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Management Implications

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USGS Patuxent Wildlife Research Center

11410 American Holly Drive

Laurel, MD 20708, USA

301-497-5622 (Tel)

301-497-5624 (Fax) 


Replacing collecting bottle for emerging invertebrates



Wetland impoundments historically have been managed with seasonally adjusted water depths to mimic natural drying and flooding cycles and provide appropriate habitat and food for nesting and migratory waterfowl. Impoundments at Patuxent Research Refuge, Laurel, MD have been managed to produce the greatest biomass of plants (especially seeds) beneficial to migratory waterfowl.  In years past, an impoundment was drained (a drawdown) when floating-leaved plants covered more than 50% of the water area.  Drawdowns encourage beneficial moist soil annual plant growth, which produced large amounts of seeds for wintering waterfowl.  The impoundment drawdown procedure became known as moist-soil management because of the large mud flats that were initially created. 

There are other factors, however, that should be taken into consideration if wetland managers want to obtain and maintain the greatest biomass and biodiversity.  Macroinvertebrates, an important protein rich food source for migratory birds, are required by migrating shorebirds.  Midge larvae (Chironomidae) found in sediment of shallow water areas are especially valuable to migrating shorebirds in spring and fall.  The objective of this study was to determine if impoundment drawdowns for plant production have an adverse effect on the production of macroinvertebrates, especially chironomids.

This experiment was conducted concurrently in 20 created impoundments and in 12 experimental pools at Patuxent.  In 1998, sampling of benthic invertebrates in study sites was conducted with a 10 cm soil auger to a depth of 10 cm in the sediment.  Samples were placed in a screen sieve, washed in the field, and analyzed in a laboratory.
Placing Core Sample in Sieve Washing Mud from Core Sample

During spring and summer 1999, macroinvertebrates in the impoundments were determined with an emergent trap, which was placed in each wetland impoundment and experimental pool.  Emerging invertebrates were captured in a bottle with alcohol, which was removed each week and replaced with another sample bottle.  Invertebrates were identified in a laboratory to the highest taxa.
Emergent trap for invertebrates

Replacing collecting bottle for emerging invertebrates

Core sample data taken in June 1998 indicated that total benthic invertebrates were not adversely impacted when impoundments were drawn down the previous year.  Chironomids were actually more numerous in the impoundments drawndown the previous year, whereas, other invertebrates (mainly odonates) were more numerous when impoundments were not drawndown.

Figures 1 and 2: Mean number of chironomids and mean number of other invertebrates based on drawdown status

Results from the 1999 emergent traps sampling in impoundments indicated that invertebrate production (chironomids and other invertebrates) was not reduced when impoundment drawdowns occurred the previous year.  Emergent trap data for the experimental pools documented that there was not a significant difference of invertebrates (chironomids or other invertebrates) when pools were drawn down in June or in August the previous year.

Figures 3 and 4: Chironomids and other invertebrates in impoundments

Figures 5 and 6: Chironomids and other invertebrates in experimental pools

Core sampling in 1998 indicated that more macroinvertebrates were found in the shallower areas of wetlands.

Figures 7 and 8: Chironomids and other invertebrates in impoundments at different depths

Contour maps were generated from water depths taken from the impoundments with GPS locations.  These maps have been entered into a GIS program that can predict the amount of exposed mud flats based on any amount of draw down conducted by refuge managers. By knowing invertebrate density at specific depths, managers can predict availability of invertebrates for migrating shorebirds.

Uhler Marsh depths (cm): Two dimensional contour map

Uhler Marsh depths (cm): Three dimensional contour map


The scheduling of drawdown can target a desirable species or multiple species depending on the life cycle data available.  Cooperative  research is presently underway between USFWS and USGS that will further refine impoundment management on a regional basis.  By analyzing data from many refuges biologists can manage impoundments more effectively for migrating birds.

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