LTER NIGHT VERTICAL ZOOPLANKTON TOWS

 

I  Preparation

 

Label 250 ml jars (8 oz.) glass jars with date (format: YYMMDD, e.g. 960824 for 24 August, 1996), lake, net mesh, tow number (1 through 5 for most lakes, see below), depth, and time (leave blank for now). Add approximately 3 ml Lugol's Iodine to each jar to preserve the sample.

 

Sampling stations are at the deep hole permanent anchor in each lake, except for Trout Lake (see below). Collect five replicate tows at the same depth from the deep hole of each lake. For Trout Lake, in addition to the five replicate tows from the deep hole, take three replicate tows from four sites with depths of approximately 10 m, 15 m, 20 m, and 25 m along a transect from the deep hole to Rocky Reef point. Thus each lake will have five samples from any given night except Trout Lake, which will have 17 total samples.

 

Here are the target depths that you should try to shoot for:

 

Lake Depth

 

Allequash (AL)    7 m

 

Big Muskellunge (BM)     19 m

 

Crystal Bog (CB)            2.5 m

 

Crystal Lake (CR)          19 m

 

Sparkling (SP)    19 m

 

Trout Bog (TB)   7 m

 

Trout Lake (TR)  32 m (plus 10-, 15-, 20-, and 25-m tows)

 

Gear needed: Sample jars, cooler, 1 m diameter 1 mm mesh plankton net with matching bucket, metered line, data sheets, pencil, watch, wash bottle, oars, motor and gasoline (for all except CB, CR, TB), boat cushions, PFDs, flashlight.

 

II  Field Sampling

 

Samples should be taken in late summer (late August to early September). Sample after dark. The time will vary throughout the sampling period, but always wait until all skylight from the sun has disappeared. This is to give Chaoborus, which performs diel vertical migrations, time to ascend into the water column.

 

Tie up to floats at the sampling stations. To maximize the accuracy of population estimates, the boat should be prevented from drifting. Lower the net so that the ring is at the prescribed depth. This allows us to sample most of the water column, while keeping the net off the bottom. If the bucket hits the lake bottom, rinse out the net thoroughly three times with lake water and resample, adjusting the depth accordingly. If you must make this adjustment, be sure to record the actual depth and take all remaining samples from that same depth. Once the net has been lowered to depth, allow it to settle. Raise the net at a rate of 0.5 m/sec for good sampling efficiency.

 

Upon reaching the surface with the net, rinse it by dipping it three times into the lake. Do no submerge the top of the net at this point. This will rinse animals trapped on the mesh into the catch bucket without introducing new animals into the sample. Reduce the volume of water in the bucket by swirling it--this unclogs the mesh and allows the water to drain. Rinse the lower 25 cm of the net in the lake again to further rinse animals into the bucket (trapped zooplankton often accumulate at the lower end of the net). Reduce the volume of water in the bucket as much as possible by swirling again. Release the butterfly clamps and pour the contents into the appropriate sample jar. Rinse the bucket using the wash bottle, swirling as before to reduce the volume, and adding the rinse water to the sample. Record the sample time (24-hour clock) on the jar and the datasheet and place into the cooler. Rinse the net and bucket in the lake between tows. Upon returning to the lab, rinse the net and bucket in the sink and hang net over the sink to dry.

 

III  Counting

 

Samples should be counted with a binocular microscope at about 60x total magnification. Count the entire sample; do not subsample. For ease of counting, use a petrie dish with a grid or superimpose a grid on the counting dish (a metal grid with 1 cm squares works very nicely). Samples are counted for the total numbers of the following animals: Chaoborus spp. (differentiating between larvae and pupae), Leptodora kinditi, and Mysis relicta. In addition, make a note of the relative abundance of Holopedium and Daphnia within each sample--these animals do not have to be counted, as the 1 mm mesh is too big to give a quantitative sample. As a general guideline for Holopedium and Dapnia: "very few" » 10, "few" » 50, "moderate" » 100, "many" » 500, "very many" » 1000 or more.