Physical Forcing of Phytoplankton Population Abundance in the Western Gulf of Maine

Dennis McGillicuddy

Project Duration: 6/1/96-6/30/98
Key Words: Gulf of Maine, modeling, phytoplankton, coastal circulation

Final Report

This investigation into the large-scale seasonal variations in phytoplankton abundance in this region was based on an adjoint data assimilation technique. This approach was used by McGillicuddy et al. (1998) to study physical and biological controls on Pseudocalanus spp. distributions in this same region. The forward problem is posed as an advection-diffusion-reaction equation for organism concentration C:



where v is the velocity, K the diffusivity and H the bottom depth. The reaction term R(x,y) represents a highly idealized parameterization of population dynamics which varies in space only. Positive R implies net growth, while negative R implies net mortality. Specifying the climatological velocity and diffusivity fields, the adjoint of the advection-diffusion-reaction equation is used to invert for the population dynamics implied by changes in organism abundance and the circulation during the intervening period. This approach has proven to be successful with the climatological phytoplankton distributions mapped from the O'Reilly and Zetlin (1996) monograph. Results are described in Wang (1999).

In short, the analysis has revealed geographically specific patterns in phytoplankton source/sink terms (see example for Jan-Feb to Mar-Apr in Figure 1). These spatial patterns vary seasonally according to the phytoplankton distributions, the climatological currents, and their orientation with respect to each other. In cases when the flow is either weak or aligned with gradients in organism abundance, changes in concentration over time are dominated by local population dynamics. In situations where the currents are normal to these gradients, complex three-way balances arise between the local tendency, advective transport, and the population dynamics source term. Diffusion does not appear to play a major role in these simulations.

References:
McGillicuddy, D.J., Lynch, D.R., Moore, A.M., Gentleman, W.C., Davis, C.S. and C.J. Meise, 1998. An Adjoint Data Assimilation Approach to diagnosis of physical and biological controls on Pseudocalanus spp. in the Gulf of Maine - Georges Bank Region. Fisheries Oceanography 7(3/4): 205-218.

O'Reilly, J. E., and Zetlin, C. 1996. Monograph on the seasonal, horizontal, and vertical distribution of phytoplankton chlorophyll a in the northeast U. S. continental shelf ecosystem. NOAA Technical Report, NMFS.

Wang, C. 1999. Diagnosis of physical and biological controls on phytoplankton distributions in the Gulf of Maine - Georges Bank Region. Masters Thesis, MIT/WHOI Joint Program, Cambridge, MA.