Phytoplankton Chlorophyll and Nutrient Studies
on Georges Bank
David W. Townsend, Keska Kemper, Maura A. Thomas,
Annette L. Brickley and Abigail M. Deitz

PROJECT OVERVIEW:

The project began in the winter of 1997 as part of the U.S. GLOBEC Georges Bank Program. The purpose of our component of that multi-institutional study is to investigate the idea that the growth and production of zooplankton and fish on Georges Bank are limited by the amount of nutrients (especially nitrogen) that is brought onto the Bank from the nutrient-rich, deeper waters around the Bank's edges (cf. Townsend and Pettigrew, 1997).

The sampling period was chosen to bracket the winter-to-spring transition, which is coincident with the Georges Bank GLOBEC broad scale cruises conducted in 1997, 1998 and 1999. The cruise dates were:

1997*January 13 - 20 (R/V Albatross) 
    *February 11 - 22 (R/V Oceanus) 
    *March 16 - 29 (R/V Oceanus) 
    *April 20 - May 3 (R/V Oceanus) 
    *May 19 - 30 (R/V Albatross) 
    *June 18 - 28 (R/V Albatross)

1998*February 7 -17 (R/V Oceanus)
    *March15 - 26 (R/V Oceanus)
    *April  16 - 26 (R/V Oceanus)
    *May 10 - 20 (R/V Albatross)
    *June 17 - 25 (R/V Albatross)   

1999*January 11 - 24 (R/V Albatross)
    *February 11 - 23 (R/V Oceanus)
    *March 10 - 23 (R/V Endeavor)
    *April 16 - 28 (R/V Oceanus)
    *May 19 - 27 (R/V Albatross)
    *June 14 - 24 (R/V Albatross)

Water samples were collected on all cruises for the analysis of phytoplankton biomass (chlorophyll a and phaeophytin). In addition, dissolved inorganic nutrient concentrations (NO3+NO2, SiO4, PO4 and NH4) were determined for four of the six Broadscale cruises in 1997, five of six in 1998, and all six in 1999. Water collections were made at various depths at all of the regular hydrographic stations (1-40 or Sta 41 after 1997) using Niskin bottles mounted on the rosette sampler. Additional surface water samples were collected at positions between the regular stations (numbered >41; refer to Station Location Map for example). Note that because data files are in some cases ordered by station number, time does not necessarily increase monotonically throughout a given data file. Detection limits for ammonia vary for each month of analysis; for details contact Maura Thomas (mthomas@maine.edu) or David Townsend (davidt@maine.edu).

Phytoplankton chlorophyll a and phaeopigments were determined fluorometrically (Parsons et al., 1984). The extracted chlorophyll measurements involved collecting 100ml from all bottle samples taken at depths shallower than 60m, filtering through GF/F filters, and extracting in 90% acetone in a freezer for at least 12 hours. The samples were analyzed at sea using a Turner Model 10 fluorometer.

Water samples for DIN were filtered through 0.45 Millipore cellulose acetate membrane filters and then frozen immediately in 20ml acid-washed polyethylene scintillation vials by first placing the vials in a seawater-ice bath for approximately 10 minutes. Samples were analyzed in the lab following the cruise using a Technicon II 4-Channel Auto-Analyzer.

In addition to dissolved inorganic nutrients and chlorophyll, in 1999 we are also analyzing samples for particulate organic carbon and nitrogen, dissolved organic nitrogen, and particulate and dissolved organic phosphorus.

Results from the vertical profiles taken at each station can be viewed as contour plots (using Surfer Software, Golden Colorado) for 1997, 1998 and 1999. Plotted variables include Temperature and Salinity for the hydrographic data, Chlorophyll a for the pigment analyses and Nitrate plus Nitrite, Silicate, Ammonium and Phosphate for the inorganic nutrient analyses. The hydrographic data were contoured only for the surface(2m). The pigment and inorganic nutrient concentrations were contoured for the surface(2m), 20meter and 60 meter depths.

REFERENCES:

Parsons, T.R., Y. Maita and C.M. Lalli. 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon, Oxford. 173pp.

Townsend, D.W. and N.R. Pettigrew. 1997. Nutrient limitation of secondary production on Georges Bank. J. Plankton Res. 19: 221-235.