The Ecological Role of Bottlenose Dolphins, Tursiops truncatus, in an Estuarine Food Web

Standards Addressed

Background

Virtual Field Trip

Classroom Activities

1. Photo-ID
2. Ecological Role of Bottlenose Dolphins

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Glossary

Teacher Guide

Objective:

Determine how much Primary Productivity is required to support the population of bottlenose dolphins in North Inlet.

This activity is broken down into five steps:

1. Constructing a food web of the North Inlet system
2. Calculating a representative trophic level for the bottlenose dolphins
3. Figuring out how much prey is available
4. Estimating the total annual prey consumption of North Inlet’s Bottlenose Dolphin population
5. Determining the percentage of primary production required to support these dolphins

Procedure and Questions:

Step 1:

 Let’s begin by looking at the food web in North Inlet.   Use the information below, and what you already know about trophic levels and estuaries, to sketch a simplified North Inlet food web leading to dolphins.  You may assume that the bulk of their diet (≈95%) is fish.

Based on studies of stranded animals & a study of North Inlet fishes, the table below lists likely candidates for dolphin food sources. 

Question 1.  What does a simplified version of the North Inlet food web look like?  Use the information below to sketch out a food web leading to dolphins at the top.

Step 2 

Our next step consists of estimating the average trophic level represented by the dolphins in the North Inlet System.  Look at your food web and the trophic level information presented for the prey. Your task is to come up with one number (it doesn’t have to be a whole number) to represent the trophic level of bottlenose dolphins in North Inlet. 

Questions

• Question 2a. What is your estimate for the trophic level of dolphins in North Inlet?  Defend your answer (How did you come up with this number?)
• Question 2b. This seems like a round-about way to figure out what a dolphin eats.  How is diet determined in studies of other animals?  Why is this more difficult with dolphins?

Step 3

Now you must figure out how much prey is available to the dolphin population in North Inlet.  If you know the total amount of primary production in North Inlet, and you know what proportion of energy is transferred from one trophic level to the next, you can calculate how much prey should be available at a given trophic level.  There are quite a few things to consider first.

Primary Production in the North Inlet system may be broken down into four categories:  phytoplankton, benthic microalgae, Spartina spp., and macroalgae. 

• The first two (phytoplankton & benthic microalgae) are grazed on directly, thus providing a more efficient path for energy to proceed up the food chain. 
• The second two (Spartina spp. and Macroalgae) are not fed upon directly; instead they "leak" organic carbon, or they die and are broken down by bacteria, and the bacteria are grazed upon by small plankton.  This is called the microbial/detrital loop.  For our model, we have added 1.5 trophic levels to these paths to represent the extra steps.

Estimated annual primary production in North Inlet is shown in the following table:

* 1 metric ton = 1 x 10⁶ grams

Once you have established the annual primary production, calculate the available prey using the following formula:

C = P x E ^(TL-1)

P is the annual primary production.

E is the transfer efficiency between trophic levels, expressed as a decimal proportion…not a percent. (Check with your teacher if you don’t remember and can’t find it.)

TL is the average prey trophic level.

C is the annual production of prey (food available for the whole year).

For your model, you will use the minimum and maximum estimates for primary production in the table above to calculate a minimum and maximum estimate for “C”.  It is common to calculate a range of estimates in ecological models.  

Questions

• Question 3a.  Look at the trophic level you calculated for dolphins in Step 2.  What is the average trophic level of their prey (TL)? 
• Question 3b.  What value will you use for E?
• Question 3c.  What is your maximum estimate and minimum estimate for C?  (In your calculations, you will have to treat the direct and indirect sources of primary production separately.  For direct sources – phytoplankton and benthic microalgae – assume the prey are supported at the trophic level given in your answer to question 3a above.  For indirect sources – Spartina and macroalgae – you must add 1.5 trophic links to your calculations to account for the microbial loop.  Combined, your results from both calculations will add up to the total annual production of prey from all sources.

Step 4

Next you will estimate the total annual prey consumption of North Inlet’s Bottlenose Dolphin population.  Consider the following information:

¹Young and Phillips (2002), ²Barros and Odell (1995), ³Odum(1971)

Questions

• Question 4a.  How much does a single dolphin eat per day, expressed as grams of Carbon?
• Question 4b.  How much does the North Inlet dolphin population eat per year, again expressed in g of C?

Step 5

We will conclude our investigation of the role of Tursiops truncatus, in the North Inlet Food Web by determining the percentage of primary production required to support dolphins in North Inlet.

In part (3) you calculated annual available prey based on primary production in North Inlet.  In part (4) you calculated annual consumption of the dolphin population in North Inlet.

Questions

• Question 5a.  What percentage of the total annual primary production in North Inlet is required to support the dolphin population?  (You should have a maximum and minimum estimate)
• Question 5b.  What percentage is required to support just one dolphin?

• Question 5c.  Is this “a lot”?  Defend your answer.  What other species are also supported by the same primary production?  Do you think it takes a comparable amount to support you?  Why or why not?

Citations

• Barros, N.B., and D.K. Odell. 1995. Bottlenose dolphin feeding and interactions with fisheries in the Indian River Lagoon system, Florida. (Abstract). Bulletin of Marine Science 57(1):278-279.

• Odum, E.P. 1971. Fundamentals of ecology, third edition. Saunders, Philadelphia, PA.

• Young, R.F. and H.D. Phillips. 2002. Primary production required to support bottlenose  dolphins in a salt marsh creek system. Marine Mammal Science 18(2):358-373.