Ocean Productivity

Marine Biology, Biol 410

Marine Biology: Ocean Primary Productivity

Abiotic factors like solar radiation and nutrients, and biotic factors like zooplankton predation may affect ocean primary productivity.

Productivity varies with the season, and also locally and globally. Variation in primary productivity, measured typically as the concentration of chlorophyll in water, is a primary determinant of all biological productivity up the food web and trophic pyramid.

In this lecture you have the opportunity to explore a number of examples of how ocean productivity varies. The images used here are made from satellites such as the Nimbus 7 satellite, with color scanner, or the Coastal Zone Scanner. These satellites were in-service in the 1970's and 1980's.

Global patterns of productivity

Look at this image of global productivity. Colors correspond to surface chlorophyll content. Red end of spectrum is highest productivity, blue/violet end is lowest. The overall pattern with latitude is clearly visible: low productivity in the tropics and subtropics, probably due to nutrient limitation brought about by strong, year-round thermocline and pycnocline.

North and South Atlantic: Large gyre systems, with low productivity. In the north Atlantic, the gyre is called the Sargasso Sea.

Pacific: Relatively high productivity, but still low in gyres. What causes these high productivity areas?

Indian: Typically low (0.2 gC/m2/day), but depends on seasons and monsoon winds, which can cause upwelling N of equator.

Southern Ocean: High productivity, high phytoplankton density can yield 1gC/m2/day! Over about 100 day season, that yields 100 gC/m2/year. Production can occur under ice!

Notice that there are a lot of exceptions to the global patterns, and local areas of high productivity in generally low productivity zones.

Benguela current of S. Africa

Equatorial Current

What causes these local areas of high productivity?

Local patterns of productivity:

High productivity can occur even at low latitudes

Shelf areas cover 15% of ocean area, but 50% of oceanic primary productivity.

--Nutrients recycle from bottom in shallow, well-mixed shelf waters

--Nutrients run off from land, a major reservoirs of P

--Light levels are highest just off coastline

Inshore tropical waters have a productivity as much as 10 times higher than offshore because of high nutrients and light.

2. Upwellingzones bring nutrient-rich deep water to the surface, fueling primary productivity

Three major mechanisms: Coastal upwelling, equatorial upwelling, island mass effects

Coastal upwelling: Winds moving water along the shore result in a curl Right or Left (N or S hemisphere) due to Coriolis effect and Ekman transport.

A perfect example is the Peru Current. You can see the effects on productivity along the coast of South America (near the bulge on the west coast).

SE tradewinds along the shore of Peru leads to Ekman transport, moving surface waters offshore to the west (left). This water is replaced by water from 200-300 m deep.

This leads to high primary productivity as seen on the image, and fuels a very dense community of zooplankton, anchovy, higher predators, birds, and mammals. Between 1964-1970, the fishery for anchovy was 20% of total world fishery

During El Nino episodes, winds switch to W, N-ward current slows, warm counter current brings warm water from the equator, and upwelling ceases. Anchovy seek cooler, water deeper, and predators can't get them. During the 1972 El Nino, anchovy fish catch went from 20 to 2 million metric tons.

Equatorial upwelling: Recall that the Equatorial Currents north and south of the equator result in a transverse ocean currents flowing away from each other due to the Corialis effect and Ekman transport. (The net flow of the westward currents are north on north side of the equator, and south on the south side, and this water is replaced by deeper water)

You can see the effects of equatorial upwelling on the first image on global productivity. Better yet, look at the Eastern Pacific in this image. You can see the coastal upwelling near California, and the equatorial upwelling west of the Galapagos Islands.

Island Mass effects: As currents encounter islands, deeper nutrient rich water is forced toward the surface.

This helps increase productivity to coral reefs. The shows the coast of Australia, and the Great Barrier Reef shows up as high productivity specks to the east.

This image of the equatorial region around the Galapagos Islands shows the possible effects of natural iron that is derived from these volcanic islands.

The Island Mass effect can also be seen around Tasmania. The polar westerlies blow around the Southern Ocean, and drive a eastward current. As this current approaches Tasmania, the upwelling of nutrient rich water fuels primary productivity.

Special example: Gulf Stream Rings

This image shows ocean productivity along the East Coast from Cape Hatteras NC to the Bay of Fundy. See if you can explain all the patterns of productivity shown here.

The deep blue path of water running past Cape Hatteras, then offshore is the Gulf Stream. Why is the Gulf Stream low in productivity?

Why are waters more productive waters near Cape Hatteras, and in and near Chesapeake Bay?

The famous fishing grounds called Georges Bank is the red color east of Cape Cod. This shows the effect of currents dragging deep waters over shallow bottom (bank).

There are rings (or lenses) of low productivity water north of the Gulf Stream. This is water from the Sargasso Sea (tropical, gyre water) trapped in a meander (looping curve) of the Gulf Stream. They form when a meander breaks off, trapping water from the OPPOSITE side of the Gulf Stream. Why is the productivity low?

(Note: The gray/brown color shows continent and clouds.)