Ecology Terms Starting With O
Ecology Glossary: O
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Ocean Acidification
/ OH-shun ah-SID-ih-fih-KAY-shun / · Scientific term used in marine ecology.
Ocean acidification is the ongoing decline in seawater pH caused by the ocean absorbing carbon dioxide from the atmosphere, which reacts with seawater to form carbonic acid and reduce carbonate ion concentrations.
Since the Industrial Revolution, ocean surface pH has dropped from approximately 8.2 to 8.1, a shift that represents a roughly 26 percent increase in hydrogen ion concentration because pH is measured on a logarithmic scale. When carbon dioxide dissolves in seawater, it forms carbonic acid, which dissociates into bicarbonate and hydrogen ions; the rising hydrogen ion concentration then reacts with carbonate ions, reducing their availability. Organisms such as corals, oysters, sea urchins, and pteropods rely on carbonate ions to build calcium carbonate shells and skeletons, and laboratory studies show that shell formation rates in Pacific oysters (Crassostrea gigas) decline by up to 25 percent at pH levels projected for 2100.
Warmer water also holds less dissolved carbon dioxide, but ocean warming and acidification together amplify stress on calcifying organisms beyond what either factor causes alone.
Some deep-sea cold-water coral reefs, such as those formed by Lophelia pertusa off the Norwegian coast, already sit in water that is undersaturated with respect to aragonite, the form of calcium carbonate these corals use to build their skeletons. These reefs are dissolving at their bases even as new growth continues at the surface, a process that may destabilize entire reef structures within decades.
Respiratory System Fun Facts →Ocean acidification does not mean the ocean becomes a strong acid. Seawater remains basic, but its pH is decreasing as dissolved carbon dioxide forms carbonic acid.
Pteropods (Limacina helicina), free-swimming sea snails found throughout polar and subpolar oceans, show visible shell dissolution within 48 hours of exposure to seawater at pH 7.9, a level projected for parts of the Southern Ocean by 2050. These animals form a major component of the diet of salmon, herring, and baleen whales, so their decline would propagate through marine food webs well beyond the pteropods themselves.
Old Growth Forest
/ OHLD GROHTH FOR-est / · Old English ald (old) + growan (to grow) + Latin foresta
Old Growth Forest is a forest that has developed over an extended period without stand-replacing disturbance, characterized by large old trees, structural complexity, and ecological processes that take centuries to develop.
Old growth forests accumulate structural features that younger forests lack, including multi-layered canopies, large standing dead trees called snags, and abundant fallen logs in multiple stages of decay. In the Pacific Northwest of North America, old growth Douglas fir (Pseudotsuga menziesii) forests typically require 200 or more years to develop the structural complexity that supports species such as the northern spotted owl (Strix occidentalis caurina) and the marbled murrelet (Brachyramphus marmoratus). Fallen logs in these forests can persist for 200 to 500 years as they decay, providing habitat for salamanders, small mammals, and hundreds of invertebrate and fungal species throughout that interval.
Carbon storage in old growth forests is also substantial; Pacific Northwest old growth stands hold an estimated 500 to 1,000 metric tons of carbon per hectare, far exceeding the storage capacity of managed second-growth forests.
Old growth forests store a disproportionate share of the world's terrestrial carbon. A 2020 study published in Nature Climate Change estimated that forests over 140 years old account for approximately 70 percent of the carbon stored in all of the world's forests, despite covering a much smaller fraction of total forested area.
Old growth forest means untouched by any disturbance. Natural disturbances such as windthrow, low-intensity fire, and individual treefall gaps occur regularly in old growth forests and contribute to their structural diversity.
The coastal redwood (Sequoia sempervirens) forests of northern California qualify as old growth where individual trees exceed 2,000 years in age and reach heights above 100 meters. These forests support a distinct canopy ecosystem, with soil accumulations on large branches hosting ferns, salamanders, and invertebrates hundreds of meters above the ground.
Oxygen Cycle
/ OK-sih-jen SY-kul / · Greek oxys (sharp, acid) + genein (to produce) + Greek kyklos (circle)
Oxygen Cycle is the biogeochemical pathway by which oxygen atoms move continuously among Earth's atmosphere, oceans, freshwater, rocks, and living organisms through processes including photosynthesis, respiration, and weathering.
Photosynthesis by plants and phytoplankton releases molecular oxygen into the atmosphere at a rate of approximately 330 billion metric tons annually, while respiration by all aerobic organisms and combustion of fossil fuels consume oxygen and return it as carbon dioxide. Oxygen dissolves in water, creating dissolved oxygen that aquatic organisms depend on; concentrations range from about 14 milligrams per liter in cold water near 0 degrees Celsius to roughly 5 milligrams per liter in warm water near 35 degrees Celsius. Geological processes also participate: oxygen reacts with iron and sulfur minerals during weathering, locking oxygen into rock and releasing it again through volcanic activity and tectonic uplift over millions of years.
Atmospheric oxygen concentration has remained near 21 percent for approximately 200 million years, reflecting a long-term balance between photosynthetic production and consumption by respiration, fire, and chemical weathering.
During the Carboniferous period, roughly 300 to 360 million years ago, atmospheric oxygen reached an estimated 35 percent, nearly double today's level. This oxygen-rich atmosphere supported giant insects such as Meganeura, a dragonfly relative with a wingspan of about 70 centimeters, because insect respiratory systems that rely on passive diffusion become more efficient at higher oxygen concentrations.
Oxygen only matters for animals that breathe it. Plants, aerobic microbes, fungal decomposers, rocks undergoing oxidation reactions, and oceanic and atmospheric chemistry all participate in the oxygen cycle.
Marine phytoplankton, including cyanobacteria and diatoms, generate an estimated 50 to 80 percent of Earth's atmospheric oxygen through photosynthesis in the sunlit surface layer of the ocean. The single genus Prochlorococcus, with individual cells measuring only 0.6 micrometers in diameter, contributes roughly 20 percent of global oxygen production despite being invisible to the naked eye.