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Zebra Mussel Monitoring Program for Texas

Methods

The ZMMP uses a suite of detection and quantification methods to determine spatial distributions, dispersal, spread, population dynamics, and reproductive phenology of both zebra and quagga mussels.

Water-Sample Collection and Analysis

Dreissenid mussel reproduction commonly occurs semiannually during distinct spawning events (typically in spring and autumn). They have a microscopic, free-swimming larval stage (veliger). Fine mesh tow-net sampling is done from boats typically in spring through autumn when veligers are expected to be present. These samples are used to monitor for the presence of dreissenid mussel veligers and DNA.

Zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena bugensis rostriformis) are both invasive species. They are closely related and are jointly referred to as dreissenid mussels.

Collection of water samples is done in areas of likely introduction of veligers (boat ramps and marinas) at reservoirs, at selected bridge crossings at riverine sites, and at raw-water sampling points at water treatment plants.

Each study area has dedicated sampling gear to prevent cross contamination.

Technique
  • Water samples are collected by using fine mesh (64-micron) plankton tow nets
  • Each water sample is collected in a 250-milliliter polyethylene bottle and preserved in 50-percent ethanol.
  • A minimum of three replicate water samples are collected at each site.
  • Samples are transported to the USGS North Texas Program–Fort Worth laboratory and analyzed by using cross-polarized light microscopy. If present, veliger densities (number per liter) are determined.
  • Select replicate samples are preserved in 70-percent ethanol and sent to a separate laboratory for DNA analysis of which species is present (zebra or quagga mussel).
Plankton tow net

Plankton tow net used to sample water for zebra mussel veligers (larvae)

Close-up photomicrograph

Close-up photomicrograph of zebra mussel veligers taken using light microscope

Artificial Substrates and Analysis

Late-stage dreissenid larvae (veligers) settle out of the water column and attach to substrates by using proteinaceous byssal threads. Often, dreissenid mussels will settle on hard substrates (e.g. pipes, screens, rocks, and other mussels) but can settle on soft substrates (e.g. plants, rope, and Styrofoam). After settlement, mussels metamorphose into juveniles.

Artificial substrates, constructed of a a series of stacked 4- by 4-inch hardboard tiles separated with 0.25-inch spacers, act as passive samplers. These samplers are deployed at most study sites, and are used to monitor the settlement and growth of dreissenid mussel juveniles and adults.

Samplers are visually inspected in the field. If mussels are detected, they are analyzed in the laboratory by using a stereomicroscope. Mussel density (mussels per square meter) is determined and the shell lengths of a subset of individuals are recorded. A handheld digital microscope with attached camera can be used to detect and document shell lengths of recently-settled mussels while in the field.

New and fully colonized substrates.

SCUBA Diving

USGS personnel certified as SCUBA divers search for juvenile and adult dreissenid mussels during dives to determine their occurrence and relative densities on water intake structures, boat ramps, marina flotation and support devices, and other submerged structures.

Annual dives are done to assess change in dreissenid mussel presence and density at multiple sites at most monitored reservoirs. Additional dives are done as necessary or as requested. Submerged surfaces are analyzed by using visual and tactile inspections.

High-definition video footage is collected to document occurrences and mussel population/settlement dynamics.

SCUBA diver inspecting a submerged screen.

SCUBA Videos

Dive video - Inspecting a submerged screen at Lake Texoma, 7/16/2013

Water-Quality Sampling

The potential distribution of dreissenid mussels in the United States and Texas is unknown. However, some water-quality thresholds (namely water temperature, pH, and calcium) may limit their distribution.

Automated water-quality data sondes are used to measure water temperature, dissolved oxygen, specific conductance, and pH. Physicochemical properties are measured and recorded at five-foot depth intervals—from the lowest depth (i.e., benthos) to the surface—providing a spatial profile of water-quality conditions. Submersible data loggers at selected reservoirs and river sites also record water temperature data every 15 minutes at five-foot depth intervals.

Additionally, water-quality samples are collected manually using standard protocols (U.S. Geological Survey, variously dated) at a minimum of two discrete points in the water column—at benthos and at the surface—at selected sites at least twice per year. At select sites, water-quality samples are collected at 10-foot depth intervals from the benthos to the surface.

Samples are stored on ice and shipped to the USGS National Water Quality Laboratory in Denver, Colo. The water-quality samples are analyzed for nutrients, common ions (such as calcium), and alkalinity.

Together with the zebra mussel occurrence and distribution data, the water-quality data enhances our understanding of dreissenid mussel ecology and potential range in temperate, low-latitude waters.