Freshwater Amphipod Survey
We had a client that required a freshwater shrimp survey. After acquiring the necessary federal permits, and bringing on a specialty taxonomist, the field work was performed during the peak high groundwater discharge period of late March and early April of 2014.
The Hay’s Spring Amphipod (Stygobromus hayi), also known as a scud, may occur in headwater spring seeps in the greater Washington, DC area. In addition, one federal candidate species, Kenk’s Amphipod (Stygobromus kenki) may also occur. Both of these amphipods are groundwater species, which are found in seeps and springs.
The Hay’s Spring Amphipod is a small shrimp-like freshwater crustacean that occurs in five springs along Rock Creek in the District of Columbia, a tributary of the Potomac River. The amphipod occurrences within the five springs are considered a single population. The amphipod measures 0.39-inch (10-mm) in length. Because it lives underground, it is white in color and eyeless.
The Kenk’s Amphipod is a small shrimp-like freshwater crustacean that occurs in the District of Columbia and Montgomery County, Maryland, within the Potomac River watershed. Its entire range is less than 40 square miles and has been found with the Hay’s Spring Amphipod. The adult female Kenk’s measures 0.22-inch (5.5 mm) in length and the male measures 0.15-inch (3.7 mm), with both the female and male of the species being smaller in size than the Hay’s Spring amphipod. Both species are visible to the naked eye.
Amphipods of the genus Stygobromus occur in groundwater or groundwater-related habitats (seeps, small springs, wells, caves and interstices) and have modified morphology for survival in these subterranean habitats. They are eyeless and unpigmented (without color) and frequently have attenuated (reduced in length and width) bodies.
Our two study amphipods occur in seepage springs in wooded areas. Seepage springs typically have a diffuse discharge of water where the flow cannot be immediately observed but the land surface is wet compared to the surrounding area. The shading, hydrologic conditions and organic matter found in these woodlands are probable factors in maintaining suitable habitat for the species. The amphipods can be found in leaf litter (dead leaves) or loose sediment submerged in the water of seepage spring outflows. When present, the species are typically found in small numbers and then only when ground water levels are high and springs are flowing freely. These conditions typically occur during the early spring season, except during especially dry years.
Suitable habitat allows amphipods to spend life in the shallow groundwater zone, moving in water that percolates among sand grains and gravel unless large volumes of water flush it up to and out of an existing spring. These subterranean species appear seasonally and sporadically in seeps and springs or may not appear even during high water flows. Underground populations may be connected to each other through groundwater drawdown to zones of deeper flow, or by moving through underground interstitial spaces (the saturated space between grains of sediment in aquifers and alluvia).
Locations where groundwater discharge was evident were flagged, surveyed and sifted/screened for amphipods. Wet leaf mats were pulled apart and organic leaf detritus was picked-through, looking for small, white-colored amphipods. All organisms were collected using tweezers and placed into sample jars, specifically labeled to the collection location. The saturated/inundated ground within the seep was also screened to see if organisms occurred within the first few inches of gravel/soil substrate.
Precipitation at our site for a six-month period from October 1, 2013 through March 31, 2014 was 26.1-inches, where the average mean is 19.3 inches. Based on these findings, the study was performed during a period of above (6.8-inches) average precipitation, with every month preceding our study, less January of 2014, being in excess of the mean. This information would suggest that shallow groundwater spring seeps should be flowing slightly above normal/typical rates for March/April of 2014.
A few locations generated samples of aquatic fauna, which were placed into jars with preservative and labeled to location. At all three sites, we picked through wet leaf pack, O-horizon organic detritus, and interstitial sands, pebbles and silts.
The most common species collected (24 total) was Caecidotea kenki (Crustacean, Order Isopoda), which is an isopod with no common name. Individuals of this species reach a length of 0.39-inch (10 mm). This species is known from southern Pennsylvania, Maryland, Northern Virginia and Washington, DC in the coastal plain and eastern piedmont. C. kenki inhabits small springs, small spring-fed creeks and seeps, all of which are superficial groundwater habitats. This species is highly adapted to seeps and springs, possess tiny eyes and is pigmented. They feed on microbial foods such as algae, bacteria and dead organic matter. This isopod is known to be relatively common in the Potomac drainage.
The second identified species (8 total) was Gammarus fascitatus (Crustacean, Order Amphipoda), which is an amphipod with no common name. G. fascitatus is a wide ranging, habitat generalist. Males reach 0.55-inch (14.0 mm) in length and females reach 0.47-inch (12.0 mm) in length. This species ranges from the Great Lakes, across much of New York and south along the coastal plain to North Carolina. G. fascitatus can be found in lakes, rivers, streams, springs and seeps. They possess small eyes and are usually whitish in color. They are filter-feeders that primarily feed on organic detritus and live plant matter.
Both photographs are of the amphipod Gammarus fasciatus, which were taken from the tailgate of my pick-up truck, with a Nikkor Micro 85mm lens. Don’t tell my wife, but I used her white Tupperware trays as a screening pan, to then place the specimens into a preservative agent.
Spring seeps and their contribution to base flow of headwater streams is an important natural resource, deserving protection. The primary threat to seepage springs and spring run habitat is modification of hydrology (water quantity) and degradation of water quality. Unfortunately, spring recharge areas may be beyond the boundaries of protection.
The following YouTube video from MD DNR, expands on the rare species of amphipods located in the greater Washington DC, Maryland area.