Instructor’s Guide
Environmental Analysis: Effects of Acid Rain on Atlantic Salmon Populations
Sections of this overall module
Identifying the problem
Sampling and Sample Preparation
pH / ISE
titrimetry (Acid Neutralizing Capacity - ANC, water hardness)
IC (Ca2+ and Mg2+)
instructor’s guide
How this module has been used:
There are several different experimental approaches that can be pursued as identified in the different hypotheses.
This project has been used as an analytical problem based project in Environmental Chemistry, and in a Quantitative Analysis leaning Analytical Chemistry course where the students cover the basic analytical methods and then
Collect water samples,
Obtain field pH measurements over time during spring snow melt,
Measure Acid Neutralizing Capacity (ANC) to identify sites where the water system will be sensitive to acid rain impacts
Measure nutrient levels (phosphorus and nitrogen containing compounds)
Measure water hardness (titration) and concentrations of Ca2+ and Mg2+ via IC
The surface waters in the Machias and East Machias watershed are very low conductivity (often about 20 µS/cm), resulting in needing the trace analysis via ion chromatography (IC) rather than titration. The Ca2+ concentrations in these watersheds often run about 2 ppm. IC is used as it was the instrument readily available for undergraduate use. Atomic absorption spectroscopy could readily be used, and this process is described in other modules.
These low ionic strength waters also make measuring pH a challenge and require very careful calibration, validation, and measurements.
There is also a remediation project where clam shells are used as a source of calcium carbonate to buffer smaller streams from episodic acid rain events. This project has been used to obtain upstream and downstream analysis to identify the changes in water chemistry from the dissolving shells.
There is sample data provided so this could also be used as a dry lab for students to walk through the data analysis process.
This module in primarily focused on the first two hypotheses spelled out in the Identifying the Problem section.
Identifying the problem
Facts, Observations and Evidence
The Atlantic salmon is an anadromous fish species, spending most of its adult life in the ocean coming back to spawn in freshwater rivers. The adult creates a redd, where it lays the eggs, in the fall. After hatching the salmon undergo several stages of their lifecycle in the freshwater. The fry leave the spawning grounds in search of food and develop into parr. After two years, the fry undergo the process of smoltification, where their physiology changes so that they can live in salt water. At this stage the smolt leaves the river and heads to the ocean, where most ultimately head to feeding grounds off of Greenland. The salmon usually live in the ocean for 2-3 years before they return to the river they were hatched in.
In the United States, the historical grounds of the Salmon ranged on the northeast coast south to the rivers of Long Island Sound. Currently, remnants survive only in 8 rivers in Maine, the largest of which is the Penobscot River. Historically in Maine, salmon landings were as high as 90 million tons in the late 1800s. (Baum 1997) By the 1940s the commercial fisheries in Maine were closed, and by the 1990s, less than 2000 adult salmon were returning annually to all of the rivers in Maine, despite over 13 million juvenile salmon stocked annually.
To identify the cause of the minimal returns, the mortality in the river and the sea run mortality were determined. To minimize the potential mortality as parr and fry, a number of smolts are stocked shortly before they are set to leave for the sea. Some of the smolts were pit-tagged with radio labels to identify near shore mortality. Data collected demonstrated a significant mortality from the time when the fish reached the estuary and made it to the open ocean. This mortality would correspond to heavy predation in the bay, or failure of the smolt to survive the transition from fresh to salt water.
During the smoltification process, the salmon has significant biological changes in the gill structure for osmoregulatory processes. In fresh water the salmon needs to maintain a higher salt content in its blood as compared to the water it is in. When in seawater, the salmon needs to maintain a lower salt content in the blood compared to the water it is in. During this change in the gill structure, the salmon is most sensitive to stressors or damage to the gill. Studies have shown that gill deformities can be caused by low pH and by high aluminum. In both of these cases the deformities occur as the cation binding to the gill structure.
The National Resource Council in 2004 specifically stated that the effects of acid rain on smolts is one of the most significant factors impeding the recovery of the Atlantic salmon; “The problem of early mortality as smolts transition from freshwater to the ocean and take up residence as post-smolts needs to be solved. If, as seems likely, that the difficulty of the transition is due in part to water chemistry, particularly acidification, the only methods of solving the problem are changing the water chemistry and finding a way for the smolts to bypass the dangerous water.”
The related Brook Trout has had similar extirpation from streams and ponds in many northeast locations due to low pH.
The question to be answered is: What is causing the increased mortality of Atlantic salmon?
Some hypotheses as to possible (and perhaps intertwined) causes include:
Low pH. Salmonids (Atlantic Salmon, Brook Trout, and related species) are known to be sensitive to pH, with stress occurring below pH 5, and mortality below pH 4.5. The pH of the aquatic ecosystem can be lowered due to natural acid inputs, such as dissolved organic carbon (DOC), or through anthropogenic inputs, predominantly acid rain. The acid rain is created from nitrogen oxides and sulfur oxides emissions. These may be episodic or chronic conditions.
High Aluminum. Aluminum will bind to the gills of the salmonids and irreversibly damage the gills, preventing the fish from uptaking oxygen. Aluminum will get into the water systems predominately from leaching from the ground when acid rain impacts a system.
Elevated Temperature Salmonids are temperature sensitive. When stream temperatures reach over 22ºC, salmon are severely compromised. When temperatures reach over 26ºC, mortality often ensues. The temperature of the stream can change due to increases in air temperature, reduction in boreal cover, or reduction in underground cold-water stream inputs.
Increased predation. There are two major predators. As the fish swim to the ocean as smolts, they are met by a host of predators such as cormorants and seals. The populations of both of these predators have increased in recent years. Another predator is humans, and overfishing is of concern. Starting in the 1950s, salmon have been caught in their ocean feeding grounds off of Greenland and Newfoundland. The catch has steadily declined with the decreasing populations.
Identifying Possible Analysis Methods
There are three hypotheses that have been put forward that could be responsible for the decline of the Atlantic Salmon. The purpose of this exercise is to identify analysis methods that could be used to test the chemical species that contribute to each of these hypotheses. Among the various analytical methods you find that may be applicable to this measurement, identify their strengths and weaknesses (e.g., sensitivity, expense, ease of use, reproducibility).
Hypothesis 1. Episodic acid rain events overcome the buffering capacity of the water system and decrease the pH leading to stress and mortality in Atlantic salmon parr and smolts.
Q1. Using available literature and other sources of information, identify possible analytical methods that could be used to monitor acidity.
Hypothesis 2. Acid rain has increased the leaching of calcium from freshwater, reducing the nutrients available for salmonids. This loss of calcium prevents salmon from properly undergoing the smoltification process resulting in mortality when the fish head to the ocean.
Q2. Using available literature and other sources of information, identify possible analytical methods that could be used to detect calcium and measure their levels in components of the water (e.g., free, organic bound).
Hypothesis 3. Acid rain has increased the leaching of aluminum, resulting in a high concentration of free aluminum ion resulting in gill damage, leading to the main cause of mortality of Atlantic Salmon during the smoltification process.
Q3. Using available literature and other sources of information, identify possible analytical methods that could be used to detect aluminum and measure their levels in components of the water (e.g., free, organic bound).
The rest of the module primarily focuses on the first two hypotheses.
References:
Baum, E. T. 1997. Maine Atlantic Salmon: A National Treasure. Atlantic Salmon Unlimited. Hermon, ME
National Research Council, Atlantic Salmon in Maine. The Committee on Atlantic Salmon in Maine, Board on Environmental Studies and Toxicology, Ocean Studies Board, Division on Earth and Life Sciences. National Research Council of the National Academies. National Academy Press. Washington, D.C. 260 pp
U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. A1-A9, available online at http://pubs.water.usgs.gov/twri9A.
Instructor’s Guide – Sampling Plan
This section of material is designed to be approached after students read through the Identifying the Problem module (or an analogous scenario), so they have some idea of what analytes and samples sources the sampling program would focus on addressing. This is meant to have students can work in groups on the questions that are provided. The answers to the questions are provided below.
I have used this by giving the question sets to the students with about 10 minutes left in a class period where I had them discussing Q1. Then they did Q2-5 between classes- so they began thinking about the questions on their own. The next class period they worked in small groups of 3-4 to discuss those answers and proceeded through much of the rest of the assignment. This ended up taking about 90 minutes of class time.
Q1. What key questions must be considered when designing a sampling plan?
Students should be given time to brainstorm on key issues to consider when designing a sampling plan. The instructor may want to provide the answers listed below only after the students may have had a chance to come up with their own answers. Eventually students should be guided to consider issues such as:
1. Where in the watershed should we collect water samples?
2. What type of samples should we collect?
3. When should we collect the sample?
4. What is the minimum amount of sample for each analysis?
5. How many samples should we analyze?
6. How can we minimize the overall variance for the analysis?
Q2. Pick eight random samples from the grid laid out above. How do you ensure you sampling is random?
One way to get random samples is to use Excel or other random number generator. To get 8 random grids label the boxes 1-8, row 1, 9-16, row 2, etc. for 64 boxes. Then use Excel to generate 8 random numbers between 1 and 64; for example: 63, 35, 25, 46, 7, 53, 43, 5.
You might ask the students to discuss whether they think a random approach represents the best way to sample. They may realize that the answer depends in part on what you may already know about the system you are sampling. If there is a specific site of concern or point source of the chemical, then random sampling might not be the best option.
Now take a look at the following grids with the analyte of interest identified (colored squares).
1 / 2 / 3 / 4 / 5 / 6 / 7 / 81
2
3
4
5
6
7
8
Grid A
1 / 2 / 3 / 4 / 5 / 6 / 7 / 81
2
3
4
5
6
7
8
Grid B
Q3. Would you consider the samples above to be heterogeneous or homogeneous?
The analyte is heterogeneous because it is not evenly distributed throughout the entire grid.
Q4. Did your random sampling affect the potential accuracy or precision of your measurement of the analyte for the samples in grid A or grid B? If so how?
The random sampling would have been better for grid B because the analyte is more spread out than in grid A where the analyte is confined or stratified.
Q5. Each of the previous grids is an example of one of these cases. Can you identify which sample is which?
Sample B exhibits constitutional heterogeneity while Sample A exhibits distributional heterogeneity.
Q6. How does distribution heterogeneity affect accuracy and precision?
The answer to Q6 is included with the answer to Q7 below.
Q7. How does constitutional heterogeneity affect accuracy and precision?
When you overlay the sampling scheme with Sample A and Sample B neither sampling scheme is effective, as shown below. The analyte in sample “A” was sampled once (35) as it was in Sample “B” (53). With either sample, unless the sampling scheme could take this heterogeneity into account, the accuracy and precision of the measurement would be compromised.
A. Distributional Heterogeneity B. Constitutional Heterogeneity