NO A A PROFESSIONAL PAPER 11 



140 



120 



too 



80 



E 



i/i 



60 



40 



20 - 



Li 



MARCH 1976 



J. 



20 m m 



STATION 3 



at 30m depth 



24m Om 



STATION 2 



at 50m depth 



STATION 1 



at 100m depth 



FIGURE 9.1-11 . — Histogram of C. tripos and Nitzschia seriata cell den- 

 sity off Long Island and extending south across the shelf to the shelf 

 break (stations 3, 2, and 1), March 1976. 



growing photosynthetically. Within 20 km of the coast, 

 and especially in the region of the Hudson River plume, 

 the C. tripos layer was usually below the 1 percent light 

 depth. These observations suggest the hypothesis that 

 coastal populations in the subeuphotic zone were main- 

 tained and possibly increased by recruitment of actively 

 growing, photosynthetic populations from farther off- 

 shore. 



Though some form of shoreward entrainment must have 

 taken place, several objections exist that question the im- 

 portance of this mechanism. 



1. A shoreward flow of bottom water would transport 

 not only C. tripos into the region where the oxygen min- 

 imum layer was pronounced but also oxygenated water. 

 (See chapter 8, table 8-2.) 



2. Estimates of photosynthetic growth rates at the 1 

 percent light depth were 0.02 to 0.04 doublings/d in both 

 late April and in May. However, the rate of increase of 

 population density from May to June off the New Jersey 

 coast was 0.04 doublings/d. If the coastal population was 

 being maintained by recruitment from offshore popula- 

 tions, the increase in cell density probably reflected an 

 increase in concentration rather than an increase in pop- 

 ulation size. 



3. Nitrate + nitrite concentrations were low throughout 

 the water column across the shelf except in the Apex (fig. 

 9.1-17). The nitrogen budget for the Apex during May- 

 July 1975 (table 9.1-1) indicates that the nitrogen supply 

 to the euphotic zone and phytoplankton uptake rates are 

 high and closely coupled, and that regenerated ammonia 

 is a major source of nitrogen. Phytoplankton blooms dur- 

 ing May and June are usually dominated by small-celled 

 phytoplankters growing at mean euphotic zone rates of 

 0.5 to 2.0 doublings/d. These blooms are localized in the 

 surface mixed layer (upper 10 m of the water column) and 

 are most pronounced off the New Jersey coast in the plume 

 of the Hudson River. There is no evidence that C. tripos 

 influenced the development of these blooms during June 

 1976, and nannoplankton chlorophyll concentrations in 

 the surface layer were similar to previous years. The nu- 

 trients required for nannoplankton growth are derived 

 from estuarine runoff and regeneration above the ther- 

 mocline (Malone 1976b). Considering the distribution of 

 C. tripos and its photosynthetic growth rate, it is unlikely 

 that it was competing (or could compete) with nanno- 

 plankton populations for these nutrients. If photoautotro- 

 phy was involved in the maintenance or growth of the 

 subthermocline population, nutrient inputs must have 

 been greater than in previous years and must have in- 

 volved onshore transport of bottom water across the shelf. 

 However, if C tripos is capable of "luxury" nutrient up- 

 take and can store nutrients for weeks or months, the 

 nutrient distributions of May and June might not be a 

 factor. (Luxury consumption of this magnitude has never 

 been reported). 



The second hypothesis involves heterotrophic growth 

 (or maintenance) by the C tripos population below the 

 Hudson River plume off the New Jersey coast. This hy- 

 pothesis is based on circumstantial evidence. 



1. Growth of C. tripos had no obvious effect on growth 

 of diatom populations during May and June in the Apex. 

 Yet, growth of C tripos during February and March in- 

 creased the POC content of the water column by a factor 

 of 2 or 3 over previous years. 



2. C. tripos did not respond (as reflected in distribution 

 of biomass) to estuarine runoff as other photoautotrophic 

 populations did. The observed downstream increase in 

 biomass (in contrast with the distribution of diatoms in 

 February and March and nannoplankton in May and June) 

 would develop if C tripos were feeding phagotrophically 

 on POM of estuarine origin or on phytodetritus. 



3. C. tripos is euryhaline, with a salinity optimum of 

 20%c to 25%c. This suggests that the decline in abundance 

 with decreasing salinity in the Apex was not related to 

 salinity per se. 



Since C. tripos may have the ability to ingest POM, the 

 observed accumulation of C. tripos in the water column 

 may have been a consequence of phagotrophic uptake of 



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