JtlDKINS ET AL.: ZOOPLANKTON IN THE NEW YORK BIGHT 



avirostris (September 1974), L. retroversa (March, 

 April), Pseudocalanus sp. (May), and Paracalanus 

 parvus (August, September 1975). 



DISCUSSION 



Previous zooplankton studies in the New York 

 Bight have been based on relatively few samples, 

 usually taken from a restricted area over a limited 

 period of time (of. review in Malone 1977). Grice 

 and Hart's (1962) study is closest to ours in 

 taxonomic coverage, net mesh size, geography, 

 and quantitative analysis. They collected a total of 

 14 samples with vertically hauled 230 /xm mesh 

 nets from New York Bight shelf waters on cruises 

 in September and December 1959 and March and 

 July 1960. These samples were part of a larger 

 study of zooplankton along a transect between 

 Montauk, N.Y., on eastern Long Island and Ber- 

 muda. Comparison of mean concentrations of sev- 

 eral abundant species of copepods in their samples 

 (table 4, Grice and Hart 1962) with our mean 

 concentration values (Table 2) is informative. The 

 eight most abundant copepods during 1959-60 (in 

 order of decreasing abundance: Pse£/c?oca/a;!i/s sp., 

 C. typicus, O. similis, T. longicornis, Paracalanus 

 parvus, Calanus finmarchicus, M. lucens, Can- 

 dacia armata) correspond closely with the eight 

 most abundant species in 1974-75 (Centropages 

 typicus, Pseudocalanus sp., T. longicornis, 

 Paracalanus parvus, Calanus finmarchicus, O. 

 similis, Acartia tonsa, O. atlantica). Furthermore, 

 the mean densities of the two most abundant 

 copepods in both studies, Centropages typicus and 

 Pseudocalanus sp., were very similar for both 

 species during the two periods (i.e., the mean den- 

 sity of C. typicus was 450/m^ in 1959-60 and 

 650/m^ in 1974-75; the mean density of Pseudo- 

 calanus sp. was 560/m^ in 1959-60 and 520/m^ in 

 1974-75). This comparison suggests that zoo- 

 plankton in the New York Bight had not changed 

 substantially in the 15 yr between the two studies. 

 The degree of similarity is somewhat surprising in 

 view of the evidence that considerable year-to- 

 year variations may occur in the timing, duration, 

 and amplitude of abundance maxima in important 

 zooplankton taxa (Bigelow and Sears 1939; Sears 

 and Clarke 1940). 



Grice and Hart (1962) observed an influx of 

 warmwater oceanic species into the New York 

 Bight in September 1959, and this is similar to the 

 high incidence of subtropical-tropical species in 

 autumn 1974 and summer 1975. This apparently 



annual phenomenon is probably associated with 

 intrusions of the Gulf Stream over the continental 

 slope which occur most frequently during the 

 warm seasons (Wright 1976; Bowman 1977). Our 

 hydrographic data reveal the occurrence of salini- 

 ties ( s=36'7( ) characteristic of Gulf Stream water 

 (Wright 1976) in the slope region during Sep- 

 tember 1974, and in June, August, and September 

 1975 (Figure 4), and the National Environmental 

 Satellite photos show Gulf Stream water imping- 

 ing along the outer edge of the study area in Au- 

 gust 1974 and in May, July, and August 1975. 



A shoreward increase in the abundance of sev- 

 eral common offshore copepods (e.g., Calanus 

 finmarchicus, O. atlantica, Clausocalanus 

 pergens, M. lucens) also occurred during warm 

 portions of the year. This onshore increase in 

 abundance of common forms and the frequent oc- 

 currence over the shelf of less common oceanic 

 species are probably the result of shoreward mix- 

 ing of slope water with shelf water. Slope water is 

 thought to move onshore along isopycnals during 

 late summer and autumn (Wright and Parker 

 1976; Gordon et al. 1977), and during September 

 1974 we observed slope water (35%o ssalinity 

 <36%o, Wright 1976) on the shelf (Figure 4). 



Limacina retroversa, Pseudocalanus sp., O. 

 similis, and Calanus finmarchicus, the species re- 

 sponsible for zooplankton abundance maxima in 

 the New York Bight during spring 1975, are low- 

 temperature forms whose distributions are cen- 

 tered north of the region (Fish 1936a, b,c; Redfield 

 1939; Bigelow and Sears 1939; Fleminger and 

 Hulsemann 1977). Their geographical distribu- 



TOTAL SAMPLE NUMBER 

 181 71 245 270 253 



132 154 , 195 147 256 261 



_ 370 



? 36.0 



< 

 1/) 



350 



i 



S'74 N 



F 75 M A 

 MONTH 



Figure 4.— Occurrences of Gulf Stream water (salinity s3e%.>) 

 over slope (sslOO ml. and of slope water (35%o<salinity<36%o) 

 over the shelf in the New York Bight, September 1974- 

 September 1975. Dots = Gulf Stream salinities over slope; x = 

 slope salinities over shelf. 



681 



