Trophic cascades in the Middle Rio Grande, NM

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Trophic cascades in the Middle Rio Grande, NM

• Hugo A. Magaña
• USDA Forest Service/UNM
• Bosque Hydrology Group
• Water Quality Summit
• Oct. 25, 2004

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Background

• The decline of native fishes in New Mexico over
  the past century has been attributed to five
  major causes
• Water impoundment
• Water diversion
• Timber harvest
• Introduced exotics
• Cattle grazing

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Rio Grande silvery minnow

• Historic range
• Mainstream habitats of the Rio Grande downstream
  from Velarde to the Gulf of Mexico
• Pecos River from near Santa Rosa downstream to
  its confluence with the Rio Grande
• Present range
• Middle Rio Grande
• Angostura diversion to Bosque del Apache

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Study Objectives

• Spatial and temporal evaluation of nutrients,
  algae, aquatic invertebrate, and fish populations
• Compare energy transfer dynamics of identified
  RGSM habitats in northern and southern portions
  of the Rio Grande
• Establish seasonal and annual baselines for
  stream and sediment nutrients
• Feeding trials with Rio Grande silvery minnow

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Field Sampling ProtocolUSGS Revised Protocol

• Water quality parameters (NAWQA)
• Water parameters and sampling
• Measure flow
• Sediment sampling
• Algae sampling
• Invertebrate sampling
• Fish sampling

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Study Sites
Angostura Diversion
Bernalillo Bridge
Rio Bravo
Shirk
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Study Sites
Los Lunas Floodplain
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Lab Methods Materials

• Nutrient analysis
• Isolate and identify
• Isotopic signature for each species
• Measure algal standing stock as chlorophyll a
• Identify and enumerate aquatic invertebrate
  species
• RGSM fecal contents
• RMSM feeding trials

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Middle Rio Grande nutrient concentrations summer
2003
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Los Lunas 5th Mesohabitat Summer 2003
May 17th 650 cfs
May 19th 275 cfs
May 16th
May 23rd190 cfs
May 28th 400 cfs
May 31 550 cfs
June 4th 350 cfs
June 7th 100 cfs
June 26th 0 cfs
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Results Summer 2003
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Diatom Algae 2003
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Filamentous Algae 2003
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Aquatic Invertebrates 2003
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Return to Rio Grande May-July 2004
June 3rd
June 9th
May 27th
June 21st
June 15th
June 17th
June 24th
July 7th
July 16th
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Los Lunas overflow channel/pool Summer 2004
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Middle Rio Grande subsurface nutrient
concentrations Summer 2004
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Diatom Community Shirk 2004
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Diatom Algae 2004
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Aquatic Invertebrates Summer 2004
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Larval light trap
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Los Lunas Ichthyofauna
River carpsucker 123
Common carp 102
Western mosquitofish 3
Rio Grande silvery minnow 231
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Productivity and respiration
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Possible questions to be answered by study

• Identify autecological characteristics for
  diatoms (bioassessment)
• Is there a nutrient gradient or nutrient flux in
  RGSM habitats?
• Do these fluxes affect algae or aquatic
  invertebrates?
• Will increased algae and invertebrates increase
  population size of RGSM?
• Do restored floodplains increase fish recruitment
  and population size?
• Does the algal community structure and standing
  stocks differ in constrained versus unconstrained
  (restored) segments of the Rio Grande?
• Does flow influence colonization of algae?
• Is the aquatic invertebrate community more
  abundant and diverse in the restored river reach
  versus constrained reach?
• Are fish numbers and diversity greater in the
  restored versus constrained river reach?