Project Description

Zooplankton play an important role in the food webs of lakes and large rivers by feeding on algae and other particulate matter and providing food to larval and planktivorous fish. Previous work from large rivers has indicated that zooplankton can sometimes be abundant and are often dominated by rotifers. The current study examined zooplankton collections over a broad range of three large rivers in the central US (the Missouri, upper Mississippi, and Ohio rivers). This web site displays photos from representative rotifer genera occurring in these rivers, being used to facilitate comparisons of taxa among collaborators in the current study. The photos may also be useful for students learning the rotifers and others working on large rivers. Along with photos and their taxonomic identifications, we include some description of techniques for sampling zooplankton in rivers and for identifying rotifers. We include a listing of useful keys and links to other web sites.

Funding for this project was provided by the US Environmental Protection Agency (EMAP-Great River Ecosystems program), through the Illinois Natural History Survey. We acknowledge the numerous sampling crews who collected zooplankton over extensive reaches of the three rivers and our collaborators in the plankton and geochemistry group: Drs. Anthony Aufdenkampe (Stroud), Paul Bukaveckas (Virginia), John Chick (Illinois), and Jeff Jack (Louisville). Jeff suggested the use of stains.

Challenges of working with river zooplankton

Large rivers have high average flow rates, but flows are highly heterogeneous. In their natural state, rivers have eddies and backwaters. They also have floodplains periodically connected by flooding. Most large rivers in North America have been modified by man, with large dams and navigation structures altering flow (Benke and Cushing 2005).

Pump samples and zooplankton size fractions

Most zooplankton collecting techniques were developed for standing water (e.g., see Downing and Rigler 1984). Quantitative sampling of flowing water requires different tricks. One of the simplest is to use a hand-driven pump connected to a tube that is suspended to the desired depth. Water is pumped through a plankton net and water volume measured in a large receiving barrel.

For this project, we collected from several random locations and integrated water from several depths. For “macrozooplankton” (cladocerans and copepod adults and juveniles), we pumped 180 L through 53 μm mesh. This ensures a large enough volume for collecting the less-common crustaceans. For “microzooplankton” (rotifers and copepod nauplii), we pumped 18 L through 20 μm mesh. Larger meshes lead to high losses of rotifers and, in the turbid water of large rivers, clogging prevents sampling larger volumes through the finer mesh.

Density estimates

We estimated densities with quantitative lab techniques. Following filtration and separation from the sediment (see “Working with samples from sites having high turbidity”, below), we placed samples in a graduated cylinder and filled it to a known volume with water. We sub-sampled 1ml of this volume with a Henson-Stempel pipette, and counted the sub-samples of microzooplankton in a Sedgewick-Rafter cell in order to determine density number per liter.

Working with samples from sites having high turbidity

Many of the rotifer photos in this web site are from the Missouri River. Like many rivers, the Missouri has high turbidity. Sampling zooplankton in such conditions tends to result in quantities of silt and sand in the sample, which can preclude seeing all of the rotifers. We used two main techniques to work around this problem:

1) Staining the sample. We introduced a cyanosine (phloxine-B) solution to our samples to differentiate between biotic material (rotifers and nauplii) and debris (i.e., fine sediment). This reduced the chance of overlooking rotifers hidden by sediment.

2) Settling out the sediment. After filtration and re-suspension in water, we placed samples into a container (beaker, graduated cylinder, or Petri plate) to settle out the heavier sediment. To dislodge animals that might be wedged in the sediment, we used a pipette to spray some of the water from the sample into the sediment and free the lighter rotifers. We also swirled and tapped the container to ensure contents settled out according to relative densities. The supernatant was carefully poured off into a graduated cylinder for sub-sampling and counts (see “Density estimates” above). The sediment was scanned to verify no animals were left behind; the pink- colored animals were visible under a dissecting microscope.