CHAPTER 9, PART 1 



PHYTOPLANKTON ECOLOGY 



Productivity 



Major studies of phytoplankton productivity include: 

 Ryther and Yentsch (1958), the outer Bigiit; Mandelli et 

 ai. (1970), the Long Island coast; and Malone (1976a, 

 1976b, 1977a, 1977b), the lower Hudson-Raritan estuary 

 and Bight Apex. The following synthesis is based on these 

 studies and on reviews by Smayda (1976) and Yentsch 

 (1977). 



Annual phytoplankton productivity generally decreases 

 with depth and distance from the mouth of the Hudson- 

 Raritan estuarine complex. Phytoplankton productivity in 

 the Apex is about 430 g C/m-/yr. or 70 to 80 percent of 

 the annual input of particulate organic carbon (POC) to 

 the Apex. The remaining inputs are derived primarily 

 from sewage wastes generated by the New York-New Jer- 

 sey metropolitan population. (See chapter 15.) Phyto- 

 plankton productivity in the outer Bight decreases from 

 160 to 100 g C/m-/yr as water column depth increases from 

 less than 50 m over the continental shelf to 1,000 m over 

 the slope. 



An important exception to this general trend is the high 

 phytoplankton biomass and productivity often observed 

 in the region of the shelf break (Fournier et al. 1977). The 

 development of phytoplankton blooms along the shelf 

 break appear to be a consequence of nutrient enrichment 

 and vertical stability provided by a frontal system sepa- 

 rating nutrient-poor shelf water from nutrient-rich slope 

 water. 



Phytoplankton productivity in the Apex fluctuates be- 

 tween 0.1 and 6.6 g C/m-/d (mean = 1.2 g C/m=/d com- 

 pared to 0.1 to 1.1 gC/m-/d (mean = 0.4 g C/m-/d) in the 

 outer Bight. Seasonal variations in the outer Bight appear 

 to be characterized by winter-spring blooms. Over the 

 midshelf area (<50 m) of the outer Bight, productivity is 

 0.5 to 1.0 g C/m-/d from November through April, but is 

 less than 0.5 g C/m-/d the remainder of the year. Offshore 

 (50-150 m), productivity exceeds 0.5 g C/m=/d during 

 March-May. 



In contrast, seasonal variations in the Apex are char- 

 acterized by two bloom periods coinciding with minimum 

 time-dependent changes in surface temperature during 

 February-March (2°-8° C) and June-July (19°-23° C). Chain- 

 forming diatoms (netplankton retained on a 20-|a.m mesh 

 screen) with mean euphotic zone generation times of 1 to 

 3 days usually dominate phytoplankton blooms in Feb- 

 ruary-March. During these months the water column 

 (20-30 m deep) is well mixed, the euphotic zone extends 

 to the bottom, and phytoplankton populations are abun- 

 dant throughout the water column. Maximum biomass 

 occurs during this period. Phytoplankton productivity is 

 generally higher during the June-July bloom period when 

 small green algae (nannoplankton with mean spherical 



diameters less than 10 (jim) growing at mean euphotic zone 

 generation times of 0.5 to 1.5 days dominate phytoplank- 

 ton blooms. At this time, the water column is well strat- 

 ified, with the thermocline between 5 and 15 m (5-20 m 

 off the bottom); the euphotic zone is 5 to 15 m deep; and 

 phytoplankton populations are concentrated near the sur- 

 face, with maximum densities along the New Jersey coast 

 within 20 km of the Hudson-Raritan estuary mouth. 



Major inputs of inorganic nutrients (in contrast to re- 

 generated nutrients within the Bight) include estuarine 

 runoff (mainly from the Hudson River) and fluxes onto 

 the shelf in the region of the shelf break. Although ob- 

 servations in the Bight as a whole are lacking in spatial 

 and temporal resolution, they do indicate that phytoplank- 

 ton biomass tends to be high and decreases slowly away 

 from sites of nutrient input before thermal stratification. 

 As the water column stratifies, the centers of maximum 

 biomass move closer to the nutrient reservoirs that supply 

 the euphotic zone. This leads to narrow zones of high 

 production along the coastline and the shelf break, and 

 to the development of a chlorophyll maximum in the ther- 

 mocline over the midshelf. 



High productivity and the occurrence of two major 

 bloom periods in the Apex reflect the 1) continuous input 

 of nutrient-rich estuarine water (table 9.1-1), 2) effects 

 of thermal stratification and coastal circulation on the dis- 

 tribution of estuarine water, 3) rapid regeneration of nu- 

 trients during summer (table 9.1-1), and 4) seasonal var- 

 iations in grazing pressure, which peaks during late spring 

 and summer. Winter diatom blooms develop, because of 

 low grazing pressure. Apparently, very little diatom pro- 

 duction is grazed, and most of the biomass produced sinks 

 to the bottom over an unknown but larger area than the 

 Apex. Thus, winter-spring diatom blooms in the Apex 

 may be a factor in the development of oxygen minima 

 below the pycnocline during summer. 



Summer nannoplankton blooms in the Apex are con- 

 centrated in the surface layer where they are rapidly 



Table 9.1-1. —Seasonal comparison of dissolved nitrogen input by es- 

 tuarine runoff and uptake by phytoplankton. proportion of phytoplank- 

 ton demand supplied by runoff, and area required to assimilate the 

 nitrogen input 



' Calculated from primary productivity (Malone 1976a. 1977b) and 

 assuming a C:N assimilation ratio of 7.0 by weight. 



195 



