Water transparency

The recent increase in the understanding of biological processes in lakes has led to the development of ecotechnical methods of manipulating the trophic status of lakes. The most widely used techniques of bio-manipulation involve artificial change in the abundance of predators to enhance grazing of phytoplankton by zooplankton. The increase in grazing pressure reduces phytoplankton densities and results in improved water transparency. 

Nielsen, S. L., Sand-Jensen, K., Borum, J., and Geertz-Hansen, O. (2002). Depth colonization of eelgrass Zostera marina) and macroalgae as determined by water transparency in Danish coastal waters. Estuaries 25, 1025—1032. 

The depth in water to which a Secchi disk (a 20 cm diameter disk with alternating black and white quadrants) can be seen from the surface. Secchi disk depth is an easy measurement of water transparency. 

Trophic state can be measured directly via indicators of phytoplankton biomass and nutrient concentrations or indirectly by measuring water transparency (Table 1). Water transparency can be an accurate indicator of phytoplankton productivity, even though it is affected by nonbio-logical particles suspended in the water column. Secchi disk depth is the oldest, simplest, and quickest quantitative measure of water clarity. A Secchi disk is a black and white disk that is... 

Biological regulation of phytoplankton biomass greatly improved water quality in Lake Washington. The appearance of the aggressive phytoplankton grazer, the microcrustacean Daphnia spp., coincided with an increase in water transparency to as much as 12.9 m by 1976. 

Reduction of the visible symptoms of eutrophication (i.e., green scum) can be achieved by various methods. An indirect method that has received considerable attention, although it is still in the experimental phase, is biomanipulation or the manipulation of aquatic food chains to reduce phytoplankton biomass. The idea is to reduce predation on aquatic grazers such as large zooplankton, which feed on phytoplankton. This method has proven successful in increasing water transparency and dissolved oxygen concentrations and reducing phytoplankton biomass (Shapiro and... 

Properties Colorless, odorless, nontoxic, water-soluble powder or granules. D 1.59, refr index 1.51, tensile strength 8000-15,000 psi, pH (1% solution) 6.5-8.0, stable in pH range 2-10. Viscosity of 1% solution varies from 5 to 2000 cP, depending on the extent of etherification. Insoluble in organic liquids. Reacts with heavy-metal salts to form films that are insoluble in water, transparent, relatively tough, and unaffected by organic materials. Many of its colloidal properties are superior to those of natural hydrophilic colloids. It also has thixotropic properties and functions as a polyelectrolyte. 

K.M. Reese, Volunteers Measure Water Transparency in 38 States, Chem. Eng. News, 73(31), 48, July 31 (1995). 

Baltic Sea region from 1955 to 1992. Also, the reduced water transparency (Secchi depth cf. 

Figure 4. Time dependence of absorbance over 1 cm path length at several wavelengths, of precipitating surfactant system of 0.070 wt % surfactant and 0.30 wt % NaCl in water, prepared by adding 3.0 wt % NaCl solution in time zero to 0.077 wt % surfactant in water (transparent). The initial total absorbance is predominantly by absorption, the increase by scattering from the separating phase.

An example of application of three-way PCA is a data set from the Venice lagoon. In it, 11 chemical variables (chlorophyll-a, total suspended matter, water transparency, fluorescence, turbidity, suspended solids, NH4, NOj", P, COD, and BOD5) have been measured monthly in 13 sampling sites (see Figure 5.5) during the period May 1987 to December 1990, for a total of 44 months. 

Furthermore, within these main types various more detailed classifications have been elaborated, taking into consideration different criteria. In general, physical, chemical and biological parameters are combined, such as content of oxygen, content of nutrients, colour and water transparency, and the richness and diversity of biological habitat. 

Historic data on Secchi disk depth in the northern Adriatic Sea in 1911 through the present, with few interruptions of data collection, provide a measure of water transparency that could be interpreted to depict surface water productivity. These data coupled with surface and bottom water dissolved oxygen content determined by Winkler titrations and nutrient loads outline the sequence of eutrophication in the northern Adriatic Sea. Similar historical data from other coastal areas around the world demonstrate a decrease in water clarity due to phytoplankton production in response to increased nutrient loads that are paralleled by declines in water column oxygen levels. 

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