Water chemistry overview 4 Oct 2001

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Water chemistry overview 4 Oct 2001

• Announcements
• Canoe trip!!
• Exam next Wednesday
• College certified drivers?
• Today's lecture
• The idea of budgets
• Factors influencing chemical composition of
  freshwater
• Some important ions

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Budgets
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Internal cycling driven by

• Seasonal changes in biological demand
• Food web dynamics
• Seasonal changes in the chemical environment
  (usually oxygenated/not oxygenated)
• Seasonal weather changes
• Availability of limiting nutrients

Internal cycling
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(Aquatic) Biogeochemistry

• Biogeochemistry is chemistry in an environmental
  context that considers the biology, geology, and
  chemical interactions of elements.
• Biogeochemistry focuses on the cycling of
  substances between living (biotic) and non-living
  (abiotic) components of ecosystems.
• (see Schlesinger, W.H. 1997 Biochemistry for more
  information about biogeochemistry)

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Salinity

• Salinity summed concentrations of the major
  cations () and anions (-) in the water (mg/liter
  or meq/liter)
• Major ions concentrations gt 1mg/liter
• Conductivity (specific conductance) surrogate
  measure of salinity, measures the ease by which
  an electrical current passes between two
  electrodes submersed in the water (microSiemens
  (?S)/cm at 25?C)
• Total dissolved solids (TDS) dry a filter of the
  water sample TDS change in weight of filter
  containing "stuff" in the water. (not accurate)

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Factors determining the occurrence of an element

• Occurrence and forms of elements in water
• Solubility
• Disassociation
• Major source pools
• Atmosphere (dry/wet deposition, etc)
• Lithosphere (rocks)
• Biosphere (humans)
• Biological demand (e.g. limiting elements)
• Liebig's Law of the Minimum

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Lithosphere contribution to major ions
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Other terms

• "Soft water" low salinity (conductivity) often
  in drainages with acidic igneous rocks (e.g.
  granite)
• "hard water" high salinity, due to large
  concentrations of carbonate (CO3), usually from
  the the drainage of calcareous deposits (e.g.,
  limestone)

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Proportional occurrence controlled by

• Types of minerals in the area
• Atmospheric inputs
• Evaporation vs precipitation

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Ion concentration (salinity) lake space

• Note simplification that works best at global
  scale
• At small spatial scales, precipitation,
  evaporation, and climate are similar, so
  differences in geology predominate

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Dominant ions

• On average
• Ca2 gt Mg2 gt Na gt K
• HCO3- gt SO42- gt Cl-
• Mostly rock-weathering
• In rich agricultural systems, NO3- can be
  important (gt 1mg/liter)
• P,N,Fe and trace elements (micronutrients) are
  biologically important (although unimportant in
  terms of total ions)

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Exceptions

• Ocean-derived spray and rain increase Na gt Mg2
• Rock-weathering sequence is less applicable to
  small drainages (local precipitation)
• Acidification leads to H gt ?Al ions gt Ca2 gt
  Mg2
• Local applications of road salt (NaCl) and
  fertilizers increase SO4-, Cl-, NO3- and decrease
  HCO3-
• Inter-year variation and climatic signals (
  changes in evaporation rates or precipitation)

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Changes in ions

• Conservative ions little change over time (Mg2,
  Na, K)
• Dynamic ions concentrations are influenced by
  organism metabolism (HCO3-, SO4-, Si, Fe, Ca2,
  P, N) gt Biological demand

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Calcium (Ca2)

• From rocks
• Critical for membrane ion exchange
• Can limit the distribution of organisms that use
  Ca2 in building exoskeletons or shells
• (e.g. crayfish uncommon at Ca2lt 2 ppm)
• Interacts with CO3- (carbonate) to form CaCO3 (
  whiting or marl) in the presence of high levels
  of photosynthesis
• (loss of CO2)

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Annual Calcium distributions
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Silica

• From rocks (igneous)
• Found in many forms
• High biological demand, primarily from diatoms
  (to build frustule), also sponges and some
  macrophytes

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Diatom (algae) frustules
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Distribution of silica (Si) under stratified
conditions
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Annual Silica
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Question of the day

• Given the above isopeth plot of calcium
  distributions in Wintergreen Lake,
• 1. plot the Ca conc. vs. depth profile for
  mid-August
• 2. relative to that Ca profile, sketch in
  plausible temperature and oxygen profiles (say
  max O2 is 8 mg/l and max temp is 25 oC)
• 3. give a plausible explanation(s) as to the
  mechanisms that increase calcium concentration in
  the water column during the winter months.