Warming and ocean biology

A research article in last week’s edition of Nature found that recent ocean warming has decreased “primary” productivity, the productivity of phytoplankton (e.g. algae), in the ocean.

This important result may seem counterintuitive. If the ocean is warmer, wouldn’t algae grow more? So I thought it warranted more of an explanation that was offered in some of the press coverage.

The trend is driven largely by changes at low latitudes where the ocean is “stratified”. There, a layer of warmer, less dense surface water sits above colder, heavier (saltier) water. The thermocline (or pycnocline) you hear about is the zone of steep temperature (or density) change between the two layers. Because of the layers have different densities, they don’t mix very well. Think oil and water, although not nearly that extreme.

The nutrients that phytoplankton require for growth are more abundant in colder, deep waters. That’s why most of the world’s greatest fisheries are in regions of upwelling. For example, think of the cold, productive Pacific off the S. American coast. El Nino got its name from a periodic warming of surface waters that hurt fishing catch. During an El Nino event, a shift in air pressure and surface winds advects the warm surface waters from the central Pacific towards to the South American coast. This increases stratification and decreases upwelling of colder, nutrient-rich waters that promote phytoplankton productivity and, in turn, the fishery.

So, in the case of climate-forced warming of the ocean, the surface warming causes greater stratification and further inhibits mixing. That means fewer nutrients, and less phytoplankton production.

The inverse is expected to occur in high latitude, less stratified, parts of the ocean (it’s cold at the top too!). There, temperature and light limit growth more than nutrient availability, so warming is expected to increase productivity.

In the Nature study, the group of scientists compared estimates of productivity derived from satellite measures of ocean colour to sea surface temperature. As was expected, there was an inverse relationship between temperature and productivity in the stratified ocean.

There’s only ten years of data -- the instrument has only been on the satellite for ten years – much too short to define a clear long-term trend in one direction or the other. The first three years there was a decrease (increase) in temperature (productivity) largely because of shift from El Nino to La Nina conditions in the Pacific; afterwards, there was an increase (decrease) in temperature (productivity). What is important for climate change research, however, is that the study appears to confirm theory and the results of previous studies using climate and ocean ecology models. It gives us an idea of one central response of ocean biology to any long-term climate warming that may occur.