The clarity or transparency of ocean water varies greatly around the world. In some areas, the water appears crystal clear, allowing visibility many meters below the surface. In other regions, the water can be quite turbid or murky, obscuring vision just below the surface. There are several factors that influence ocean water clarity, from the amount of phytoplankton to the type of seafloor. Understanding what makes some ocean waters clearer than others provides insight into ocean health and processes.
Phytoplankton
One of the biggest influences on ocean water clarity is the amount of phytoplankton present. Phytoplankton are microscopic, photosynthetic organisms that drift throughout the upper sunlit layer of the ocean. They form the base of the marine food web and account for about half of all photosynthetic activity on Earth. When phytoplankton are abundant in an area of the ocean, they give the water a greenish, muddy appearance and severely reduce water clarity. Areas with minimal phytoplankton typically have clearer water.
Phytoplankton thrive when there are adequate sunlight and nutrients. Upwelling zones, where deep, cool, nutrient-rich waters rise to the surface, foster phytoplankton blooms that cloud the water. Similarly, nutrient runoff from land supports dense plankton growth in coastal areas. Water clarity decreases as the phytoplankton population expands.
Sediments
Suspended sediments are another determinant of ocean water clarity. These sediments originate from land-based runoff and erosion or the resuspension of seafloor particles. Fine muddy particles remain suspended in the water column longer than sand and silt. Rivers transport huge loads of suspended sediments into the ocean. Nearshore coastal areas tend to have lower clarity due to sediment plumes from rivers and shoreline erosion.
Turbidity currents along the seafloor can also stir up bottom sediments. These underwater currents develop from sediment overloading and descend down the continental slopes, creating nepheloid layers of suspended particles that reduce water clarity. Areas with high tidal currents and wave action generally have greater resuspension of sediments and decreased visibility.
Water Depth
Water depth influences clarity because of how light attenuates or fades through the water column. The euphotic zone, the upper layer of the ocean where light levels are adequate for photosynthesis, may extend 200 meters or more in very clear, offshore waters. However, in turbid coastal areas, the euphotic zone can be less than 10 meters deep. With increasing depth, red wavelengths of light are absorbed first, making objects appear bluish or green.
In shallow waters, sunlight reflects off the seafloor and travels back up through the water column, improving visibility. Sunlight is rapidly absorbed in deeper waters, limiting light penetration and clarity. Exceptionally clear waters are found in some of the deepest parts of the ocean, like the western Pacific, where there is little vertical mixing to suspend particles.
Water Composition
Variations in water density, salinity, and temperature all influence the scattering and absorption of light underwater, affecting clarity. Dense, salty water sinks, while warmer, fresher water floats. Sharp temperature and salinity gradients can create density barriers that inhibit mixing and keep clearer water separated from murky water. For example, river outflow creates a lens of brackish water that floats atop the denser ocean water, limiting light transmission.
Dissolved and particulate organic matter, like decaying plants or animal waste, absorb and scatter light, reducing clarity. However, some dissolved organic compounds like tannins act as filters, preferentially absorbing shorter wavelengths and making the water appear bluer and clearer. Inorganic compounds can also impact light penetration. For instance, iron oxides give the Red Sea its distinctive color and turbidity.
Biological Processes
The natural biological processes of ocean organisms affect clarity. Almost all marine animals contribute waste and detrital material to the water column. Whale fecal plumes and discarded houses of larvacean tunicates are visible examples of marine snow, organic debris that attenuates light.
The feeding methods of animals also influence turbidity. For example, bottom-dwelling rays stirring up sediments and zooplankton vertical migration patterns that bring food to the surface both decrease clarity. Mass spawning events or coral bleaching that leads to debris accumulation can also reduce visibility.
Human Activities
Many human activities negatively impact ocean water clarity:
- Fertilizer and sewage runoff increase nutrient loads, fueling algal blooms.
- Mechanical destruction from dredging and trawling damages seafloor habitats, resuspending sediments.
- Shipping and boating generate waves that stir up bottom sediments in shallow areas.
- Coastal development and deforestation lead to higher sedimentation.
- Ocean waste like plastics degrades water quality.
- Global warming causes ocean acidification and coral bleaching.
By reducing pollution and protecting marine ecosystems, water clarity can be improved. Some initiatives like marine protected areas help safeguard habitat quality and visibility.
Geographic Factors
Geographic location is a major determinant of ocean water clarity. Proximity to river outflows, coastal erosion, upwelling zones, and areas of high biological productivity all decrease water transparency. Semi-enclosed seas surrounded by land tend to have more suspended sediments and phytoplankton growth.
Island coasts frequently have exceptional visibility due to clearer oceanic water and obstruction of turbid land runoff. The tropical western Pacific around islands like Palau boasts some of the clearest water due to minimal surface runoff and mixing. Higher latitude polar waters also have high clarity owing to colder temperatures and less organic matter.
Conclusion
Ocean water clarity varies around the world due to differences in phytoplankton and sediment concentrations, water depth, light absorption, natural biological processes, human activities, and geographic factors. Understanding the causes of reduced visibility provides insight for protecting marine ecosystems and aesthetic coastal resources vital for tourism. Monitoring water transparency gives a window into ocean health and changing conditions. With growing coastal populations, maintaining prime visibility through runoff management and marine stewardship is essential.
