Limited visibility diving requires that greater attention be committed to navigation techniques and buddy skills. In addition, every diver should understand what diving in limited visibility entails.
Water visibility is determined, in great part, by the amount and size of particles (particulate) suspended in the water. Areas that generally have low visibility are usually near the mouths of rivers or outlets, where water is consistently flowing into the sea or lake. Such water typically consists of sediment and waste particulate that will, given enough time, settle to the bottom and form a substrate of fine, lightweight silt. So light in fact, that any small disturbance will stir it up and cause the water to cloud, and once suspended, it takes a good amount of time to settle again. Sea floors and lake bottoms that are formed by heavier sediments, such as gravel and coarse sand, are more difficult to stir up. However, if they were, the sediment would sink rapidly and clear the water quicker. The presence of under water vegetation also tends to hold down sediment, and filters nearby water, which help to keep waste particulate low. Consequently, it is safe to say that the bottom characteristics of a dive site are one of the primary determining factors of the surrounding water’s visibility, although, not the only factor.
Water, or dive sites that typically have good visibility can periodically turn to poor visibility for a number of reasons, usually due to atmospheric changes. Hard rain can cause runoff that temporarily disrupts visibility. Rains in tropical areas that have been deforested have caused such bad particulate runoffs that covered reefs and consequently suffocated and destroyed them. Strong winds can cause large waves and heavy seas, raising more sediment than normal and keeping the particulate suspended for longer periods. In addition to the lowered visibility, these winds and seas make diving more demanding due to increased water forces and potentially changing currents.
Thermoclines, plankton blooms, and naturally occurring watercolor changes due to mineral bleaching, are other factors that can impact water visibility, independent of the composition of the bottom.
A thermocline is a transitional layer between warmer and colder water, in which a warm water mass sits above a colder water mass. There can be more than one thermocline in a water column based on a number of factors. And, because water density is different based on the temperature of the water, one layer of water may hold more particulate, while the layer directly above or below remains clear.
Many rivers around the world have a high concentration of tannin. Tannin is released into groundwater by the roots of some trees and has a distinctive brown tea color. The ground water eventually ends up in a river, which empties into the ocean or lake. Many centuries ago, ship builders used tannin to dye the sails of ships a uniform light brown color. Diving in water that has a high concentration of tannin is compared to diving in a cup of tea; near visibility is good, but visibility quickly diminishes as a diver moves farther from an object.
Plankton blooms occur in certain areas when phytoplankton — tiny microscopic plants, congregate in the upper layers of the ocean. Typically, blooms occur in late spring and summer months when the temperature rises. When this occurs, the color of the water surface changes, lowering the visibility.
Knowing what can affect, and ultimately determine the water clarity will help you hypothesize what the water visibility may be like in a certain area based on the surrounding conditions. You and your buddy should plan your dive, and the equipment you may need based on the conditions that may manifest themselves during the dive.