CHAPTER 9, PART 1 



considerable organic loading over the past two decades, 

 and the development of oxygen minimum layers and local 

 anoxia have occurred previously during summer (ch. 1). 

 Based on the effect of C. tripos on the content of POC 

 in the water column and on the development of large, 

 subthermocline populations, it is likely that C. tripos made 

 a very significant contribution to the oxygen demand re- 

 quired to account for the oxygen minimum layer. In the 

 latter context, a flocculent suspension of organic matter 

 at least 1 cm thick coated the bottom during July between 

 Sandy Hook and Atlantic City from 5 to 50 km offshore. 

 The floe consisted primarily of phytoplankton cells dom- 

 inated by C. tripos. Microscopic examination indicated a 

 steady increase in the decomposition of C. tripos cells 

 during July. (See chapter 9, part 2.) 



A computer simulation model was used to explore the 

 combined effects of benthic respiration and C. tripos res- 

 piration on the rate of oxygen depletion below the ther- 

 mocline. C. tripos respiration rates were calculated from 

 the expression R = aW^ (Banse 1976) where a and h are 

 temperature dependent constants and W and R are the 

 weight of the cell in picograms (pg) of carbon and the 

 respiration rate in picograms of carbon/cell/hour, respec- 

 tively. The carbon content of C. tripos was calculated from 

 both CHN analysis and regression of netplankton chlo- 

 rophyll a on netplankton carbon. Values ranged from 

 1 20,000 to 30,000 pg/cell; a mean value of 25,000 was cho- 

 I sen for calculating respiration rates. Using Q,,, = 2.3, the 

 i carbon specific respiration rate of a single cell was cal- 

 • culated to be 0.003/h at 10° C ( = 1.4 x 10 V' O./cell/ 

 h). In 1977, C. tripos respiration was measured in the field 

 using an oxygen polarographic electrode and an electron 

 transport system assay. The resuhs of these direct meas- 

 urements suggested a respiration rate of 1.39 ± 0.17 x 

 10 -^ |xl O./cell/h at 10° C, agreeing well with the rate 

 calculated according to the expression given by Banse 

 (1976). 

 ij The following information was input: 



1 . A mean benthic respiration rate of 11 ml 0_,/nr7h( 1 .0 

 mg-at 0,/m-/h) was calculated from Thomas et al. ( 1976) 



I for an "average" community in the Bight. 



2. Eddy diffusion coefficients of 1.0 cm-7s across the 

 thermocline and 10 cm7s below the thermocline were 

 used. 



3. The thermocline was placed 25 m above the bottom. 

 This situation existed off the coast of Long Island, but the 

 thermocline was much closer to the bottom off the coast 

 of New Jersey. 



4. The overlying water was nearly saturated with oxy- 

 gen, starting with 6.72 ml/l(= 0.6 mg-at 0;/l). 



5. Using data collected on six cruises in New York Bight 

 (Brookhaven National Laboratory data base), the follow- 

 ing numbers of cells were placed in the bottom 20 m: (a) 

 0-5 m (above the bottom) 2 x 10^ cells/m' (consuming 



2.8 X 10-^ ml Oj/l/h); (b) 5-10 m, 4 x 10^ cells/m' (con- 

 suming 5.6 X 10' ml Ojyh); (c) 10-15 m, 6 x 10' cells/ 

 m' (consuming 8.4 x lo" ' ml 0,/I/h; (d) 15-20 m, 2 x 

 10'*cells/mMconsuming2.8 x 10"- ml O./l/h). (Cells from 

 the upper 5 m were excluded because they may be at or 

 above the compensation depth and do not contribute sub- 

 stantially to oxygen depletion.) 



The model output indicated that within two months the 

 oxygen concentration in the bottom 5-m layer reaches a 

 steady state concentration that is 45 percent of the initial 

 oxygen concentration. The simulated rate of oxygen de- 

 pletion below the thermocline is extremely sensitive to 

 changes in eddy diffusivity, and small changes in diffusivity 

 are sufficient to cause simulated anoxia. 



These calculations show the potential metabolic influ- 

 ence of C. tripos. The water column integrated C. tripos 

 respiration rate exceeds the benthic oxygen consumption 

 rate by a factor of 20. Therefore, C. tripos biomass is a 

 large potential source of BOD. Oxidation of the C. tripos 

 biomass (3,255 mg-at C/m-) within 20 m of the bottom 

 would require 8,463 mg-at OJm- or 71 percent of the 

 initial oxygen content. Thus, the combined effects of res- 

 piration and subsequent death and decay of the biomass 

 were more than sufficient to produce anoxia. 



The occurrence of an oxygen minimum layer and local 

 anoxic waters off the New Jersey coast in contrast to the 

 Long Island coast may reflect differences in bottom to- 

 pography, residence time of water in the bottom layer, 

 and turbulent mixing. The shelf within 50 km of the coast 

 is much flatter and shallower off New Jersey than off Long 

 Island. Consequently, the C. tripos layer between the 20- 

 and 40-m isobaths off New Jersey was distributed over the 

 bottom surface in a subthermocline water column 5 to 15 

 m thick, whereas the C. tripos maximum off Long Island 

 intersected the bottom along an isobath and was well off 

 the bottom (>30 m) over most of its extent. In addition, 

 high cell densities occurred over larger areas off New Jer- 

 sey. These observations and the possibility that the resi- 

 dence time of bottom water is longer off New Jersey than 

 off Long Island could explain the development of a more 

 intense and widespread oxygen minimum layer off New 

 Jersey. 



Nannoplankton productivity per se was probably not a 

 major factor in the 1976 oxygen depletion even though it 

 normally accounts for most of the input of POM to the 

 region. With the exception of winter-spring diatom blooms, 

 which apparently go ungrazed, there is no evidence that 

 a significant portion of phytoplankton production nor- 

 mally accumulates below the thermocline during summer. 

 The dominance of small-celled phytoplankton (usually less 

 than 10 M-m in diameter), vertical chlorophyll-a distribu- 

 tions, the importance of ammonia as a nitrogen source for 

 phytoplankton, and the rapid increase in zooplankton 

 grazing pressure during May and June are consistent with 



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