APES Mod 3 & 23: Ocean currents & Aquatic biomes

Ocean gyre

A large-scale system of circulating ocean surface currents that rotates clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere due to winds and the Coriolis effect.

Equatorial ocean surface elevation

High solar insolation warms and expands ocean water, raising the sea surface level by approximately $8\,\text{cm}$ at the equator and initiating gravity-driven circulation toward higher latitudes.

Gulf Stream

A powerful Atlantic surface current that transports warm tropical water from the Gulf of Mexico up the eastern coast of North America to Western Europe, significantly moderating the regional climate.

East Australian Current ($\text{EAC}$)

A major western boundary ocean current that is approximately $100\,\text{km}$ wide and $1.5\,\text{km}$ deep, transporting warm tropical waters southward along the eastern coast of Australia.

Thermohaline circulation

A global deep-ocean circulation pattern, known as the global conveyor belt, driven by differences in water temperature and salinity that mix surface and deep ocean layers.

Mechanism of deep water formation

Cold surface water in the North Atlantic experiences evaporation and ice formation, becoming colder, saltier, and denser until it sinks to the ocean floor and flows southward.

Impact of climate warming on thermohaline circulation

Melting northern glaciers introduces substantial amounts of low-density freshwater, reducing ocean salinity and preventing surface water from sinking, which can slow or halt deep circulation.

Coastal upwelling

An oceanographic process where winds push surface waters offshore, causing cold, nutrient-rich deep water to rise to the photic zone and stimulate high primary productivity.

Primary producers of open ocean food chains

Phytoplankton, which perform photosynthesis in the sunlit photic zone and support marine food webs.

El Niño-Southern Oscillation ($\text{ENSO}$)

A periodic cycle of shifting sea surface temperatures, atmospheric pressure, and ocean circulation across the equatorial Pacific Ocean between South America and Australasia.

Equatorial Pacific conditions during El Niño

Trade winds weaken or reverse, allowing warm western Pacific water to move eastward toward South America, suppressing coastal upwelling, damaging fisheries, and bringing heavy rainfall to the Americas.

Equatorial Pacific conditions during La Niña

Trade winds blow stronger than normal toward the west, producing intense coastal upwelling, cooler sea surface temperatures off western South America, and increased precipitation in Southeast Asia and Australia.

Inverse relationship between water temperature and dissolved oxygen ($\text{DO}$)

Colder water holds higher concentrations of dissolved oxygen, whereas warmer water holds lower concentrations of dissolved oxygen.

Turbidity

A measure of the cloudiness or haziness of water caused by suspended sediment, which limits sunlight penetration for photosynthesis and can physically clog fish gills.

Distribution of Earth's total water reserves

More than $97\%$ of Earth's water is marine saltwater, leaving less than $3\%$ as freshwater.

Distribution of Earth's freshwater reservoirs

Nearly three-quarters (approximately $75\%$) is stored in ice caps and glaciers, roughly one-quarter (approximately $25\%$) exists as groundwater, and less than $1\%$ is readily accessible surface freshwater.

Watershed

A drainage basin where all precipitation that falls onto the land drains into a common body of water, such as a stream, river, wetland, or ocean.

River source zone

The headwater zone of a river characterized by shallow, cold, clear, fast-flowing water with high dissolved oxygen and low nutrient concentrations.

River transition zone

The middle reach of a river system featuring a wider stream bed, warmer water, moderate stream velocity, lower dissolved oxygen, and higher biological productivity.

River floodplain zone

The low-elevation river reach characterized by a wide stream bed, warm water temperatures, high turbidity, the lowest dissolved oxygen, and the highest biological productivity.

Riparian zone

The vegetated ecosystem and land directly bordering the bank of a stream, river, or other moving freshwater channel.

River delta

A wetland formed where a river empties into a standing body of water, depositing nutrient-rich sediment and creating fertile soils.

Littoral zone (lakes)

The shallow freshwater zone near the shore where sunlight penetrates to the sediment, allowing emergent rooted plants like cattails and reeds to grow.

Limnetic zone (lakes)

The open, sunlit offshore water layer of a lake down to the depth of light penetration, where phytoplankton serve as the primary producers.

Profundal zone (lakes)

The deep, dark aphotic layer of a lake beneath the limnetic zone where bacteria decompose sinking organic matter, consuming oxygen and resulting in low dissolved oxygen levels.

Benthic zone

The sediment-covered muddy bottom of a freshwater or marine water body inhabited by decomposers and burrowing organisms.

Oligotrophic lake

A lake characterized by low nutrient levels ($N$ and $P$), clear water, and low primary productivity, typically replenished by cold snow or glacial melt.

Mesotrophic lake

A lake with moderate nutrient availability, intermediate clarity, and a balanced level of biological productivity supporting varied fish species.

Eutrophic lake

A shallow body of water with high nutrient concentrations, murky green water, and high primary productivity.

Freshwater swamp

A freshwater wetland that is submerged or saturated for part of the year and dominated by emergent woody trees.

Freshwater marsh

A freshwater wetland dominated by non-woody emergent herbaceous plants, including cattails, sedges, and grasses.

Bog

An acidic, low-nutrient freshwater wetland dominated by thick sphagnum moss and characterized by slow decomposition rates.

Ecological services of wetlands

Water storage that mitigates flooding and droughts, natural filtration of sediments and pollutants by plant roots, and vital breeding and nursery habitat for wildlife.

Estuary

A semi-enclosed coastal biome where freshwater from rivers mixes with marine saltwater, creating a brackish salinity gradient between $0\,\text{ppt}$ and $35\,\text{ppt}$.

Mangrove swamp

A coastal marine wetland in tropical and subtropical regions characterized by salt-tolerant woody trees that buffer coastlines against storm erosion and shelter juvenile fish.

Intertidal zone

The narrow marine shoreline band between high and low tide marks, where organisms face harsh environmental fluctuations including wave crash, desiccation, and solar heating.

Coral polyp and zooxanthellae symbiosis

A mutualistic relationship where coral polyps provide shelter and $CO_2$ to photosynthetic algae, and the algae provide up to $90\%$ of the coral's organic sugars.

Coral bleaching

A phenomenon triggered by elevated sea temperatures or pollutants that causes corals to expel their symbiotic zooxanthellae algae, turning colonies white and risking mortality.

Marine photic zone

The upper oceanic layer extending down to approximately $200\,\text{m}$ that receives sufficient sunlight to support photosynthesis.

Marine aphotic zone

The deeper oceanic layer below approximately $200\,\text{m}$ that receives insufficient light for photosynthesis, relying on sinking marine snow or chemosynthesis.

Chemosynthesis

The process by which deep-sea bacteria produce organic compounds without sunlight, utilizing energy released from inorganic compounds such as hydrogen sulfide ($H_2S$) or methane ($CH_4$).