ESCI 104 Oceanography

Unit 4: Lecture Notes

Life in the Oceans and Energy Flow Through Marine Ecosystems


There are two fundamentally different ways of looking at the biology of marine organisms

Evolutionary Approach

This view focuses on the ancestor-descendant relationships among the plants, animals and protists in the oceans. 

Ecological Approach

Ecology is the study of the interactions between organisms and their environment.  The environment includes both physical factors (temperature, wave action, nutrient levels, substrate, etcetera) and biological factors (predator/prey relationships, competition, symbiosis). 

In this course we will emphasize marine ecology, and will only touch briefly on evolution concerning the evolution of whales.  For this first unit on marine biology we want to introduce some basic concepts, and then look at three benthic ecosystems located on rocky, subtidal surfaces (coral reefs, kelp forests and vent communities) to apply some of these concepts.

Distribution of Life in the Oceans

Primary Productivity

Definition:  The energy captured from inorganic sources by autotrophs, plants and bacteria that can make their own food.  These organisms are called primary producers.  In this course when we refer to primary productivity we will usually be referring to net primary productivity, which is the total energy captured by the autotroph minus the amount they burn up in order to live.  Net productivity represents the energy available to consumers at the next level of the trophic pyramid.

Chemical Pathways

1. Photosynthesis

2. Chemosynthesis

3. Nutrients

Secondary Productivity

Organisms that get food energy from other living things are called heterotrophs. There are three major categories of heterotrophs:

Energy Flow

There are two ways in which scientists represent the flow of energy through marine ecosystems.  One way is to group all primary producers, primary consumers, secondary consumers, etcetera together and form a trophic pyramid.  This simplifies calculations of productivity and helps to clarify the concept of transfer efficiency.  The second, more complex representation is drawing a diagram of a food chain or food web, in which the roll of specific plants and animals can be identified


Trophic Pyramids

Energy flows upward through a trophic pyramid.  The primary producers are grouped at the base.  The shape is a two-dimensional "pyramid" because most of the energy is lost going from one level to the next.

One example of a trophic pyramid is provided as Figure 13-20 in your textbook.  Click here to see a second example of a trophic pyramid for the Central Pacific.

The trophic pyramid is particularly useful for illustrating the important concept of the transfer efficiency of an ecosystem.  The transfer efficiency is the percentage of energy from one level of the trophic pyramid that is converted to biomass on the next level of the pyramid (see Figure 13-17 in your textbook).  In the oceans, typical transfer efficiencies range from 10-20 %.  We will use the Central Pacific to illustrate the importance of transfer efficiency as a critical element in describing the productivity of marine ecosystems.  We will attempt to answer the following question:

"How many kilograms of tuna can be produced from 500 kilograms of phytoplankton if the transfer efficiency of this ecosystem is 15%?"

Begin at the base of the trophic pyramid, and calculate how many kilograms of small zooplankton will be produced from 500 kilograms of phytoplankton.  To calculate 15% of 500 kg, convert 15% to a decimal fraction and multiply:

500 kg (phytoplankton) x .15 = 75 kg (small zooplankton)

Now repeat this process for every step on the pyramid:

75 kg (small zooplankton) x .15 = 11.25 kg (large zooplankton)

11.25 kg (large zooplankton) x .15 = 1.69 kg (small fish)

1.69 kg (small fish) x .15 = 0.25 kg (large fish)

.25 kg (large fish) x .15 = 0.038 kg tuna

As you can see, very little of the energy that falls as sunlight on the Central Pacific Ocean is converted into tuna.

Food Chains and Food Webs

A food chain and a food web are illustrated as Figure 13-19 in your textbook.  These diagrams show more of the complexity of marine ecosystems and the specific rolls played by particular types of organisms.  The arrows on these diagrams point in the direction of energy flow.  In other words, each arrow points from the organism that is eaten to the animal doing the eating.  For example, you can tell at a glance from Figure 13-19b that mollusc larvae eat phytoplankton (diatoms and dinoflagellates).

Compare the herring in Figures 13-19a and 13-19b.  For a given amount of energy falling on the ocean, the Newfoundland Herring will be more abundant (productive) than the North Sea Herring because (on average) there are fewer steps in the trophic pyramid.  What would happen to the two ecosystems if a disease killed most of the Calanus copepods?  What is the trade-off between productivity and stability illustrated by these two diagrams?

Rocky-Bottom Ecosystems

We will use the concepts introduced in Unit 4 throughout the semester to describe ecosystems in different parts of the ocean.  For this first section of the course we will focus on ecosystems found in rocky, subtidal areas of the ocean.  Most of the floor of the ocean is covered with loose material called sediment, so these rocky-bottom areas only form under special conditions.

Rocky Areas Below the Lowest Tide (Subtidal)

There are two common reasons the floor of the ocean will be rocky rather than covered with sediment:

Kelp Forests

View the Video clip "Unit 4 Sea Otters as Keystone Predators" from the Nature episode "Seasons in the Sea." This clip illustrates what happens when sea otters are eliminated from a kelp forest ecosystem. 

Click here for a discussion of the food web for a west coast kelp forest.  Be sure you make note of where kelp forests occur, what types of plants and animals are found there, and the definition of a keystone predator.

Coral Reefs

Review the video clip from "Unit 2 Coral-reefs"  Note that plants appear to be relatively rare on coral reefs compared to kelp forests.  Do a web search under zooxanthellae to find out why. 

Consult Figure 15-18 in your book, and answer the following question:

Coral reefs are found in which of the following climate zones?

A) Tropical  B) Temperate  C) Polar  D) All of these

Vent Communities

Vent communities are discussed in your book, and also in the streaming video "Dive to the Edge of Creation."  Complete the questions based on the video, and make sure you can compare and contrast vent communities with coral reefs and kelp forests.