Spatial Patterns of Native Freshwater Mussels in the Upper Mississippi River

Science Center Objects

Impact of UMESC Science

This research aims to quantify spatial patterns of adult and juvenile (≤5 y of age) freshwater mussels across multiple scales based on systematic survey data from 4 reaches of the Upper Mississippi River (Navigation Pools 3, 5, 6, and 18). Resource managers can use this critical information about spatial structure to make informed river management decisions.   

What do we know about the spatial structure of native mussel populations?

  • Multiple physical and biological factors structure freshwater mussel communities in large rivers, and their distributions have been described as clumped or patchy.
  • However, few surveys of mussel populations have been conducted over areas large enough and at resolutions fine enough to quantify spatial patterns in their distribution.

How did we explore spatial patterns?

  • We used indicators of spatial autocorrelation to quantify spatial patterns of adult and juvenile (≤5 y of age) freshwater mussels across multiple scales based on survey data from 4 reaches (navigation pools 3, 5, 6, and 18) of the Upper Mississippi River.
  • Native mussel densities were sampled at a resolution of ∼300 m and across distances ranging from 21 to 37 km, making these some of the most spatially extensive surveys conducted in a large river. 

How did spatial patterns vary across river reach, age group, and scale?

  • Patches of mussels were evident at large spatial scales in three of the pools evaluated; these patches varied in size and location within and among pools.
  • Multiple scales of patchiness were detected, with smaller patches of mussels nestled within larger patches in most pools.
  • In all pools, some patches of adults overlapped patches of juveniles, suggesting that good mussel habitat has persisted over time and space.
  • In pools 3 and 5, patches of juveniles were found where there were few adults, suggesting that conditions may have recently become suitable for juveniles.
  • In pools 3, 5, and 6, some patches of adults were found where there were few juveniles, suggesting a lack of recruitment of young mussels into the population.

How can these results inform managers?

  • Our results suggest that: 1) the detection of patches of freshwater mussels requires a multi-scaled approach, 2) insights into the spatial and temporal dynamics of structuring mechanisms can be gained by conducting separate analyses of adults and juveniles, and 3) maps of patch distributions can be used to guide restoration and management actions and identify areas where mussels are most likely to influence ecosystem function.

 

Results of the spatial pattern analysis in juvenile and adult mussels

This table shows the results of the spatial pattern analysis in juvenile (≤5 y of age) and adult mussels across 4 navigation pools (3, 5, 6, and 18) in the Upper Mississippi River.  Mussel distributions were classified as patchily distributed, randomly distributed, or having a gradient pattern.(Public domain.)

 

Maps showing hot spots of adult (left) and juvenile (right) mussels

Example of hot-spot maps of adult (left) and juvenile (right) mussels in Navigation Pool 5 of the Upper Mississippi River.  The size of the dots represent the number of mussels per square meter of the river bottom—with larger dots representing more dense mussel assemblages.  Hot spots (red) indicate where sites of high mussel density were surrounded by sites of high mussel density.  Cold spots (blue) are where sites of low density were surrounded by sites with low density.  There are also sites of high density surrounded by low density (HL, orange) and sites of low density surrounded by high density (LH, green).(Public domain.)