WEIGHING FILTERS

Handle the filters by the edges with a flat tipped forceps, placing each into a plastic petri dish. Label five dishes as control filters and the rest consecutively from YY-01 to YY-###, so that each filter-dish combination has a unique identification number. Since the filters themselves cannot be labeled, the filters must remain in the labeled petri dish except when in use.  A complete year of LTER sampling requires about 400 filters.

 

The day that filters are to be weighed, set all filters out on the counter by the balance, with the petri dish lids partially open and the entire set covered to prevent any accumulation of dust on filters. Allow the filters to equilibrate with the atmospheric humidity near the microbalance for several hours before weighing. Calibrate the balance before each use. After every ten filters weighed, weigh a 10 mg weight to determine if the balance is consistent. If the 10 mg weight varies by greater than 5 micrograms from the initial weight, recalibrate the balance. Record filter weights to hundredths of milligrams. 

 

Differences in humidity between weighing dates can greatly affect TPM values. The five control filters are used to correct for humidity effects. These filters are weighed every time TPM or trap filters are weighed. The average difference between the ‘before’ and ‘after’ weights of the five control filters is used to correct sample filter weights for humidity.

 

At the end of the year, store all filters in a dark, cool, dry area. Archived filters may be analyzed for a variety of constituents. Carbon cannot be determined, but particulate phosphorus, nitrogen, and some metals can be extracted from the filters at a later date. Filters can also be examined under the microscope using regular or Cyto-Clear slides.

 

 

 TPM SAMPLES

Total particulate material (TPM) is an estimate of the mass of particles suspended in the water column. For this method, particles are defined as material not passing through a polycarbonate track-etched (PCTE) filter having a 0.4um pore size.

 

SAMPLE COLLECTION

Before going into the field, load preweighed filters into numbered inline filter holders, recording filter number and filter holder on the datasheet. Collect TPM samples at the same depths as the other LTER chemistry samples using an inline filtration setup.  Filter directly into a graduated cylinder, pumping until the flow is reduced to a drip or until you have filtered 500 ml, whichever comes first. Record the volume filtered on the field data sheet. Remove and reattach the filter holder backwards on the line, then reverse the peristaltic pump in order to draw the excess water through the filter. At the lab, remove the filters from the filter holders folding them in half with the sediment side in, and return them to the numbered dishes. Store them in the freezer until they can be dried.

 

PROCESSING SAMPLE FILTERS

Dry the filters in a 40-60°C oven, in the petri dishes with the covers slightly open, for 48 hours. After drying, close the dishes and store the filters at room temperature until they can be weighed. Follow the same weighing procedure that is used for weighing filters before use.

 

CALCULATIONS

To determine the mass of particles in the water column sample:

 

                mass on filter (mg) / volume filtered (L) = TPM (mg/L)

 

The calculation is automated in the “wrkfile” spreadsheet found on the Trout lake server. TPM data are then entered into the web based data form for inclusion in the LTER database.

 

 

SEDIMENT TRAP SAMPLES

The sediment traps are pvc cylinders with an interior funnel which focuses sedimenting particles into a detachable 250 ml plastic bottle.  We set duplicate traps at the deep hole in Crystal, Trout, and Sparkling Lakes during the open water season.

 

TRAP AND LINE PREPARATION

Two mooring lines extend upward from an anchor to a subsurface float wrapped in colorful tape. The total length of the trap rig should be about 1m less than the depth of the lake where the traps are deployed. We use subsurface buoys to keep the mooring lines taut and to avoid disturbance of the trap by passing boats. Knots on the mooring lines to support the trap are placed so that the mouth of the trap will be 7m off the bottom of the lake for Crystal and Sparkling lakes and 8m off the bottom for Trout Lake. It is important that the two support knots are exactly the same distance from the anchor or the trap will not hang perfectly vertically in the water.

 

Place approximately 0.2g NaN³ into a 4ml vial with septum cap. NaN³ is a toxic substance and should be handled accordingly: read the cautions on the bottle and MSDS, and wear gloves. Perforate the septum several times with a hypodermic needle, fill the vial with milliQ water and drop it into the collection bottle. Fill the collection bottle with milliQ water and attach to the threaded collar on the bottom of the trap.

 

TRAP DEPLOYMENT AND COLLECTION

The two traps should be placed about 5m from each other and at least 10m to the north of the sampling buoy. It is critical that they are far enough apart and especially that they are far from the permanent anchor. If they are too close they become entangled with one another or the permanent anchor and can become impossible to retrieve.

 

In the field, slip the mooring lines through the slots on the base of the trap so that the knots are on the inside of the slots supporting the weight of the trap. Run the lines up the side of the trap and fasten into the clips near the top of the trap. Lower the anchor into the water, holding on to the mooring lines above the trap. Push the trap into the water and allow it to fill, holding it down on the mooring line knots until it is full. If the trap is released without being filled, it will float off the knots as it is lowered and will not hang vertically in the water column. When the trap is full, lower it into the water adjusting the length of the mooring line if necessary so the float is about 1m below the surface.

 

Sediment traps are left in the lake for approximately one month, from one chemistry sampling date to the next. Retrieve the traps after all other sampling has been completed. Snag the mooring line with a hooked pole, pulling the line into the boat until the trap reaches the surface. Cap the trap and remove it from the lines, fastening it into the rack in the boat for transport. Replace with a fresh trap. Upon returning to the lab, process the traps as soon as possible. Do not allow them to sit for an extended period of time exposed to light and heat.

 

 

SAMPLE PROCESSING

We collect two sample bottles from each trap: the collection bottle attached to the funnel, and a subsample of the above-funnel water. Seal the funnel opening with the long handled cork, and remove the collection bottle from the bottom of the trap. Place the trap in a plastic washbasin and remove the cork to drain the trap into the basin. Stir the water in the basin and scoop a subsample into a clean 250ml trap bottle. The remaining water in the basin can be discarded. The two traps from each lake are arbitrarily designated Trap 1 and Trap 2. Label the collection bottle and the above-funnel bottle from each trap with a letter designation and the lake name. Refrigerate until further processing.  Rinse the traps with tap water, scrubbing with a soft brush if necessary to remove any particles remaining on the sides of the trap. Store traps clean and dry.

 

Final processing of the sediment trap samples should be done as soon as possible after collection. Material in the collection bottle is split into two size fractions by pouring it through a 210um nylon mesh circle fitted into a filter funnel. The 210um separation removes the extra variability in sedimentation rate due to large zooplankton and large detritus that may be found in a sediment trap. There are chironomids in traps from most lakes, and Mysis in Trout Lake traps. Since these things are not really settled material (they migrate in and die), we want to remove them. To facilitate filtration, milliQ water may be squirted into the filter funnel to loosen particles covering the mesh, and very gentle suction may be applied. Rinse the sample bottle with milliQ water and pour the rinse water through the mesh.

 

Weigh the twelve plastic bottles and record the empty weight in Column A on the data form. Pour the sample fraction that passed through the mesh into the appropriate 500ml bottle. Wash the material retained on the mesh through a glass funnel into the 250ml bottle with milliQ water. Weigh the sample bottles with sample and record in Column B on the data form. Also weigh and record the weight of the above-funnel bottle with sample.

 

Filter subsamples of each of the three samples (<210um, >210um, and above-funnel) through preweighed 0.4um pore PCTE filters using suction <15 psi. Shake the sample bottle prior to pouring so that the subsample filtered is representative of the entire sample. In general you can only filter 10-20ml of the <210 sample, 40-60ml of the >210 and ~100ml of the above-funnel sample before the filter is clogged. As long as particles are visible on the filter, there is enough to weigh. Do not filter too much because if the mass on the filter is too great, it will flake off after drying causing a loss of particulate matter. Return each filter to its numbered petri dish, folded in half with the sediment side in. Freeze the filters until they can be dried and weighed. Record the number of the filter used in Column D on the data form. Weigh the three sample bottles after filtering and record the weight in Column C on the form.

 

Discard the remaining above-funnel sample.  Centrifuge the entire remainder of the <210 and >210 samples at 7000rpm for 30 minutes at 4°C. [Note: at Trout Lake we do not have a chilled centrifuge or one that spins at 7000rpm. Use 3000rpm for 30-50 minutes.] Decant and discard the supernatant and place the muck pellet in a small, labeled plastic vial and freeze. Wash all trap bottles with milliQ water, allow to dry, and reuse them. Freeze dry the centrifuged samples, and store in a cool, dry place.

 

 

 

CALCULATIONS

To determine the deposition of particles into the lake for each of the three fractions :

 

          mass on filter (mg) / volume filtered (ml) = particle concentration in subsample (mg/ml)

 

          particle concentration (mg/ml) x total sample volume (ml) = total particle mass in sample (mg)

 

          total particle mass / area of trap mouth / length of deployment = particle mass flux (mg/cm²/day)

 

These calculations are automated in the “wrkfile” spreadsheet found on the Trout Lake server. Add the fluxes of the three fractions together to determine the total flux of particulate matter (mg particles / cm² / day) for each trap deployment.

 

The traps are 44cm from the top of the collection funnel to the top of the trap. The collection area, calculated from the internal diameter of the trap opening, is 165cm² on the older Plexiglas traps and 185cm² on the newer pvc traps. Total trap volume is 7420ml for the pvc traps used beginning in 2002. Before 2002, we measured the volume of sample in the trap at the time of processing. The leak proof caps with the new traps eliminated spillage during transport, so we now use a constant trap volume for calculations.

LTER Keywords

Protocol Format

Process

Protocol ID

sediment_traps1

Protocol Type

field & laboratory