ASSOCIATION    BETWEEN    ANNUAL    HYDROLOGICAL    PATTERNS    AND
INVERTEBRATE PRODUCTIVITY IN A LARGE RIVER

Denise Bruesewitz and Michael Delong.
Large River Studies Center, Biology Department, Winona State University, Winona, 
MN  55987.  

The annual hydrograph of the upper Mississippi River exhibits a predictable pattern of 
greatly elevated discharge during the spring and a smaller rise in the fall in response 
to late-summer rains.  Although the autumnal rise in discharge does not inundate the 
floodplain, it reconnects slack water areas to the river.  Previous studies have 
suggested that reproduction and secondary production of some invertebrates reaches 
a peak in the fall, coinciding with increased river discharge.  This study examined data 
collected over 5 years to determine if recruitment of zebra mussels was correlated 
with the magnitude of increased discharge in the autumn.  We compared the number 
of 0-4 mm long zebra mussels colonizing 15x15 cm unglazed ceramic tiles from 15 
August-15 November in each year to average daily discharge over the same period 
using linear regression.  Results indicate there is a strong positive relationship 
between magnitude of autumn discharge and the number of new zebra mussels 
colonizing the tiles.  We propose that the small autumn increase in discharge provides 
food supplements by reconnecting highly productive areas of the river to less 
productive habitats, thus allowing for greater mussel production following this event. 
These benefits may be evident among other riverine invertebrates. 

Keywords:  hydrograph, abiotic factors, zebra mussel, Mississippi River, experiment
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