Upwelling

John K. Horne University of Washington

LO: interpret effects of upwelling on production of marine ecosystems

Effects of Upwelling

- Upwelling enhances biological productivity, which feeds fisheries.

- Cold upwelled water alters local weather. Weather onshore of upwelling regions tend to have fog, low stratus clouds, a stable stratified atmosphere, little convection, and little rain.

- Spatial variability of water transport in the open ocean results in upwelling and downwelling, which results in redistribution of mass in the ocean, which results in wind-driven geostrophic currents through Ekman pumping.

U velocity, f Coriolis parameter ( 2Ωsin(Φ)), L Length scale

Coriolis Force - combined effects of gravity and a rotating planet - due to reference frame: Eularian - to the right in the northern hemisphere - determined by Rossby number (low Ro, high effect) - ratio of centrifugal to Coriolis accelerations

fLURo =

ocean ~1, atmospheric cell 0.1, playing catch ~6000

angular velocity, latitude

Ekman Transport

- thin (~100 m), wind driven frictional layer at the top of the ocean

- transport is at right angles to wind but variation in wind causes convergences and divergences

Upwelling Fronts

- pycnocline intersects sea surface in upwelling zone during favorable winds

- eastern boundary currents, with wind blowing equatorward

- inshore cool, dense, nutrient rich waters to euphotic zone to replace Ekman transported surface layer water

- winds relax, surface waters shift back inshore, front disappears

Coastal Upwelling Locations

Micro-Turbulence Enhances Contact Rates

Mackenzie & Leggett 1991

- enhances contact rates with prey by order of magnitude

- contours show increase in contact rate as a function of u and v

Rothschild & Osborn 1988

Gerritsen & Strickler 1977

Upwelling Examples

- Humbolt Current: perfect storm of location (latitude, eastern terminus of equatorial wave guide), upwelling, and ENSO effects

- California Current: eastern limb of subtropical gyre in N. Pacific

Others: Benguela, Canary Islands, Somali

Dominance of Peruvian Anchovy Catch

- feed on plankton

- fish biomass supports many predators

- used for fish meal

- El Nino fish left, bird pops crashed

- birds consume 1.5 million tonnes - MSY did not work over long term

- worked as long as good recruitment

1970: 1/6 world’s catch

Peruvian Anchovy S-R

- catch and effort recorded since start of fishery

- Schaefer model assumed

- f = total effort (GRT year-1)

Anchovy -- Sardine Cycle Peru

- decadal trend in anchovy and sardine landings

- El Ninos: 69-70, 72-73, 75-76, 82-83, 86-87,91-92, 94-95, 97-98, 02-03, 06-07, 09-10

- closed fishery in 1972

- recovered in 1990’s

Peru: Anchovy, Hake, Sardine

deVries & Pearcy 1982

- fish scale accumulation #(1000 cm)-2 y-1

- hake and anchovy in phase, sardine out of phase (?)

California Current Sea surface temperature Summer flow patterns

Anchovy - Sardine Competition?

California Current

- sardine largest single species fishery

- sardine collapse 1950

- replaced by anchovy

- closed fishery in 1970

sardine

anchovy

Soutar & Isaccs 1974 Did anchovy replace sardine?

Sardine Stock-Recruitment

Spawning Biomass

1932-1948

Murphy 1966

- fishery reduced spawning stock biomass - anchovy filled the niche

- some increased survivorship at low stock size

Sardine Recovery

California Current

- numbers estimated from DEPM methods

- stock recovered in 1990’s

Deriso et al. 1996

Not so Fast… Longer Temporal Scale

- scales in cores of anaerobic sediments

- shows that abundance changed dramatically over 2000 years

- collapses and recoveries without fishing pressure

- periodicity of 30 to 35 years

- similar life histories, competition, reduced predation - fishing enhanced environmental effects to magnify and accelerate collapses - collapses occurred during rapid environmental changes

Baumgartner et al. 1992

Large-Scale Factors Influencing Populations?

- sardine rise and fall in eastern boundary upwelling regions

- sardine – anchovy dominance switch observed over time and space

- peaks getting higher and narrower

Bakun 1996

Annual Sardine Landings

Sardine Pacific Synchrony?

Kawasaki 1983

- Californian and Japanese fisheries in phase 1920’s – 1930’s

- crashed for 3 decades

- 1970’s recovery + South America

- Benguela out of phase

- indirect reactions to El Nino and regime shifts

Increased Upwelling? Bakun (1990) Hypothesized that global warming, due to CO2 increases, leads to lower nighttime cooling and increased daytime heating of land, intensifying the pressure gradient between land and sea

Result: intensified alongshore winds and upwelling; positive feedback from increased ocean cooling

Mote and Mantua (2002)

Results from 3 models showed no increased upwelling through the 21st C

Lecture Summary

- started with a review of upwelling physics

- upwelling as a physical factor influencing growth and survival

- examples of sardine and anchovy in Peru and California

- combination of biological and physical factors

- multiple factors operating over different temporal scales