NO A A PROFESSIONAL PAPER 11 



9.1-13), resulting in a uniform distribution of cells across 

 the euphotic zone. Productivity at this station was 3.5 g 

 C/m-/d, giving a mean euphotic zone growth rate of 0.2 

 doublings/d. A sample from 30 m (1% light depth) in the 

 maximum layer in May had a productivity of 8 mg C/mV 

 d and a growth rate of 0.04 doubhngs/d. Thus, cells in the 

 maximum layer in the lower reaches of the euphotic zone 

 were probably growing photosynthetically at very slow 

 rates (20- to 30-day generation times). 



Within about 20 km of the New Jersey coast and 80 km 

 of Sandy Hook, the bulk of the C. tripos population was 

 below the compensation light depth (compensation inten- 

 sity = 100-150 (xE/m^/d) between the thermocline and 

 the bottom. Two independent estimates of respiration 

 rates (from measured photosynthesis-light curves and 

 from the carbon content of the cells) indicate that C. tripos 

 respires about 3 percent of its cell carbon/d at 10° C. Con- 

 sequently, some form of heterotrophic metabolism or con- 

 tinuous recruitment from offshore photosynthetic popu- 

 lations must have occurred to account for the observed 

 increase in population density after the water column strat- 

 ified. 



Suspended Particulate Organic Matter and 

 Phytoplankton 



Levels of particulate organic carbon (POC) in the Apex 

 water column from September 1973 through November 

 1975 fluctuated about a mean of 9.8 g C/m- (1 standard 

 deviation = 2.9). The maximum turnover time of this 

 organic matter is 2 to 15 days (annual mean = 8 days) 

 and reflects the fact that particulate organic matter (POM) 

 does not tend to accumulate in the water column under 

 most circumstances. 



This rapid turnover of POM was not observed in Feb- 

 ruary and March 1976 (fig. 9.1-15). During this period. 

 POC accumulated in the water column to levels two to 

 three times higher than previously observed. This incfease 

 coincided with the initial phases of the C. tripos bloom 

 (fig. 9.1-3). C. tripos accounted for 25 to 45 percent of 

 suspended POC until the end of March when it accounted 

 for 64 percent. Elimination of the carbon accounted for 

 by C. tripos from the suspended POC pool gives water 

 column POC concentrations that reflect the diatom bloom 

 in early March and are in the range of values previously 

 reported (fig. 9.1-15). 



The influence of C. tripos on the pool of phytoplankton- 

 C in the Apex was significant (fig. 9.1-16). Before 1976, 

 phytoplankton-C accounted for 15 to 45 percent of sus- 

 pended POC, with proportions of 35 to 45 percent typical 

 of phytoplankton blooms regardless of time of year and 

 dominant species. During February and March 1976. how- 

 ever, phytoplankton-C increased from 56 to 84 percent of 

 the suspended POC pool. Removal of C. tripos brings the 

 proportion of phytoplankton-C back into the range usually 



observed in the Apex and shows the diatom bloom peaking 

 in early March (fig. 9.1-16). The gradual increase in the 

 biomass of C. tripos and the subsequent accumulation of 

 POC in in the water column did not appear to influence 

 the typical development of the winter-spring diatom 

 bloom. 



Temporal variations in copepod abundance and grazing 

 rates indicate that very little of the diatom bloom is grazed 

 at temperatures below 10° C (Chervin 1978). Above 10° C 

 selective grazing could become important, because es- 

 tuarine copepods (the major particle grazers in the Apex) 

 do not eat C. tripos (Chervin 1978), and increased co- 

 pepod grazing pressure during spring is probably a factor 

 in transition from netplankton to nannoplankton-domi- 

 inated phytoplankton blooms. C. tripos appears to be a 

 slow-growing species subject to low predation pressure. 



Accumulation of Ceratium tripos off the New Jersey 

 Coast 



The temporal and spatial distributions of C. tripos in 

 the New York Bight show an increase and a shift in max- 

 imum abundance from offshore before stratification to 

 inshore as the water column stratified. The increase in cell 

 density was most pronounced off the New Jersey coast. 

 Two hypotheses, not mutually exclusive, have been sug- 

 gested to account for these distributions. 



The first hypothesis is similar to the accumulation mech- 

 anism demonstrated for Prorocentrum micans and other 

 dinoflagellates in Chesapeake Bay (Tyler and Seliger 

 1978). It requires a two-layered circulation pattern with 

 an onshore flow of bottom water and an offshore flow of 

 surface water, organisms that aggregate in the bottom 

 layer, and an ability to survive for extended periods of 

 time at low light levels. A two-layered, thermohaline cir- 

 culation has been described for New York Bight (Ketchum 

 and Keen 1955; Bumpus 1964), and it has been well-doc- 

 umented in this report that the C. tripos population ag- 

 gregated near the upper boundary of the bottom layer. 

 Possibly, most of the increase in population size occurred 

 before stratification when the population was distributed 

 throughout the euphotic zone and nutrients were plentiful. 

 Once the water column stratified, C. tripos aggregated 

 near the base of the thermocline throughout the Bight and 

 the onshore movement of bottom water resulted in a shift 

 in the location of maximum abundance from offshore to 

 inshore. This process took place over 3 months (April- 

 June), and, though we cannot determine whether the ob- 

 served increase in cell density was a consequence of 

 growth or an aggregation of cells, some form of anabolic 

 metabolism was required to satisfy cellular respiratory 

 demands during this period. Because the C. tripos layer 

 was between the 1 and 3 percent light depths over most 

 of the outer Bight more than 20 km from the New Jersey 

 coast, it is likely that the population in this region was 



204 



