Marine Biology Terms Starting With U
Marine Biology Glossary: U
Jump to Marine Biology Term
Understory Algae
/ UN-der-stor-ee AL-jee / · Understory from under plus story meaning layer or level, algae from Latin alga meaning seaweed.
Understory Algae is the layer of smaller seaweeds and algae growing beneath the kelp canopy in marine forests, adapted to low-light conditions.
Understory algae inhabit the shaded zone between the kelp canopy and the seafloor, receiving only 1 to 10 percent of surface light depending on canopy density. These communities include red algae like Plocamium, foliose brown algae, and multiple coralline species that form foundational crusts on rocky substrate. Understory diversity often exceeds 100 species per kelp forest site, with each species occupying specific depth and light niches.
Many understory species possess specialized photosynthetic pigments like phycoerythrin that absorb blue-green wavelengths penetrating through the canopy, maximizing photosynthetic efficiency in dim conditions. When canopy kelp is removed by storms or herbivores, understory algae rapidly expand, increasing biomass by 300 percent within weeks until canopy regeneration restores shading conditions.
Some understory algae benefit from moderate grazing by sea urchins and abalone because herbivores remove competitive overgrowth and create clearings where understory species can colonize. This intermediate disturbance maintains higher diversity than completely protected or heavily grazed areas.
Understory algae are epiphytes growing on kelp fronds. These species root independently on the seafloor or rocks and grow in the shade cast by canopy kelp rather than attaching to the kelp plants themselves.
In the kelp forests of southern Australia, the understory layer hosts over 150 red algae species, including commercially harvested Plocamium species. Surveys of these communities record juvenile abalone sheltering among the algal structures during daylight, with densities reaching 12 individuals per square meter in structurally dense patches.
Upwelling
/ UP-wel-ing / · Old English up + well (spring)
Upwelling is the oceanographic process by which wind or diverging surface currents cause cold, nutrient-rich water from depth to rise toward the surface, dramatically increasing biological productivity.
Coastal upwelling occurs when wind-driven Ekman transport pushes surface water offshore, drawing cold water up from depths of 100 to 500 meters. Equatorial upwelling occurs where trade winds drive surface water away from the equator, exposing deep nutrient reserves. Eastern boundary upwelling systems occupy a small fraction of the ocean surface yet support about one-fifth of global marine capture fisheries because nutrient-rich cold water fuels dense phytoplankton blooms that sustain anchovies, sardines, hake, seabirds, and marine mammals.
The California Current system off the western United States is one of the most studied upwelling regions, where seasonal winds from the north drive offshore Ekman transport from spring through early fall. When upwelling weakens during El Niño events, phytoplankton biomass collapses and fish populations like anchovies and sardines decline sharply within a single season.
The Humboldt Current upwelling system off Peru and Chile covers a tiny fraction of the global ocean surface but can produce one of the largest single-region fish catches on Earth in productive years. Its anchoveta fishery has historically reached annual landings above 10 million metric tons, showing how concentrated upwelling can convert deep nutrients into harvestable fish biomass.
Cold coastal water is always poor for marine life. Upwelled cold water delivers nitrates, phosphates, and silicates from depth that fuel phytoplankton blooms, making some of the coldest coastal waters among the most biologically productive on Earth.
Off the coast of Monterey, California, seasonal upwelling draws water from depths near 200 meters to the surface between April and August. Surface chlorophyll concentrations during peak upwelling can exceed 30 micrograms per liter, supporting dense aggregations of anchovies, seabirds, and humpback whales that converge on the bloom.
Urchin Barren
/ ER-chin BAIR-en / · Urchin from Old French herichun meaning hedgehog, barren from Old English bæren meaning unproductive or sterile.
Urchin Barren is a marine ecosystem state where overabundant sea urchins have consumed kelp and other macroalgae, leaving behind barren rock substrate with minimal biodiversity.
Urchin barrens form when predator populations collapse, allowing sea urchin densities to increase from typical levels of 2 per square meter to over 50 per square meter in extreme cases. Purple sea urchins (Strongylocentrotus purpuratus) in California can completely deforest kelp ecosystems within months once their populations explode. These barrens support less than 10 percent of the species diversity found in healthy kelp forests, transforming productive three-dimensional habitats into biological deserts.
The transition from kelp forest to urchin barren represents an alternative stable state that can persist for decades without intervention. Recent marine heatwaves along the Pacific coast triggered massive barren formation, with over 90 percent of kelp forests between Oregon and California disappearing between 2014 and 2019.
Urchin barrens can spontaneously reverse to kelp forests when environmental conditions favor kelp recruitment and growth, but this transition often requires urchin populations to crash from starvation or disease after they have consumed all available algae, a process that may take years.
Urchin barrens are permanent ecosystem states. They represent dynamic phase shifts that can reverse when predator populations recover or urchin numbers decline through starvation, disease, or management interventions like culling programs.
Along the coast of Tasmania, extensive urchin barrens now cover more than 50,000 hectares where long-spined sea urchins (Centrostephanus rodgersii) overgrazed kelp beds following declines in rock lobster populations. Surveys conducted between 2000 and 2020 recorded a greater than 95 percent reduction in kelp canopy cover across affected areas, with urchin densities in some barrens exceeding 40 individuals per square meter.