A NOVEL APPROACH TO EVALUATE HABITAT QUALITY IN THE UPPER MISSISSIPPI RIVER -- 
QUANTITATIVE ANALYSIS OF ESSENTIAL FATTY ACIDS
 
Michelle Bartsch, Lynn Bartsch, Brent Knights, Jon Vallazza, Steve Gutreuter, 
William Richardson, and Teresa Newton

U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, 
WI  54603.

Quantitative analysis of fatty acids (FAs) has emerged as a powerful tool to 
determine food sources and quality in aquatic food webs.  Polyunsaturated fatty 
acids (PUFAs), which includes omega-3 and omega-6 FA, are almost exclusively 
synthesized by vascular plants and algae, yet play an important role in cellular 
function and hormone metabolism of animals.  PUFAs that cannot be biosynthesized 
de novo, such as alpha-linolenic acid and linoleic acid, or in sufficient 
quantities to ensure optimal physiological performance, such as eicosapentaenoic 
acid (EPA) and docosahexaenoic acid (DHA), are called essential fatty acids 
(EFAs).  EPA and DHA have been shown to regulate aspects of growth, development, 
reproduction, or immunological response in marine mollusks and fish, and 
freshwater zooplankton.  In addition to indicators of consumer health, FAs have 
been used as diet biomarkers to elucidate prey type and dietary importance for 
marine and freshwater fish, marine carnivores, and freshwater zooplankton.  We 
have begun using FAs as potential indicators of food sources and habitat quality 
in Upper Mississippi River food webs.  Preliminary data showed that 
concentrations of EFAs in consumers are higher in channel habitats than in 
backwaters.  For example, concentrations of EPA and DHA were significantly 
higher in bluegill, unionid and zebra mussel tissues from channel habitats 
compared to backwaters.  In contrast, concentrations of arachidonic acid (ARA, a 
known precursor of stress prostaglandins in fish and humans) were significantly 
higher in bluegill and zebra mussel tissue from backwaters compared to channel 
habitats.  ARA concentrations were similar in unionid mussel tissue between 
habitats.  Preliminary results suggested that channel habitats provided 
consumers with a better mix of EFAs than backwaters, implying that consumer 
health was better in channels than backwaters.  We hypothesize that low EFA 
concentrations in backwaters may result from: 1) the long hydraulic retention 
times that inhibit replenishment of depleted EFA supplies; 2) lower production 
of EFA-rich phytoplankton; or 3) abundant populations of cyanobacteria (due to 
long retention times and late summer nitrogen depletion of backwaters).  Future 
research will more explicitly examine the spatial and temporal patterns of EFAs 
in food webs in these habitats including relations between: 1) EFA 
concentrations in algal groups and consumers; 2) EFA concentrations and rates of 
secondary and tertiary production; and 3) nutrient concentrations, habitat 
connectivity, and food web production.  Finally, these initial studies will 
identify baseline patterns of food web interactions and food sources against 
which the effects anthropogenic perturbations (e.g., invasive species, habitat 
restoration, and climate change) can be measured.

Keywords:  fatty acids, food webs, habitat quality, Mississippi River, channel 
and backwater