Atlantic Forest

Exercise 11: Water Salinity and Animal and Plant Community Composition
Module 11: Chemical and Hydrological Cycles

Your Questions

1. Does salinity influence aquatic animal and plant community composition?
2. Are there commonalities in freshwater vs. brackish vs. salt water communities?

Background

Today’s exercise is a two-day endeavor that will allow you to demonstrate how well you've mastered the basic tenents of the scientific method, experimental design, and the techniques that we've been studying for the past three weeks. We are interested in not only how you design the experiment, but also in how you interpret the data and how you could extend your study in the future with more time and resources.

Aquatic animals and plants are largely limited by the water in which they live. Those that live in fresh water have to deal with having more dissolved salts in their tissues than in the surrounding water. As a consequence, their bodies are hypertonic relative to the water - much of their physiological mechanisms are geared towards pumping out the excess water that is being absorbed by their tissues and retaining the salts that are in their tissues. If the salt levels in the water significantly change, they can quickly die because of dehydration or due to a loss of essential salts and minerals (electrolytes) in an effort to maintain their proper water and salt balance.

In contrast, those animals that live in water with salt levels that approximate that found in their body tissues, such as those living in marine environments, are isotonic relative to the water. Therefore, these organisms do not need to spend as much energy maintaining the salt levels in their tissues. If an aquatic organism that normally lives in fresh water is brought to marine water and dropped in, they often hypotonic relative to the water, as they have become so efficient at pumping out the water that they will over-concentrate the salts in their tissues and die.

As an example of these phenomena, your hands do not wrinkle up as quickly in salt water as they do in fresh water. That is because you absorb water more quickly in fresh water (and expand the tissues so much that they wrinkle with thes extra cell thicknesses) than you do in salt water. A nifty and simple observation.

Clearly, the appropriate salt concentrations are essential for maintaining life in aquatic organisms. Those organisms that can move quickly and easily between very different types of water are remarkable. For example, salmon live most of their life in salt water, but breed in small rocky fresh water streams, a lifestyle called anadromous - the inverse of which is called catadromous.

We would therefore expect that most species live in only one of three water types: fresh, brackish (a mixture of salt and fresh water), and marine or saline. How well do the coastal Atlantic Forest ecosystems adhere to this expectation? Determining the answer to this question is the goal of this field practical.

Your Assignment

Our goal is to provide you the ability to succeed at this evaluation. We are not interested in seeing anyone do poorly, but groups will get the grade that they earn.

Some of the variables that should be considered are:

1. Independent variables to manipulate or compare - What are the most appropriate independent variables to keep track of so that you can answer your question?
2. Dependent variables to record - What are the data points that you will record that may change in response to the independent variables?
3. Location of study - Where will you do it?
4. Number and distribution of study sites - How often will you replicate it? Why is replication important?
5. Methodology - Be explicit and thorough
6. Types of controls - Are these controls or standards for comparison?
7. Modes of analysis - What types of statistical analyses should you use for this study?
8. Limitations - Given that you will not be able to conduct a thorough study in only two days, what obstacles did you encounter? How could they have affected your results? How would you get around them in an ideal world with enough time to mitigate their impacts?
9. Pseudoreplication - Is this a problem with your study? If so, why? If not, why not?

Evaluation will be based on:

1. Design and execution of experiment
2. Ability to collaborate with your team - be supportive but still push each other to do your best!
3. Oral summary of each group's analyses the day before.
4. Participation in the evaluation of each group's presentation in the second day of the activity. Based on your analyses, do you agree or have additional insights?

Objectives

1. Be able to apply what you have learned to designing experiments to answer a specific question.
2. To flex your scientific skills and knowledge that you've learned thus far in the class.
3. Allow the instructors to evaluate your progress to date in the course.

Key Skills

1. Improve your ability to work in groups, which is the basis of nearly all science at present.
2. Increased competence collecting field measurements.
3. Designing, carrying out, and analyzing the results of a field experiment

Timetable

Given the unique set-up of this activity, it is not possible to assign time to specific tasks. However, the bulk of your in-class time should be toward designing, conducting, analyzing and writing up your answer to the question. Times below are only gross approximations.

1. Total elapsed time to perform the experiment: 2 days.
2. Total time: 13 hours.

• Lecture and discussion (1 hr day before we leave)
• Preparation for field work and experimental design (3 hrs)
• Discussion of design with instructors (1 hr)
• Field work (6 hrs)
• Analysis of results (1 hr)
• Write-up and discuss results (2 hr).

Procedural Notes

1. This lab will serve as a field practical.
2. This is a two-day activity that we will carry out when we go to the coast. Students will be responsible for beginning to design the field experiment the day before we leave to go to the shore.
3. Each group in the class will be comprised of 5 students and all students should take the opportunity to design their own experimental system to answer the above question. Use the time that we spend in transit to the coast to refine what it is that you would like to do to answer your question.
4. Each group will present their initial design over lunch for discussion with the instructor and TAs - we'll discuss each design individually with each group then and will then discuss how to improve the experimental set up.
5. The afternoon and morning of the next day will be spent collecting and analyzing the requisite data.
6. Presentations will occur in the afternoon of the second day of the activity, with each group of students presenting their results to the class and everyone discussing these presentations.
7. As a point of suggestion, try to focus what part of the question you will answer as much as you can. This will ensure that you will be able to succeed.

Materials Needed

1. Materials needed will be determined by the students on Sunday afternoon before we leave for the shore.
2. Because we will not be bringing all of the field equipment that we have, you should give some serious thought to how you wish to do this experiment over the weekend to ensure that we will have all the materials your group will need to collect the data that you will need.