TM99-08 Method Change for the Determination of Phytoplankton Biomass

The National Water Quality Laboratory (NWQL) historically has used USGS method B-6560-85 Biomass/Chlorophyll Ratio for Phytoplankton, (Britton and Greeson, 1989, p. 231-234) to determine chlorophyll and biomass in phytoplankton. In this method the chlorophyll and other organic materials are extracted before the biomass is determined. Biomass is determined by drying the extracted residue, weighing on a balance, ashing the residue, wetting, redrying, and reweighing. The dry weight and ash weight are reported to the data base. The customer then subtracts ash weight from dry weight to determine biomass. Phytoplankton biomass is supposed to be the biomass from the algal community. When the samples are taken, the water is filtered through glass-fiber filters. The filters not only collect phytoplankton but also zooplankton, any plant material present, and organic carbon and inorganic salts contained in suspended sediments. This procedure can produce an inaccurate representation of the phytoplankton biomass if significant quantities of zooplankton and other carbonaceous materials are collected on the filter. This determination is more aptly titled "Plankton Biomass." Method B-6560-85 has produced a significant low bias because the sample is extracted with diethyl ether before the biomass determination is started. The technique is also susceptible to analytical error because of the many steps in the procedure. The Nineteenth Edition of Standard Methods for the Examination of Water and Wastewater (American Public Health Association, 1995) describes a more accurate method for the analysis of plankton biomass. The title of this method is "Determination of Biomass (Standing Crop)" and the method number is 10200I. In this method the plankton is collected on a filter, and the filter is dried and weighed to determine "dry weight." The dried filter and plankton then are burned at 500oC, cooled and wetted, dried and reweighed to determine "ash weight." The Standard Methods 10200I (SM 10200I) method is procedurally the same as the USGS method for the analysis of periphyton biomass by method B-3520-85 (Britton, and Greeson, 1989, p. 139 and 140). The two methods (USGS B-6560-85 and SM 10200I) were compared to determine bias and to describe its significance. The study concluded that the results for dry weight and ash weight produced by USGS method B-6560-85 were significantly lower than those produced by SM 10200I. The USGS method B-6560-85 results for biomass also were less than the SM method 10200I, but a paired T-test indicated that the bias was insignificant at the 0.05 significance level. An F-test also indicated that the difference in variability was insignificant at the 0.05 significance level. The SM method 10200I will replace USGS method B-6560-85 because it is more accurate and less susceptible to analytical error than the USGS method. The labcode for dry weight by the SM method 10200I is 2190, the WATSTORE code is 81354, and the method code is B. The labcode for ash weight by the SM method 10200I is 2189, the WATSTORE code is 81353, and the method code is B. The price of SM method 10200I is less than the price of USGS method B-6560-85 because it requires considerably less handling than the USGS method. The SM method 10200I will require that field personnel send a separate filter for phytoplankton chlorophyll determination and a separate filter for phytoplankton biomass determination. The new method will produce less accurate results if small sample volumes are filtered. The NWQL recommends filtering sample volumes sufficient to produce 100 mg (milligrams) of wet sample on the filter to increase the accuracy of the method. One way to estimate when the filter contains an adequate amount of material is to continue passing sample through the filter until the filter flow-rate decreases by 50 percent or more.