Lab 7 – Identify factors that control Primary Production in the western temperate Atlantic Ocean
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) and abiotic variables (surface water temperature, 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: 45-60 minutes for each of the three activities
- 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 temperature 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||Guided practice||Applied|
What students should know before this activity
We assume that the students have had an introduction to basic data literacy skills, particularly reading time series charts and distinguishing trends in “messy data” as introduced in Lab 2.1.
We also assume that students previously have been introduced to the following basic concepts:
- Phytoplankton have chlorophyll that they use for photosynthesis; we can measure chlorophyll concentration as a proxy for phytoplankton abundance
- Phytoplankton need C, N, P, and other elements (nutrients) for growth; dissolved inorganic nitrate (NO3) is a source of N for many phytoplankton; nitrate (and other nutrients) are usually found at higher concentrations in deeper water
- 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
- The basics about marine food webs: phytoplankton are eaten by zooplankton, which are eaten by larger organisms including filter–feeding whales
What instructors should know before this activity
- This location (Pioneer Array on the eastern U.S. continental shelf) 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.
Phytoplankton are the most abundant marine primary producers. They are found at the base of the marine food web, directly and indirectly supporting most of the life found in the oceans, including large animals such as whales. Because phytoplankton are photosynthetic, they contain the green pigment chlorophyll a (simply referred to as “chlorophyll” hereafter and in the student lab instructions). The concentration of chlorophyll in the water can easily be measured using sensors and serves as a proxy for the abundance of phytoplankton: higher chlorophyll concentration indicates higher phytoplankton abundance, which in turn indicates higher primary production and more food availability for the rest of the food web.
Phytoplankton require sufficient sunlight to photosynthesize (note: we use “sunlight” and “irradiance” interchangeably in the student instructions; the data used for this lab technically refer to downwelling shortwave irradiance). Phytoplankton 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 the sinking of organic matter and 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 and has been omitted from the student lab to reduce complexity). The seasonal change in stratification is apparent in the Pioneer profiles in Lab 5.3 (compare the thermoclines in December-January to July-August).
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 result in similar water density and increased mixing due to the absence of a thermocline. This mixing allows nutrients from deeper water to become available in the surface water. However, because sunlight (irradiance) is low, phytoplankton are unable to grow and primary production also remains low, allowing nutrients to accumulate.
- Spring: Surface temperature and irradiance begin to increase, gradually reducing mixing as the thermocline begins to form. Initially, nutrients are still plentiful from the winter months. Increasing irradiance and high nutrient concentrations result in high phytoplankton growth and primary production (spring bloom). Later in the spring, nutrients decline because they are used up by phytoplankton, and the water column becomes increasingly stratified due to warmer surface temperatures and the establishment of a thermocline. This prevents mixing of new nutrients into surface waters and leads to an eventual decline in phytoplankton primary production.
- Summer: High light availability and warm surface temperatures result in a strong thermocline and keep the water column stratified. Nutrients are low in surface waters because stratification prevents the deep water from reaching the surface. Despite high light availability, phytoplankton and primary production are low because of insufficient nutrient availability.
- Fall: Light availability and water temperatures start to decrease, breaking down the stratification and thermocline (with the help of winds). This allows nutrients to be mixed into surface waters. Phytoplankton abundance and primary production may increase again if enough sunlight and nutrients are available in surface waters (fall bloom) but decrease when light availability becomes too low in late fall.
An idea for shortening the required time frame for Activity 7.2 would be to subdivide students into groups, with each group looking at chlorophyll 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.
Activity 7.2 was designed to walk students through each data set and comparison to chlorophyll individually. More advanced students may not need this level of scaffolding and could be asked to describe relationships and explain patterns without the scaffolding. Some suggested questions for upper level students:
- Compare chlorophyll to each of the abiotic variables; do you notice any relationship at a seasonal scale? For each of the following variables, describe any relationships with chlorophyll you observe.
- Synthesize your observations by explaining which factors lead to the pattern of primary production shown. Explain why chlorophyll and the three abiotic variables are related in this manner. What is responsible for these patterns and relationships? How do the different variables affect each other? How does that result in the pattern of chlorophyll (primary production) shown?
If Activity 7.3 is not used, consider asking students to explain the seasonal patterns of chlorophyll at the end of Activity 7.2 by using a slightly modified version of Question 4 from Activity 7.3 (focusing on just one year of data instead of four):
- Look at the data for summer. Describe what you notice about the magnitudes of irradiance, nitrate, and temperature during that season. Using evidence from the graph and concepts from the previous questions, explain why chlorophyll concentrations are low during summer.
- Look at the data for winter. Describe what you notice about the magnitudes of irradiance, nitrate, and temperature during that season. Using evidence from the graph and concepts from the previous questions, explain why chlorophyll concentrations are low during winter.
- Chlorophyll concentrations are highest during spring and also increase again slightly in the fall. Based on your analysis of irradiance, nitrate, and temperature patterns, what happens during spring and fall to result in higher chlorophyll concentrations? Using evidence from the graph and concepts from the previous questions, explain why chlorophyll concentrations are highest during spring and show another slight increase in the fall.
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:
- True or false: Phytoplankton biomass is highest during the summer.
- In a temperate ocean, such as along the US northeast coast, during which season is primary production at its highest? A) Winter, B) Spring, C) Summer, D) Fall
- For a temperate ocean, such as along the US northeast coast, order the seasons from highest to lowest expected primary production in the (eu)photic zone. Fall, Spring, Summer, Winter
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 the real-world data supported?
- The four-year data set could be used to discuss variability and replication in more detail by asking the following discussion questions: were your predictions based on the data from one year still supported by the data from four years? If they were not supported, does the data from three years change anything? What is the importance of collecting and evaluating data from more than one year?
- 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.
- Salinity was removed from the dataset and widget for this lab to reduce complexity. It is included in this original widget if a more complex data set is desired for advanced students. This widget also contains wind speed data, which is related to water column mixing.
- For upper level students, other potential sources of nitrate (i.e. runoff) can be discussed. How could we determine whether nitrate concentrations are due to runoff? 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 runoff as well as seasonal mixing of the water column?
- For a deeper discussion of the effect of climate change on North Atlantic right whales (addressed briefly in the reflection questions of Activity 7.3), students could read and discuss this article about climate-induced circulation changes and their effect on North Atlantic Right Whale conservation: Record, N.R., J.A. Runge, D.E. Pendleton, W.M. Balch, K.T.A. Davies, A.J. Pershing, C.L. Johnson, K. Stamieszkin, R. Ji, Z. Feng, S.D. Kraus, R.D. Kenney, C.A. Hudak, C.A. Mayo, C. Chen, J.E. Salisbury, and C.R.S. Thompson. 2019. Rapid climate-driven circulation changes threaten conservation of endangered North Atlantic right whales. Oceanography 32(2):162–169, https://doi.org/10.5670/oceanog.2019.201. For introductory level students not accustomed to reading in-depth scientific articles, this article from the New York Times provides a good discussion of the original article.
Learn more about the Coastal Pioneer array here.
These OOI lab activities relate to skills or topics in this lab activity:
- OOI Lab 2 addresses some of the necessary skills required to interpret time series data
- OOI Lab 5 explores density and stratification in the ocean
A short (12 minutes) video about marine food webs and factors influencing marine primary production can be found here. This video also mentions right whales.
These links provide more information about right whales.