Phytoplankton Chlorophyll and Nutrient Studies
on Georges Bank

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

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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).

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.

Contour plots and tabulated data are available here.

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.