casts, and hydrographic data) must be taken with sufficient resolution to address the specific 

 objectives of the field program. Based on our preliminary data from July, 1993, phytoplankton 

 community structure and chlorophyll concentration are fairly conservative properties (relative to 

 temperature and salinity) on the continental shelf off Cape Hatteras in the summer. In July, 

 1993, 16 stations positioned five minutes apart in a hydrographic grid on the mid-shelf region 

 off Cape Hatteras (between 36.0 and 36.5 °N, and 75.02 and 75.17 °W) revealed a large, 

 chlorophyll-rich pool of very cold (7-9 °C) water approximately 15 meters thick lying over the 

 bottom of the shelf. Given that the average shelf depth in this grid was 32 meters (s.d. 2.8m), 

 this could be interpreted as a benthic boundary layer occupying roughly half the water column. 

 However, the phytoplankton present in this water mass were planktonic diatoms, primarily from 

 the genera Leptocylindrus, Corethron, and Pseudonitzschia. Data collected using molecular probes 

 suggests that these cells were healthy, ie. in a physiological state associated with balanced growth 

 rather than senescence. This is not improbable given the fact that the Secchi depth on the shelf 

 was approximately 18m; assuming a 1% light level 2.5 times the Secchi depth, the entire water 

 column, and the sediment surface, was well within the euphotic zone. 



METHODS AND PLATFORMS 



None 



STRENGTHS AND LIMITATIONS OF PROPOSED RESEARCH 



This approach has the advantage of providing information on both the average character 

 of phytoplankton populations, as well as the variance of individual cells. Further, in cases where 

 the populations of interest are bloom-forming diatoms, the data collected using molecular probes 

 combined with chlorophyll fluorescence allow us to determine whether or not the population is 

 in exponential growth, nearing stationary phase, or highly senescent. The main weakness of our 

 approach has been the need to use fresh material, (this problem has been solved for most kinds 

 of samples), and the time it takes to analyse samples by microscopy and image analysis. The 

 latter problem can be solved for many types of samples using flow cytometry. 



STATUS OF RESEARCH 



The methods needed to process samples for the OMP field year are nearly all in place. 

 This includes chlorophyll size-fractionation, phytoplankton enumeration and staining with lipid 

 probes and esterase activity probes. The membrane potential probe is still under development, 

 as are additional methods for working with preserved samples and flow cytometry. The highest 

 priority for this project is completing the analysis of the data from our July, 1993 cruise, 

 particularly the cross-shelf grid. If the preliminary evidence that the chlorophyll-containing cold 



132 



