![]() ![]() Unlike in the atmosphere, where visibility can be on the order of miles, the visual range in the underwater environment is rather limited, at best on the order of tens of meters. The ability to see better and farther has always been a central goal of underwater imaging projects. Underwater vision is vital to many Navy applications involving mine detection, diver visibility, and search and rescue. These calculations, however, are unreliable due to substantial uncertainty regarding the relationships between Secchi depth and chlorophyll concentration and chlorophyll and primary production. Extending the calculations to represent primary production indicates an increase of slightly < 1% yr-1. Calculations of changes in chlorophyll concentrations suggest a yearly increase of ~1%. Both tests showed that Secchi depth decreased by ~0.05 m yr-1 during both periods. We performed a step trend test to compare the data from the two periods and applied a monotonic trend test to the later series. We present the results of trend tests applied to Secchi depth values recorded during two discrete time periods: 91. A more appropriate type of data in this area is Secchi disk measurements. However, the evidence of such a rise is circumstantial, and most of the available findings concerning primary production cover only brief time periods. Nutrient loads to the Baltic Sea have increased during the last century, and primary production has probably also risen. ![]() Furthermore, documenting the variation caused by slightly disturbed lake surface conditions relative to calm surface conditions and among trained observers ensures consistent interpretation of the long-term data base. Documentation of the extensive variation in Secchi disk clarity is needed to determine 'normal' conditions and to judge the long-term status and trends of lake clarity. Clarity readings using a 100-cm disk were 7.0 m deeper than were 20-cm disk readings. Waves from tour boats, drips of water from the research vessel, and wind generated ripples and chop decreased disk readings as much as 5 m relative to readings recorded when the lake surface was calm. When the lake surface was calm and skies were clear or had high haze, differences between descending and ascending observations decreased slightly with increased disk depth. On average, variation in Secchi disk readings among observers was 1 m or less. Average monthly disk readings exhibited considerable variation among years (9.4 m in June, 13.4 m in July, 13.0 m in August, and 8.2 m in September). Secchi disk readings were deepest in June and shallowest in August. Secchi disk depths typically were in the high 20- to low 30-m range. Secchi disk data were collected from June through September, 1978-1995. An additional objective was to determine differences between descending and ascending disk readings (the averages of the two readings representing the disk readings) relative to the depth of the Secchi disk readings, lake surface conditions, and disk size. ![]() The primary objective was to document summer monthly and annual variability in Secchi disk readings, and variation caused by observers, disk size, and surface condition of the lake. ![]() The variability of Secchi disk readings was explored in Crater Lake, an exceptionally clear and deep subalpine caldera lake. ![]()
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