RAREFACTION ANALYSIS TO DETERMINE SUB-SAMPLING EFFORT FOR ESTIMATING CHIRONOMID 
DIVERSITY IN MINNESOTA FARM PONDS.

Benjamin D. Campbell1, Roger J. Haro1, Melinda G. Knutson2, and William B. 
Richardson2.  1Rivers Studies Center, University of Wisconsin-La Crosse, La 
Crosse, WI 54601.  
2U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, 
WI 54601.

Farm ponds experience a variety of disturbances from agricultural practices.  
Sediments, pesticides, herbicides, and fertilizers are transported by run-off 
from surrounding-agricultural watersheds and deposited into receiving ponds.  
Non-biting midges (Chironomidae: Diptera) are one of the largest families of 
aquatic insects, with representatives found in almost every aquatic habitat.  
The wide geographical range and diverse composition of chironomid assemblages 
makes this group very useful for assessing ecological change in a variety of 
aquatic habitats.  Chironomid larvae have been used as biological indicators for 
lakes and rivers, but little work has focused on ponds. The objective of this 
study is to determine how patterns in chironomid richness, relative abundance, 
and instantaneous growth vary across farm ponds under different land uses.  
Larvae were collected from four types of farm ponds, based on wetland type and 
adjacent land uses: (1) natural wetlands, and constructed agricultural ponds 
adjacent to (2) row crop agriculture, (3) grazed grassland, and (4) non-grazed 
grassland.   Constructed ponds were classified based on the land-use practices 
within the first 100 m of their surrounding drainage basin.  The epi- and in-
faunal chironomid assemblages were sampled in each pond over a three-month 
period during the summer of 2001.  A pilot study was conducted with samples 
collected from the summer of 2000 in order to determine sub-sampling effort.  
Rarefaction was used to determine the minimum number of larvae needed to 
adequately estimate chironomid diversity across pond type.  The rarefaction plot 
showed a horizontal asymptote around 14 genera that corresponded with 70 larvae 
per sample whereas at 100 larvae per sample, only one additional genus was 
represented.  Thus, it proved more efficient to measure, mount and, identify up 
to 70 larvae from each sample.  Taxonomic richness, relative abundance, and 
instantaneous growth within the chironomid assemblages will be compared to 
develop an index for monitoring disturbances in farm ponds.

Key words: chironomids, farm ponds, southeastern Minnesota, rarefaction, 
taxonomic richness