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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Photic zone | 3/3 | https://en.wikipedia.org/wiki/Photic_zone | reference | science, encyclopedia | 2026-05-05T07:36:08.714083+00:00 | kb-cron |
Phytoplankton are unicellular microorganisms which form the base of the ocean food chains. They are dominated by diatoms, which grow silicate shells called frustules. When diatoms die their shells can settle on the seafloor and become microfossils. Over time, these microfossils become buried as opal deposits in the marine sediment. Paleoclimatology is the study of past climates. Proxy data is used in order to relate elements collected in modern-day sedimentary samples to climatic and oceanic conditions in the past. Paleoclimate proxies refer to preserved or fossilized physical markers which serve as substitutes for direct meteorological or ocean measurements. An example of proxies is the use of diatom isotope records of δ13C, δ18O, δ30Si (δ13Cdiatom, δ18Odiatom, and δ30Sidiatom). In 2015, Swann and Snelling used these isotope records to document historic changes in the photic zone conditions of the north-west Pacific Ocean, including nutrient supply and the efficiency of the soft-tissue biological pump, from the modern day back to marine isotope stage 5e, which coincides with the last interglacial period. Peaks in opal productivity in the marine isotope stage are associated with the breakdown of the regional halocline stratification and increased nutrient supply to the photic zone.
The initial development of the halocline and stratified water column has been attributed to the onset of major Northern Hemisphere glaciation at 2.73 Ma, which increased the flux of freshwater to the region, via increased monsoonal rainfall and/or glacial meltwater, and sea surface temperatures. The decrease of abyssal water upwelling associated with this may have contributed to the establishment of globally cooler conditions and the expansion of glaciers across the Northern Hemisphere from 2.73 Ma. While the halocline appears to have prevailed through the late Pliocene and early Quaternary glacial–interglacial cycles, other studies have shown that the stratification boundary may have broken down in the late Quaternary at glacial terminations and during the early part of interglacials.
== Phytoplankton == An increase in the amount of phytoplankton also creates an increase in zooplankton, the zooplankton feeds on the phytoplankton as they are at the bottom of the food chain. Phytoplankton are largely restricted to the photic zone, as their growth is primarily dependent upon photosynthesis. This results in phytoplankton primarily occupying the uppermost 50-100 m of the water column. However, diatoms can survive after sinking, and cells have been observed alive thousands of meters down in the water column. They are generally not actively photosynthesizing at these depths due to lack of light availability, but are able to persist in a resting stage. Phytoplankton growth within the photic zone can also be influenced by terrestrial factors, like the weathering of crustal rocks or nutrients from the respiration of plants and animals on land that are carried to the ocean via runoff or riverine input. Phytoplankton move within the photic zone, and many sink over time, with an average sinking rate of 150 m per day. Light access can affect phytoplankton sinking rates through its involvement in photosynthetic activity and energy regulation. Phytoplankton utilize light energy for growth, and where that energy is allocated has been found to change as light availability increases. In areas of higher light, phytoplankton can invest energy in storage compounds, such as lipids. Lipids have a lower density and are involved in buoyancy regulation. With more lipids present, the cellular weight decreases, enabling the sinking rate of the phytoplankton to slow. While migration is more common in zooplankton, some motile phytoplankton will engage in diel vertical migration (DVM), where they migrate upwards in the water column during the day to maximize photosynthetic activity, and descend in dark hours.
Phytoplankton play a central role in the biological carbon pump (BCP). They fix CO2 at the surface, and through sinking, transport this carbon out of the photic zone. About 10–20% of this carbon sinks below the photic zone, and less than 1% reaches the seafloor. Dimethylsulfide loss within the photic zone is controlled by microbial uptake and photochemical degradation.
== See also == Aquatic photosynthesis Electromagnetic absorption by water Epipelagic fish Mesophotic coral reef
== References ==