Nitrates and Phosphates and Algae, Oh My!

Part B: Algal Blooms in the Gulf of California

Real-color and chlorophyll concentration images of an algal bloom in the Gulf of California on August 10, 2003. Images courtesy of NASA.

An algal bloom is a rapid increase in the population of algae or phytoplankton in an aquatic system, and can be recognized by discoloration in the water. The discoloration arises from the presence of chlorophyll, a green pigment used for photosynthesis. Because phytoplankton contain chlorophyll, the distribution and abundance of phytoplankton in oceans, lakes, and seas can be determined through satellite measurements of chlorophyll concentration.

In Part A, you saw how adding nutrients to algal cultures increased their reproduction rates. Fertilizer, much like what you added to your lab cultures, finds its way into lakes and oceans through runoff from agricultural farms, golf courses, and suburban lawns. Other nutrients get added from the atmosphere, soil erosion, upwelling, aquaculture facilities, and sewage plants. In this part of the investigation, you will use images and data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project, to look for correlations between phytoplankton blooms in the Gulf of California and the addition of nutrients through agricultural runoff from the surrounding land.

  1. The maps below show the concentration of chlorophyll in the Gulf of California, averaged over the 8-day period from August 4-11 (left) and December 10-17, 2006. Concentrations are measured in milligrams per cubic meter (mg/m³). As the legend shows, red areas contain the greatest concentration of chlorophyll (and therefore the greatest concentration of phytoplankton), while the purple areas have very low chlorophyll concentrations. As you examine the images, familiarize yourself with the legend and look for areas of high and low chlorophyll concentration on each map.

  2. Go to the Giovanni Ocean Color Time-Series Online Visualization and Analysis website.
  3. Select the area surrounding the Gulf of California.
  4. Set the rest of the parameters to create an animation of the chlorophyll a concentration in the Gulf of California from July 12, 2005 through July 19, 2007.
  5. Click on the > button to begin the animation.
  6. Play the animation all the way through at least once to watch how the chlorophyll levels in the Gulf change over time.
    • To reset the animation to the first slide, use the S(0) button.
    • To step through each 8-day range one at a time, use the +1 and -1 buttons.

    Stop and Think

    1:Describe the variation in the chlorophyll levels in the Gulf of California during the two year period covered by the animation. Are there certain times of year during which chlorophyll levels are particularly high or low? Explain why you think these differences exist.

    2:In general, in what area(s) of the Gulf are chlorophyll levels highest? Explain why you think this occurs.

  7. Close the animation window and go back to the page on which you entered the data parameters. Change the Plot Type to Time-Series, Area-averaged. Leave all other parameters as they are.
  8. Click on the Generate Plot button at the bottom of the page. A new window will open, this time showing a scatter plot (with data points connected by smoothed lines) of chlorophyll a concentration vs. time.
    • Chlorophyll concentration is given in units of milligrams per cubic meter (mg/m³). Each data point represents the average chlorophyll concentration for the entire area within your selection box on a given date.
    • The latitude and longitude boundaries of your selection box are shown above the plot area.

    Checking In

    Describe how the average chlorophyll concentration in the Gulf of California varies over the given time period.
    • During what times of year are average chlorophyll concentrations highest?
    • During what times of year are average chlorophyll concentrations lowest?
  9. Read the article Researchers discover direct link between agricultural runoff and massive algal blooms in the sea for an explanation of what you have just observed in the satellite data.

    Stop and Think

    3:According to a study published in Nature, irrigation typically took place in Mexico's Yaqui Valley four times a year from 1998-2002: November, January, March, and April. Do the data from 2005-2007 indicate the same trend in irrigation events? Support your answer with evidence from your graph.

Although there is currently no evidence that the blooms seen in the Gulf of California are toxic or creating oxygen-depleted "dead zones," the threat is real. Continue to Part C to learn more about these potentially deadly consequences of excessive algae growth.