The effect of freshwater run-off on biological …magan/amce608/lecture6.pdf(Continued of lecture 5)...

(Continued of lecture 5)

•The effect of freshwater run-off on biological production in estuaries

Stratified: (+ effect) lead to the oxygen-depleted while nitrogen (e.g. from benthic organism) and phosphorus (from sediment particles )content rose;(-effect) limit the mixing between the surface and bottom

Mixing: (+ effect) replenishing the O2 at depth and upwelling nitrogen and phosphorus; (-effect) turbidity cause light to be limiting.

Alternating between these two process provides the conditions for very high primary production (e.g. York River Estuary). This ‘alternating’ can occur, for example, between spring and neap tides or other physic processes affected it.

Neap Tide bloom

•The biological effects of tidal mixing

Tidal front: the place where intensity of turbulent mixing was just enough to continuously overcome the barrier to mixing presented by the stratification.

E=lg(h/Dt)=1.9 is the place where front is located. (H is water depth, Dt is depth-averaged rate of dissipation of energy from tides.

Control by factors of (a) stratification; (b) mixing; (c) light

For Phytoplankton: Potentially, tidal mixing may have adverse effects on phytoplankton productivity more than compensated for by the increased nutrient flux to the water column from the sediments.

For Zooplankton:

Tidally mixing may delay warming of the water column due to the lack of stratification and prevent upward migration of a large biomass of adult and late stage copepods.

Tidally mixing waters tend to have a relatively slow growth of the zooplankton population

Large flux of nutrient from sediments and rivers in the tidally mixed water column

Poor penetration of light

• Biological effects of river and estuarine plumes

(a)Materials carried by the river on biological production in the plume;

(b)Entrainment and consequent upwelling of nutrient-rich water;

(c)Enhancement of the stability of water column (+: enhance productivity;-: inhibit vertical mixing and hence reduce primary productivity.

Mississippi plume in Gulf Mexico (effect a):

River-borne nutrients inputs enhance primary production and sinking of organic matter

The increased phytoplankton production and sinking of phytoplankton biomass increase bacterial activity and formation of zones of low oxygen (hypoxia) or zero oxygen (anoxia).

The plume in Amazon river (effect b)

The entrainment of salt water into upper fresh water layer lead to the compensatory shoreward flow of high nutrient bottom water as river plume moves offshore.

Algal blooms on the Amazon shelf receive 83% nitrogen, 69% of phosphorous and 59% of silicon.

Fresh water run-off in the coast of Iceland (effect c)

Fresh water input forms a great resistance for thermal stratification to be breakdown by the tidal or wind-induced mixing, which lead to earlier spring bloom.

Lecture 6: Coastal Upwelling

• The Physics of coastal upwelling• Biological responses

Coastal UpwellingWind Stress

Coastal Upwelling Jet

Bottom Ekman Layer

Surface Ekman Layer

Processes in Coastal UpwellingWhen winds have a component blowing parallel to the coast, Ekman layer transport directed 90° to the right

Offshore surface transport lows sea level near the coastal, produces a pressure gradient which is directed normal to the shore and drives a geostrophic current along the coast

Flow field related to upwelling is directed at an angle away from the coast near the surface, parallel to the coast at mid-depth (below the Ekman layer but above the bottom boundary layer) and at an angle towards the coast in the frictional boundary layerat the bottom.

colder nutrient-rich waters from the deep ocean areuplifted onto the shelf and coastal upwelling jet is formed.

More than half of the world’s annual commercial fishing occur in the upwelling zone.

Five major coastal currents associate with upwelling area besides numerous coastal region, e. g. North South China Sea, coast off Vietnam

113 114 115 116 117 118 119

113 114 115 116 117 118 119

20

21

22

23

24

20

21

22

23

24

Temperature DIstribution on 10m layer in summer,2002

wind

Horizontal distribution of water temperature at 10 m showing strong alongshore variability in upwelling on the shelf off NSCS (Guo, SOA)

Wind-driven Ekman flow

What is the response of ocean to the wind forcing?

Adjustm

ent (three stages) of motion at the surface

during onset of eastward w

ind

Transit stages (I and II)(forces are not balanced)

Equilibrium stage (III)(forces are not balanced)

Similarly, the water beneath the surface, although not dragged directly by the wind, is dragged by the surface water (45o to the right) and causes the flow to the right of surface current, i,.e at an angle > 45o.This process continues downward … and forms the Ekman spiral.

At depth De, the direction of velocity is opposite to the surface velocity. De is called Ekman depth

2/1)/(sin4.4 ϕWDE ≈

W is the wind speed and φ is the latitude.

At the bottom, the drag force is bottom stress which generates a‘reverse’ Ekman spiral.

Ekman drift and coastal upwelling

What is the net transport in the wind-driven Ekman layer and its consequence in the coastal

region?

The net movement of the wind-driven flow, after averaging over the Ekman layer, is 90o to the right of the wind.

(averaging effect over De)

Transport by Ekman drift, i.e. Ekman transport (Me) is given by

fM E

τ−=

τ is the wind stress at the surface , f is Coriolis parameter.

Relationships between the wind, Ekman drift and coast, as illustrated below, lead to the formation of Coastal Upwelling

Negative pressure gradient, dp/dx<0 (Pa>Pb), formed by the density contrast and sea level difference across the upwelling area, generates southward geostrophiccurrent.

xy

Pa Pb

The width of coastal upwelling-the Rossby deformation scale

D: The distance of the upwelling front from shore, which moves offshore as upwelling progresses.

Ri (Rossby internal deformation scale): The width of the region where the interface rises to the sea surface.

Ri=(g’H)1/2/f

g’ is the reduced gravity, H is depth of the upper layer and f is Coriolis parameter.

ggρρρ 12' −

=

Variations in upwelling

• Time dependent• Space dependent

Spatial variability of upwelling:Ocean Surface Temperature on Sep. 5, 1994 as measured by satellite.

•By spatially varied alongshore upwelling favorable winds

Sea Surface Temperature

•By bottom topography:Topographically controlledupwelling

• By coastline variation

•By combined effects of time-dependent wind stress and coastline variation-------upwelling relaxation

Wind Stress

1 Dyn.

Day 10 Day 13 Time

•Upwelling and primary production

wind

PRE E. HK

Offshore direction

Density

NO3

Cross-shelf section in line normal to Pearl River Estuary

Density

NO3

Cross-shelf section in line normal to the coast of east of HK

PRE

East of HK

Onshore velocity in sections of PRE and east of HK

Remarks about the upwelling-primaryproduction

oThere is a time delay between the onset of the wind and the arrival of nutrient-rich water.

oThere has to be a quantity of nutrient-limited phytoplankton cells ready to respond the availability of nutrients

oThe high productivity results from the alternation of upwelling events. (upwelling brings nutrient, calm period, stratification develops, phytoplankton grows and multiplies)

•Upwelling and zooplankton

o Zooplankton organism, e.g. copepods, require longer time (weeks) to complete life cycle.

o Process is: adult copepods is upwelled to the surface from deep water during upwelling, their offspring thrive on the abundant phytoplankton and tend to be carried offshore. The peak value of zooplankton occurs when upwelling is weakened so that they can stay over the shelf.

Lower nitrate and chlorophyII-a, and higher zooplankton concentration are found during the periods after upwelling (or relaxation of upwelling and downwelling, e.g. June 22-30)

•Upwelling and fish

Upwelling favorable wind

Upwelling circulation

Upwelling nutrients

Bursts of Phytop.

Relaxing of upwelling winds

Build up of Zoo.

Landing of fish

Example of upwelling in California current

Bakun upwelling index:

fM

vC

E

da

τρτ

=

= 2

aρ is air density 0.00122 g cm-1, Cd is drag coefficient as 0.0026. The index is the simply the time average of Me for different part of coastal zone.