Aquatic Chemistry, Redox, Dissolved Oxygen, and Carbon Cycling

Flashcards covering freshwater chemical composition, redox potential thermodynamics, oxygen dynamics, photosynthesis-respiration parameters, carbon speciation, anaerobic degradation, and nutrient cycling pathways from Chapters 12 and 13.

How is the operational distinction between dissolved and particulate materials in freshwater established?

It is an arbitrary distinction based on the ability of material to pass through a filter of a specified size, typically a $0.45\,\mu\text{m}$ or $1\,\mu\text{m}$ screen.

How are dissolved, colloidal, and gravitoidal materials distinguished from one another conceptually?

Dissolved materials are completely dissolved into solution; colloidal materials are not dissolved but do not settle out due to gravity; gravitoidal materials are not dissolved and are large enough that gravity causes them to settle to the bottom.

What are the definitions of Total Dissolved Solids (TDS), salinity, and conductivity in freshwaters?

TDS is the total mass of material dissolved in water; salinity is the amount of inorganic salts dissolved in water; conductivity is the ability of water to conduct electricity as a function of the concentration of dissolved ions.

What is the difference between chemical and mechanical weathering in freshwater systems?

Chemical weathering releases dissolved matter via dissolution, whereas mechanical weathering releases particulate matter through physical processes such as freezing, thawing, and wind or water erosion.

What is the general relationship between temperature and chemical solubility in water, and what are the major exceptions?

Solubility generally increases with increasing temperature, with major exceptions being calcium carbonate ($\text{CaCO}_3$) and calcium sulfate ($\text{CaSO}_4$).

What is the mathematical definition of pH, and how does hydrogen ion activity change per unit change in pH?

$\text{pH} = -\log_{10}[\text{H}^+]$. Each single-unit change in $\text{pH}$ corresponds to a $10$-fold change in the activity/concentration of hydrogen ions, where a smaller $\text{pH}$ indicates greater hydrogen ion activity.

What is the difference between alkalinity and water hardness?

Alkalinity is the acid-neutralizing capacity represented by the sum of all titratable bases (usually carbonate, bicarbonate, and hydroxide), whereas hardness is the sum of magnesium and calcium ions and indicates the ability of water to precipitate soap.

What is Oxidation-Reduction Potential (Redox Potential), and what do high versus low electrode readings signify?

Redox potential is the relative concentration of electrons for chemical reactions in solution, measured in $\text{mV}$. A low reading denotes a large number of transferable electrons, while a high reading denotes a small number of transferable electrons.

What determines whether a chemical compound has high potential energy in an aquatic environment?

A compound possesses high potential energy when its redox state is significantly different from its surrounding environment, such as a low-redox compound in a high-redox environment or a high-redox compound in a low-redox environment.

In this potential energy diagram, what determines whether a transformation yields energy or incurs an energy cost across oxidizing versus reducing environments?

Transformations go with potential energy and produce a net energy yield when converting a reduced compound (like ammonium) to an oxidized compound (nitrate) in an oxidizing environment, or nitrate to ammonium in a reducing environment. Reversing these directions goes against potential energy and requires a net energy cost.

Why is oxygen gas ($\text{O}_2$) considered a master variable in determining redox potential in aquatic systems?

Oxygen has a very high affinity for electrons. When $\text{O}_2$ is present, redox values are high (few available electrons), which causes reduced species like soluble ferrous iron ($\text{Fe}^{2+}$) to convert readily to ferric iron ($\text{Fe}^{3+}$) and precipitate out.

How can living organisms carry out chemical reactions that go against potential energy gradients or high activation energies?

Organisms utilize enzymes to lower the activation energy of reactions and use metabolic energy from $\text{ATP}$ to proceed against potential energy differentials.

What environmental and physical factors govern the saturation concentration of dissolved oxygen ($\text{O}_2$) in water?

Temperature (lower temperature yields higher $\text{O}_2$ concentration), atmospheric pressure (higher pressure yields higher $\text{O}_2$ concentration), and dissolved solutes (higher solute concentration yields lower $\text{O}_2$ concentration).

What is the formula defining Net Primary Production ($\text{NPP}$), and what do positive, zero, and negative $\text{NPP}$ values indicate?

$\text{NPP} = \text{GPP} - \text{Respiration}$. Positive $\text{NPP}$ indicates an environment dominated by photosynthesis, $\text{NPP} = 0$ indicates equal photosynthesis and respiration, and negative $\text{NPP}$ indicates an environment dominated by respiration.

On this Photosynthesis-Irradiance (P-I) curve, what do the parameters $\alpha$, $P_{\text{max}}$, and the compensation point represent?

$\alpha$ is the initial linear slope indicating photosynthetic efficiency at low light; $P_{\text{max}}$ is the maximum photosynthetic rate; the compensation point is the irradiance level where gross primary production exactly equals respiration.

How do water velocity and temperature each affect the rate of aquatic photosynthesis?

Every $10^\circ\text{C}$ increase approximately doubles photosynthesis up to a detrimental threshold. Increased water velocity decreases the thickness of the diffusion boundary layer, thereby increasing the diffusion rate of $\text{CO}_2$.

What is the Van't Hoff-Arrhenius equation for respiration, and what does each variable signify?

$R = e^{-E/(kT)}$, where $R$ is the rate of metabolism, $E$ is activation energy, $k$ is Boltzmann's constant, $T$ is absolute temperature in Kelvin, and $e$ is the mathematical constant approximately equal to $2.718$.

What are the four dissolved inorganic carbon forms present in freshwater that comprise total inorganic carbon ($\sum \text{CO}_2$)?

Carbon dioxide ($\text{CO}_2$), carbonic acid ($\text{H}_2\text{CO}_3$), bicarbonate ($\text{HCO}_3^-$), and carbonate ($\text{CO}_3^{2-}$).

What is the chemical equilibrium expression that describes the bicarbonate buffering system in natural waters?

$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^- \rightleftharpoons 2\text{H}^+ + \text{CO}_3^{2-}$.

According to this inorganic carbon speciation diagram, which chemical forms dominate below $\text{pH } 5$, between $\text{pH } 7$ and $9$, and above $\text{pH } 11$?

Below $\text{pH } 5$, free $\text{CO}_2$ ($\text{H}_2\text{CO}_3$) dominates; between $\text{pH } 7$ and $9$, bicarbonate ($\text{HCO}_3^-$) dominates; above $\text{pH } 11$, carbonate ($\text{CO}_3^{2-}$) dominates.

How are dissolved organic carbon (DOC), fine particulate organic matter (FPOM), and coarse particulate organic matter (CPOM) categorized by size?

DOC consists of organic particles less than $0.45\,\mu\text{m}$; FPOM ranges from $0.45\,\mu\text{m}$ to $1.0\,\text{mm}$; CPOM consists of particles greater than $1.0\,\text{mm}$.

What is Biochemical Oxygen Demand (BOD)?

BOD is the total demand for oxygen exerted by chemical and biological oxidative reactions in a water body.

How do humic compounds differ from non-humic compounds in dissolved organic carbon (DOC)?

Humic compounds are high-molecular-weight breakdown products of celluloses, tannins, and lignins that impart a brown color and resist biological degradation. Non-humic compounds are low-molecular-weight, readily degradable molecules like sugars, amino acids, urea, proteins, and lipids.

How are humic acids, fulvic acids, and humin chemically distinguished by their solubility?

Humic acids are soluble in alkaline solutions and precipitate in acids; fulvic acids remain soluble in acidic solutions; humin is insoluble and cannot be extracted by acid or base.

In the absence of oxygen, in what thermodynamic order are alternative inorganic electron acceptors utilized to oxidize organic carbon?

They are utilized in descending order of redox potential: nitrate ($\text{NO}_3^-$) reduction/denitrification first, followed by manganese ($\text{Mn}^{4+}$) reduction, iron ($\text{Fe}^{3+}$) reduction, and sulfate ($\text{SO}_4^{2-}$) reduction.

In this Winogradsky column diagram, how do the microbial groups zone themselves relative to the opposing vertical gradients of oxygen and sulfide?

Cyanobacteria and heterotrophic bacteria occupy the upper aerobic/low-sulfide zone; iron-oxidizing, purple non-sulfur, purple sulfur, and green sulfur phototrophs occupy intermediate depths; sulfate-reducing chemotrophs inhabit the bottom anaerobic/high-sulfide mud layer.

Why is microbial syntrophy required during anaerobic polymer fermentation, and why do peat bogs accumulate organic material?

No single microbial species can ferment organic polymers all the way to $\text{CO}_2$ and $\text{H}_2$, requiring syntrophic consortia to process intermediate organic acids. Accumulating organic acids lower the $\text{pH}$, which progressively slows and ultimately arrests degradation, leading to peat accumulation.

What is the difference between autotrophic methanogenesis and acetoclastic methanogenesis (disproportionation of acetate)?

Autotrophic methanogenesis produces methane by reducing $\text{CO}_2$ using hydrogen ($\text{CO}_2 + 4\text{H}_2 \rightarrow \text{CH}_4 + 2\text{H}_2\text{O}$), whereas acetoclastic methanogenesis splits acetate into methane and carbon dioxide ($\text{CH}_3\text{COOH} \rightarrow \text{CH}_4 + \text{CO}_2$).

What distinguishes methylotrophs from methanotrophs?

Methylotrophs are bacteria that metabolize simple organic compounds containing methyl groups as well as carbon monoxide, whereas methanotrophs are specialized bacteria that specifically utilize methane ($\text{CH}_4$).

In biogeochemical nutrient cycling, what is the conceptual difference between assimilation and remineralization (regeneration)?

Assimilation is the uptake and incorporation of nutrients into organic matter (which occurs across oxic and anoxic conditions), while remineralization/regeneration is the release or excretion of nutrients back into inorganic forms.