FISHERY BULLETIN: VOL. 86, NO. 2 



Mexico and by comparing these with oceano- 

 graphic and hydrographic indicators of water 

 column stratification and productivity. 



METHODS 



Samples were collected during 5 winter cruises 

 between November and March, during 1981-84, 

 (Table 1). Microplankton samples and hydro- 

 graphic data were collected from 3 general re- 

 gions of the northern Gulf of Mexico: off of Galve- 

 ston, TX, in the region of the Mississippi River 

 delta, and off of Cape San Bias, FL (Fig. 1). Sam- 

 ples were not collected from all 3 regions on each 

 cruise. 



Water samples for chlorophyll and nutrient 

 analyses were collected with Niskin bottles. For 

 each chlorophyll analysis, between 25 and 150 

 mL of sea water was filtered onto a GF/F or GF/C 

 glass fiber filter and homogenized by grinding in 

 90% aqueous acetone. Fluorescence of the filtrate, 

 brought up to a volume of 10.0 mL, was deter- 

 mined before and after acidification with 2 drops 

 of 10% HCl using a Turner Designs'' Model 10 

 fluorometer. Chlorophyll and pheopigment con- 

 centrations as chlorophyll equivalents were de- 

 termined from 



chlorophyll (fxg/L) = 



Table 1.— Station information, 1981-84. 



pheopigment (|JLg/L) 



where K is the machine calibration constant, [„ 

 and fa are the fluorescence readings before and 

 after acidification, R is the acid ratio, and u is the 

 volume of seawater filtered, in mL (Strickland 

 and Parsons 1968). 



Samples for nutrient analyses were frozen. Ni- 

 trate and nitrite were analyzed according to 

 method number 353.2 described by EPA publica- 

 tion number EPA 600/4-79-020 (Environmental 

 Protection Agency 1979). 



Temperature and salinity were measured by 

 several methods. During cruise I, temperature 

 was measured by the temperature sensor on the 

 MOCNESS^ net, and salinity was measured with 

 a refractometer. During cruises II and III, tem- 

 perature was measured with expendable bathy- 



'^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



^Multiple opening-closing net and environmental sensing 

 system. 



thermographs (Sippican Instruments). Salinity 

 was measured with a YSI Model 33-S-C-T sali- 

 nometer on cruise II, and with a Beckman Model 

 RS5-3 salinometer on cruise III. For both of these 

 cruises, bottle samples, analyzed in the labora- 

 tory with a Guildline Model 8400A Autosal, were 

 used to check the shipboard salinity measure- 



320 



