Dissolved oxygen refers to the concentration of oxygen molecules (O2) dissolved in seawater. Unlike the oxygen in water molecules (H2O), this oxygen is readily available for marine organisms. Its presence is a fundamental indicator of water quality and is essential for the survival of most marine life. The primary biological mechanism introducing this oxygen into the ocean is photosynthesis, predominantly carried out by microscopic organisms known as phytoplankton.
Phytoplankton: The Ocean’s Oxygen Factories
Phytoplankton are tiny, single-celled, plant-like organisms that float suspended in ocean waters. These microscopic entities are diverse, encompassing various groups such as photosynthetic bacteria like cyanobacteria and unicellular protists, notably diatoms. They are found in the sunlit upper layers of oceans, lakes, and other aquatic environments, known as the euphotic zone.
Phytoplankton are the primary producers in marine ecosystems, converting inorganic carbon into organic compounds and forming the foundation of the aquatic food web. Despite making up only about 1% of global plant biomass, these organisms generate a significant portion of Earth’s oxygen. They produce approximately half of the oxygen in our atmosphere annually, a contribution comparable to all land plants combined.
The Process of Photosynthesis in the Ocean
The oxygen generated by phytoplankton enters ocean water as a byproduct of photosynthesis. This process mirrors how land plants produce oxygen, utilizing sunlight, carbon dioxide, and various nutrients from the surrounding water. Phytoplankton absorb carbon dioxide from seawater and, with sunlight’s energy, transform it into organic matter, such as carbohydrates, for their own growth and sustenance.
During this conversion, oxygen is released as a gaseous byproduct into the water. Phytoplankton require nutrients like nitrates, phosphates, silicates, and calcium to fuel their growth and photosynthetic activity. The availability of these nutrients, along with sunlight and carbon dioxide, directly influences their growth rates and the amount of oxygen they produce. This continuous biological process ensures a steady replenishment of dissolved oxygen in the ocean’s surface waters.
Why Dissolved Oxygen is Essential for Marine Life
Dissolved oxygen is essential for the survival of most marine organisms, from fish and invertebrates to bacteria and plants. Aquatic animals extract oxygen from the water for respiration, similar to how terrestrial animals breathe air. They absorb this oxygen into their bloodstream through specialized organs like gills or directly through their body surfaces.
Without sufficient dissolved oxygen, aquatic organisms experience stress, impaired growth, and can die. For instance, sensitive fish species may struggle to reproduce or avoid areas with low oxygen levels. Prolonged exposure to low oxygen conditions, known as hypoxia, can lead to widespread mortality events, disruptions in food webs, and a decline in ocean biodiversity.