Lab 7.2 – What is the relationship between chlorophyll and other abiotic variables?

Fundamental concept: Interpret the relationship between primary production, nitrate concentration, irradiance, and surface water temperature
Estimated time to complete: 45-60 minutes
Data skills preparation: Lab 2.1 – Time series
Materials needed: None

In Activity 7.1, you determined that phytoplankton abundance (chlorophyll) varies seasonally in this location in the temperate Atlantic Ocean. Phytoplankton abundance is influenced by a number of abiotic variables, including sunlight, nutrients, and degree of density stratification. In this activity, you will look at data of irradiance (sunlight), nitrate (a nutrient) and surface water temperature to investigate why phytoplankton abundance changes over the course of a year.

The following is a summary of how primary productivity and phytoplankton abundance change over the course of a year in temperate oceans. More detailed information may have been provided by your instructor and can be found in any oceanography textbook.

Phytoplankton require sufficient sunlight (irradiance) to photosynthesize; thus, phytoplankton are only abundant near the ocean surface, and only at times of the year when there is sufficient sunlight.

Phytoplankton also require a range of nutrients, including nitrogen and phosphorus. For many phytoplankton, one of the main sources of nitrogen is dissolved inorganic nitrate (NO3–). Nitrate becomes available through the decay of organic matter but is quickly utilized by phytoplankton in surface waters. In deeper water, due to the sinking of organic matter and a lack of uptake by phytoplankton, nitrate is usually found in higher concentrations.

Thus, while sunlight is available at the surface, nutrients, including nitrate, tend to be more concentrated in deeper water.

When surface water temperature is warm, the large density difference between warm surface water and cold deep water results in a thermocline and density stratification, which prevent nutrients from reaching the surface waters. However, when the surface temperature is cold and the thermocline is weak or absent, mixing can bring the nutrient-rich deeper water to the surface, fueling phytoplankton growth if sufficient sunlight is also available.

Because sunlight, water temperature and nutrient availability vary seasonally in temperate locations, their interactions will lead to changes in phytoplankton abundance, and thus chlorophyll concentrations, over the course of the year.

Click on the diagram below and scroll through each slide to see how these abiotic and biotic variables change through the seasons:

Graph of seasonal cyclesFigure 7.2.1. Typical changes in solar irradiance, water stratification, nutrients and phytoplankton biomass over the year in the temperate ocean.
Graph of seasonal cycles
Graph of seasonal cycles

The summary above is based on the classic textbook version of how primary productivity changes in temperate oceans. Use the data in the graphs below to further explore this explanation. You can interact with the data by

  • turning on and off different variables to compare to chlorophyll
  • hovering over a data point to view the data values of the different variables on that day
  • turning on and off the different seasons to highlight winter, spring, summer and fall

Orientation questions

Answer the following questions to check your understanding of the datasets in the interactive graph.

  1. Examine each variable and fill in the table below (for chlorophyll, you can consult your answers from Activity 7.1, Questions 1 & 2). For each variable, indicate the following information:
    1. Approximate minimum and maximum values for the season
    2. Magnitude of the variable compared to other seasons (e.g., is it high, low, intermediate, etc.)
    3. Change in the variable over the season (e.g., does it increase, decrease, remain about the same, etc.)
Winter Spring Summer Fall
Chlorophyll a.
b.
c.		 a.
b.
c.		 a.
b.
c.		 a.
b.
c.
Irradiance a.
b.
c.		 a.
b.
c.		 a.
b.
c.		 a.
b.
c.
Nitrate a.
b.
c.		 a.
b.
c.		 a.
b.
c.		 a.
b.
c.
Temperature a.
b.
c.		 a.
b.
c.		 a.
b.
c.		 a.
b.
c.

Interpretation questions

  1. Phytoplankton need sunlight for photosynthesis. Therefore, you might expect to see higher chlorophyll concentrations when irradiance is higher, and lower chlorophyll concentrations when irradiance is lower.
      1. Looking at the different seasons, did you find a positive relationship between irradiance and chlorophyll? Justify your answer by listing examples of the data values and trends of irradiance and chlorophyll within each season.
      2. What does your answer to Question 2a  tell you about the impact of irradiance on the pattern of chlorophyll throughout the year? Is irradiance the only factor affecting chlorophyll concentration? Justify your answer.
  1. Phytoplankton need nutrients for growth. Therefore, you might expect to see higher chlorophyll concentrations when nutrients, including nitrate, are higher.
    1. Looking at the different seasons, did you find a positive relationship between nitrate and chlorophyll? Justify your answer by listing examples of the data values and trends of nitrate and chlorophyll within each season.
    2. What does your answer to Question 3a tell you about the impact of nitrate on the pattern of chlorophyll throughout the year? Is nitrate the only factor affecting chlorophyll concentration? Justify your answer.
  1. Many biological rates, including growth, are positively correlated with temperature (warmer temperatures lead to higher rates, at least up to a certain optimum temperature threshold). Therefore, at the temperature range experienced at this location, you might expect to see higher chlorophyll concentrations at warmer temperatures, when phytoplankton might grow faster.
    1. Looking at the different seasons, did you find a positive relationship between temperature and chlorophyll? Justify your answer by listing examples of the data values and trends of temperature and chlorophyll within each season.
    2. Does your answer to Question 4a indicate that the positive effect of temperature on growth rates is responsible for changes in chlorophyll throughout the year? Justify your answer.

Application questions

  1. The effect of temperature on phytoplankton abundance is much more complex than the direct effect on biological rates. This is due to temperature’s effect on water density and thus the three-dimensional physical structure of the ocean environment. Temperature differences between surface and deeper water affect the stratification and mixing of the water column (for more information on density stratification, see background information to this lab activity at the top of this page, additional information provided by your instructor, or the introduction to Lab 5 – Investigating Density and Stratification in the Ocean).
    1. The temperature plotted in the graph is the surface temperature at this location. Assuming that the water temperature at deeper depths generally remains cool throughout the year, what does the surface temperature tell you about the potential degree of stratification during the different seasons? During which season is this location likely the most stratified? During which season is it likely the most mixed (least stratified)? Explain your reasoning.
    2. Stratification and mixing affect the availability of nutrients, including nitrate, in the surface water (see background information at the beginning of lab activity 7.2 or additional materials provided by your instructor). Based on your answer to Question 5a above, during which season would you expect high nitrate concentrations at the surface? During which season would you expect low nitrate concentrations? Does this agree with what you found in the graph? Justify your answer and list actual data values from the graph or table.

Reflection questions

  1. Take a closer look at the nitrate concentration at the beginning of April: over the course of a few days, it drops sharply. Nitrate then remains fairly low throughout April despite the colder surface water temperatures indicating some degree of mixing. What might most likely be responsible for this drop in nitrate and the continued low nitrate values despite mixing?
  1. The area around the Coastal Pioneer Array recently has been identified as a high-use area for the North Atlantic Right Whales, especially during certain times of the year. During which season(s) would you expect the whales to spend time near the array, and why? In your explanation, include a detailed discussion of a) the factors leading to seasonal variability of phytoplankton abundance (from activity 7.2), b) the effect this has on the availability of food for the whales (from introduction to Lab 7 and activity 7.1), and c) how these factors informed your choice of season.
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