NAUI Master Scuba Diver 206 Limited Visibility and Night Diving LEARNING GOALS In this chapter, you will: 1. Be introduced to the terms presented in bold letters. 2. Learn six causes of low visibility. 3. Learn how to use a buddy line. 4. Learn the proper techniques using a dive light. 5. Learn 4 methods to orient yourself in limited visibility. 6. Learn why colors are more vibrant under the illumination of a dive light. 7. Identify the items essential to night diving, aside from normal basic diving equipment. 8. Read about the different types of dive lights and batteries that are available to the diver. 9. Learn the maintenance procedures for dive lights. 10. Demonstrate the three main signals used with dive lights. What is limited visibility diving? Underwater visibility is considered limited when you cannot see another diver at a distance of 3 m (10 feet) or less in a horizontal direction. Why would one ever want to dive in water with a horizontal visual limit of less than 3 m (10 ft)? There are so many reasons that only a small number of the available answers will be mentioned here. Among the reasonable answers are: the only water in the area has limited visibility; to explore wrecks located in such water, to earn the NAUI night diver specialty rating, to search for lost objects, to explore a previously undived site, artifact hunting, to locate clearer water at greater depths, to examine marine life (which may be abundant in such water), and for many other equally valid reasons. Whatever your personal requirements, make sure that you have had adequate advanced training before diving in any limited visibility setting. FACTORS DETERMINING WATER VISIBILITY Pure water is transparent, but does not occur many places in nature. The range of underwater visibility is controlled primarily by particles suspended in the water. Other factors affecting visibility include: darkness (night) or overcast during daylight hours; overhead environments that limit the available light (cavern zones, caves, or the interior of sunken wrecks), stirred up silt or debris (many times by the divers themselves), plankton blooms, suspended silt, mud, chemicals, or polluted water. In the case of diver-induced low visibility water conditions, simply being trained in anti-silting swimming techniques can mean clearer water, and thus more enjoyable, clearer water diving. Other causes of turbid water are related to weather conditions. During the rainy season in many areas, rivers and streams can transport large quantities of sediment laden water into lakes and oceans. Water visibility can be drastically reduced following intense rainstorms. You will be able to see a “mud line” from the shore. During the dry season, clearer water can be expected due to the lack of runoff. The type of bottom sediment is an important factor that influences water visibility. Coarse-grained sand is dense and quickly settles down to the bottom when disturbed. Fine-grained sediments, like silts and clays, can be kicked up by divers fins and create clouds that can reduce the visibility to zero in a short time. In these habitats, divers must practice good buoyancy control and swim off the bottom to avoid stirring up the sediment. You should choose dive sites that lie upcurrent from the mouths of streams and rivers. Obviously, diving on calm days or nights will enhance your chances of finding clearer water. Your best potential visibility will occur around reef areas away from the shore. You should avoid cove or bay areas that will usually have lower visibility. Further, the lee shore of islands is usually clearer, as are drop-offs or rocky shoreline areas. Periods of tidal change are to be avoided, as the radical movement of the water during these times may suspend more matter with consequent lower visibility. Thermocline interface areas may sometimes be multi-layered, under which conditions you may find
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