Quick questions on Oceanic environments and abiotic factors (QCE Marine Science Unit 3)

Light is absorbed and scattered as it passes through water, so it falls exponentially with depth. The epipelagic (euphotic) zone, roughly 0 to 200 m, has enough light for photosynthesis and contains almost all marine primary production. Below it the mesopelagic (twilight) zone (200 to 1000 m) has faint blue light too weak for net photosynthesis, and the bathypelagic zone below 1000 m is permanently dark. Red wavelengths are absorbed first, which is why deep water looks blue and why many deep animals are red (red appears black where no red light reaches them).

Surface temperature is set by latitude and season, warmest at the equator and coldest at the poles. With depth, temperature falls sharply through the thermocline (a zone of rapid temperature change around 200 to 1000 m in the tropics) to a near-constant 2 to 4 degrees Celsius in the deep ocean. The thermocline acts as a density barrier that limits mixing between warm surface water and cold deep water.

Salinity is the mass of dissolved salts, averaging about 35 parts per thousand (ppt) in open ocean. It rises where evaporation exceeds rainfall and falls near river mouths and in high-rainfall tropics. In Queensland estuaries such as the mouth of the Fitzroy River, salinity can drop close to zero after monsoonal floods, which stresses corals and seagrass. A permanent salinity gradient with depth is called a halocline.

Hydrostatic pressure increases by about one atmosphere for every 10 m of depth. At 1000 m the pressure is roughly 100 times that at the surface. Deep-sea organisms have biochemical and structural adaptations (flexible bodies, no gas-filled spaces) that let them tolerate it.

Dissolved nitrate, phosphate and silicate are the limiting nutrients for phytoplankton. They are scarce in sunlit surface water because organisms take them up, and abundant at depth where sinking dead matter is broken down. This is why upwelling, which brings deep nutrient-rich water to the surface, creates highly productive fisheries (for example off western South America). The clear, low-nutrient water of the Coral Sea is why coral reefs there are so transparent but support modest open-water productivity.