A Little Experiment
Sample processing is one of the LMVP volunteer's most important duties. While some lake water is retained for nutrient analyses, the bulk of the volunteer-collected sample is passed through filters to allow analyses of particulate matter such as algae and sediment. The use of filters not only allows the particulate matter to be preserved through freeze-drying, as a bonus the filters take up much less space in the freezer than a two-liter sample bottle.
Most LMVP volunteers process two types of filters. The first filter type is for chlorophyll analysis and helps the LMVP estimate the amount of algae in the lake water (CHL, see Fall 2003 Water Line). The second filter is used to measure total suspended solids (TSS, see Winter 2003 Water Line). This analysis provides an estimate of the total amount of material in the water (TSS), as well as a break-down of how much of the material was inorganic sediment (referred to as ISS in the Data Report) and how much was organic material such as algae.
Because the method for chlorophyll analysis is quite sensitive, only a small amount of material needs to be retained on the filters for accurate measurements. The analytical method for TSS is less precise, so more material is required on the filter to ensure good results. This is why volunteers are instructed to pass twice the volume of water through the TSS filters (500 mL) than the chlorophyll filters (250 mL).
Occasionally, a TSS filter will become clogged before 500mL of water can pass through it, indicating a high concentration of particulate matter suspended in the lake. In these situations the volunteer is instructed to discard the filter and begin again using a new filter and a smaller volume of water. Other times the 500mL volume passes easily through the filter leaving little material. We wondered if TSS results might vary with the volume of water used to process the TSS filters, so this summer we conducted an informal experiment.
Three lakes were sampled by MU lab technicians with TSS filters being processed using different volumes of water (24 – 30 filters were processed for each lake, 6 for each water volume). The largest volume filtered for each lake was roughly the maximum amount that could be pulled through the filter without clogging.
The three lakes differed in TSS concentrations, with Little Dixie Lake having the lowest levels at around 5mg/L (see table). In contrast, Long Branch and Dairy lakes had substantially higher TSS concentrations at 24 and 29 mg/L, respectively. The lakes also differed in terms of the make-up of the TSS. In Little Dixie Lake the TSS was about 60% inorganic sediment and 40% organic matter. Long Branch Lake was dominated by inorganic sediment (80% of TSS) while Dairy Lake had slightly more organic matter than inorganic sediment (55% organic vs. 45% inorganic).
Filters were processed using four different volumes for Little Dixie Lake. Upon initial review, the results seem quite comparable, with average TSS values ranging between 4.9 – 5.4 mg/L. While the differences were small, they were statistically significant. Differences in inorganic sediment concentrations were not statistically significant, indicating no notable change in concentration as volume varied. Organic matter did increase and the differences were significant. These results suggest that some of the organic matter in Little Dixie Lake was small enough to pass through the filters at low volumes. As filter volume increased the filters started to pack with material and more of the small organic particles were retained on the filter. This would explain the slight increases in both the organic matter and TSS values with volume.
Results in Dairy Lake were similar to those in Long Branch Lake; the more water filtered, the higher the concentrations of TSS and inorganic sediment. Filters that received the maximum volumes had 27% more TSS and 59% more inorganic sediment than the filters that received the lowest volume. These differences were statistically significant, while variations in organic matter were not. As with Long Branch Lake, it seems the more material we packed onto the filters, the better the filters captured small inorganic particles that might otherwise have passed through.
We found that the volume of water filtered through TSS filters can influence the analytical results if there are small particles (<1.2µm) present in the lake water. Sometimes these small particles are in the form of algal cells (Little Dixie Lake) while other times the particles are mostly inorganic matter (Long Branch and Dairy lakes). Results suggest that the filter does not have to be packed to the point of clogging to attain a valid measurement. For example, in Little Dixie Lake a filtered volume of 700mL through the TSS filters led to the same results as 900mL.
Volunteers who experience TSS filter clogging should consider the amount of water that passed through the filter prior to clogging when choosing a volume for the new filter. For example, if the first filter clogged at 300mL, then 250mL should provide enough material for an accurate measurement without forcing the volunteer to spend extensive time filtering. However, it is important to remember to record the new volume on the data sheet and filter houses so we can make the appropriate adjustments in lab.
Note to volunteers: To prevent contamination, completely enclose the filter inside its "house."