Organic matter dynamics in the Kuparuk River Influence of Temperature and Light on Rates of Inorgani

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Organic matter dynamics in the Kuparuk
RiverInfluence of Temperature and Light on
Rates of Inorganic Nitrogen Transport by Algae in
an Arctic Lake
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• Since the late 1970s, research has been conducted
  on the upper 24.6 km of the river, from its
  headwaters in the foothills of the Brooks Range
  to its intersection with the pipeline and Dalton
  highway. Minimal terrestrial plant cover,
  streamside patches of dwarf birches and willows,
  often less than 1 m.
• Entire watershed is underlain with permafrost,
  greatly reduces seepage. Thaw depth beneath
  riverbed has not been thoroughly investigated.
• Frozen solid from Sept-May, Air temp winter -30
  to -40, summer 10 to 18
• Mean annual water 1.7C, open water period 9C
• Average annual precip 18cm

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Inputs

• Stream rocks with intact epilithic communities
  placed in closed, artificially-lighted chambers,
  measure dissolved oxygen concentration as index
  of primary production
• Over the 5-year study, GPP averaged 22.5
  mg/(m²h)
• 3-month growth season, production occurs 24
  hours/day and not at all during the rest of the
  year, yielding an average of 49 g/(m²y)
• Net primary prod., by subtracting epilithic
  respiration from GPP, 11.4 mg/(m²h), or 25
  g/(m²y)
• Bryophyte moss GPP 55 g/(m²y) NPP 39
  g/(m²y), however, production may be
  overestimated due to fertilizer runoff.
  Allocthonous litter was almost exclusively peat
  and tundra plant fragments. Meandering path
  erodes streambanks, the major source of
  particulate organic matter
• Hyporheic rock communities GPP 5 g/(m²y), NPP
  1 g/(m²y)
• DOM and POM values are not available

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Outputs

• Respiration of epilithic algae measured in
  darkened chambers
• Averages 24 g/(m²y) Bryophyte moss 16 g/(m²y)
• Respiration 24 hours/day during growth, near
  zero rest of year
• Respiration may also be overestimated due to
  fertilization of land
• No data for heterotrophic respiration
• Exports of DOM, POM, and CPOM gt300 µm, complicated

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Standing crops

• Four categories of BOM UFBOM, FBOM, LBOM, and
  VLBOM
• FBOM and UFBOM cylinder placed on stream bottom,
  rocks inside scrubbed, gravel stirred up to
  resuspend organic matter
• LBOM filtered with 300 µm mesh
• VLBOM hand-collected, mostly peat and woody
  debris
• Algal standing crop on stream bottom convert
  total chlorophyll content into carbon units using
  a 501 CChl weight ratio 1.1 g/m²
• Bryophyte biomass estimate percentage of stream
  bottom covered select plots, remove, dry, and
  weigh plot against estimated cover total
  estimated bryophyte biomass 20 g/m²
• Benthic invertebrates primarily insects mean
  summer 1.3 g/m²
• Arctic grayling (Thymallus arcticus) only fish
  present in Kuparuk about 75 adults/km, 2
  young/m², total standing crop 0.2 g/m²

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Chlorophyll and Bryophtye
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Insects
(Cadisfly)
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Fish
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Conclusions 1

• Dominant source of organic matter is allocthonous
  DOM
• Wet, cold soil rich in biomass, underlain by
  permafrost
• 2nd largest peaty soil organic matter, eroded
  from stream banks
• judged to be of low quality due to slow decay
  rate
• Native primary production contributes much less
  organic matter
• higher quality material for bacteria and
  consumers
• Bryophyte production may equal epilithic algal
  production
• Important moss tissues are not as easily
  decomposed as diatoms and algae
• Key unknowns in organic matter budget rates of
  biological mineralization of allocthonous DOM and
  POM.
• Especially how it affects higher trophic levels,
  such as insects and fish

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Effects of Light and Temeprature

• Concentrations during ice-free June-Sept in
  µmol/L
• Nitrate 0.10 Ammonium 0.15 total dissolved
  Phosphorous 0.15
• Soluble reactive Phosphorous lt0.03
  (undetectable)
• Thermal stratification July-August, surface water
  15C
• Kinetic experiments indicate uptake is limited by
  external DIN concentration
• 24 hour bioassay experiments show that
  N-enrichment stimulates photosynthesis as often
  as P-enrichment

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Inlet mixing
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Injection of traceable dye Rhodamine
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Toolik Lake bathymetric map
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Weather Summary
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Temperature Dependence Experiments

• Ambient water temp range 8-14C
• Data pooled, no significant relation found
  between increasing values of Topt and increasing
  values of Tamb
• DIN transport rates at Tamb were temp. limited
• NH4 transport rate far exceeded that of NO3- in
  every experiment

Optimal temp range for NO3- and NH4 16-22C,
19-23C
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Ratio of transport rates

• Values of ratio ranged from 1.8 to 5.8, but
  remained generally constant at 2-3 in all
  experiments until temp became supraoptimal.
• At that point, relative transport rates of
  NO3-declined more dramatically than those of
  NH4, leading to the sharp increase in ratio

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Light Dependence Experiments

• Typical response curves show that at all levels
  of irradiance, rates of NH4 transport were
  always greater than corresponding rates of NO3-.
• Suggested by MacIsaac and Dugdale that dark DIN
  transport is not necessarily a constant
  proportion of light transport, but rather a
  distinct process that occurs in the dark or under
  low light.
• Ignore dark DIN transport and fit
  Michaelis-Menten relationship to light
• Rate Ratedark Ratemax (I/(KLT I))

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Light Dependence Experiments

• Depressed transport was noted in clear bottles in
  experiments LT3 and LT4 perhaps photoinhibition,
  eliminated from curve so as not to affect
  calculations of Ratemax and KLT

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Chlorophyll-specific NO3- transport profile for
Toolik Lake

• Ambient NO3- levels were so low throughout that
  limitation of NO3- rate/Chl was probably never
  achieved solely by light.
• Decreased chlorophyll-specific transport rate
  with depth must be interpreted in terms of the
  combined stress of NO3- limitation and
  sub-optimal light.
• Conclude ambient concentration of DIN is most
  important, light plays secondary role in
  regulating DIN transport by phytoplankton

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Conclusions 2

• Maximum rates of DIN transport in algae are
  light-dependent from marine and temperate lake
  systems to a high-latitude lake where light is
  continuous throughout the growing season.
• NH4 is the preferred inorganic nitrogen source
  for algal nutrition to include arctic freshwater
  phytoplankton as well.
• First to document the influence of temperature on
  nitrogen-saturated rates of DIN transport in
  natural plankton population
• Results can be used to predict phytoplankton
  response to unplanned disturbances through
  nutrient addition to arctic lakes, especially as
  public access to the Alaskan arctic increases