. 194

From

1

Parts

I

and

the

II

Vol. II, No. 4, 1933

Woods Hole Oceanographic

Institution

appeared in Papers in Physical Oceanography and Meteorology,
and Vol. IV. No. 1, 1935.



CONTENTS
Page

Introduction

189

Acknowledgments
Geographic limits and subdivisions

190
of the area

190

History and sources of information

190

Methods

192

Collection

192

Measurements and calculations

193

Expression of results

196
197

Validitj^ of calculated results

Part

I.

The volume

The water column

of

zooplankton

as a whole

200
200

Seasonal cycle

200

February

200

April

202

May

203

June

206

July

209

Autumn

210

Annual

differences

212

Vertical distribution

214
214

Diurnal stratification
^'ertical stratification other

than diurnal

Comparison with other areas
Relative abundance of different species

Monthly succession

-

221

228
230

February

230

April

231

May

233

June

237

July

241

Autumn

243

Annual

differences

Sources of the local plankton

The

217

area as a feeding ground for plankton-eating fish

245

246
253


memoir: museum of comparative zoology

184

Page

261

Conclusions

The plankton

as a

261

whole

Horizontal distribution

261

Vertical distribution

264

Annual

265

differences

Comparison with other areas

265

Relative importance of different species

265

Feeding conditions for plankton-eating
Part

II.

268

fishes

270

Volumetric distribution of individual species

270

Chordates

270

Doliolum sp

272

Salps
Fritillaria sp

•

273

Oikopleura dioica

273

Oikopleura labradoriensis

274
275

Molluscs
Clione limacina

275

Frequency

275

Abundance

277

Source of the local stock

278

Limacina

reiroversa

279

Frequency

279

Abundance

280

Vertical distribution

284

Relation to temperature

284

Annual variations

284

Source of the local stock

284

Other molluscs

Decapods
Crab and hermit crab larvae

286

286
286

Crago sp

289

Palinurid larvae

290

Lucifer typus

Stomatopods
Euphausiids

Meganyctiphanes norvegica

290
290
291
291


BIGELOW AND SEARS! NORTH ATLANTIC ZOOPLANKTON STUDIES

185
Page

Nematoscelis megalops

293

Thysanoessa inermis

294

Thysanoessa gregaria

295

Thysanoessa longicaudata

296

Euphausiid larvae

297

Other euphausiids

298

Mysids

298

Amphipods

299

Euthemisto compressa

299

Frequency

299

Abundance

300

Annual variations

302

Other amphipods

302

Copepods

303

Acartia

303

Anomalocera pattersoni

304

Calanus finmarchicus

304

Frequency

304

Abundance

305

Breeding periods

310

Vertical distribution

310

Diurnal migration

312

Distribution in relation to temperature

314

Annual fluctuations

316

Calanus hyperboreus

317

Candacia armata

318

Centropages hamatus

320

Centropages typicus

321

,

Frequency

321

Abundance

321

Annual variations

324

Vertical distribution

324

Centropages violaceus

326

Corycaeus sp

326

Eucalanus sp

326

Euchirella rostrata

328


memoir: museum of comparative zoology

186

Page

Mecynoccra clausi

328

Metridia lucens

329

Frequency

329

Abundance

331

Annual variations

333

Oithona sp

333

Oncaea sp

334

Pleuromamma

sp

334

Paracalanus parvus

336

Paraeuchaeta norvegica

337

Pseudocalanus minutus

338

Frequency

338

Abundance

339

Annual variations

340

Bhincalanus nasutus

340

Frequency

340

Abundance

342

ScolecUhrix danae

343

Temora

longicornis

343

Temora

stylifera

Other copepods

345

345
346

Cladocerans

Podon and Evadne

346

Penilia

346

Chaetognaths

Eukrohnia hamaia
Sagitta elegans

347

347

347

Frequency

347

Abundance

349

Annual

differences

351

Sources of the local stock

351

Vertical distribution

352

Sagitta enflata

356

Sagitta serratodentata

356

Frequency

356

Abundance

358


niGELOW ANO sears: north ATLANTIC ZOOPLANKTON STUDIES

187
Page

Annual variations
Other chaetognaths
Annelids

360

360
361

Tomopteris sp

361

Other annelids

361

Medusae

362
digitale

362

Regional distribution

362

Seasonal and annual variations

363

Aglantha

Leptomedusae
Other medusae

365

366

Siphonophores

366

Agalmidae

366

Muggiaea kochii
Other siphonophores

367

367

Ctenophores

368

Beroe sp

368

Pleurobrachia pileus

369

Other ctenophores

370

Protozoans

371

Bibliography

372

INTRODUCTION
111

April, 1!)29, the U. S.

Bureau

commenced an

of Fisheries

investigation of

the distribution of the eggs and larvae of the mackerel in the waters of the continental shelf

between Cape Cod and Latitude about

36°, a

ing the vernal half year, in the three subsequent years.
of

work continued dur-

And

while the collection

plankton was only a secondary goal, samples were systematically obtained on

all cruises,

and turned over to us

for study.

Plankton investigations, we conceive,

and distribution studies

lation

of the relationship of

fall

into

two

of particular species or

one species to another

(e.g.,

chief groups: (a)

popu-

groups of species, or studies

feeding habits of fishes) and (b)
;

attempts to assay the richness of one part of the sea or another, either
production, or as a feeding ground for larger animals.

in organic

Studies of the

first of

these categories depend on enumerations of the specimens present, whether of
different species or of different

have been presented

have been

growth stages

And

of given species.

in the great majority of recent publications

of this sort. If,

the data that

on zooplankton

however, we attempt to approach the matter from the

other angle, information as to the mass of organic matter that

is

present in the sea

from time to time and from place to place becomes of prime import. And very
seldom can this be deduced from a knowledge of the numbers of units of which
this

mass

is

composed, because different planktonic animals, whether different

species or different stages in the

growth

of

one species, vary so widely

counts of total numbers are apt to prove very deceptive,
to total mass.

Part

I of this

if

in size that

interpreted as indices

report attempts to give at least a rough picture of

the latter for the part of the sea in question, together with the proportions in

which the leading species enter into

it.

Part II includes such information as to

the distribution of individual species as our

own

analyses have yielded.

In studies of this sort, the vegetable and animal fractions of the plankton

can be considered either as a unit, or separately: the latter course has seemed to
us preferable, because of the basic difference between the nutritional require-

ments

of the

two

categories.

We must realize, however, whether for the zooplank-

ton or for the phytoplankton, that measurements of mass

may

be

—are only one step

in

our path toward a knowledge of the productivity

of the sea in organic substance.

ment

is

—however precise these

A

second, coincident, and equally vital require-

come only from populaindividuals combined with classification

a knowledge of the rate of overturn, which can

tion studies based

on enumerations

of age-frequencies.

A third essential step is the chemical analysis of the groups of

of


memoir: museum of comparative zoology

190

organisms concerned, for these

differ

one from another,

com-

in the proportional

and sundry other compounds.
a survey of the mass distribution of the

ponents of proteins, carbohydrates, fats,

The

present report, confined to

animal plankton,

is

offered in the

hope that

may

it

larger

serve as a preliminary ap-

proach to the broader field outlined above.

The
on

detailed data (which cannot be reproduced here for lack of space) are

Woods Hole Oceanographic

at the

file

Institution.

ACKNOWLEDGMENTS
Mr. O. E.

Sette, Dr.

Roderick Macdonald, and Dr. George L. Clarke

col-

laborated in identifying the plankton caught during 1929, Miss Alice Beale in
identifying that caught during 1930 and part of 1931.

And

Dr. Th. von Brand

has assisted us by making the ether extracts and the weighings used in our discussions of the area as a feeding ground for plankton-feeding

fish.

GEOGRAPHIC LIMITS AND SUBDIVISIONS
OF THE AREA
The account
from the

tour,

is

ofl[ing of

One

Latitude 36°.

confined to the continental shelf, out to the 200-meter con-

Martha's Vineyard, westward and southward to about

cruise only (February 1931) extended south of

Cape Hat-

Neither was the number of hauls seaward from the 200-meter contour

teras.

large enough, for

any general estimate

For convenience

of plankton

in regional comparisons,

volumes

we have

in the slope water.

arbitrarily divided the

area into a northern sector north from (and including) the Atlantic City profile,
a southern sector, south of the latter, an inshore belt extending 30 miles out from

the coast, and an offshore belt, thence seaward to the 200-meter contour (Fig.

1).

HISTORY AND SOURCES OF INFORMATION
So

far as

we have been

able to learn, the

first

published account of the

zooplanktonic communities of the region in question was Rathbun's (1889) report on tow-net catches

made in April-May

investigations of that year.

1887, in connection with the mackerel

During subsequent years, several notices appeared

of the occurrences of individual planktonic
of the

copepods of the

New Jersey coast

copepods, and amphipods,

etc., of

the

groups for restricted

localities, e.g.,

(Fowler, 1912), of medusae, ctenophores,

Woods Hole

region (Fish, 1925; Hargitt,

1905; Hohnes, 1905; Mayer, 1912; ^Vlieeler, 1901; Wilson, 1932), of copepods


BIGELOW AND SEARS: NORTH ATLANTIC ZOOPLANKTON STUDIES

Fig.

1.

Charts of the area: A, Locations of
present report.

profiles; B, Sub-divisions of the area as

employed

191

in the


192

memoir: museum of comparative zoology

and other groups

for

Chesapeake Bay (Cowles, 1930; Wilson, 1932a). But

it

was

not until 1913 that any systematic survey of the plankton of the continental shelf

was undertaken. In that summer, the U.
lected samples at a grid of stations

S. Fisheries

between the

Schooner "Grampus"

offings of

col-

Cape Cod and

of

Chesapeake Bay, as well as in the Gulf of Maine (Bigelow, 1915). And she surveyed these same waters again in 1916, both in August and in November (Bigelow, 1922).

No

data had, however, been obtained for any other year until 1929,

when "Albatross
were repeated

11" carried out cruises in April,

July.

And

these

months and years:

in the following

— February, April
May,
1931 — February, May, June, July;
June
1932 — February, May
1930

May, June, and

(2 cruises).

June

(2 cruises), July;

(3 cruises).

(4 cruises),

Woods Hole Oceanographic Institution, also made a cruise in
And periodic collecting was carried out from the Institution,

"Atlantis" of the

October 1931.

both with a pump-filter and with tow nets, at a station a few miles off Martha's
Vineyard, from late June 1935 to September 1936 (Clarke and Zinn, 1937).

We

have drawn

by the U.

S.

Bureau

freely

from

all of

the foregoing.

But the

collections

made

of Fisheries during the years 1929-1932, as outlined

above

which forms a sequel to

earlier

have been the chief basis

for the present report,

accounts of the temperature and of the salinity of the same region (Bigelow,
1933; Bigelow and Sears, 1935).

METHODS
Collection

The observing

stations (with few exceptions) were located along the profiles

indicated on Figure

1,

so spaced that on each profile the inshore belt, the mid-

and the outer edge of the shelf were sampled at points seldom more than
15-20 miles apart, and often much closer (for position of stations, etc., see Bige-

belt,

low, 1933, p. 104).

The plan was

the same locaUties, on
six

in

made

belt,

at approximately

the cruises, and this was adhered to as far as practicable,

hundred and four plankton stations on the continental
all,

>

shelf being occupied

377 of them in the northern sector, 227 in the southern, 330 in the inshore

and 274

Most

offshore.

of the hauls

by "Albatross H",
'

all

for observations to be

made by

"Atlantis" in October 1931 were vertical, those

in 1929, horizontal, at the surface,

Bad weather sometimes

interfered,

and some

and (except when

of the cruises were abbreviated.

in very


BIGELOW AND SEARS: NORTH ATLANTIC ZOOPLANKTON STUDIES
shallow water, or

when

at each station.

In subsequent years, on "Albatross 11", oblique hauls were

193

the sea was very rough) at one or more subsurface levels

em-

ployed to obviate the chief shortcoming of horizontal towing, namely, that

may

it

be a matter of chance whether the net hits or misses the strata where the

planktonic population

is

richest or poorest."

Ideally, hauls of this type should be

made by lowering

the net close to the

bottom and by towing in a diagonal direction up to the surface. This, however,
is seldom practical, the best substitute
being to tow in a series of short horizontal
steps at frequent intervals from

The standard procedure
Mo), was to tow

for

bottom to

surface.

in the present case (seldom,

two minutes

at each five

meter

level,

abbreviated to one meter of towing at every ten meter
in order to shorten the time required.

The column

however, attained in
the hauls often being

level, at the

strained in this

deep stations,

way

diverged,

on the average, by about 15° from the horizontal. At a towing speed averaging
1.2 knots or 37 meters per minute (as observed repeatedly), the obUque column
fished through was thus about 370-518 meters long, at stations where the vertical
depth was 20-30 meters, or about 777 meters where the depth was 200 meters.

The

drawback to hauls

chief

of this sort,

from near bottom to surface,

is

that they

give no information as to the degree of stratification of the communities at differ-

ent levels.
at a time,

To meet

this difficulty,

two or more nets were attached on the wire

20-35 meters apart, on the cruises subsequent to February 1931. Un-

fortunately, a considerable proportion of the hauls

next the bottom, equalling as

much

as

50%

of the

left,

unsampled, a stratum

whole vertical distance in ex-

treme cases.

The

nets were either 1-meter, or J^-meter in diameter, of

silk,

the forepart

with 29-38 meshes per linear inch, the rear part with 48-54 meshes per inch.

Nets of these meshes and diameters

may

be expected adequately to sample

planktonic animals of the various groups from the size of the copepod, Centropages,

up

Many

to fish fry.

smaller larval copepods of

And

failure to

sample these

or most of the smaller adult copepods (Oithona),

all species,
is

and other minute animals pass through.

mentioned repeatedly in the following pages.

Measurements and Calculations
Dry weight would
of the plankton,
'

if

be the most reliable index, easily obtainable, to the mass

the desiccation and weighing could be done soon after the col-

For a recent discussion

of the

advantages of oblique towing, see Walford, 1938.


memoir: museum of comparative zoology

194
lections were

made. But,

much more

quired

copepods

number

for so large a

was

assistance than

of samples, this

And

available.

so

much

would have
of the oil

re-

from

— and other substances as well, both from these and from other groups —

dissolves out into the preservative that long preservation of the samples robs

much

dry weighing of

which

is

a

much

We may

of its initial

advantage over volumetric measurement,

simpler procedure.

"volume"

also point out that

is

usually translatable into "wet

(preserved) weight" within a reasonable limit of error, for while the specific
gravities of different groups of planktonic animals differ considerably
life

and

this

—both

in

between shelled pteropods and ctenophores),

after preservation (e.g., as

comparativel}^ constant within each of the major groups.

is

Selected samples, from the collections of 1929, showed the following weight-

volume

relationship,

when weighed

in water:

by displacement; "wet weight," 18.35 gms;
Sagitta elegans, 20 c.c. by displacement; "wet weight," 21.34 gms;
Limacina, 20 c.c. by displacement, "wet weight," 20.18 gms.
Calanus, 20

c.c.

The method

of volumetric

measurement that has most often been used

in

the past consists simply of allowing the catch to settle for a given length of time,

graduated cylinder of convenient

in a

size,

and

of

then measuring

its

bulk.

It

been universally appreciated that the resultant measurements
have only a comparative value, because they include the interspaces between the
has, however,

animals, as well as the latter themselves. Savage (1931, p. 5) has, in fact,
that such measurements
for the
tests,

may

same samples by the

that

we have made

shown

average more than twice as large as those obtained,
so-called displacement method.'

And comparative

for the entire series of "Albatross II" catches for the

year 1929, have similarly shown a wide disparity, with volumes averaging 2-4
times as large

by "settlement"

as

by "displacement,"

for all types of

plankton

combined, the difference being greatest for plankton dominated by sagittae

(extreme case, 105

c.c.

by displacement; 935

ton consisting chiefly of Calanus (210

c.c.

c.c.

by

settlement), least for plank-

by displacement 260
;

c.c.

by settlement).

All measurements in the present report have, therefore, been made by displace-

ment

as follows:

The sample

meshes as

mass
1

is

of

plankton

is first

fine as that of the nets in

then added to a

See Johnstone, 1908,

p.

drained, through a bolting silk strainer with

which the catch was taken. The semi-dried

known volume

of water,

when

the resultant increase in

130 for a general discussion of these methods.


volume

BIGELOW AND SEARS: NORTH ATLANTIC ZOOPLANKTON STUDIES

195

equal to that of the sample, plus the few drops of liquid that

still

is

may

adhere to the latter after draining.

In the preliminary catches of 1929, the respective percentages of the several
constituent species or groups were determined either after these
out, or in

some

cases,

roughly estimated. In the subsequent catches, the volumes

of the species represented

The

total catch

had been sorted

was

most readily separable,

were arrived at as follows:
first

sorted into such of

its

constituent parts as were

e.g., into the chaetognaths, euphausiids, larger medusae,

adult amphipods, pteropods, and "residue," this last consisting chiefly of copepods, with other forms of similarly small size.

Each

of these groupings

was then

measured by the same displacement method as used in obtaining the total volume.
A random and well mixed sample of at least 100 specimens' was then taken from
each grouping, and the number of specimens counted, for each species represented. In order next to calculate the proportional volumes of the several species
included in each grouping, and so to arrive at the total volume of each species in
the total catch,
its

members.

was necessary to weight each according to the average

it

size of

In the cases of Calanus finmarchicus, Centropages typicus, and

Thysanoessa inermis, the weightings were based on actual measurements of the

volume per 100 specimens
range.

of large series of adults, covering the average size

Volumetric comparison with these, by eye, then gave a rough ratio for

weighting the younger stages of these same species, as well as for the other species.

The

following actual example

procedure

may

serve

more graphically

to illustrate the

:

Station,

Montauk

V, June 12, 1930, total volume of catch, 60 c.c, or 480

c.c.

per 20 minutes towing with a 1-meter net; volume of "residue," 59 c.c, or 472
c.c, per standard tow;

number

of individuals of each species represented

119 counted specimens in sample of "residue", 61 Calanus finmarchicus,
tropages typicus, 52 Metridia lucens, 5 Pseudocalanus minutus.

1

Cen-

Weights derived

as above, Calanus, 25; Centropages, 4; Metridia, 25; Pseudocalanus, 2.

lated percentages

among

Calcu-

(volumetric), in "residue", Calanus, 53.7%; Centropages,

0.1%; Metridia, 45.7%; Pseudocalanus, 0.3%. The volume

of "residue" being

472 c.c for the standard haul, the calculated volumes for the several species

work out
1 c.c. of

at 253 c.c. of Calanus;

Fewer,

1

c.c of Centropages; 216

c.c. of

Metridia, and

Pseudocalanus.

Comparison
'

<

if

of catches

there were not that

made

many.

in horizontal hauls at the surface

and deeper


memoir: museum of comparative zoology

196

and

in 1929,

column

through shoaler and deeper sections of the water

in oblique hauls

in 1931

and 1932, afford some information as to the degree

of stratifica-

open nets were used, it is obvious
that the volumes taken in the deeper tows represent not only the abundance prewere
vailing at the towing level, but also whatever was caught while the nets
tion of the plankton in different months. Since

And

being lowered and hauled in again.

this

contamination might well be great

—

enough entirely to obscure the picture in individual case.s
chanced that the net was hauled up through a dense swarm
another.

But

at

most

of the stations, inside the

if,

of

for example,

it

one organism or

200-meter contour, the vertical

sectors averaged less than 1/7 as long as the horizontal sectors in the cases of the

horizontal hauls, and only about 1/15 as long as the horizontal sectors in the
cases of obliques.

It

may,

therefore, be

an important factor for our purposes

numbers

of catches are averaged.

correct for

assumed that

this

contamination

in these shoal waters,

when

is

not

considerable

Consequently, we have not attempted to

it.'

In order to render the catches
standpoint of depth) with those
the catches of the latter

may

the water columns sampled.

made

made

in horizontal

in obliques,

tows comparable (from the

we have

further assumed that

be accepted as representative of the mid-depths of

And

the depths stated in the following discussion

are so derived.

We have credited a value of 1 to catches <
volumes and

1

c.c, in the calculations of average

ratios, while in the case of the latter,

as equivalent to

1

c.c, for the sake of simplicity.

we have

also treated values of

Likewise, in the tables giving

subdivision was
percentages and abundance, a dash signifies that the particular
that the species was not detected in that
not visited on a particular cruise a
;

particular subdivision, though taken in another; a blank that

any subdivision on a particular

in

it

was not detected

cruise.

Expression of Results

The

fact that the catches

were taken

in nets of different sizes, as well as in

hauls of different lengths, some obUque, some vertical, and some horizontal,

makes

it

necessary to reduce

all

the measurements to one

common

basis in order

to render the results comparable, one with another.

The elements on which such
haul are:—
(c)
'

a reduction must be based for

any particular

haul in time,
(a) the diameter of the net used, (b) the duration of the

the average speed of the vessel (or else the linear extent of the haul), and (d)
For a case

of such correction, see

Bigelow and Sears, 1937,

p. 69.


BIGELOVV AND SEARS

NORTH ATLANTIC ZOOPLANKTON STUDIES

:

the efficiency of the purticuhir net used.
hauls, the

first

In

tlie

cases of horizontal

two elements are precisely known, hence the

first

197

and oblique

reduction

is

to

"catch per unit time per unit net-opening"; the standards here adopted being 20

minutes of hauling with a net
is

meter in diameter.

1

But

precise mathematically.

it

Up to this

point, the reduction

does not yet provide a standard of comparison,

unless "time" can be translated directly into "length of water

Close attention was, therefore, paid to the speed of the ship, on

column
all

fished".

the cruises of

"Albatross 11" and of "Atlantis", and records by R.P.M. of the propeller, and

by ship's log, show that this was close to 1.2 knots for the series as a whole. But
whoever has had experience with the differences in the velocities and directions
of currents at different levels in coastwise waters

where the circulation

by the tide, will appreciate that the rate at which a ship
surface water

may

differ

considerably from the rate

is

moving,

some deeper

of

each individual tow, we have, therefore, thought

Rather than attempt the calculation

average speed of 1.2 knots in

all

governed

relative to the

of a net relative to the

at

level.

is

it

water

of the linear extent

preferable to assume an

the calculations, and the catches of the horizontal

and oblique hauls were reduced to the common standard accordingly.
On this basis, the linear extent of the standard tow of 20 minutes would
average 741 meters.

And

there would be as good justification for expressing the

volumes caught "per unit volume
these expressions includes the

impress upon

tlie

translation into

volume
on

of

water" as "per unit time," for the one of

same probable

errors as the other.

reader that neither of these expressions

"volume

of

is

But we must

susceptible of direct

plankton present," whether per unit time or per unit

of water, because of uncertainty as to the efficiency of the nets (discussed

Hence, to avoid any possible misconception as to the rehabihty

p. 198).

observations,

we have endeavored

to

draw a sharp

of the

distinction, in this respect

throughout the descriptive sections of the present report.

It

has also been sug-

gested to us that "catch per unit volume of water" might suggest a higher degree
of precision
able.

We

than "catch per unit tow", whereas actually the two are interchange-

have, therefore, adopted the latter.

All the results, then, are expressed as catch, in c.c. per 20 minutes towing

with a 1-meter net at an assumed speed of

1.2

knots (2222 meters per hour),

unless otherwise stated.

VALIDITY OF CALCULATED RESULTS

No modern student of

we

would claim that quantitative calculations, based on tow nettings, can be any more than approximations to the truth.
plankton,

fancy,


memoir: museum of comparative zoology

198

To

begin with, we must face what

herent in the

be termed the "catching error"

may

as to the rehabiUty of the latter that gave planktonologists so

ing the last quarter of the nineteenth century.

remind the reader that a tow

It

loss

concern dur-

net, of the usual conical form,

and

of

mesh appropri-

somewhat

than the

less

water that would pass through a simple hoop of equal diameter, the

of

the

(i.e.,

much

seems pertinent, however, to

ate for the capture of planktonic organisms, filters

amount

in-

We have no intention of reviving the controversies

tow net method.

amount

regurgitated) depending on the shape of the net, on the

proportionate areas occupied by the threads and by the spaces between the
latter,

and on the pressure,

i.e.,

on the velocity with which the net

through the water. The slower the towing, the more complete
planktonologist knows from observation.
#20) the loss
,

may

drawn

filtration, as

In the case of fine meshed

every

silk (e.g.,

be so serious at ordinary towing speeds as to necessitate special

types of net, to increase the filtering surface relative to the

even with

is

meshed as #0

silk as coarse

the fore parts of our nets, only about

mouth opening. And

(38 meshes per linear inch), such as used in

30%

as

much water would

pass through a

given area, drawn transversely at 2 knots, as through an open cyhnder of the

same

area,

according to the relationship between pressure and filtration in

Hensen's (1895) experiments^
This

loss is so

minimized by the increase

opening, resulting from the conical form, that
at the usual towing speed for our nets,

in straining area relative to
it

probably averaged

less

as

10-30%

or even

recent discussion). Neither

more
is

used, except on rare occasions

(see

Winsor and Walford, 1936,

of the
fails

Much more

plankton that

to capture,

it

serious
is

may
p.

vary as

190, for a

clogging likely to be serious, for nets such as those

when diatoms

or Phaeocystis

the gelatinous bodies of ctenophores, appendicularians,

meshes.

than 10%,

though experiments at various hands have

proved that the catches made in parallel hauls with similar nets

much

mouth

is

may swarm, or when
etc., may block the

the uncertainty as to what relation the fraction

adequately sampled bears to the remainder that the net

being

common knowledge

that no one type of net will equally

well sample the various size categories of planktonic animaLs.

The

nets used in

the present studies (meshes, 38-54 per linear inch) being comparatively coarse,

the failure of small copepods such as Oithona, or the young of others, to figure

more
'

largely in the following volumetric

lists,

does not necessarily

"
.

—

/v«

= \/
Pressure at the given velocity calculated from the formula,
ila, p
•'

V

2k
2g

.

mean

that


BIGELOW AND SEARS NORTH ATLANTIC ZOOPLANKTON STUDIES

199

:

not actually have been present in considerably greater abundance

may

they

(numerically, at least) than the catches would suggest.

In extended surveys, errors also creep in through the fact that
to classify the hauls

by the time

no precise measure

cases,

tance,

i.e.,

by

the duration of the tow

of the speed of the net

remarked

available, as already

is

—

WTiile errors of the sorts just

More

may

vital is the question

generalization for the included area as a whole.

seems to us

of results

necessary

for in

,

through the water,

most

the dis-

i.e.,

reach extreme proportions in

by combining a

sufficiently ex-

whether the grid formed by the

any particular plankton survey, has been

stations, in

—

(p. 196).

mentioned

individual cases, they can usually be minimized
tensive series of data.

it is

close

enough to warrant

In this respect, the consistency

sufficient warrant, as already

argued by Walford (1938) for

a similar survey carried out on George's Bank.
If these

shortcomings of various kinds should chance to be cumulative for a

given haul, the calculated volume for the latter

—perhaps more.

may

much

very likely be as

as

plus or minus error

But, when so many observations are in hand, the
no doubt averages much less than this perhaps not more

than 20-30%, which

is

100%

in error

—

far smaller

that form the basis of study.

than the variations observed among the values

And since the latter also show very clear consistency,

both regional and secular, not only for the volumes of plankton as a whole, but
also for the

ment

may

is

more abundant

of the constituent species,

we think no

further argu-

needed in justification of our conclusion that the picture they have yielded

be accepted as representative (within reasonable limits) of the larger zoo-

plankton from place to place within the area, from season to season, and from
year to year, for the period 1930-1932.

The

basis for comparison

between

this

group of years and the year 1929

not so soUd, because of the use in that year of horizontal hauls. The best
do, in this case,

that were

made

is

to

assume that the average catch,

at different levels at

rect average for the water

that this

marked

is

But from June

temperature tends to become increasingly

tally, at

any particular

thick the stratum
level,

surface.

through the period February-April,

stratification has developed.

how

two or more

is,

we can

of these

each station, at least approximates the cor-

column as a whole, bottom to

close to the truth

doubtful

of the

is

for

It is
i.e.,

probable

before any

on, as the plankton like the

stratified,

which the yield

it

becomes increasingly

of a net

working horizon-

can be accepted as representative.


memoir: museum of comparative zoology

200

PART

I.

THE VOLUME OF THE ZOOPLANKTON AS A UNIT
The Water Column as a Whole

The volumes

in c.c. per

standard haul for each month of the series are shown

on Figures 2-6, 8-9, and average
subdivisions in the following table

Month

:

and

maximum volumes

for

the several

BIGELOW AND SEARS: NORTH ATLANTIC ZOOPLANKTON STUDIES
community

in the waters in question, as in boreal seas in general,

is

at

its

201
lowest

ebb, at the end of winter, or in early spring.

The

Fig.

2.

three surveys for this

month

(1930, 1931, 1932) agree in showing the

of plankton, per standard haul: A, February 5-13, 1930 and February 13-March 5,
1931 (underlined); B, February 10-March 1, 1932, contour lines 100, 500, 1000, 1500, 2000 c.c.

Volumes

and to the south, decreasing
though with great irregularity from station to

volumes as largest over the inner half
offshore

and to the north

station, as

is,

(Fig. 2)

,

of the shelf,

in fact, the case throughout the year.


memoir: museum of comparative zoology

202

The magnitude
perature

may

normal tem-

of this inshore-offshore gradient in winters of

be illustrated by the fact that, of the 37 tows

was 294

1931, the average of the ten closest to shore

made

in 1930

c.c, four yielding

250 c.c, with fourteen hauls over the mid-belt of the

and

more than

shelf averaging 104 c.c,

whereas thirteen along the continental edge averaged only 72 c.c, with four alone

of the latter yielding as

much

The plankton thus averaged roughly

as 100 c.c.

twice as voluminous inshore as along the mid-belt of the shelf, four times as

voluminous as along the outer edge of the
offshore relationship

was

of this

And

most cases the inshore-

in

same order along individual

with occasional exceptions as in 1930 off
c.c.

latter.

at the inshore station, but 191 c.c at the station next seaward,

in 1931, off

though

profiles,

Cape May, where the volume was 136
and again

Martha's Vineyard where the catch closest to land was only 35 c.c,

but 108 c.c farther out.

The

contrast prevaihng at this season between small volumes in the north-

by the New York

eastern sector (bounded

profile)

and

large in the southern

is

February volumes having averaged only about 1/3 as great
for the former as for the latter in 1930 and in 1931 combined, and l/lO as great

even more

striking,

in 1932. Furthermore,

no catch as great as 200

c.c.

was made

in the eastern sector

any February, whereas fourteen such February catches were recorded to the
south and west. But this abundance seems not to extend south of Cape Hatteras
in

"which

may

be regarded as the southern boundary in winter to the cold boreal

water" (Bigelow, 1933,

having yielded only

the advance of spring,

is

tows made in 1931 in the vicinity of the Cape

and 205

c.c.

respectively.

of events in the

development

15, 80, 92,

The sequence

April.

of the plankton, with

obscured in our data by the long period that elapsed be-

and second surveys of each year, no coUectioas having been made
March. In 1930, which we must perforce accept as representative, being the

tween the
in

p. 11),

only year

first

when

collections were

made both

in

volumes had increased about eight or nine fold

February and in April, average
from the one month to the other

over the offshore belt, northward and eastward from the Barnegat
the catch averaged 325

c.c.

of

magnitude

where

in early April, contrasting with 39 c.c. in February.

In the southern sector, however, the volume of plankton

same order

profile,

in April (244 c.c) as

it

had

in

still

averaged about the

February (175

c.c).

This combination of relatively stationary conditions in the south, with

marked augmentation

in the north resulted

—in the year

—

in question

versal of the north-south relationship that existed in February, so that

in a re-

by early

April, the volume of plankton averaged nearly three times as great (419 c.c)


BIGELOW AND SEARS: NORTH ATLANTIC ZOOPLANKTON STUDIES
eastward from the

New York

only about 1/3 as great, as

profile, as

had

it

south of the hitter (157 c.c), instead of

end

at the

203

where

of the winter, this being a case

the north-south gradient would have been hidden, had the comparison been

made between

sectors separated

by the boundary

(Atlantic City profile) that

has usually proved significant.

was not possible to include the immediate offing of Chesacalculation, lacking an oblique haul there in that April. A very

Unfortunately,

peake Bay

in this

it

was made

month, but we
have no information as to whether the average volume had or had not increased

rich surface catch (1200 c.c.)

at this particular locahty

there,

it is

meantime. And volumes

true, in that

for April similarly

averaged

larger in the northern sector than in the southern in 1929, as well.

Vernal augmentation spread southward in 1930, between the

weeks of April as

far as the

ofRng of

And
the

there

Bay

is

where the catches were greater than 500

third

by the expanding
c.c.

(Fig.

3A, B).

evidence of some slight alteration of the same order to the south of

as well,

where the

hauls), contrasting with 98
in the

and

Chesapeake Bay, causing a five-fold increase

since February, along the mid-belt of the shelf, as illustrated

outlines of the area

first

late April average
c.c. (3 stations, 1

was 181

c.c.

(6 stations, oblique

oblique, 2 horizontal hauls), early

month. Consequently, the plankton averaged much the richest along the

mid-belt of the shelf by mid-April of that year (16 stations, average, 496

whereas the coastal waters

c.c.),'

—which had been richest in February—were now

rela-

tively barren (8 stations, average, 192 c.c), as was also the case along the con-

tinental edge (8 stations, average, 133 c.c).

The

facts, (a) that the richest

(Fig. 2, 3)

have not been at the most easterly stations, and

of the Gulf of

Maine

low, 1926; Fish

dominant

aggregations recorded for February and April

is

on the whole sparse in

and Johnson,

species, individually,

1937),

late winter

(b) that the

combined with the vernal

is suflficient

plankton

and early spring (Bigehistories of the

evidence that in years

when volumes

increase significantly in early spring, this results chiefly from local reproduction,

not from immigration from waters farther to the east. Neither have

we any

evi-

dence of mass immigration from oiTshore, or from the south, at this season.

May.

In one of the years (1930)

the area that had been well populated

(

when

> 500

April can be compared with
c.c.)

expanded seaward to the continental edge, by the
of

Chesapeake Bay

to that of

latter, all

along from the

Montauk (though apparently no

oflfing

farther eastward),

In early April, only two stations yielded more than 500 c.c. (Fig. 3A), both of them east of
In late April this was the case at nine stations scattered as far south as Chesapeake Bay.
'

May,

during the earlier month, had

New York.