Lab 7 – Identify factors that control Primary Production in the western temperate Atlantic Ocean
Instructor Guide

North Atlantic right whales are one of the most endangered whales in the world. They were traditionally hunted and remain endangered today, with only a few hundred individuals surviving. Scientists are interested in learning more about right whale migration patterns to help protect them and save them from extinction. Recently, scientists discovered that these whales spend significant time feeding along the coast south of Cape Cod, Massachusetts, during March and April while they are moving from the south, where they breed during the winter, to the north where they feed during the summer and fall. These whales are filter feeders and eat copepods, which in turn eat phytoplankton. In this lab activity, students will investigate why the whales are present in this part of the ocean at this time of the year.

To do so, students will work with data collected from the Coastal Pioneer Array, which is located in the same general area that the whales have been found to frequent in the spring. Students will describe and interpret the relationships between primary production (phytoplankton abundance, as indicated by chlorophyll-a) and abiotic variables (sea surface temperature, salinity, nitrate, and irradiance) over the course of several seasons. They will explain how seasonal changes in a temperate ocean lead to seasonal differences in primary production. They will also predict patterns over multiple years using knowledge gained from working with a year of data. At the end of the lab, students will apply this knowledge to explain the connection between primary production and the timing of whale migration.

Approximate time involved: 45-50 minutes for each of the three activities

Learning outcomes

  • LO1. Describe patterns in individual data sets and correlations between the different data types presented.
  • LO2. Interpret the provided data and hypothesize about how these variables influence each other and why.
  • LO3. Explain the relationship between primary production, nitrate concentration, irradiance, and sea surface temperature (SST) in the western Atlantic Ocean using evidence and relevant scientific concepts to support the hypotheses.
  • LO4. Predict patterns over multiple years using knowledge gained from working with a year of data.

 

Learning outcome Activity 1 Activity 2 Activity 3
Outcome 1 introduced introduced reinforced
Outcome 2 introduced introduced reinforced
Outcome 3 introduced reinforced
Outcome 4 introduced

Materials needed: none

What students should know before this activity

We assume that the students have worked through the initial lab in this lab notebook that introduces them to basic data literacy skills, particularly reading time series charts and distinguishing trends in “messy data.” We also assume that students previously have been introduced to the following basic concepts:

  • Phytoplankton have chlorophyll-a that they use for photosynthesis; we can measure this as a proxy for their abundance
  • Phytoplankton need C, N, P, and other elements (nutrients) for growth; dissolved inorganic nitrate (NO3) is a source of N for many phytoplankton
  • Phytoplankton need sunlight to grow; the availability of sunlight (irradiance) changes seasonally
  • In some parts of the ocean (such as in this temperate ocean example), seasonal changes in water column stratification occur: the surface ocean becomes warm and stratified during the late spring and summer; it becomes cooler and mixes with nutrient-rich deeper water during stormier fall and winter months
  • Seasonal changes in ocean stratification bring on seasonal changes in nutrient availability.

What instructors should know before this activity

  • This location was chosen to highlight the role that seasonal mixing and stratification of water masses, combined with changes in light availability (irradiance), plays in controlling primary production. This location contrasts from other regions where upwelling might play a more important role.

Optional Pre-Lab Activities:

  • Discuss the biology of North Atlantic right whales. What do they eat? How do they fit into the food web? What are their migration patterns? Watch this video about right whales to get students interested. 
  • Explore general trends in global primary production using this website. Students could use the slider bar to examine how sea surface temperatures and the abundance of chlorophyll-a vary across the different oceans, hemispheres, and seasons.

Pre/post-lab Assessment Questions:

  1. True or false: Phytoplankton biomass is highest during the summer.
  2. True or false: Salinity plays an important role in influencing primary production.
  3. In a temperate ocean, such as along the US northeast coast, during which season is primary production at its lowest? A) Winter, B) Spring, C) Summer, D) Fall
  4. For a temperate ocean, such as along the US northeast coast, order the seasons in order of the expected primary production in the photic zone, from most to least. Fall, Spring, Summer, Winter (Spring, Fall, Summer, Winter)

Scientific Background

While we can’t directly measure the abundance of phytoplankton, we can measure the abundance of chlorophyll-a, the pigment involved in photosynthesis. Phytoplankton require sufficient sunlight to photosynthesize; they also require a range of nutrients, including nitrogen. 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 a lack of phytoplankton, nitrate is usually found in higher concentrations.

If the water column is stratified (layered), noticeable differences in nitrate concentrations may be present from surface to deep water. At this study site, the water becomes stratified during certain seasons (most pronounced during summer). At other times, seasonal mixing occurs (most pronounced during winter). These differences are related to temperature changes, which in turn affect the density of the seawater (Note: salinity also affects density of seawater; however, based on the data, it plays a lesser role at this location). Larger differences in density between surface and deep water (e.g., warm, less dense surface water vs. cooler, denser deep water) result in increased stratification, while similar density throughout the water column (e.g., cool, dense water at the surface and at depth) leads to mixing. When mixing of the water column occurs, nutrients from deeper water become available in the surface water. If increased nutrient availability coincides with seasonal increases in available sunlight, a sudden increase in phytoplankton abundance may be observed. Seasonal stratification and mixing are the strongest influences in nutrient availability at this site, although students may point out that runoff of nitrate from land can also have a seasonal influence.

Here is a summary of how irradiance, nutrient availability, and primary production vary seasonally in this temperate ocean (also see diagram below).

  • Winter: colder temperatures throughout the water column lead to increased mixing; nutrients are abundant in surface waters but sunlight (irradiance) is low; this leads to low primary production because of light limitation
  • Spring: sea surface temperature (SST) and irradiance begin to increase, gradually reducing mixing; nutrients are still plentiful from the winter months; increasing irradiance and high nutrient concentrations result in high primary production (spring bloom); later in the spring, nutrients decline because they are used up by phytoplankton, and surface water becomes increasingly stratified due to warmer SST; this prevents mixing of new nutrients into surface waters and leads to an eventual decline in primary production (note: some of that decline may also be due to grazing by zooplankton)
  • Summer: high light availability and warm SST keep the water column stratified; nutrients are low in surface waters; primary production is low because of nutrient limitation
  • Fall: light availability and SST start to decrease, breaking down the stratification (with the help of winds); this allows nutrients to be mixed into surface waters; primary production may increase again if enough sunlight and nutrients are available in surface waters (fall bloom) but decreases as soon as light availability becomes too low

Teaching notes

An idea for shortening the required time frame for Activity 2 would be to subdivide students into groups, with each group looking at chlorophyll-a and one other variable. Each group could report on their analysis before the entire class worked on synthesizing the relationships between all of the variables. A class discussion could then tackle the idea of which variables were most important in controlling the distribution of chlorophyll-a.

Salinity is included in this dataset even though its seasonality is less pronounced at this location. It is included to give students an opportunity to distinguish between datasets that correlate significantly and those that do not. To shorten Activity 2, it could be eliminated from the lab.

Extensions

These questions, discussion topics, or activities could be used for further discussion or assessment:

  • Ask students to reflect on the data used in this exercise compared to the standard textbook figure of seasonal changes in primary production in a temperate ocean in the Northern Hemisphere. How does the real-world data that they examined in this exercise support the textbook’s explanation of this concept? What additional learning has this real-world data supported?
  • A similar “widget” without an accompanying lab is available for the Southern Ocean. This resource could be used to do a follow-up exercise on a polar ocean so students can compare and contrast patterns in two very different parts of the ocean. It also requires students to think about how the seasons differ in the southern hemisphere.
  • Seasonal changes in global primary production and sea surface temperature could be explored using this website. Students could use the slider bar to look at how sea surface temperatures and the abundance of chlorophyll-a vary across the different oceans, hemispheres, and seasons. They could be asked to summarize, explain, or discuss the global patterns, using the knowledge gained in this exercise. The interactive map set does not show irradiance or nutrients, thus students could make inferences about these variables.
  • For upper level students, other potential sources of nitrate (i.e. run-off) can be discussed. How could we determine whether nitrate concentrations are due to run-off? What sorts of data might we need that might not be available from OOI data (i.e. weather data)? We looked at OOI data from three locations, offshore, central, and inshore. Which location is most likely to be affected by nitrate from run-off as well as seasonal mixing of the water column?

Associated resources

Learn more about the Coastal Pioneer array here.

These links provide more information about right whales.

Answer Key

[Will be supplied to instructors upon request]