L17- Living Near the Surface: Ecology of the Epipelagic Zone

Overview of the Epipelagic Zone

• Definition of the Pelagic Zone: This refers to the entire water column of the ocean, which is distinct from the benthic zone (the ocean floor).

• The Epipelagic Zone: This is the top surface layer of the ocean, extending from the surface down to a depth of approximately $200\,\text{meters}$.

• Importance of Light: The epipelagic zone is characterized as the euphotic zone, where there is generally plenty of light, especially at the very surface. This abundance of light has significant implications for biology and the structure of food webs.

• Spatial Separation of the Epipelagic:

  • Neritic Zone: This is the part of the epipelagic close to the coastline. It lies over the continental shelf and is typically less than $200\,\text{meters}$ deep. These waters are well-mixed and do not easily stratify. They interact heavily with the bottom and receive significant inputs of nutrients and sediments from land and rivers.

  • Oceanic Zone: This is the part of the epipelagic located further away from the coastline. It is far from the bottom (limiting mixing) and distant from riverine nutrient inputs, resulting in a "clear water" system.

• Global Scale: Earth is a "water planet" with $71\%$ of its surface covered by ocean. Within the epipelagic zone, the neritic part accounts for only $8\%$, while the oceanic part accounts for $92\%$. Most of the planet consists of distant, offshore, clear oceanic water.

Scientific Methods for Studying the Epipelagic

• Net Collections: Scientists have used nets to collect plankton since the early $1800\text{s}$. Nets come in various sizes and mesh densities and are deployed from ships of all sizes. However, very small organisms can be squeezed through or escape the mesh.

• Microscopy: Essential for viewing the microscopic algae and small organisms that constitute the bulk of life in the upper ocean.

• Water Samplers (Containers): To capture the "real small stuff" that escapes nets, scientists use water samplers lowered into the water column. A "messenger" (a weight) is sent down a cable to close the containers at specific depths, providing an uninterrupted piece of water for precise density and species estimates.

• Fishing Methods:

  • Historical Context: Archaeological evidence suggests humans have used fishing nets for over $10,000\,\text{years}$, and possibly up to $20,000\,\text{to}\ 30,000\,\text{years}$.

  • Standardization: While commercial fishers aim to maximize catch, scientists use standardized fishing methods. This allows for data comparison over long periods (e.g., comparing data from $10\,\text{years}$ ago to today). Scientists often do not catch as many fish as commercial fishers because they must follow these strict protocols.

• Observations and Remote Sensing:

  • SCUBA: Allows for behavioral and observational studies, though the open oceanic zone is often less interesting for divers due to the sparsity of visible life (aside from rare shark encounters).

  • Satellite Remote Sensing: Used for aerial photography and monitoring the surface. These cannot penetrate deep into the water but are excellent for tracking surface chlorophyll and productivity.

  • Underwater Drones and Gliders: These devices scan and take photos beneath the surface, providing a more comprehensive view of the water column.

• Biogeochemical and DNA Analysis:

  • Environmental DNA (eDNA): A modern technique where water is filtered to capture traces of DNA dropped by organisms (cell material). Metabarcoding in a laboratory can then identify the list of species present in the system, from fish to bacteria. This is becoming a common method for sampling remote environments.

Classification and Size of Epipelagic Organisms

• Femtoplankton: The smallest category, consisting primarily of viruses. They are too small for standard microscopes and nets. Whether they are "living organisms" is debated, as they evolve and reproduce but lack other typical characteristics of life.

• Picoplankton and Nanoplankton: Organisms smaller than $2\,\mu\text{m}$. These require water container samples and high-magnification microscopy ($1,000\times$) to observe.

• Micro-, Meso-, and Macroplankton: Range from $20\,\mu\text{m}$ to $20\,\text{cm}$. These are the types typically seen in student labs and caught with traditional nets.

• Megaplankton: Organisms larger than $20\,\text{cm}$ that drift with currents.

  • Colonial Tunicates: An example is the Pyrosoma, which can reach lengths of $10\,\text{meters}$.

  • Sargassum: High-level seaweed. While most seaweeds must attach to rocks to complete their life cycle, two species—Sargassum natens and Sargassum fluitans—have never been observed attached to a substrate. They have drifted in the Sargassum Sea for millions of years, propagating through fragmentation. They support unique evolved animal communities, such as the Sargassum frogfish and Sargassum crabs, which are perfectly camouflaged. In $2018$, these floating mats covered $6,000\,\text{km}^2$.

Primary Producers of the Epipelagic

• Cyanobacteria: Extremely important because they can perform nitrogen fixation. They take inorganic nitrogen ($N_2$), which is inert and unusable by most plants, and convert it into usable forms like nitrates or ammonium.

• Diatoms: Known for being very productive and fast-growing; they are major contributors to primary production.

• Dinoflagellates: Some are mixotrophic, meaning they can photosynthesize like plants or eat other organisms like animals.

• Coccolithophores: Characterized by calcium carbonate ($CaCO_3$) shells. When they bloom and settle, they contribute to the formation of limestone sedimentary rocks.

Zooplankton and Nekton

• Holoplankton: Organisms that spend their entire life cycle in the water column (e.g., copepods, arrow worms, some isopods, snails, and worms).

• Meroplankton: Organisms that live only a portion of their life as plankton, usually the larval stage (e.g., sea stars, crabs, octopuses, and many fish).

  • Advantages: Increased dispersal (moving kilometers rather than meters) and "cheap" production (producing many microscopic larvae).

  • Disadvantages: High vulnerability to predators and the risk of being carried by currents away from suitable habitats (offshore loss).

  • Direct Development: An alternative where organisms skip the larval stage (e.g., some snails hatch directly from eggs as miniatures of the adult).

• Nekton: Organisms that are large and strong enough to swim against currents. They can move where they want. Most are predators, and they are generally streamlined and fast.

Adaptations to Life in the Water Column

• Staying Afloat (Buoyancy):

  • Increased Surface Area: Organisms may be flat or have spines/projections to increase drag and slow their sinking. This also makes them harder for predators to swallow if the predator's mouth is smaller than the projections.

  • Regulating Density: Many use lipids, swim bladders, or gas pockets.

  • Colonial Structures: The Portuguese Man of War (Physalia physalis) uses a gas-filled float (pneumatophore) to stay at the surface.

• Protection and Camouflage:

  • Transparency: Many organisms are nearly invisible in clear water.

  • Bioluminescence: Used for signaling mates or confusing predators. This has evolved independently $40\, \text{to}\ 50$ different times.

  • Countershading: Typical in nekton; dark on top to blend with the depths when seen from above, and light on the bottom to blend with the sky when seen from below.

  • Predator Specialties: The nudibranch Glaucus atlanticus eats the Portuguese Man of War and steals its stinging cells (nematocysts) to use for its own defense.

Diel Vertical Migration (DVM)

• The Movement: This is the largest migration of biomass on Earth, involving over $10 \times 10^9\,\text{tons}$ of organisms moving up and down the water column daily.

• The Cycle: Zooplankton move toward the surface at night to feed on phytoplankton and retreat to deeper (approx. $100\,\text{meters}$), darker, colder water during the day.

• Reasons for DVM:

  • Predation Avoidance: Staying in the dark during the day to avoid visual predators like fish.

  • Energy Conservation: Respiration is lower in colder, deeper water.

• Magnitude: A $1\,\text{mm}$ copepod traveling $200\,\text{meters}$ in a day moves $200,000$ times its body length. For a human, this would be equivalent to running $340\,\text{kilometers}$ every day.

• The Biological Pump: This migration accelerates the transfer of carbon from the surface to the deep ocean. Organisms eat carbon at the surface and transport it down, where it can sink or support deeper-dwelling life.

Food Webs and The Microbial Loop

• Historical View: Previously, it was believed the food web was simply phytoplankton (\rightarrow) zooplankton (\rightarrow) nekton.

• The Microbial Loop: Discovered in the $1950\text{s}$ and $60\text{s}$, this loop involves dissolved organic matter (DOM), bacteria, picoplankton, and protozoan grazers. This loop accounts for about $50\%$ of all primary production in the ocean.

• Complexity: Food webs are non-linear. Trophic status often changes with size; a fish larva might eat jellyfish, but as an adult, it might eat other fish.

Productivity and Oceanography

• Productivity Comparison: On a per-square-meter basis, the epipelagic ($40\, \text{to}\ 100\,\text{g}\,\text{C}\,\text{m}^{-2}\,\text{yr}^{-1}$) is much less productive than benthic systems like mangroves or coral reefs (which reach the thousands). However, due to its massive global area, the epipelagic is more important for total global primary production.

• Nutrient Limitation: Phytoplankton require light and nutrients (nitrogen and phosphorus). In the water column, light is at the top, but nutrients often peak at depth (where things decompose). Surface waters are often nutrient-depleted because the primary producers use them up.

• Upwelling and Coriolis Force:

  • Coriolis Force: Due to Earth's rotation, moving objects appear to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

  • Coastal Upwelling: When surface currents are driven away from the coast (Ekman flow), they are replaced by nutrient-rich bottom water. This results in highly productive fisheries.

  • Equatorial Upwelling: Diverging currents at the equator also pull up nutrient-rich deep water.

• Latitudinal Patterns of Productivity:

  • Tropics: Low productivity year-round. The water is thermally stratified (warm on top, cold on bottom) with little mixing, leading to perpetual nutrient starvation.

  • Polar Regions: A single massive peak in primary production during the summer when light is available. Nutrients are generally high.

  • Temperate Regions: Characterized by a large spring bloom (diatoms) and a smaller fall bloom (often dinoflagellates triggered by autumn storms that mix the water).

Questions & Discussion

• Question (Student): What is the difference between "hollow" and the other one?

• Response: The professor explains that the student is referring to "holoplankton" (plankton for their whole life) versus "meroplankton" (plankton for only part of their life).

• Question (Student): Does the floating Portuguese Man of War have air inside it?

• Response: Yes, it has a gas-filled structure (pneumatophore). Some organisms have air bubbles inside them, while some float on the very top of the surface with parts exposed to the air.

• Course Logistics Discussion:

  • A student asks about taking a special topic course (307/384) because of a conflict with Cell Biology 2 and a field deployment in October.

  • The professor mentions he hasn't supervised a 307 before but is willing to look into requirements.

  • They discuss the difficulty of doing a project on coral reefs since they aren't locally available, and the professor notes that pure literature reviews can sometimes become "an AI exercise," whereas field work is preferable.

  • A student mentions they are having trouble making sense of demography data from "Jungkook" [likely a misspelled locality or project name] for a report.