Chapter 8- Limited Visibility and Night Diving Limited Visibility and Night Diving 207 layers of clearer water sandwiched between more turbid layers. During spring and fall months, temperate zone lakes and quarries are isothermal, or the same temperature from the surface down. Suspended sediments are carried throughout the water column by natural wind-induced circulation. This means that the visibility is essentially the same regardless of depth. As the surface waters warm, a thermal stratification develops. There is a narrow zone of rapid temperature change with less sediment influx. The upper layer of water may have very limited visibility, while the colder deep water will be clear. It is not uncommon to have visibility of 1 m (3 ft) at the surface and 7 m (21 ft) below the thermocline. Tannin is a brownish chemical released by trees that line the banks of springs and waterways in Florida and other southern states. Although dark, tannin-stained fresh water is sometimes low in suspended particulate matter and thus, even though the visibility is low, a light may dramatically increase your visibility under such conditions. Peat stains water similarly in Ireland, Scotland, and in parts of northern Europe. In the ocean, plankton, which are small free-floating plants and animals, can remain suspended in the water column for long periods of time. The quantity of plankton depends on the season, the amount of sunlight, nutrients, water movement, and thermal structure. A plankton bloom commonly occurs in spring and summer months and can turn the water almost into a green soup, limiting visibility. Prolific blooms of some plankton are called red tides, which are not actually related to the tides. The water can turn a reddish-brown color and virtually eliminate all light below the plankton layer. TECHNIQUES FOR LOW VISIBILITY DIVING Many of the specific techniques used in low visibility diving are also appropriate for use in normal open water diving settings. Among the most important factors to consider are visibility, navigation or direction of your swim, and careful dive planning. Divers would generally enjoy all types of diving more if they would consistently avoid stirring up the silt and sediments which form the substrate of almost all open water diving sites. Even in very low visibility water, it is still important (for both safety and enjoyment) to minimize the addition of visibility-limiting substances to the water. Therefore, do not make it worse than it already is by using poor diving techniques that churn up the bottom and further reduce visibility. Your body trim while swimming near the bottom is very important. Ideally, you should be swimming with your head slightly down and your feet slightly up from the horizontal. A bent-knee cave diving kick can be used to avoid disturbing the bottom. Buddy Diving The most obvious concern in limited visibility diving is diver separation. The major increase in risk caused by low visibility diving is the reduction in your ability to communicate and interact with your buddy under water. You can, however, compensate for this reduced ability to communicate with your buddy and lessen the increased risk. Remaining within touching distance of your buddy is a necessary skill to be mastered. Staying in sight of your buddy, frequent visual buddy referencing and, in very low visibility conditions, actual contact with your buddy are important considerations. It is also important to recognize that stress and personal task loading increase as water visibility decreases. You should plan your low visibility dives for shorter durations and less mobile, shorter distance swims. In addition, it is a good idea to monitor your instruments and direction more frequently than in clearer water diving. If you swim slower than usual, the increased number of tasks can be accomplished in a timely manner, and you can enjoy your dive more. Diver separation can be stressful to some individuals. If, in spite of your slower, more regulated pace you become separated from your buddy; stop swimming, breathe normally, relax, and listen. You might hear bubbles nearby. Visibility can be much improved a few feet off the bottom. Carefully rise upward a few feet off the bottom and scan the area, circling slowly. Look for your buddy, bubbles, or a silt trail that might indicate a diver’s presence. Search for no more than one minute, then surface, continuing to slowly circle at the surface. If your buddy does the same,
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