Marine Biology: Life Near the Surface

Overview of the Epipelagic Zone

• Definition and Scope: The epipelagic refers to the water column from the surface down to a depth of $200\,m$.

• Global Significance: It covers approximately $71\%$ of the Earth’s surface.

• Divisions of the Epipelagic:

  • Neritic Zone: The portion of the epipelagic located over the continental shelf. It accounts for approximately $8\%$ of the total epipelagic area.

  • Oceanic Zone: The portion beyond the continental shelf, making up the remaining $92\%$ of the epipelagic.

• Sampling the Pelagic Environment: Methods for studying this zone are complementary and include:

  • Plankton nets and underwater video.

  • Sonar and acoustic sensors.

  • Direct observation through diving.

  • Satellite remote sensing.

  • Autonomous robots, gliders, and sensors.

  • Environmental DNA (eDNA) analysis.

Classification of Pelagic Organisms

• Size Categories: Organisms range from microscopic to massive.

  • Size spectrum: Includes femtoplankton ($0.02$ to $0.2\,\mu m$), picoplankton ($0.2$ to $2\,\mu m$), and extends to megaplankton (greater than $20\,cm$).

• Megaplankton (Macro-scopic Life):

  • Sargassum: Specifically Sargassum natans and Sargassum fluitans.

  • Statistics: In 2018, the "Great Atlantic Sargassum Belt" covered approximately $6000\,km^2$, representing over $20\,\text{million tons}$ of biomass.

• Phytoplankton (Primary Producers):

  • Responsible for approximately $50\%$ of global primary production ($PP$) and oxygen ($O_2$) production.

  • Nearly $95\%$ of all ocean primary production originates from pelagic phytoplankton.

• Zooplankton (Animal Plankton):

  • Holoplankton: Organisms that spend their entire life cycle in the pelagic zone.

  • Meroplankton: Organisms that spend only a portion of their life in the plankton, typically the larval stage. This phase facilitates dispersal and can last from minutes to an entire year.

• Nekton: Organisms capable of swimming against water currents, predominantly acting as predators.

Adaptation and Survival in the Epipelagic

• Mechanisms for Staying Afloat: Because the epipelagic lacks solid substrates, organisms have evolved strategies to avoid sinking.

  • Increased Resistance: Morphological adaptations such as flattened body shapes or long projections to increase surface-to-volume ratios.

  • Buoyancy Regulation: Use of lipids (oils), swim bladders, or the storage of gases.

  • Specific Surface Dwellers:

    • Neuston: Organisms living right at the surface but staying underwater.

    • Pleuston: Organisms that reside at the surface with part of their body projecting into the air.

    • Examples: Glaucus atlanticus (sea slug) and Physalia physalis (Portuguese man o' war).

• Predator-Prey Adaptations:

  • Coloration and Camouflage: Development of translucency or countershading (dark on top, light on bottom) to blend with light coming from above or the darkness below.

  • Sense Organs: Large eyes for visibility in clear water and lateral lines to detect vibrations/movement.

  • Swimming Performance: Streamlined bodies and specialized muscles to achieve high speeds.

• Diel Vertical Migration (DVM):

  • Many planktonic organisms migrate from deep water to the surface at night to feed and return to depth during the day to avoid visual predators.

  • The Scale of Movement: A copepod measuring $1\,mm$ ($0.001\,m$) may travel $200\,m$ in a $24\text{-hour}$ period.

  • Magnitude: This distance is $200,000$ times its own body length. For a human, this is equivalent to running $340\,km$ every day for their entire life.

  • The Biological Pump: This migration acts as a primary mechanism for transferring organic carbon and nutrients from the surface (top) to the deep ocean (bottom).

Epipelagic Food Webs and Productivity

• Trophic Efficiency: Pelagic food webs are generally more efficient than terrestrial ones, often involving up to five levels of consumers.

• The Microbial Loop:

  • A significant portion of energy flows through microscopic organisms rather than the traditional "phytoplankton to fish" chain.

  • Up to $50\%$ of pelagic primary production is processed through the microbial loop.

  • Flow: Dissolved Organic Matter ($DOM$) is consumed by bacteria/picoplankton, which are then eaten by protozoan grazers.

• Factors Governing Primary Production:

  • Light: Photosynthetically Active Radiation ($PAR$) is measured in $W\,m^{-2}$. Light availability decreases with depth.

  • Nutrients: Primary production is often limited by the availability of Dissolved Inorganic Nitrogen ($N$) and Phosphorus ($P$).

  • Dynamics: Nutrients are depleted at the surface by high production and sinking of organic matter, resulting in a nutrient profile where concentrations are higher at depth where respiration dominates over production.

• Global Patterns of Production:

  • Tropics: Light and temperature remain high year-round, but production is often low due to permanent stratification (lack of nutrient mixing).

  • Temperate and Polar Regions: Exhibit seasonal peaks (e.g., diatom blooms) driven by changes in light and seasonal mixing of nutrients.

  • Upwelling Hotspots:

    • The Coriolis Effect and Ekman Transport move surface waters, allowing nutrient-rich deep water to rise to the surface.

    • Types: Coastal upwelling (near continents) and Equatorial upwelling.

    • These zones create highly productive biological hotspots despite the general nutrient limitations of the open ocean.

Summary of Principles

• The pelagic zone is vast ($71\%$ of Earth) and studied through diverse technological methods.

• Organisms are classified by size (femto- to mega-), trophic level, life history (holo- vs. mero-), and mobility (plankton vs. nekton).

• Surviving in clear, open water requires specific adaptations for buoyancy (gases, lipids) and concealment (countershading).

• Daily mass migrations (DVM) drive the ecological "biological pump."

• While global offshore production is often limited by nutrients and light depth, upwelling zones provide essential exceptions that support high biomass.