Biological Oceanography, Plankton Blooms

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Biological Oceanography, Plankton Blooms the
Gulf of Maine

• Basic Principles Governing the General
  Oceanography of the Gulf of Maine Region
• Overview Examples of Basic Concepts
• Basics of Biological Oceanography Ecology
• Some New Angles

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The Gulf of Maine

• Major Features
• Morphology Deep Basins, Limited Access
• Freshwater Input
• Tides and Mixing.
• Processes
• Circulation
• Vertical Mixing and Upwelling
• Water Masses
• Nutrient Injections Cycling

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Broad Continental Shelf System
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1.
Morphological Features of the Gulf of Maine and
Georges Bank

• Jordan Basin
• Georges Basin
• Wilkinson Basin

Isolated from Open North Atlantic by
Deep-water Access Northeast Channel
Browns Bank Georges Bank
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2.
Freshwater Sources
Average River Discharges
(From Apollonio, 1979)
Scotian Shelf Water (50)
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Surface Salinity (Winter)
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3.
TIDES Mean Tidal Ranges Throughtout the The
Region
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The North Atlantic Tidal Wave and Amphidromic
Point
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Trough of the North Atlantic Tidal Wave
Sweeping by the Gulf of Maine... (low
tide) Water empties...
Crest of the North Atlantic Tidal Wave Sweeping
by.. (high tide) Water fills
Note how Wave is focused into Bay of Fundy
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Reason for large tides Resonance
Resonant Period
(Determined by Basin Size)
Coffee cup ca. 0.2 sec.
Bathtub ca. 1.5 sec.
Gulf of Maine Semi-enclosed Basin 12.4
hours (Very Close to the M2 tide)
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Rectification of Rotary Tidal Currents (on
Georges Bank)
Etc.
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Actual drifter tracks on Southern Flank
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4. Vertical Mixing 1. Winter Convection
Cold water sinks BUT Not strictly true
for Fresh Water
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Watch what happens to a lake from summer to
winter
Ice can now form at the surface
Ice can form only after the water temperatures
reach 4C from top to bottom
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But, when you add SALT
Maximum density of sea water is well below zero,
and equal to the freezing point
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Now watch what happens to sea water
Now ice will form only after the water is ca.
-1.9C from top to bottom
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Vertical Mixing by Tides From the bottom up

• Depends on
• Tidal Current Speed
• Bottom Roughness
• Depth.

Typical Summer SST
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Tidal Mixing can be quite dramatic (e.g., SW
Nova Scotia Shelf)
Sand grains
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5.
Penetration of Slope Water along the bottom and
into the Gulf (Bigelow, 1927)
Along with tidal mixing and freshwater fluxes,
determines density distributions
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Why this is all important
Average Density Field and baroclinic currents
Typical Offshore Profile in Summer
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Residual Circulation That Results
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Satellite-tracked Drifters
(from Pettigrew, unpubl.)
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6.
How it all fits together to give us Life in
the Sea
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Photosynthesis

• Makes only carbohydrate
• CO2 H2O Light CH2O O2

• But cells need more than simple sugars
• They need other stuff e.g., Nutrients

• The Macronutrients
• C, H, O, N, Si, P, Mg, K, Ca

• The Micronutrients
• Fe, Mn, Cu, Zn, B, Na, Mo, Cl, V, Co

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A Phytoplankton Cell
Growth Photosynthesis plus Biosynthesis
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Three important (limiting) nutrients for
phytoplankton
Nitrogen (N) Important in structural components
of the cell (such as proteins) limiting in the
ocean Phosphorus (P) Important in the energy
cycle of the cell (e.g., ATP,ADP) and is
continually being turned over limiting in fresh
waters Silicon (Si) Si(OH)4 is required by
diatoms, and at times it limits phytoplankton
growth, such as following the spring
phytoplankton bloom on Georges Bank and in parts
of the Gulf of Maine.
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For Growth (vs. Photosynthesis)

• Approx. chemical composition is
• (CH2O)116 (NH3)16 H3PO4
• (which is basis of the Redfield Ratio)

So, sinking phytoplankton other biomass will
eventually remineralize and release dissolved
forms of these elements
Note well This does NOT include Si (e.g.,
Si(OH)4)
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7.
Sources of Nutrients to the Gulf of Maine
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The Nitrogen Cycle (one version)
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Light Seasonal Water Column Structure(Their
Role in Plankton Production)
Summer
So, light is near the surface
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How light levels affect photosynthesis and
respiration
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Combining these processes
When the Critical Depth criterion is met,
either by increasing light levels, or by shoaling
depth of mixing, we will see Net Positive
Phytoplankton Primary Production.
Depth where P R Compensation Depth
Depth where Integrated P Integrated R
Critical Depth
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Giving The Spring Phytoplankton Bloom
The Gulf of Maine and Georges Bank are Productive
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A Summertime Picture After the Spring Bloom
The Nutrients are at depth
Once in the Gulf, they must somehow reach the
surface for phytoplankton to use them

• Tidal Mixing
• Ekman Upwelling
• Winter Convection

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Combinations of Deep Water Penetration, Tidal
Mixing, and Residual Circulation, create
Nutrient-Rich Waters in NE Gulf
Surface NO3
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A closer look at the Gulf of Maine Coastal
Current system
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Alexandrium fundyenseThe Red Tide Dinoflagellate
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And the Sandwich Phenomenon...
Historically, an absence of PSP from mid-Maine
coast.
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Example of Results
June Results Alexandrium cell densities in
relation to hydrography
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Variable Trajectories of the Eastern Maine
Coastal Current (and whatever those waters carry
with them)
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Water Mass Flows INTO and OUT OF the Gulf of Maine

• Important to
• Nutrient Loads
• Freshwater flux
• Bottom Temp.

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Large-scale Circulation and Offshore Source Waters
June 11, 1997
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Cold, Low-Salinity Scotian Shelf (Surface)
Water Crossing the NE Channel and Carrying Cod
Eggs to Georges Bank AND BUOYANCY
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Scotian Shelf Water penetrating the interior of
the Gulf of Maine, Spring 2000 Buoyancy
important in initiating the spring phytoplankton
bloom
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Seasonal Time Course of Scotian Shelf Water
Penetration into and across the Gulf of Maine
Calendar day for minimum surface layer salinity
(modified from Mountain and Manning, 1994)
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March 1997
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March 1998
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March 1999
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March 2000
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1997
1998
Springtime (March) Scotian Shelf Water Variability
1999
2000
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Inter-annual Variability in Importance of
Low-Salinity Scotian Shelf Water
Surface layer salinity anomaly (PSU) in Wilkinson
Basin (modified from Smith et al., 2001)

• Scotian Shelf Water important in initiation of
  spring phytoplankton blooms, and
• Impeding winter convective mixing (which mixes
  nutrients upward)

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North Atlantic Oscillation
Controls on Water Masses (surface deep) in the
Gulf of Maine
NAO Index
From Drinkwater et al. (ms)
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• Effects Downstream
• Position of the North Wall of the Gulf Stream
• Penetration of Labrador Slope Water to the
  Southwest

NAO Low
NAO High
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Surface-water Temperatures
Far-ranging NAO Tele-connections?
Surface-water Temperatures
Bottom-water Temperatures
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Two contrasting years 1998 1999
Warm surface water Adjacent to Shelf Break
Cold surface water Adjacent to Shelf Break
A normal year
1-1.5 years following the 1996-1997 NAO Low
From Townsend et al. (in press)
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1998 Labrador Slope Water Penetrates Gulf of
Maine (along the bottom)
Percentages of Labrador Slope Water in the deep
layer (150-200 m) in the Gulf of Maine, January,
April, June, and August of 1998 (from D.
Mountain).
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• Properties of
• Labrador Slope Water
• NAO Low (1998)
• Warm Slope Water
• NOA High (1999)

Northeast Channel
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Laborador Slope Water vs. Warm Slope Water and Ph
ytoplankton Production
Higher Nutrient Loads, and Higher Production
with Warm Slope Water in 1999
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