NO A A PROFESSIONAL PAPER 11 



over netphytoplankton above the pycnocline. The rapid 

 growth of these small, nonsiliceous nannoplankton (e.g., 

 Nannochioris atomus) was primarily responsible for the 

 high productivity and low nutrient concentrations ob- 

 served above the pycnocline during August-September 

 1976. 



PHYTOPLANKTON PRODUCTIVITY 



Integral daily rates of phytoplankton productivity were 

 generally high throughout the Bight, ranging from 0.1 g 

 C/m-/d at station 76 in the upper Hudson Shelf Valley to 

 12.7 g C/m-/d at station 34 in the central Christiaensen 

 Basin (figs. 10-8 and 10-9A). Of the 21 stations, daily 

 productivity exceeded 1 g C/m-/d at 11 stations and 3 g 

 C/m-/d at 5 stations. Photosynthetic efficiencies per unit 

 light energy and estimated growth rates (based on pro- 

 ductivity to chlorophyll-fl ratios) were very high (table 

 10-1). At many stations, growth rates of phytoplankton 

 exceeded two divisions per day. At highly productive sta- 

 tions (i.e., station 34) adjacent to the estuary the euphotic 

 layer was less than 10 m. whereas the euphotic layer at 

 offshore stations was 20 to 40 m deep (figs. R)-8 and 

 10-9A). Outside the Apex, adjacent to the New Jersey 

 coast (i.e., stations 200 and 213). the euphotic layer oc- 

 cupied the entire water column (figs. 10-8 and 10-9). At 

 these stations the typical vertical profile (i.e., station 34, 

 fig. 10-9) with the highest productivity at or near the 

 surface is not seen. Instead, the simulated in situ (SIS) 

 and photosynthetic capacity (PC) primary productivity of 

 these stations (i.e., station 200) is maximal below the pyc- 

 nocline (thermocline, fig. 10-9A and B) and near the bot- 

 tom (table 10-2 and fig. 10-9A). In the oxygen-depleted 

 area (i.e., stations 213, fig. 10-9A) phytoplankton biomass 

 was high below the pycnocline as evidenced by PC meas- 

 urements. Vertical profiles of SIS and PC data for all 

 stations can be found in Thomas et al. (in preparation). 



The levels of primary productivity observed in the Apex 

 and the estuary are comparable to summer values reported 

 by Malone (1976) and O'Reilly et al. (1976). The value 

 of 12.7 g C/m-/d observed at station 34 near the sewage 

 sludge disposal site seems anomalously high (fig. 10-8). 

 However, the average concentrations of euphotic chlo- 

 rophyll a were high (16 mg/m', fig. 10-6) and the ratio of 

 integral daily productivity to integral chlorophyll a (79.8 

 g C/m^/d:g Chla/m") was within the range observed for 

 the estuary and Apex (table 10-1). The productivity ob- 

 served outside the Apex and in and around the low D.O. 

 area seems high when compared with the values (0.2-0.3 

 g C/m^/d) estimated from chlorophyll and light data by 

 Ryther and Yentsch (1958) for stations off Barnegat Inlet 

 in late summer. However, comparison of our August-Sep- 

 tember 1976 and June 1977 data for the same area shows 



that total primary productivity was about the same in both 

 years for the entire area studied, but was slightly higher 

 in June 1977 than in August-September 1976 for the low 

 D.O. area. The euphotic layer did not occupy the entire 

 water column in June 1977 in contrast to August-Septem- 

 ber 1976. 



Photosynthetic capacity (at saturating artificial light in- 

 tensities) in surface waters ranged from 50 to 100 mg C/ 

 mVh in the Apex and near Sandy Hook to between 1 and 

 5 mg C/mVh along the 50-m isobath at the eastern edge 

 of the sampling area off Delaware Bay (fig. 10-6). The 

 largest change in this estuarine-offshore gradient was near 

 stations 86, 76, and 51 at the outer perimeter of the Apex. 



Photosynthetic capacity and simulated in-situ produc- 

 tivity of nannoplankton was much greater than that ob- 

 served for netplankton (figs. 10-8 and 10-9A). Note that 

 at station 200 (fig. 10-9A) netplankton productivity is 

 greater than nannoplankton productivity below the pyc- 

 nocHne. The predominance of netphytoplankton in bot- 

 tom water was previously noted in the discussion on the 

 distribution of phytoplankton species. The lowest nan- 

 noplankton/netplankton productivity ratios observed, 1.0 

 to 1.5, were found at stations 41. 34. and 200 (fig. 10-8). 

 During the June 1977 survey, nannophytoplankton also 

 dominated primary productivity. 



Throughout our study area, phytoplankton above the 

 pycnocline were metabolically active and community 

 growth rates were high (table 10-1 ). Assimilation numbers 

 of 10 and above (fig. 10-7) indicated that phytoplankton 

 were not nutrient limited (Curl and Small 1965) even 

 though low and near zero concentrations of nutrients (am- 

 monium, nitrate, nitrite, and silicate) were observed in 

 surface water. The high values for community primary 

 productivity and high photosynthetic efficiencies above 

 the pycnocline were due in part to the small size of the 

 nannoplankters. high surface area to cell volume ratios, 

 and high nutrient uptake rates, which are often (but not 

 necessarily always) associated with smallness (Taguchi 

 1976). These actively growing populations may have con- 

 tributed to maintenance of the depressed D.O. episode 

 by continually loading bottom water with a portion of the 

 surface layer production or organic matter derived from 

 phytoplankton (i.e., fecal pellets. Malone 1978). 



Phytoplankton below the pycnocline, where light inten- 

 sity was low (\'7c to 3% of surface intensity), had low 

 assimilation numbers (fig. 10-7) and low photosynthetic 

 efficiencies. High rates of photosynthetic capacity under 

 optimal light conditions (fig. 10-9A), low assimilation 

 numbers, and low chlorophyll/phaeopigment ratios where 

 chlorophyll a was elevated indicate that the subpycnocline 

 phytoplankton, though very abundant, were probably 

 physiologically debilitated. They were not nutrient lim- 

 ited, because nitrogen, phosphorus, and silica were abun- 

 dant. 



244 



