CHAPTER 9, PART 1 



occurs between midnight and sunrise, according to El- 

 brachter ( 1973) who reported that C. tripos divides at rates 

 of 0.03 to 0.3/d in coastal waters. He also reported dark 

 survival times of 41 days in filtered seawater. 



Most species of Ceratium are phototactic and probably 

 tend to aggregate at depths where light is optimal. C. 

 tripos is usually most abundant near the bottom of the 

 euphotic zone. Hasle (1950) reported light-dependent ver- 

 tical migrations, but Nordli (1957) did not observe pho- 

 totaxis in the laboratory. 



Large populations of C. tripos have been observed be- 

 low the euphotic zone, but the extent to which this reflects 

 photosynthetic growth at subeuphotic zone light intensi- 

 ties, long dark survival times, or heterotrophic metabolism 

 is unknown. Circumstantial evidence suggests that some 

 ceratia may have the ability to metabolize organic particles 

 (Hofender 1930; Von Stosch 1964; Norris 1969). Fal- 

 kowski observed inclusions of exogenous origin in ceratia 

 from the Long Island shelf, which suggest phagotrophic 

 assimilation (Plate I). 



The extent to which C. tripos is subject to grazing mor- 

 tality is not well documented. Elbrachter ( 1973) reported 

 that recently divided ceratia were vulnerable to grazing 

 by isopods and ciliates. However, grazing by copepods 

 (the dominant grazers in New York Bight) appears to be 

 minimal (Chervin, in press; Dagg, Brookhaven National 

 Laboratory, personal communication). 



Finally, endoparasites, which inhibit cell division, have 

 been observed in C. tripos (Von Arndt 1967; Elbrachter 

 1973). Consequently, parasitism may be one means by 

 which population size is limited naturally. 



PLANKTON AND BIOLOGICAL OXYGEN 

 DEMAND: JANUARY-SEPTEMBER 1976 



Water-Column Stratification and Dissolved Oxygen 



Seasonal variations in water column stratification and 

 dissolved oxygen (D.O.) in bottom water parallel each 

 other off the New Jersey coast. (See chapter 2.) During 

 winter, the water column is well mixed and D.O. concen- 

 trations are near saturation (6-8 ml/I). As the water col- 

 umn begins to stratify in April, D.O. concentration in the 

 subpycnocline layer begins to decline so that concentra- 

 tions are usually 10 to 40 percent of saturation (2^ ml/I) 

 by July-August. Local anoxic conditions occasionally de- 

 velop below the thermocline in the Christiaensen Basin 

 (head of Hudson Shelf Valley) adjacent to the sewage 

 sludge dumpsite (fig. 9.1-1) in the Bight Apex (National 

 Marine Fisheries Service 1972). During summer 1976, the 

 oxygen minimum layer was more widespread, existed over 

 a longer period of time, and was characterized by lower 

 oxygen concentrations than generally occur that time of 

 year. 



Time-Course of the Ceratium Bloom 



C. tripos was abundant in the Apex at least as early as 

 February 7, 1976, but did not increase substantially during 

 February (fig. 9.1-3). Cell densities increased steadily 

 from a geometric mean of 5.8 cells/ml to 29 cells/ml by 

 the end of March. The growth rate of 0.06 doublings/d 

 calculated from these changes yields a mean water column 

 cell density of 240/ml by late May, which is within the 

 range of densities reported from the layer of maximum 

 cell density in the Apex at this time (fig. 9.1-3). 



A similar pattern was observed at Fire Island Inlet (fig. 

 9.1-3) where cell density increased from less than 0.1/ml 

 in January to 22/ml by the end of March, a rate of 0.05 

 doublings/d. The population remained relatively stable 

 through April and May and declined from a maximum of 

 50/ml in May to less than 0.1/ml by the end of July. This 

 pattern roughly paralleled variations at a station 8 km 

 south of Fire Island Inlet where cell density peaked in 

 May and June and declined rapidly thereafter to near zero 

 in August (fig. 9.1-4). 



Mean cell densities along the New Jersey shore peaked 

 near mid-June (fig. 9.1-3). In the New York Harbor re- 

 gion, cell densities were highest in March (29-75/ml), de- 

 clined to 10/ml by the end of May, and remained constant 

 at 10/ml through mid-July. 



Cell densities in the outer Bight increased from 1 to 60/ 

 ml (mean = 10/ml) near the end of March to 10 to 400/ 

 ml by mid-June (mean = 240/ml). Based on qualitative 

 net phytoplankton samples collected from 10 m with a 

 Hardy continuous plankton recorder (225 by 234 |xm 

 mesh), C. tripos was present throughout the Bight in Jan- 

 uary and increased to a maximum in May (fig. 9. 1-5). The 

 decrease from May to June was probably a consequence 

 of an aggregation of cells below the thermocline, as dis- 

 cussed later. 



Vertical Distribution 



Vertical profiles of temperature, chlorophyll a, and C. 

 tripos cell density showed little stratification from January 

 through March when netplankton (phytoplankton re- 

 tained on a 20-fjLm mesh screen) accounted for more than 

 80 percent of chlorophyll a in the water column. As the 

 water column began to stratify in April, vertical distri- 

 butions of chlorophyll (figs. 9. 1-6 and 9. 1-7) and C. tripos 

 (fig. 9.1-8) began to show patterns of stratification, which 

 varied systematically across the shelf. 



Based on continuous vertical profiles between April 30 

 and May 5, seaward of the shelf break (stations 93, 94) 

 maximum chlorophyll-w concentrations occurred in the 

 upper 25 m and diatoms dominated the phytoplankton 

 (fig. 9.1-6). Across the shelf break (stations 95, 96, 97) 

 a broad maximum between 10 and 35 m was observed, 

 which was dominated by diatoms near the surface and by 

 C. tripos at depth. Farther inshore (stations 98, 99, 101, 



197 



