chemical properties of freshwater

increased CO2 in the atm is acidifying freshwaters

freshwaters are variable but it’s less obvious compared to oceans - less so

the composition of dissolved substances in freshwater is mostly driven by some of the least soluble substances

true - NaCl in rocks being weathered - NaCl is very soluble so it comes out of the rocks easily

water has a dipole moment which leads to what other properties of H2O?

hydrogen bonding, high specific heat, and high latent heat, surface tension

where do freshwaters matter

in the global carbon cycle - wasn’t considered important for a while though

behavior of freshwater systems

is totally coupled to the physical-chemical properties of water - in terms of atm and vegetation, bog or peatland have different compositions than groundwater wetland in southwestern Mn

what determined freshwater composition

determined by where water comes from: atm vs soils and rocks

ions that are not very soluble

dominate the chemistry of freshwater

freshwater on the global carbon cycle image

almost ½ Pg of dissolved C and ½ Pg of inorganic C delivered to the sea

asymmetric covalent bonds

give water a dipole moment - have partial positive on H and partial negative on O

dipole moments lead to

H-bonding and many unique properties of water

• lots of energy required or given off in phase transitions (high latent heat)

• water can hold a large amount of heat (high specific heat)

• unique aspects of surface tension and density

ice crystals

less dense than liquid water so ice floats

Density characteristics of water

water is 800x more dense than air and more viscous and pure water reaches its max density at 3.98 C

where is the warm water in a lake when it starts to thaw

at the bottom of the lake - density of water changes with temperature - warm water is at surface and becomes less dense and colder water is more dense

water temp in a lake outside of winter

temperature decreases the further down you go - thermal stratification

epilimnion

warm isothermal mixed layer

metalimnion

region where temperature changes rapidly (thermocline) - usually between freezing and liquid and occurs in the middle of the lake depth wise

hypolimnion

deep, cool, high-density water mass

dimitic lake

mixes twice a year in fall and spring

annual temperature changes in dimictic lakes (Lawrence Lake in Michigan)

• in Jan., the lake is under ice and the temperature is close to zero - gets warms the deeper you go under ice

• thermocline begins to rise up in April and mid to late may is mixed

• temp is uniform from top to bottom when lake is mixing

• the contours go from top all the way to bottom so it’s mixing and doing gas exchanges with the atm and nutrient exchanges with sediments

Polymictic lake

Clear Lake, CA but similar to Mn lakes - not very deep (less than 4 or 5 m) can mix any time during the year with wind

monomictic lakes

UK Lakes - lakes that don’t form ice so mixes only once - usually between May and September to about 20 m

importance of mixing in lakes

• enables gas exchange with atm (methane, CO2 can be release and CO2 can be absorbed)

• brings up nutrients from below and promotes production (biomass falls to bottom where it produces nutrients and mixing brings the nutrients back up)

• the depth of mixing delineates habitats for organisms/processes in freshwater

Lotic freshwater

rivers, streams, etc. - dynamic physics and chemistry

Lentic freshwater

Lakes, ponds - more stable

wetlands

bogs, fens, peatlands, swamps, etc. - shallow water, lots of plants

littoral zone

the shallow, near-shore area of a lake or pond where sunlight can reach the bottom, allowing aquatic plants (macrophytes) to grow

photic zone

the uppermost layer of a body of water (like an ocean or lake) where sunlight penetrates, allowing for photosynthesis.

Pelagic zone

encompasses the entire open water area of the ocean, excluding the seafloor and coastal areas. It's the largest habitat on Earth, divided into different zones based on depth and sunlight penetration

aphotic zone

the region where there is little or no sunlight

profundal zone

the deepest area in a lake or pond, below the limnetic zone, where sunlight no longer penetrates. - general territory

do organisms sink?

yes - inevitable - they make structures that are denser than water and are negatively buoyant

what will sink in water

density of pure water at 4 degrees C is 1000, so anything greater will sink

does algae sink

algae make fats and oils but they make things that prevent them from fully being at the surface because they can sunburn

sinking and life at low Re

stokes equation - little sings sink slowly, big things sink faster

solvent properties of water

because water has a dual charge, it is chemically attracted to ionic crystals like salt and will bring them into solution

solubility of gases in water

solubility increases as temperature decreases - less molecular activity at low temp decreases the likelihood that gases will escape to the atm

• gases dissolve in water better the colder the temps

pressure effect on solubility

solubility increases as pressure increases - think the angler fish story that came to the surface to die

• pulling a deep sea fish up to the surface fast explodes its bladder and kills it

annual oxygen dynamics - Lake Minnetonka July through August

oxygen stays at top of lake and makes its way to the bottom likely during a storm only once - no oxygen on the bottom so fish wouldn’t survive here

CO2 sources

atmosphere, soils, rocks and respiration

• concentration in atm: 410 ppm and rising

CO2 in water

atmospheric CO2 diffusion into water in described by Henry’s law (c=Kp)

• c = concentration

• K = constant

• p = partial pressure in atm (the pressure pushing it into the liquid)

as the push increases, the concentration increases

is the CO2 concentration in equilibrium with the atmosphere

not necessarily because the CO2 concentration in water is also partially controlled by equilibria with rocks and biological processes such as photosynthesis and respiration

• mixing is like shaking a soda can - releases CO2 into the atm

how do CO2 concentrations change throughout the summer

CO2 concentration at water surface is below equilibrium concentration in daytime but then photosynthesis is taking up more than can be replaced - opposite at night - CO2 levels are above equilibrium at night because there is no photosynthesis at bottom but there is respiration

CO2 concentration on surface

below saturation because there is so much photosynthesis

most freshwater with dissolved inorganic carbon

inorganic carbon in freshwater has a pH between 6 and 9 so you have more free carbon (CO2 + H2CO3-)

water is a weak acid/base

a small fraction of water molecules dissociates into protons and hydroxyl groups to form acid and base with equal activities so do not control pH

• CO2 is an acid when dissolved in water so it lowers the pH

carbonate precipitation

If CO2 is removed (as in photosynthesis or due to temperature increases), CaCO3 precipitates. If CO2 is added, it will dissolve solid CaCO3. When CO2 is above the equilibrium concentration, it is referred to as “aggressive” CO2.

generalized DIC profiles in summer

oligotrophic CO2 controlled by equilibrium with the atmosphere but eutrophic lakes may not be

seasonal dynamics

note drawdown of free CO2 in summer and buildup in the hypolimnion

cations in water and water hardness

water harness is caused by the presence of multivalent ions especially C2+ and Mg2+

• hardness is expressed by mg/L of CaCO3 and meq/L

regional water hardness

western and Northeastern US tend to have softer water - midwestern, central, and some southern areas tend to have harder water - hard water is usually linked to limestone or dolomite-rich regions which dissolves Ca2+ and Mg2+

anions - where does alkalinity come from

MeCO3 + CO2 + H2O = Me++ + 2HCO3- where Me refers to “metal
and is usually Ca2+ or Mg2+

• other ions contributing alkalinity: borate, phosphate, silicates but these aren’t typically very important because their concentrations are too low

solubility of different ions in water

Ca2+, Mg2+, and CO32- are less soluble than Na+ and Cl-

• abundance does not equal solubility - there is more Ca2+ and Mg2+ in rocks and soils, but their salts are less soluble so they precipitate out

  • Na+ and Cl- are dominant in oceans and often in rivers, while Ca2+ and Mg2+ are more regulated by precipitation reactions and biological cycling

conductivity

electrical conductivity is normalized to 25 degrees C - conductivity of pure water ca. 5 microS - conductivity increases with weatherability

climate and geology

the influence of geology and precipitation on ion dominance - freshwater have less than 0.5% ppt`

rivers and watersheds

shallow soils have short GW residence times while deep soils have long GW residence times

importance of flow paths

deep groundwater has a higher concentration of calcium than saturated soils which has slightly more than unsaturated soils (vadose zone)