Marine Ecology Notes (Video Transcript)
Abiotic and Biotic Factors in Marine Ecology
Marine ecology: the study of interrelationships among organisms and between organisms and their environment.
Key questions: how do different water-dwelling organisms depend on each other? who preys on whom? who assists whom?
Healthy marine ecosystem: determined by both abiotic (non-living) and biotic (living) factors.
Abiotic factors include: and other non-living determinants of life.
Biotic factors include living organisms that influence who lives where and why.
Habitat and Niche
Resources are limited in most communities (marine or terrestrial), leading to competition for resources such as food, water, and shelter.
Intraspecific competition: within a single species. Example: many humans vying for a single parking space.
Interspecific competition: between different species. Example: humans and squirrels competing for the same parking spaces (as an analogy).
Niche: the sum of all the conditions, resources, and requirements that a species needs to survive in its habitat; essentially the species’ job or role in the environment.
In a stable community, no two species have exactly the same niche; competition can lead to one being more dominant or others shifting.
Narratives around marine ecosystems often focus on the niche of a species (its competition, requirements, and success).
Two different communities can have species filling the same ecological role (niche) albeit with very different organisms.
Example on coral reefs vs kelp forests:
Coral reef sea urchin: niche = algae grazer that keeps algae from overgrowing coral.
Kelp forest abalone: niche = grazer on smaller rock-attached algae (does not eat the kelp itself).
Note: sea urchins in kelp forests also eat kelp, which can be problematic for kelp; abalone are mollusk (not an echinoderm) and serve a different but functionally similar grazing role.
Endemic vs native distributions:
Endemic species: occur only in one geographic location; higher extinction risk due to limited range.
Example: banded angelfish restricted to deep reefs below ~75 m depth in the Northwest Hawaiian Islands; this makes them more vulnerable to localized disturbances.
Native species: historically present in an area; can have broad distributions (worldwide or broad regional presence).
Endemic vs native interplay: a species can be native to a region without being endemic to it; conversely, an endemic species is not found elsewhere.
Exotic (non-native) vs native vs invasive:
Exotic/non-native: introduced to a new area (often by humans) but not necessarily disruptive.
Invasive: exotic species that spread and disrupt the ecosystem, causing harm and imbalance.
Example: lionfish commonly cited as an invasive species on some Caribbean reefs, aggressively preying on a wide range of native species.
Species Distributions and Risk
Endemic species: high risk of extinction due to restricted geographic range.
Native species: historically found in an area; can have broad or limited distribution.
Exotic/non-native species: introduced to new areas by human activity or natural events; not automatically invasive, but some become invasive.
Invasive species: cause ecosystem disruption and outcompete native species; example given: lionfish.
Marine Zonation and Habitat Depths
Humans are accustomed to a mostly two-dimensional environment; the ocean adds a three-dimensional complexity.
Pressure increases with depth: every 10 meters of depth, atmospheric pressure doubles (roughly). This has important implications for animals and equipment.
If surface pressure is 1 atm, then at depth meters: (approximately; with surface P0 ≈ 1 atm).
Terminology for zones and habitats:
Benthic: organisms living on the bottom.
Pelagic: organisms living in the water column (not on the bottom).
Neritic: zone over the continental shelf, nearshore waters.
Littoral (intertidal) zone: between high and low tide lines; experiences drying and submersion.
Sublittoral (subtidal): offshore, always underwater, shallow.
Continental shelf: shallow, nearshore region before the open ocean.
Continental slope and beyond: deeper regions into the open ocean (oceanic zone).
Abyssal zone: deep, flat ocean floor, roughly around (≈ ).
Hadal zone: trenches; depths up to (≈ ).
Organism types by bottom association:
Benthic organisms: live on or near the bottom.
Epifauna: animals living on the surface of the bottom.
Infauna: animals that burrow into the sediment.
Myofauna: tiny organisms living among sand grains (less commonly used term in this course).
Seascape by zones:
Epipelagic: top layer where light penetrates; sunlit zone.
Mesopelagic: twilight zone; deeper than epipelagic but still within the water column.
Bathypelagic: dark zone; deeper water where light is minimal.
Abyssalpelagic: deep abyssal water column in the deepest regions; very little light.
Note on terminology: hadal trenches are not typically called hadalpelagic because trenches are characterized by steep canyons and depth extremes.
Nekton vs Plankton:
Nekton (swimmers): organisms that actively swim, e.g., fish.
Plankton (drifters): organisms that float or drift with currents, e.g., jellyfish.
Light and depth in the pelagic zone:
Light diminishes with depth; colors are absorbed at different depths:
Red light is absorbed first (shallowest depth, about or ).
Orange, green, and blue wavelengths are absorbed at increasing depths.
By about ~ (≈ ), much of the red/orange spectrum has been absorbed, leaving predominantly blue wavelengths that give deep waters their characteristic appearance.
Practical example from personal diving: at ~ depth, blood appeared dark because red wavelengths were absorbed.
Color and camouflage:
Deep-water organisms may appear black or reddish because the relevant wavelengths for reflection are absorbed; coloration can serve as camouflage in low-light environments.
Dimensional Anatomy and Adaptations
Example organisms and functional roles:
Sea urchins on coral reefs: niche = algae grazer that controls algal growth to protect slower-growing corals.
Abalone in kelp forests: niche = grazer of smaller rock-attached algae; does not graze on kelp itself.
In contrast, sea urchins in kelp forests may also feed on kelp, illustrating how similar groups can occupy different roles depending on community context.
Temperature effects:
Some species tolerate wide temperature ranges; others are temperature-limited (e.g., corals bleaching when temperatures rise).
Arctic cod possess antifreeze compounds in their blood, enabling survival at sub-zero temperatures.
Leatherback sea turtles and elephant seals migrate from Arctic to tropics, indicating broad thermal tolerance.
Salinity and osmoregulation:
Salinity strongly affects water balance; organisms must manage water loss or gain due to osmotic differences.
Some species tolerate wide salinity ranges (e.g., bull sharks); most cannot switch easily between salt and freshwater.
Bull sharks have been documented far inland in freshwater systems (e.g., 4{,}000\ km from coast, Lake Nicaragua). They are exceptional in their salinity tolerance and partly responsible for their higher human encounter rates.
Pressure as a limiting factor:
Deep-sea environments subject organisms to high pressures; rapid ascent can be catastrophic (e.g., causing gas expansion in swim bladders or hulls of submersibles).
Historical examples connect deep-sea pressure to structural failures when rapid decompression occurs (e.g., submersibles or hull failure under extreme pressure).
Substrate and bottom complexity:
Bottom type strongly influences biodiversity: rocky bottoms and complex substrates provide more hiding places and niches than mud or sand.
Higher habitat complexity generally correlates with higher species richness.
Wave action and habitat zones:
Swash zone (high-energy nearshore): organisms must be able to anchor or be strong swimmers to resist pounding; jellyfish are less common here due to exposure to force.
Oxygen availability and dead zones:
Most organisms require oxygen; some bacteria are anaerobic and can thrive without it.
Areas with thick mud or dense vegetation can create low-oxygen (hypoxic) conditions; some zones become dead zones where little life persists.
Dead zones can form from nutrient runoff (fertilizers) fueling algal blooms; decomposition of algae consumes dissolved oxygen, leaving little for other organisms.
Example marine dead zone: Gulf of Mexico, where nutrient runoff has created a large hypoxic area, roughly the size of the state of Delaware.
Nutrient enrichment and ecosystem consequences:
Excess nutrients drive algal blooms; when algae die, bacteria decompose them and consume oxygen, reducing habitable space for other organisms.
Real-World Relevance and Implications
Coral bleaching is a direct consequence of elevated water temperatures reducing the symbiotic algae that corals rely on.
Invasive species like lionfish illustrate how non-native organisms can alter predator-prey dynamics and reduce native biodiversity, especially in coral reef systems.
Understanding abiotic factors helps explain why some habitats are more diverse and resilient than others, and why certain species are restricted to particular zones or depths.
Human activities (pollution, nutrient runoff, climate change) alter abiotic conditions and can cascade through biotic interactions, affecting ecosystem structure and function.
Quick Reference: Key Terms and Concepts
Abiotic factors: non-living environmental factors (e.g., temperature, salinity, pressure, light, substrate).
Biotic factors: living components (organisms, interactions).
Habitat: physical environment where a species lives.
Niche: the role and requirements of a species within its habitat; the sum of resources and conditions needed for survival.
Intraspecific competition: competition within a species.
Interspecific competition: competition between species.
Endemic species: restricted to a single geographic location.
Native species: historically present in a region; broad or regional distribution.
Exotic (non-native) species: introduced to a new area.
Invasive species: exotic species that spread and disrupts ecosystems.
Benthic: bottom-dwelling (on or near the seafloor).
Pelagic: life in the water column (not on the bottom).
Nekton: swimming organisms (e.g., fish).
Plankton: drifting organisms (e.g., many small organisms; jellyfish are planktonic in some contexts).
Epifauna vs Infauna: surface-dwelling vs burrowing organisms in sediment.
Myofauna: tiny organisms that live among sand grains.
Epipelagic, Mesopelagic, Bathypelagic, Abyssalpelagic: light-related vertical zones in the pelagic ocean.
Littoral, Sublittoral, Neritic, Oceanic, Abyssal, Hadal: broad geographic and depth-related zones.
Coral bleaching, algal blooms, dead zones: key ecological processes driven by abiotic factors and human impacts.
Numerical References (examples from the transcript)
Depths and zones:
Shallow reef depth example: around 75 m depth (or slightly shallower) for some coral communities; but in context, the banded angelfish is restricted to deep reefs around ~75 meters and deeper in the NW Hawaiian Islands.
Wreck depth example: about 65 ft deep (
$\approx 20$ m) for a dive anecdote.Abyssal depth: about ().
Hadal depth: up to ().
Pressure amplification:
Every 10 m depth, pressure approximately doubles: (assuming surface pressure ~1 atm).
Light wavelength attenuation:
Red wavelength absorption occurs first, at shallow depths (roughly ; quoted as ~10 ft).
Gulf of Mexico dead zone:
Size described as “the size of Delaware.”
Salmon, bull sharks, and freshwater reach:
Bull sharks observed up to ~ from coast into freshwater systems (e.g., Lake Nicaragua).