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Below Whitewash Rock: Bird Droppings, Seaweed and Tide Pools

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Below Whitewash Rock: Bird Droppings, Seaweed and Tide Pools

Model how matter from the sea cycles through tide pools while energy flows.

Four pools on the point

Read the passage and study Table 1. Then answer the questions.

A Field Trip to Whitewash Rock

1Each spring, Ms. Yolanda Petrakis takes her seventh-grade class from Pinecrest Middle School to a marine field station on a rocky point. Just off the point stands a rock stack that the station staff call Whitewash Rock. About 50 gulls and cormorants rest on it every day, and its top is streaked white with their droppings.

2The station's biologist, Dr. Mireille Toussaint, explained that the birds feed at sea. Cormorants dive for small fish, and gulls catch fish and scraps at the surface. Each bird leaves about 20 g of droppings (dry mass) on the rock every day. The droppings are rich in nitrogen, a nutrient that living things need to build proteins. Rain and splashing waves wash some of the droppings down into the tide pools at the foot of the stack.

3The class studied four pools on the sheltered side of the point. All four are about the same size and depth and sit at the same height on the shore, so the tide covers and uncovers them at the same times. They lie at different distances from the stack. Students measured the nitrogen dissolved in each pool's water, the percent of the rock covered by seaweed, and the number of limpets, small snails with cone-shaped shells that scrape seaweed off the rock (Table 1).

4Back at the station, staff wearing gloves set up a tank test. Pieces of the pools' most common seaweed, a thin red seaweed, were weighed and placed in six tanks: two with water from Pool 1, two with water from Pool 4, and two with water from Pool 4 plus 1 g of dried droppings from the rock. All tanks had the same light and temperature. After 7 days, the class weighed the seaweed again (Table 2).

5Dr. Toussaint also described other living things in the pools. Shore crabs eat limpets and other snails. When seaweed, limpets or crabs die, bacteria and other decomposers break down their remains, along with the animals' wastes, and release nitrogen back into the water. Out at sea, the small fish that the birds eat feed on tiny drifting animals, which feed on drifting algae that use sunlight to make their food.

Table 1. Four Tide Pools on the Point
PoolDistance from stack (m)Nitrogen (mg/L)Seaweed cover (%)Limpets per m²
121.68060
2100.85540
3400.23022
4800.12520
1.

The water in Pool 1 held how many times as much nitrogen as the water in Pool 4? times as much

2.
Which explanation of the nitrogen and seaweed data in Table 1 fits the passage best?
Question 2 options
3.
Pool 1 had 60 limpets per m², but Pool 4 had only 20. Which explanation fits the passage and Table 1 best?
Question 3 options

The tank test

Use paragraph 4 and Table 2 to answer the questions.

Table 2. Seaweed Tank Test (average of 2 tanks; same light and temperature)
TanksWater usedSeaweed at start (g)Seaweed after 7 days (g)
APool 1 water57.4
BPool 4 water55.6
CPool 4 water + 1 g dried droppings57.2
4.

How many grams did the seaweed in Tanks A gain in 7 days? Do not round. g

5.

The seaweed in Tanks A gained how many times as much mass as the seaweed in Tanks B? times as much

6.
Circle the two results that, together, best show that matter from the droppings made the seaweed grow faster.

7.
The seaweed in Tanks C gained 2.2 g, but only 1 g of droppings was added and most of it stayed in the water. Where did most of the seaweed’s new mass come from?
Question 7 options

Modeling matter and energy at the rock

Use paragraphs 2 and 5 and everything on this sheet.

8.
Use paragraph 2.

About how many kilograms of droppings (dry mass) do the birds leave on Whitewash Rock in one week? kg

9.
Match each living thing to the part it plays in moving nitrogen around this ecosystem.
10.
Which statement about energy in this ecosystem do the passage and the tank test support best?
Question 10 options
11.
Suppose boats start landing on Whitewash Rock, and the birds stop resting there for several years. Which prediction fits the model and Table 1 best?
Question 11 options
12.
A visitor says, “The bird droppings are just waste. The tide pools would be the same without them.” Do you agree? Write a claim, use evidence from Table 1 and Table 2, and explain your reasoning about how matter cycles and energy flows in this ecosystem.