Welcome to Feeding Relationships: The Marine Food Web!
Hello future marine biologists! This chapter, Feeding Relationships, is the engine room of marine ecology. Understanding who eats whom, where the energy comes from, and how it flows is absolutely essential for grasping how marine ecosystems function—from the smallest plankton to the largest whales.
Don't worry if some of the terminology seems dense. We'll break down these concepts step-by-step, using clear definitions and marine examples, so you can confidently analyze any food chain thrown your way!
1. Key Terminology: Defining Roles in the Ecosystem (Syllabus 3.2.1)
Before diving into how energy moves, we need to know the players. Every organism fits a specific role in the feeding hierarchy.
Producers vs. Consumers
- Producer (or Autotroph): An organism that creates its own organic food (biomass) from inorganic substances (like CO\(_2\) and water), usually using an external energy source like sunlight or chemicals.
Example: Phytoplankton (microscopic algae), seaweeds. - Consumer (or Heterotroph): An organism that obtains energy by feeding on other organisms.
The Consumer Hierarchy (Trophic Levels)
Consumers are classified by what they eat, which also defines their trophic level (TL). A trophic level is simply the feeding position in a food chain.
Memory Tip: Trophic levels start counting from 1 (Producers).
We classify consumers based on their level:
- Primary Consumer (TL 2): Eats only producers (plants/algae). They are Herbivores.
Example: Zooplankton grazing on phytoplankton. - Secondary Consumer (TL 3): Eats primary consumers.
Example: Small fish eating zooplankton. - Tertiary Consumer (TL 4): Eats secondary consumers.
Example: Tuna eating small predatory fish. - Quaternary Consumer (TL 5): Eats tertiary consumers.
Example: Killer whales eating tuna or sharks.
Organisms that eat both plants and animals are called Omnivores.
Other Important Feeding Roles
- Decomposer: Breaks down dead organic material (detritus) and waste products, recycling nutrients back into the ecosystem.
Example: Marine bacteria and fungi. - Predator: An animal that hunts, captures, and kills other animals for food.
- Prey: An animal that is hunted and eaten by a predator.
Food Chains and Food Webs (Syllabus 3.2.2)
A Food Chain shows a simple, single path of energy flow.
Example: Phytoplankton \(\rightarrow\) Zooplankton \(\rightarrow\) Herring \(\rightarrow\) Seal \(\rightarrow\) Killer Whale
A Food Web is much more complex, showing all the interconnecting food chains in an ecosystem. It is a more realistic representation of feeding relationships in the marine environment because most organisms eat (and are eaten by) multiple species.
Key Takeaway (Section 1)
Understanding these terms allows you to place any marine organism into its correct Trophic Level, which is the starting point for tracing energy flow.
2. Energy Production: Photosynthesis and Chemosynthesis (Syllabus 3.2.3, 3.2.4)
The foundation of almost every food chain is the producer, which must fix energy into organic compounds (biomass).
Photosynthesis (The Sunlight Energy Path)
In the surface waters (where light can penetrate), producers like phytoplankton use sunlight to convert inorganic substances into energy-rich organic molecules (sugars).
The energy captured from sunlight is made available to the rest of the food chain.
The process is summarised by the word equation (3.2.4):
Carbon Dioxide + Water \(\xrightarrow{\text{light/chlorophyll}}\) Glucose + Oxygen
(Note: You are only required to know the word equation for AS Level, not the balanced chemical equation or the detailed stages.)
Chemosynthesis (The Chemical Energy Path)
Some unique ecosystems, like those found around hydrothermal vents in the deep ocean, have no sunlight. Producers here are bacteria that use energy from dissolved inorganic chemicals (like hydrogen sulfide) to fix carbon and produce organic material.
This process is called chemosynthesis. This forms the base of the deep-sea food web.
Using the Produced Energy: Biomass and Respiration (3.2.6, 3.2.7)
Once glucose is produced (via photo- or chemosynthesis), producers use it in two main ways:
- Building Biomass: Glucose is converted into other organic substances (like cellulose, proteins, and lipids) that make up the organism’s body. This stored chemical energy is the biomass available to the next trophic level.
- Respiration: Glucose is broken down to release usable energy (ATP) needed for life processes (movement, reproduction, growth).
Respiration is summarised by the word equation (3.2.7):
Glucose + Oxygen \(\rightarrow\) Carbon Dioxide + Water + Usable Energy
Investigating Photosynthesis Rate (PA 3.2.5)
You may investigate how factors like light intensity affect the rate of photosynthesis. Generally, as light intensity increases, the rate of photosynthesis increases up to a certain point (when another factor, like CO\(_2\) or temperature, becomes limiting). (Although the syllabus mentions fresh water plants are acceptable for this practical, the principles apply directly to marine phytoplankton.)
Quick Review (Section 2)
Producers make the food (biomass) using two methods (photo- or chemosynthesis). They use this food either to grow (biomass) or to live (respiration).
3. Productivity and Energy Transfer Efficiency (Syllabus 3.2.8, 3.2.9)
Defining Productivity (3.2.8)
Productivity is defined as the rate of production of biomass per unit area or volume per unit of time.
It tells us how quickly producers are converting energy into food.
Units usually look like: \(\text{g m}^{-2} \text{year}^{-1}\) (grams per square meter per year) or \(\text{kJ m}^{-2} \text{year}^{-1}\) (energy per square meter per year).
- Primary Productivity: Specifically refers to the rate of biomass production by producers (phytoplankton, algae).
Why is high primary productivity important? High primary productivity means more food is available at the bottom of the food chain, allowing the ecosystem to support a greater number and variety of consumers. This is why highly productive areas like upwelling zones often have huge fisheries.
High primary productivity is mainly influenced by the availability of light and dissolved nutrients.
Energy Loss along Food Chains (3.2.9)
When a consumer eats an organism from a lower trophic level, the energy transfer is inefficient.
Only a small fraction of the energy from one trophic level is incorporated into the biomass of the next level. This is often simplified to the 10% rule (though the actual percentage can vary from 5% to 20%).
Why is so much energy lost?
The total energy assimilated (eaten) by a consumer is lost through several processes before it can be converted into new biomass:
- Not fully consumed: Parts of the prey may not be eaten (e.g., bones, shells, roots).
- Waste: Some food is indigestible and is excreted as faeces.
- Heat Loss (Respiration): A large portion of energy is used in respiration to fuel movement, maintain body temperature, grow, and reproduce. This energy is ultimately lost to the environment as heat.
- Death: Organisms die and are transferred to the decomposer food chain rather than the next consumer trophic level.
Calculating Energy Losses
If you are asked to calculate the energy loss, remember it is the energy available at the current level *minus* the energy transferred to the next level.
Example: If a Primary Consumer (Zooplankton) has 1000 J of energy available, and transfers 100 J to the Secondary Consumer (Small fish).
Energy transferred = \(100/1000 \times 100 = 10\%\)
Energy lost = \(1000 - 100 = 900 \text{ J}\)
Did you know? Because energy loss is so high (around 90% per step), marine food chains rarely exceed five trophic levels. There just isn't enough energy left to support higher-level predators!
Key Takeaway (Section 3)
Productivity measures the rate of biomass creation. Energy transfer is highly inefficient (lots of energy is lost as heat via respiration) resulting in short food chains.
4. Representing Relationships: Ecological Pyramids (Syllabus 3.2.10)
Ecological pyramids are diagrams used to show the structure of an ecosystem, representing the quantities of organisms or energy at each trophic level.
1. Pyramid of Energy
A Pyramid of Energy represents the total amount of energy (usually measured in \(\text{kJ m}^{-2} \text{year}^{-1}\)) available at each trophic level over a specific period of time.
- Appearance: Always pyramid-shaped.
- Why? Because energy is lost at every trophic transfer (due to respiration and waste), the base (producers) must always contain the most energy. This pyramid can never be inverted.
- Advantage: This is the most accurate representation of energy flow.
2. Pyramid of Biomass
A Pyramid of Biomass represents the total dry mass (biomass) of the organisms at each trophic level at a specific point in time (known as standing stock).
- Appearance: Usually pyramid-shaped, but can sometimes be inverted (upside down).
- Units: Usually mass per area (\(\text{g m}^{-2}\) or \(\text{kg m}^{-3}\) in the water column).
When Biomass Pyramids are Inverted (Plankton Blooms)
This is a common exam point! In aquatic ecosystems, especially during a plankton/algal bloom, the pyramid of biomass can be inverted.
- The biomass of the Producers (phytoplankton) may be low at any one time, but they reproduce *very* rapidly.
- The biomass of the Primary Consumers (zooplankton) may be higher than the phytoplankton they feed on, because the phytoplankton are being consumed almost as fast as they reproduce. The zooplankton have a high standing stock compared to their food source.
3. Pyramid of Numbers
A Pyramid of Numbers shows the number of individual organisms found at each trophic level.
- Appearance: Often irregular or inverted.
- Why? The size of the organism matters. If the producer is very large (like a single large marine macroalga or tree) supporting many small insects or copepods, the count of producers is very low, making the base look tiny compared to the next level.
- Special Case: Parasites: If a secondary consumer is hosting many tiny parasites (e.g., copepods on a fish), the parasitic trophic level will have a huge number of individuals compared to the host, resulting in an inverted top section of the pyramid.
Summary Comparison of Pyramids
The best way to represent energy flow in an ecosystem is always the Pyramid of Energy, as it factors in the time taken for energy conversion and cannot be misleadingly inverted.
Quick Check: Trophic Pyramid Stability
- Can the Pyramid of Energy be inverted? No.
- Can the Pyramid of Biomass be inverted? Yes, often due to plankton dynamics (high turnover rate).
- Can the Pyramid of Numbers be inverted? Yes, usually because the producer is huge (e.g., a single kelp plant) or due to parasites.
Summary of Feeding Relationships
We have learned that marine ecosystems are structured by energy flow:
- Producers (TL1) use photosynthesis (sunlight) or chemosynthesis (chemicals) to create biomass.
- This biomass is transferred up the food chain through various Consumers (TL2, TL3, etc.).
- The rate of biomass creation is called Productivity, influenced by light and nutrients.
- Energy transfer is highly inefficient (often ~10% transfer rate), meaning large amounts of energy are lost primarily as heat during respiration.
- We visualize this energy flow using Ecological Pyramids, recognizing that pyramids of biomass and numbers can sometimes be irregular or inverted in marine systems.
Keep practicing defining the vocabulary and explaining *why* energy is lost—these are core examination requirements!