SEASONAL FLUCTUATION OF ZOOPLANKTON POPULATIONS IN LOWER DELAWARE BAY

'Don MAURER,' Les WATLING, 3 Rose LAMBERT, ' Ann PEMBROKE

'College of Marine Studies, University of Delaware, Lewes, DE 19958;' Ira C. Darling Center, University of Maine, Wal-pole, ME 04573;' Graduate School of Oceanography, University of Rhode Island, Kingston, RI 02881 ;' Pandullo, QuirkAssociates, 545 Tilton Road, Northfield, NJ 08225 .

Received November 29, 1977

Keywords : Seasonal, zooplankton, Delaware Bay

Abstract

Quantitative zooplankton samples were obtained monthly or bi-monthly 15 times from June 1974 to May 1975 at three stationsin lower Delaware Bay . Two 12-hour cruises were also conductedat one of the stations .

Arthropods dominated the samples in terms of number ofspecies and number of individuals . The number of zooplanktonfrom surface samples ranged from 58/m 3 in August to 21,092/m 3 in June, while bottom samples varied from 259/m3 in Augustto 30,395/m3 in October . In general, larger concentrations of in-dividuals were found in bottom samples .

Only on three occasions did meroplankton exceed the holo-plankton, and these occurred at the shallow water stations . Mero-plankton comprised a larger percentage of the bottom samplesthan surface samples . Zoeae of Neopanope sayi and Uca sp . con-tributed mainly to the large proportion of meroplankton in July1974, veligers of Mytilus edulis in January 1975, and nauplii ofBalanus s p . in May 1975 .

Copepods were the largest component of the populationthroughout most of the year. At all stations and depths, Arcticatonsa dominated most of the summer samples . In the spring of1975, A. tonsa was replaced by Centropages hamatus, Temoralongicornis, and Pseudocalanus minutus .

During the 12-hour cruises there were higher numbers of indi-viduals in the bottom waters in the day with migration to surfacewaters in the afternoon and evening . Based on cluster analysis,five time-related assemblages were discerned : June, July-August,September-November, December, January-May . Comparisonof Delaware Bay zooplankton with other estuarine systems indi-cated that the densities obtained locally were most similar tothose reported in the York River, Virginia .

Dr. W. Junk b. v . Publishers - The Hague, The Netherlands

Introduction

In a review of zooplankton research, Jeffries & Johnson(1973) stated that the zooplankton from Cape Cod toCape Hatteras has been studied more or less continuous-ly, in one way or another, since the turn of the century .Because of differences in sampling and in methods forquantifying data, comparisons and syntheses of variousauthor's research presented a formidable task . In the De-laware Bay area several zooplankton studies have beenconducted . These included studies in and near the Chesa-peake and Delaware Canal (Ferrante, 1971 ; Raytheon,1975), along the length of the lower bay (Cronin et a!.,1962), in the bay mouth region (Deevey, 1960), and in theadjacent coastal waters (Van Engel & Tan, 1965 ; DuPontet a!., 1972; Sandine & Swiecicki, 1975) . Deevey (1960),using six years of samples gathered from 1929-1935, qua-litatively analyzed the fauna of the surface waters in thebay and near coastal waters . A fairly coarse mesh [No. o(0.515 mm average aperture) or No . 2 (0 .342 mm averageaperture)] was used which limited collection of smallercopepods and larvae unless net clogging occurred .

Methods

Zooplankton data were obtained monthly or bimonthly15 times from June 20, 1974 to May 28, 1975, always athigh slack tide from three stations (Fig . 1) . Stations wereselected because they represented a rapid, horizontalgradient from shallow to deep water in the lower bay .Station I was 20 .8 m deep and Stations II and III were 6 .1

Hydrobiologia vol . 61, 2, pag. 149-160, 1978

1 49

1 5 0

Fig . i . Map of Delaware Bay showing location of hydrographic and plankton stations .

m deep . Moreover, the study area was near an importantanchorage site for oil lightering . Two 12-hour cruises werealso conducted at Station II on February 2o and April 14,1975. Zooplankton samples were taken with a 30 cm dia-meter Clarke-Bumpus semi-quantitative plankton sam-pler fitted with a No . 6 (0.241 mm) plankton net. Twotows were taken at each of the three stations . Samplingwas done at the surface and at the bottom. Duration oftows was approximately 15 minutes . The plankton sam-ples were preserved in a i : to dilution of 40% bufferedformalin for later analysis . To quantify the zooplankton,the total volume of the sample, plankton plus fluid, wasrecorded. In most cases additional 40% buffered formalinsolution was added to adjust the volume of the sample toan even number . This additional volume added was alsorecorded. Before taking aliquots, any large organismssuch as medusae were removed, and these were noted . Inthe manner suggested by Frolander (1968) 1 ml aliquotsof the plankton sample were taken until a total of 300 in-dividuals or greater had been counted . Prior to removingthe aliquots with a Stempel pipette, the samples werevigorously shaken to disperse the organisms .

Most copepods were identified to species using Wilson(1932) and sheets of the `Fiches d'Identification' pub-lished by the Conseil International pour l'Exploration dela Mer. Copepodites were identified to genus in somecases or simply referred to as copepodites . Certain cope-pods, for example, Corycaeus and Paracalanus, wereidentified only to genus because of the difficulty in dis-tinguishing among the species . No attempt was made toidentify medusae since they were generally badly distortedby the nets and formalin. With respect to the meroplank-ton, attempts were made to identify to species all crablarvae. Some of the veligers were also identified fromOctober 15 to December 13, 1974. Operational problemswith the Clarke-Bumpus meter may have caused overestimates of population densities for this period .

Results

Seasonal CruisesA list of all species collected was presented in Table i .Nine phyla were represented . Arthropods dominated thesamples in terms of number of species and number of in-dividuals .

The number of surface zooplankton ranged seasonallyfrom 58/m 3 in August to 21,092/m 3 in June . Bottomsamples varied seasonally from 259/m 3 in August to

30,395/m' in October (Table 2). Stations had peaks inOctober and May ; and Station I had a peak in June andJuly, Station II in December, and Station III in August .In general, larger concentrations of individuals werefound in bottom samples ; this was particularly true forStations I and III .

Meroplankton and HoloplanktonThe percent composition per sample represented bymeroplankton, holoplankton, and copepods for eachcruise was presented in Table 3 . Only on three occasionsdid meroplankton exceed the holoplankton, and theseoccurred at the shallow water stations . Meroplanktoncomprised a larger percentage of the bottom samplesthan surface samples at Stations I and II . The shallowwater stations tended to have a larger proportion of mero-

Table i . List of species obtained in lower Delaware zooplanktonsamples .Phylum Protozoa

Order ForaminiferidaForaminifera sp .Globergerina sp .

Phylum CnidariamedusaCeriantharian larva

Phylum Ctenophora

Phylum Ectoprocta

Phylum AnnelidaClass Polychaeta

Phylum MolluscaClass Pelecypoda

Class Gastropoda

Ctenophora sp .

Cyphonautes larvae

polychaete larvae

veligerMytilus edulis Linnaeus (veliger)

Limacina inflataunidentified pteropod

Phylum ArthropodaClass Crustacea

Order CladoceraPenilia avirostris DanaPodon sp .Evadne sp .

Subclass CopepodaAcartia clausi GiesbrechtAcartia tonsa GiesbrechtAcartia copepodite)Calanus finmarchicus GunnerusCentropages_hamatus (Lilljeborg)Centropages t picus KroyerCentropages (copepodite)Corycaeus sp .Eucalanus attenuatusEurytemora affinis (Poppe)Euterpina acutifrons (Dana)

1 5 1

15 2

Table 1 (continued)

Phylum Chaetognatha

Phylum Chordata

Subclass Copepoda (continued)

Labidocera aestiva WheelerLabidocera copepodite)Microcalanus sp .Oithona brevicornis GiesbrechtOithona similis ClausOithona spinirostris ClausParacalanus sp .Pseudocalanus minutus (Kroyer)Pseudocalanus copepodite)Pseudodiaptomus coronatus WilliamsTemora longicornis (Muller)Temora (copepodite)Tortanus discaudatus (Thompson and Scott)harpacticoid copepodcopepod naupliiunidentified copepoditeunidentified copepodcopepod species A

Order 1ysidaceaMysidacea sp .

Order CumaceaMancocuma altera Zimmer

Order AmphipodaCorophium sp .Gammarus sp .Hyperia sp .unidentified amphipod

Order DecapodalarvaePalaemonetes sp . (zoea)Crangon septemspinosa (Say) (zoea)

Suborder ReptantiaCallianassa sp . (zoea)Pagurus longicarpus Say (zoea)Emerita talpoida (Say) (zoea)Ovalipes ocellatus (Herbst) (zoea)Hexapanopeus angustifrons (Benedict and Rathbun)

(zoea)Neopanope_sayi (Smith) (zoea)Panopeus herbstii H . Milne-Edwards (zoea)Pinnixa sayana Stimpson (zoea)Pinnotheres maculatus Say (zoea)Uca sp. (zoea)Upogebia affinis (Say) (zoea)unidentified crab zoeae and/or megalops

Cancer irroratus Say (zoea)Libinia sp . (zoea)Reptantia larvaeNantantia larvae

Subclass CirripediaBalanus sp . naupliusBalanus sp . cyprid

Sagitta sp .

Oikopleura sp .Fish' eggsFish larvae

Table 2

Total numbers per m3 of zooplankton individuals .

Date

Station I

Station II

*Values may be over estimates due to Clarke-Bumpus.meter malfunction .

plankton than did the deep water stations . The large pro-portion of meroplankton which occurred on July 19, 1974was mainly due to zoeae of the crab, Neopanope sayi, atStation II and to Uca zoeae at Station III which illustratedthe kind of variability that occurred in the meroplanktonover a short distance . Veligers of Mytilus edulis were res-ponsible for the large proportion of meroplankton atmost stations on January i6 . The fairly high percentageof meroplankton reported for May 28, 1975 at the shallowwater stations was due to fish eggs, Balanus sp . nauplii,and cyprids. Copepods were the largest component of thepopulation throughout most of the year, except when vastnumbers of Oikopleura sp . occurred on October 15, 1974 .Then the copepods were consistently less than 50% of thesample .

Dominant Species

The seasonal cycle of the dominant organisms were pre-sented in Figures 2 and 3. Data from Station II were con-sidered representative for all three stations . For graphicalconvenience, numbers of individuals were converted toproportion of the total sample. However, this distortedthe importance of some species, particularly A. tonsa inlate summer, when almost no plankton was found in thewater.

Station III

At all stations and depths the copepod, Acartia tonsa,dominated most of the summer samples (Figs . 2 and 3) .There was some residual population of A. tonsa at someof the stations year-round, but in very low numbers .During the summer months A. tonsa was occasionally

replaced as the dominant species by Pseudodiaptomuscoronatus in some of the bottom samples . This replace-ment occurred on August i, 1974 for Stations I and II,and on July i9, 1974 at Station III-bottom . Pseudodiap-tomus was found only in significant numbers in the bot-tom samples, and they were found only in the summerand autumn months (Fig . 3) .

Larvae of bottom invertebrates comprised a significantpercent of the population during the summer months .

Crab zoeae were particularly abundant in the early sum-mer samples (Fig . 2) . Cyphonautes larvae (ectoprocts)appeared in early August and disappeared from the sam-ples at the end of October . Veligers identified as Mytilusedulis significantly contributed to the zooplankton popu-lation during December and January (Fig . 3) . Polychaetelarvae could generally be found in low numbers duringany time of the year .

In the spring of 1975, A. tonsa was replaced by the cope-pods, Centropages hamatus, Temora longicornis, andPseudocalanus minutus (Figs . 2 and 3). Also, in these

1 53

Surface Bottom Surface Bottom Surface Bottom

June 20, 1974 21,092 14,755 1,197 3,475 2,953 7,363July 19 279 20,511 807 2,824 2,031 2,954Aug . 1 1,046 1,971 3,946 805 5,680 9,432Aug . 21 121 1,806 59 259 58 -Sept . 17 669 4,806 160 1,541 1,128 2,403Oct . 15* 1,745 1,795 1,049 2,616 2,859 5,475Oct . 30* 8,977 30,395 12,547 2,804 4,674 8,259Nov . 14 3,325 3,935 2,643 1,780 1,723 1,493Dec . 13* 6,148 7,336 7,545 3,304 7,061 5,444Jan . 16, 1975 1,303 1,009 1,052 1,644 957 1,046Feb . 20 - - 1,908 505March 18 4,145 6,571 5,234 4,375 13,436 4,561April 14 - - 3,451 3,686May 9 2,223 14,035 3,146 10,681 5,919 14,587May 28 4,565 12,120 391 651 695 1,623

Table 3 (continued)

Station III

1 5 4

- = Missing data points

Holoplankton/meroplankton ratios and percent composition of copepods(including medusa as meroplankton) .

Station I

same spring samples, but not figured in the graphs, a highpercentage of unidentified copepodites was found .

Twelve-Hour CruisesChanges in zooplankton numbers/ M3 during the t2-hour cruises (Station II) were illustrated in Figures 4 and5. On February 20, 1975 the zooplankton numbers in-creased in the surface waters and decreased in the bottomwaters from the preceding cruise . By April numbers hadincreased over the first 12-hour cruise by a factor of aboutthree . Both cruises had higher numbers of individuals inthe bottom waters during the day, with migration to sur-face waters occurring in the afternoon and evening .During both cruises diurnal vertical migration towardsthe surface began about i 50owith the zooplankton begin-ning to leave the surface again about igoo . This large in-crease in individuals may have been augmented by the

Table 3

Station II

tidal regime . This could be true for the February cruiseswhere increasing salinity was noted about 1500, but on theApril cruise weather conditions retarted the normal tide-related changes . With fairly brisk northeasterly windsblowing, high salinity waters remained in the bay thewhole day. As a result, the day was probably spent sam-pling the same water mass on the April cruise . In the ab-sence of any sizable surface tidal movement, the migra-tion of individuals to the surface occurred at about thesame time as previously . This migration pattern wasagain significant because the dominant copepods weredifferent during each cruise (Figs . 4 and 5) . On the Febru-ary cruise Pseudocalanus minutes, Acartia tonsa, andTemora longicornis were most abundant . On the Aprilcruise the dominant copepods were Centropages hama-tus, T. longicornis, and Centropages sp . copepodites .

Cluster AnalysisCluster analysis techniques were used to assess the pres-ence of recurrent 'time-groups' in the zooplankton data .The methods for this analysis were described in Watling& Maurer (1976) . Five time-related assemblages werediscerned and are listed below with their characteristicspecies :Time-Group

Characteristic Species

June

Podonsp .Acartia tonsaPseudodiaptomus coronatusCentropages hamatus

Date Hol ./Mer .Surface

% CopepodsSurface

Hol ./Mer .Bottom

% CopepodsBottom

June 20, 1974 96 .8/ 3 .2 89 .1 98 .9/ 2 .0 95 .96July 19 27 .6/72 .4 27 .0 97 .2/ 2 .8 96 .3Aug . 1 94 .2/ 5 .8 94 .2 91 .3/ 8 .7 98 .9Aug . 21 50 .0/50 .0 37 .9Sept . 17 99 .7/ 0 .3 79 .5 82 .2/17 .8 73 .3Oct . 15 80 .8/19 .2 29 .2 86 .8/13 .2 57 .6Oct . 30 90 .3/ 9 .7 86 .9 79 .1/20 .9 78 .4Nov . 14 76 .7/23 .3 67 .9 68 .5/31 .5 66 .6Dec . 13 91 .7/ 8 .3 91 .7 91 .1/ 8 .9 90 .5Jan . 16, 1975 89 .9/10 .1 89 .9 58 .5/41 .5 58 .5Feb . 20 - - - -Mar . 18 99 .0/ 1 .0 98 .8 99 .0/ 1 .0 98 .8April 14 - - - -May 9 99 .6/ 0 .4 99 .0 97 .5/ 2 .5 97 .3May 28 74 .1/25 .9 73 .5 78 .5/21 .5 75 .7

Date Hol ./Mer .Surface

% CopepodsSurface

Hol ./Mer .Bottom

% CopepodsBottom

Hol ./Mer .Surface

% CopepodsSurface

Hol ./Mer .Bottom

% CopepodsBottom

June 20, 1974 72 .9/27 .02 47 .88 97 .7/ 2 .3 94 .49 71 .1/28 .8 54 .8 92 .3/ 7 .6 71 .30July 19 79 .9/20 .0 79 .9 99 .6/ 0 .4 99 .2 39 .0/61 .0 37 .9 95 .9/ 4 .1 95 .7Aug . 1 88 .8/11 .2 88 .5 93 .6/ 6 .4 91 .1 92 .3/ 7 .7 92 .3 69 .6/30 .4 60 .7Aug . 21 90 .9/ 9 .1 71 .0 91 .4/ 8 .6 89 .7 76 .2/23 .8 49 .1 71 .4/28 .6 57 .1Sept . 17 98 .5/ 1 .5 70 .2 88 .6/11 .4 82 .1 95 .0/ 5 .0 28 .1 92 .1/ 7 .9 79 .4Oct . 15 91 .5/ 8 .5 36 .1 61 .1/38 .9 11 .1 86 .6/13 .4 23 .7 87 .2/12 .8 31 .6Oct . 30 89 .8/10 .2 81 .3 84 .2/15 .8 82 .3 92 .5/ 7 .5 92 .3 84 .0/16 .0 78 .2Nov . 14 87 .7/22 .3 83 .1 84 .1/15 .9 79 .2 85 .0/15 .0 83 .7 83 .9/16 .1 81 .1Dec . 13 84 .1/15 .9 83 .9 66 .6/23 .4 66 .3 88 .4/11 .6 88 .4 70 .2/29 .8 70 .2Jan . 6, 1975 94 .0/ 6 .0 94 .0 78 .0/22 .0 77 .7 60 .2/39 .8 60 .2 50 .4/49 .6 50 .4Feb . 20 - - - - 99 .5/ 0 .5 99 .5 91 .8/ 8 .2 91 .8Mar . 18 99 .5/ 0 .5 98 .8 98 .0/ 2 .0 98 .2/ 1 .8 97 .8 92 .9/ 7 .1 89 .0April 14 - - - - 97 .7/ 2 .3 97 .1 93 .8/ 6 .2 91 .6May 9 98 .6/ 1 .4 98 .6 99 .6/ 0 .4 99 .0 99 .6/ 0 .4 99 .6 98 .9/ 1 .1 98 .0May 28 90 .9/ 9 .1 89 .7 84 .1/15 .9 83 .1 67 .7/22 .3 58 .5 51 .6/48 .4 43 .9

50%Acartia tonsa

50%aCentrooaoeg hamatus

i

50%Centrooooes sp .

copepodite 50%Temora lonoicornis

50°/aPseudocalanus minutus

50%Bivalve veliger

Crab zoeae

Qikooleura sp .

Fish eggs

50%•

50%

50%

50%Acata tgnsa

5O%

Centroooaes hamatus

5001,rd-- , ,' ' 1119 22' 13 20'

19 1I

21May

J

J

A

/974

/975SAMPLING DATE

Fig . 2 . Seasonal changes in relative abundance of dominant zooplankton species at Station 2-surface .

I

50%rCentroooaes sp .

copepodite 50%e•Temoro lonaicornus

50%Pseudodiaotomus ~~caonatus

509Pseudocaanus minutes • STATION II

50010 BOTTOMParocolanus sp.

50%Bivalve veliger

50%-Other larvae

I

50 19 22' 13 20'

19 r1 21 F 17 ' 15 3~ 14May

J

J

A

S

0

N

STATION II - SURFACE

17 1 153~ 14 1 13

j

16 1

20'

18' 14 '9 281S

0

N

D I J

F

M

A

May

-90

' 13 j 16 '

20'

18 1 14 1 9 2B1

D I J

F

M

A

May

/974

/975SAMPLING DATE

Fig. 3 . Seasonal changes in relative abundances of dominant zooplankton species at Station 2-bottom .

1 5 5

1 5 6

0700 0800 0900 1000LowTide

T I ME (hours)Fig . 4 . Changes in total zooplankton numbers during i 2-hour cruise of February 20, 1975 .

0700 0800 0900 1000

FEBRUARY 20, 19750-o Surface*- --0 Bottom

APRIL 14, 1975o-o Surfacee--• Bottom

1100 1200 1300 1400 1500 1600 1700 1800 1900HighTide

)k,

1100 1200 1300 1400 1500 1600 1700 1800High

LowTide

Tide

T I M E (hours)Fig . 5 . Changes in total zooplankton numbers during 12-hour cruise of April 14, 1975 .

ti

1900

12,000ME

10,000

z0H 8,000YzQJ 6,000a.00N 4,000JQHo 2,0001-

0

July-August

Acartia tonsaPseudodiaptomus coronatus

Neopanope sayi (zoea)Pinnixa sayana (zoea)copepodites

September-November

Oikopleura sp .Paracalanus spp .veliger larva

polychaete larva

December

Acartia tonsaveligerAcartia copepodite

January-May

Centropages hamatusCentropages copepoditesTemora longicornisPseudocalanus minutes

Based on the cluster dendrogram, the stations occupiedduring any one cruise were more similar among them-selves than they were to samples taken during othercruises . Often, surface and bottom samples at onestation were not paired together because of widely differ-ing numbers of individuals in these samples .

Discussion

In the Chesapeake and Delaware Canal, Raytheon (1975)reported that zooplankton abundances increased fromJanuary through May and then declined over the follow-ing months . In the coastal waters off the mouth of Dela-ware Bay, highest zooplankton numbers were reported inthe spring and early summer by DuPont et al. (1972), inthe winter and fall by Van Engel & Tan (1965), and in thespring and fall by Sandine & Swiecicki (1975) .

Deevey (,960) found that the greatest numbers of zoo-plankton occurred in Delaware Bay during the winter-spring period . In Contrast, Cronin, et al. (1962) foundthat spring and summer numbers exceeded their fall andwinter numbers. In this study every station showed con-sistently high numbers in the winter-spring, and relativelylow numbers in August )Table 2) . Another peak mayhave occurred in the autumn. Moreover, there was anindication from Station I that the bay can annually sus-tain three major peaks in zooplankton biomass (winter-spring, summer, and fall). This fact would make Dela-ware Bay distinct from more northern estuaries where

bimodal peaks in biomass occur (Deevey, 1956 ; Sage &

Herman, 1972 ; Martin, 1965) . However, it was uncertain

whether the October 1974 peak was real or due to me-chanical failure .

Based on four years' data, Deevey (1960) did not find aconsistent cyclical, annual rhythm in biomass numbers

such as had been found in the more northern areas . Her

findings and our results suggested that any pattern ofrhythmicity which occurred in the latter half of the yearwas due almost entirely to A. tonsa and secondarily to themeroplankton.A. tonsa was the dominant species from June until

October. Its maximum density occurred at Station I(17,272/m) on June 20, 1974 (water temperature 21°C) .Its numbers were drastically reduced in January (watertemperature 4 .5°C) and at the end of the study in May, A.tonsa had not reappeared in very large numbers (watertemperature 18°C) . A similar pattern was noted forA . tonsa in Great Bay, New Jersey and lower NarragansettBay (Sandine & Swiecicki, 1975 ; Hulsizer, 1976) . In thisstudy, A. tonsa occurred as the dominant organism oftwo major peaks ; one occurred in early summer and onein late fall. During the summer A. tonsa generally com-prised 50% of the sample . Major biological activities inthe water column during the summerfall period musthave been greatly affected by the production of A. tonsa .

The winter-spring peak in zooplankton had a relatively

high proportion of Centropages hamatus and Temora

longicornis. Also Pseudocalanus minutes was relativelyabundant at some stations. These species were also strongwinter-spring dominants in nearby coastal waters inprevious years (Van Engel & Tan, 1965 ; DuPont et al.,1972). In contrast, even though the summer sampleshad more species, one species (A . tonsa) comprised thebulk of the samples.

Jeffries (1967) noted the significance of congenericspecies replacement in which case the summer-fallspecies was replaced by a winter-spring species of the

same genus . Some authors noted this replacement withrespect to A. tonsa and A. clausi (Deevey, 1956 ; Conover,1956; Jeffries, 1962), but in a more recent study this replace-ment was not as marked (Hulsizer, 1976) . Deevey (1960)found A. clausi to occur fairly regularly in Delaware Bay,but seldom in as large numbers as A. tonsa. In the presentstudy, A. clausi was first found in low numbers in Decem-ber 1974 and occurred at its highest density (529/m 3) inApril ; at no time did this boreal-temperate species ap-proximate the biomass of A. tonsa .

Bowman (1961) pointed out a geographical trend for

1 57

the A. tonsa-A . clausi replacement. The farther souththe area, the less important A. clausi becomes both innumbers and seasonal range . Jeffries' (1967) observationof congeneric species replacement is valid in a narrowlatitudinal range where optimal conditions for eachspecies occur during some part of the year .

Jeffries (1967) also suggested congeneric species re-placement for Oithona similis-the summer form-andO. brevicornis-the winter form. This was not observedduring this study or in the shallow bays of New Jersey(Sandine & Swiecicki, 1975) . O. similis was found nearlyyear-round while O. brevicornis was found only duringDecember to May. Probably due to warmer winter tem-peratures, the summer form was able to maintain asmall population year-round while the winter form wasonly evident during the shortened winter season . This wasadditional evidence to suggest that Delaware Bay shouldnot be classed with the more northern estuaries in termsof zooplankton composition .

There was an important difference in distribution of amajor species between Cronin et al. (1962) and our re-sults . Our study area occurred in the polyhaline zone(18-3o%o), but Cronin et al. (1962), did not list A. tonsa asone of the principal species of this area although it was byfar the dominant species in our samples . This differencemay be due in part to the fact that a different species com-position may exist in the more offshore polyhaline baywaters examined by these authors or the major speciesmay change from year to year .

Salinity data from the two 12-hour cruises demon-strated that the study area was usually always within thepolyhaline zone . The fact that none of the estuarine cope-pod species were found at ebb tide indicated that move-ment downbay by the estuarine species to this area didnot occur. With the occurrence of the more coastal spe-cies, particularly C. typicus, it was suggested that thestudy site was more influenced by the coastal oceanwaters than by the river waters (Jacobs, 1968) . However,the low numbers or absence of most coastal water cope-pods such as Labidocera and Tortanus suggested that thestudy area may be in that zone of the estuary where theplankton population can generally, by their reproductivecapabilities, maintain themselves (Barlow, 1955) .

In examining the annual species succession in Dela-ware Bay, nearly the same patterns were observed 40years ago (Deevey, 1960) . The dominance of A. tonsa inthe summer, with contributions by the warm watercladoceran, Penilia avirostris, and invertebrate larvae wasrecorded by Deevey (1960). Also, she found the warm

1 58

water copepod, Corycaeus sp., and Cronin et al. (1962)found the other warm water species noted in this study,Euterpina acutifrons . Both these copepods contributedsignificant biomass in November . Deevey (1960) andCronin et al. (1962) also noted high numbers of Oiko-pleura sp . in the fall. In the present study, they dominatedthe mid-October sample . All studies agreed reasonablywell on the significant contribution of the cold waterspecies, C. hamatus, P. minutes, and T. longicornis,during the winter-spring period .

Differences were noted in the abundance of the cope-pods, Paracalanus sp. and P. crassirostris, during the fall .Deevey (1960) and Cronin et al. (1962) both suggested thatParacalanus spp. were relatively rare in the bay and thatthey were more prevalent in the coastal waters . But,in the present study, Paracalanus sp. was found at allstations in the fall. It was one of the dominant organismsat the end of October . This difference among the studiescan probably be explained by the use of a smaller meshsize here, thereby increasing the likelihood of catchingParacalanus.

Deevey also did not record large numbers of veligers inthe winter . These were identified in this study as larvae ofMytilus edulis . Densities as high as 36oo/m 3 were record-ed with the larger numbers of veligers found in bottomsamples. It is likely that the magnitude of the densityobserved here was not an annual occurrence .

Little variation occurred between species compositionand depth . Though there were higher densities andspecies richness in bottom samples, dominant specieswere similar to those at the surface . One notable excep-tion was the marked vertical distribution of Pseudo-diaptomus coronatus, which was found consistently inbottom samples . Jacobs (1961) has suggested that P . co-ronatus is not a truly planktonic copepod since it readilyclings to the substrate . This suggested a reason for itsbottom distribution. Also, there was some indication inthe same bottom samples that P. coronatus can replacethe dominant A. tonsa.

In considering what differences occurred among thestations, the same water mass was probably being sam-pled at each station . Station I-bottom consistently hadhigher numbers of individuals than any other station .This greater abundance of individuals in bottom sampleshas been previously documented (Cronin et al., 1962;Sage & Herman, 1972) . Using cluster analysis, stationsgenerally clustered together within each cruise, showingthat they were highly similar . The few exceptions gener-ally occurred when Station I was dissimilar to Station II

and Station III, or the surface samples were dissimilar tothe bottom. But this dissimilarity was only noted in twoof the 15 cruises . The clusters also illustrated the seasonaltrend of high species diversity in the summer and fall,consecutive cruises displaying less similarity than duringthe winter .

Comparison of Regional Zooplankton NumbersIn terms of total numbers of zooplankton individualsthroughout the year, Station I-bottom was the mostproductive station with the highest mean value of 9311individuals/m 3 . The mean value for all stations was

4649/m3 . Delaware Bay had lower zooplankton num-bers than the Sandy Hook area where Sage and Herman(1972) found a mean value of 8502/m 3 , which wouldcompare only with the mean for Station I-bottom . Her-man et al. (1968) obtained much lower mean values inthe Patuxent Estuary (4325/m) . Sage and Herman(1972) used a smaller mesh net and Herman et al. (1968)a larger mesh net than that used in the present study . ThePatuxent Estuary was also a lower salinity area . Jeffries(1964) found biomasses in Raritan Bay higher than here,but he used a smaller mesh net and collected the wholelength of the bay . In lower Narragansett Bay Hulsizer(1976) reported mean values of 15,900/m 3 in 1972 and28,I0o/m 3 in 1973 . Zooplankton densities averaging44,000 individuals/ M3 were reported for Little EggHarbor and Brigantine Bays, New Jersey by Sandine andSwiecicki (1975) who used a No. 2o net. Heinle's (1966)study on copepod production showed that any directcomparisons of data require similar sampling techniques .Using a No. 2o net on the Patuxent Estuary he founddensities of copepods, consisting primarily of develop-mental stages, exceeding 100,oo0/m3 . The densities ob-tained in this study most nearly approach those foundby Burrell (1972) in the York River, Virginia estuarywhich ranged from 500-13,00o/m 3.

Summary

In this study, all stations showed consistently high num-bers in the winter-spring and relatively low numbers inAugust. Any pattern of rhythmicity which occurred inthe latter half of the year was due almost entirely to A .tonsa and secondarily to the meroplankton .

In contrast to more northern estuaries, A . clausi did notapproximate the density of A. tonsa . Again, the con-generic species replacement for O. similis (summer) and

O. brevicornis (winter) reported in northern estuaries wasnot marked in Delaware Bay .

There was an important difference in distribution of amajor species between an earlier study (17 years ago) andout results . In the polyhaline zone, A . tonsa was a domi-nant species in our study, but was not listed as a principalspecies in the earlier research .

In examining the annual species succession in Dela-ware Bay, nearly the same patterns were observed 40years ago. Dominance of A . tonsa in the summer withcontributions from P. avirostris and meroplankton andsignificant contribution of cold water species, C. hama-tus, P. minutus, and T. longicornis during the winter-spring .

Comparison of Delaware Bay zooplankton with otherestuarine systems (Sandy Hook area, Raritan Bay, Patu-xent River, Narragansett Bay, Little Egg Harbor, Brigan-tine Bay, and York River) showed that local densitieswere more similar to those in the York River, Virginiathan any other area .

References

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