Secchi disk depth

Productivity Trophic status Mean total P. M-g/L Mean annual chlorophyll, pg/L Oxygen concentration, as % of saturation Secchi disk depth, m... 

Berman, T., WaHine, P. D., SchneUer, A., Rotherberg, J., and Townsend, D. W. (1985). Secchi disk depth record A claim for the eastern Mediterranean. Limnol. Oceanogr. 30, 447-448. 

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... 

Figure 4 Changes in Lake Washington water quality in relation to sewage diversion, (a) Mean (July-August) lake surface (epilimnetic) chlorophyll a concentration, (b) Mean (July-August) Secchi disk depth. The vertical lines show the range, (c) Bars = annual mean of total phosphorus in the epilimnion. Circles = mean (January-March) bio-available phosphorus (phosphate) concentrations in the epilimnion. (d) Relative amount of sewage as indicated by the capacity of the treatment plants emptying into the lake, maximum taken as 100% (reproduced by permission of American Society of Limnology and Oceanography from Nutrients and Eutrophication, 1972, pp. 172-193).

Figure 6 Mean annual primary productivity and mean monthly Secchi disk depth over time in Lake Tahoe (reproduced hy permission of Backhuys Puhhshers from The Great Lakes of the World (GLOW) Food-web, Health...

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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