Modeling Studies of the Physical-biological Processes Controlling Spring Bloom Dynamics on Georges Bank:

1-D and 2-D experiments

R. Ji, C. Chen, P.J.S. Franks, D.W. Townsend, E.G. Durbin, R.C. Beardsley, and R.W. Houghton

Compiled from Dr. Townsend’s data

Inter-annual variability of the spring bloom:

Timing Magnitude Location

“Second” bloom in June

Near the front?

Jan

Mar

Apr

May

Jan

Jun

Feb

1997 1998 1999

Chlorophyll a (g/l)

Jan

Mar

Apr

May

Jun

Feb

1997 1998 1999 1997 1998 1999

Nitrate+Nitrite (M) Silicate (M)

Townsend&Thomas, 2002, MEPS

Diatom

Dinoflagellate

Other

Phytoplankton Composition

SeaWiFS data from GOMOOS website By Dr. Andrew Thomas, UMaine

Inter-annual variability of seasonal variation and spatial distribution of phytoplankton and nutrients during spring-early summer time

Seasonal succession of phytoplankton community

Depletion of silicate earlier than nitrogen. Implication of nutrients limitation

“Second” bloom near front area in June, may be corresponding to silicate recovery.

Advection importance. GOM water, SSW, slope water, exchange between central bank and flanks.

What we learned from data?

Model structure

Ammonia Silicate

SmallPhytoplankton

LargePhytoplankton

SmallZooplankton

LargeZooplankton

DetritusNitrogen

DetritusSilica

Predation

Mortality

Remineralization

UptakeUptake Uptake

Dissolution

Fecal

Mortality

Grazing Grazing

Mortality

Mortality

Nitrate

Grazing

Mortality

1

2

3

1-D 2-D 3-D

Seasonal dynamicsSensitivityModel behavior

StratificationFrontal structureCross-section variation

AdvectionEvent level

ECOM-si ECOM-si FVCOM

A

B

1. The central bank in which water is shallow, vertically well-mixed, and relatively self-contained;

2. The mid-bank region characterized by a seasonal tidal mixing front;

3. The outer-flank between the seasonal tidal mixing front and the permanent shelf break front.

1-D

Site A Site B

Biological variables 1-D Model

Site A Site B

Model-Data Comparison 1-D Model

Light1-D ModelSite A

N/Si in diatom1-D ModelSite A

N/Si = 1.5 N/Si = 0.8

Sensitivity analysis

0 0.4 0.8 1.2

VplVpsKpl1Kpl2Kps1Kps2KslRzlRzsRsdRlsSzlSzsSsdSlsGzlGzsGsdGlsEplEpsEzlEzsEdnEdsRnsAfaBetalBetas

/ˆParameter / Parameter

F FS

1-D ModelSite A

What is learned from 1D

What is missing in 1D

Light environment controls the onset of the bloom in the shallow region, while stratification plays a more significant role in the deep region.

Magnitude of bloom is modified by both light and nutrients.

N/Si ratio is an important parameter for the nutrients limitation process and succession of phytoplankton community.

Second bloom in shallow area is not reproduced in the 1D model, although silicate is recovered from later spring depletion with increased water temperature.

The site with deep water didn’t capture the basic pattern of springtime phytoplankton bloom, indicating the significance of advection process.

2-D

2-D model grid

The early spring bloom occurring in the central bank: controlled by biological process associated with light intensity.

The recurrent bloom event during late spring and summer is mainly driven by the seasonal tidal mixing front.

The timing of the bloom on the southern flank is determined by (1) timing of the formation of the tidal mixing front and onset of vertical stratification, and (2) front-ward nutrient flux from the deeper region.

Sectional view

Verticalmixing

Verticalmixing

Critical depth

High NutrientsDeep water

Nutrientsflux

TMF

Verticalmixing

Verticalmixing

light

Critical depth

Wind

Before Stratification

After Stratification

Tide excursion

PMA

Biological field 2-D Model

2-D Model

Sensitivity of bloom to heat flux

What we learn from 2D

What is missing in 2D

The basic pattern of lower level trophic food web dynamics in shallow and deeper area mirrors the site A and site B in 1D model. A unique pattern is developed in tidal mixing frontal zone.

If no impact from advection process, the development of weak stratification is critical for the springtime bloom; wind and heat flux can regulate this process.

The frontal zone is the possible area for the “second” diatom bloom

Like 1D, the deep-water site didn’t reproduce the basic pattern of springtime phytoplankton bloom, indicating the significance of advection process.