Module 11 Lecture 4: Plankton and the Global Ocean Food Web

Introduction to Plankton and Global Food Webs

• Plankton are critical to the food web of the entire planet, not just the ocean.
• The process of the food web for Earth begins with these organisms.
• There are two primary types of plankton discussed: phytoplankton (plants) and zooplankton (animals).

Primary Producers: Phytoplankton

• Definition: Phytoplankton are primary producers that consist mostly of microscopic organisms.
• Physical Characteristics:
  • Most cannot be seen without a microscope.
  • Some may reach the size of a pinhead, making them visible to the naked eye.
• Ecological Role:
  • They serve as the absolute base of the ocean food web.
  • They utilize photosynthesis to generate energy from carbon dioxide, water, and sunlight.
• Significance to Human Life:
  • Phytoplankton are responsible for producing $50\%$ of the oxygen that humans breathe.
  • They are classified as plants because they produce energy through photosynthesis rather than consuming other organisms.

Primary Consumers and Secondary Producers: Zooplankton

• Definition: Zooplankton are considered animals because they feed on other organisms (phytoplankton).
• Physical Characteristics: They can often be easily seen with the naked eye.
• Ecological Roles:
  • Primary Consumers: They occupy this level because they eat the primary producers (phytoplankton).
  • Secondary Producers: They serve as the primary food source for larger marine animals, such as fish and baleen whales.
• Food Web Dynamics: As the food web progresses from plankton to larger organisms, the biology and food sources become increasingly large and complex.

Marine Snow and the Deep Ocean Food Web

• The Decomposition Process: When plankton die and decompose, they fall through the ocean water column toward the seafloor.
• Marine Snow:
  • This downward movement of decomposing material is highly visible in the water column and resembles falling snow.
  • It is technically referred to as marine snow.
  • It is the primary contributor to the downward sedimentation of material in the oceans.
• Detritus:
  • Once marine snow reaches the ocean floor, it is termed detritus.
  • This layer of detritus provides a food source for scavengers living in the deep ocean, completing the food web.
• Sediment Life: Even when sediment looks empty to the naked eye, under a microscope it is teeming with:
  • Tiny single-celled organisms.
  • Larger organisms such as bristle worms.
  • Researchers have observed that the seafloor becomes covered in a new layer of detritable material every year.

Scale and Density of Life in Seawater

• Dr. Lampett’s Room Analogy: To visualize the density of life, imagine expanding $1\,\text{cm}^3$ of seawater to the size of a room.
  • Viruses and Bacteria: These would still be very small even at this expanded scale.
  • Phytoplankton: At this scale, they would appear to be the size of a pinhead, with each organism separated by approximately $10\,\text{cm}$.
  • Zooplankton: Larger, shrimp-like organisms would be scattered around the room.
  • Marine Snow: These dead particles are much rarer than living organisms and are scattered throughout the mixture.

Methods of Observation and Study

• Scuba Diving: While most divers focus on the behavior of beautiful fish, plankton behavior can be observed by looking closely through a diving mask.
• Plankton Nets: Scientists like Dr. Lampett gather samples by skimming the water with nets in the upper layers, where life is most abundant due to sunlight.
• Mud Sampling: Research teams take samples of mud from the ocean floor to study how plankton continues to play a role in the ecosystem as it decomposes in the sediment.