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Day 1: Modeling the Interaction of Salinity and Diatom Populations in the Hudson Estuary

Unit Plan: Hudson Data Literacy ActivitiesTime: One 40 minute class period Setting: Classroom
9-12Hudson River Ecology
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Students will learn about salinity in the Hudson River Estuary and graph changes in salinity across time and space. They will collect diatom samples and compare diatom communities from their sampling site with salinity levels.


  1. Students discuss what it means to be an estuary (in Algonquin, “Mohicanituk,” "The  River That Flows Both Ways”) and construct a model of the Hudson Estuary to deepen their understanding of the interaction of various biotic and abiotic factors.

  1. Students use data from the Hudson River Environmental Conditions Observing System (HRECOS) to graph and map salinity data at various sites throughout the lower Hudson Estuary.


  • Powerpoint presentation Modeling Interaction of Salinity & Diatoms

  • Large display surface (e.g. butcher paper, large magnetic whiteboard and magnetic pieces, Smartboard) for mapping biotic and abiotic factors

  • Large map of Hudson River (poster, navigation maps, or drawn model on butcher paper) may use only lower Hudson River, from mile 0-90. A map of the Hudson River showing miles is available from the NYDEC here.

  • HRECOS data files and graphs

  • A computer with access to HRECOS and Excel/Google sheets OR printouts of HRECOS datasets and graph paper, pencil, ruler

  • Post-it notes or something similar e.g. magnetic piece for adding components to model

  • Student worksheet Worksheet Day 1 HRECOS Salinity & Diatoms 

NOTE: if students are working in Google docs directly, they will first need to make a copy of the worksheets with their last name in the filename.

DAY 1:  Graphing Salinity Data from HRECOS


  • Ask students to brainstorm names of organisms that they associate with the Hudson Estuary. Students can write their ideas in the chart on the Day 1 student worksheet (question 1). Have students report out, and write a list on the board.
  • Have students characterize each organism as a producer, consumer, or decomposer on their worksheet. Remind students that a food web must contain all three kinds of organisms. If this reveals a lack of organisms, see if students can fill in those gaps. Depending on student level of prior knowledge, students can be provided with names of key organisms. Sample food webs are available under resources.
  • Students can then draw the organisms on a magnetic sheet or Post-it note and place on the board. Discuss links and differences between them. Have students work together to draw arrows between organisms in order to create a food web.
  • Brainstorm abiotic factors (possibilities include oxygen [DO and atmospheric], carbon dioxide, salt, etc.). Students can list these in the table on the Day 1 student worksheet (question 2). Draw arrows to show locations and movements of these abiotic factors in the ecosystem on the board. If students struggle with generating ideas, teacher can provide a list of factors and ask students to place them on the diagram and describe how a selected one would move through or affect organisms. Possibilities include acidity, temperature, water depth, dissolved oxygen, and turbidity. Data on each of these parameters can be found on the HRECOS Website.


  • If students have not mentioned salinity as an abiotic factor, introduce the idea that the lower Hudson is an estuary, a body of water where fresh and saltwater mix. Give each student HRECOS salinity data from one of the Hudson River stations, which can be found in this folder. Have each student graph the change in salinity at their station, and use the data chart to calculate the average salinity at their station. Data can be graphed by hand, or using programs such as Google Sheets or Excel, but be sure to draw attention to scale if graphed by computer. If graphed by hand, teacher may want to have all students use the same scale to make differences in patterns more clear.
  • Discuss the answers to the questions, “Does the salinity at your station change over time? If so, is there a pattern? What do you think might cause this pattern?” (question 3 on the student worksheet for day 1). Student answers may differ depending upon their assigned station. Review the idea of ocean tides as a class if necessary. An information sheet on tides in the Hudson is available here.


  • Ask students to locate their station on the Hudson River map, and place their graph at their assigned station. Students will walk along the length of the river map, observing the relative locations, salinity graphs, and average salinities for those locations. On the large display surface, write “Different” and “Same.” Give each student two Post-it notes. On one, have the student write one way in which the graphs are different at each station. Possible answers include the average salinity (decreasing from south to north), and the graph shape (flatter as you move north away from the ocean and the impact of high and low tide decreases). On the other, have the student write one way in which the graphs for each station are the same. Possible answers include the cyclical increase and decrease in salinity caused by the tide, and the presence of some amount of salt even in freshwater. Report out and discuss the answers.
  • Have students answer the question, “How does the average salinity change as you travel from north to south on the Hudson River? What causes this change?” (question 4 on student worksheet for Day 1). Look at the “Graphic breakdown of water salinity, defining freshwater, brackish water, salt water, and brine water” sheet. Students have graphed salt concentration in practical salinity units (psu), which is equivalent to parts per thousand (ppt).
  • Have students mark on the Hudson River map areas where water is fresh and brackish by comparing the average salinities they calculated for each station to the graphic sheet. Students may label each area of the map, color each area, or label each area with a Post-it note. Ask students to find the border between brackish and freshwater. The salt front is the leading edge of salt water entering an estuary. Have students locate the salt front on the Map, and answer questions 5 on the student worksheet for Day 1.
  • Discuss with students what differences they would expect to find between freshwater and brackish ecosystems (question 6 on the student worksheet for Day 1). Encourage students to think about the differences in fresh and saltwater (conductivity, buoyancy, tonicity, etc), and what adaptations organisms would need to live in each. If students have completed the NYS Living Environment lab “Diffusion through a Membrane,” this discussion can be linked with what they observed of the movement of substances in and out of cells and the questions on the lab.



As an extension, student can use the HRECOS site (, “Current Conditions”) to graph data on other abiotic factors at their site (depth, DO, water temperature, acidity, etc.). These graphs can be saved or copied by right-clicking on the graph. Students can then print them, place them at the appropriate station on the map, and compare other abiotic measures at the locations.


Students will construct a food web, showing relationships between organisms. Students will be able to name abiotic factors, and show their interactions with biotic factors. Students will accurately plot data and calculate averages. Students will identify and explain cyclical patterns from graphed data.

Lesson Files

Answer Key Salinity & Diatoms
Worksheet Day 1 HRECOS Salinity & Diatoms
Modeling Interaction of Salinity & Diatoms
Next Generation Science Standards

Science and Engineering Practices

Developing and using models, Analyzing and interpreting data, Using mathematics and computational thinking

Cross Cutting Concepts

Systems and system models, Stability and change

Disciplinary Core Ideas

LS2A: Interdependent Relationships in Ecosystems
New York State Science Learning Standards

Performance Expectations

HS-LS2-2. Use mathematical representations to support and revise explanations based on evidence about factors affecting biodiversity and populations in ecosystems of different scales.

This lesson was created by teachers Donna Light and Kelly Czermerys