Biogeochem - Chapt 7: Biogeochemical methods

Question 1

advantages and disadvantage of benthic flux chamber

Answer 1

+ solute exchange between bottom water and sediment

+ can be used on any compound for which analytical procedure available

• no depth gradient or spatial resolution

Question 2

advantages and disadvantage of porewater profiles (eg peepers, gel probes, sliced sediments)

Answer 2

+ can be used for any compound for which analytical procedure available

• depth resolution not always sufficient (eg if gradients are very steep)
• accessible volume quite small (may not be sufficient for analysis)

Question 3

advantages and disadvantages of electrodes and optodes (typically used for O2, CO2, pH and H2S)

Answer 3

+ solute exchange sediment/water, depth profiles (oxygen, DBL)

+ high spatial resolution (vertical and 2D if planar optode)

• not for all important compounds available
• not suitable where no distinctive gradient, dominated by a single process and with the counter-process being absent
  (doesn’t work where sulphate reduction occurs and where sulphide is immediately re-oxidised)

Question 4

What tool/method can be used to determine biological/biogeochemical rates

Answer 4

radioactive isotopes

Question 5

what isotopes are commonly used

Answer 5

^3H, ^14C, ^32P, ^35S, ^55Fe

Question 6

advantages and disadvantages of using isotopes

Answer 6

+ extremely sensitive
- not for all atoms available (eg no N or O isotopes)
- special lab needed, high costs
- some isotopes with very short half-life not suitale for biogeochemical analysis

Question 7

How can non-volatile compounds (eg fermentation products) be separated and what does it allow?

Answer 7

using liquid chromatographic methods

it allows the separation and collection of different fractions of dissolved compounds which can be measured separately in LSC (Liquid Scintillation Cocktail)

Question 8

how do you check the primary production levels?

Answer 8

add ^14HCO3- to water sample and incubate in situ or under in situ conditions. Filter the sample and determine the amount of isotope i the particulate (cellular) fraction (biomass) compared to the amount of isotope remaining in the carbonate fraction

Question 9

How do you determine which compounds are used by which group of bacteria?

Answer 9

You can inhibit a specific group of bacteria and check whether a certain substrate concentration increases.

Question 10

How do you tell if other organisms are potential consumers of a compound?

(hint: links to inhibition)

Answer 10

If compounds accumulate in the environment with higher than normal concentrations, other metabolic groups can be potential consumers

Question 11

What happens if sulphate reducing bacteria (SRB) are inhibited?

Answer 11

This means their substrates are accumulating as the fermenters are not inhibited so keep producing their products.

The SRB outcompete methanogens normally for H2, but when inhibited the methanogens activity increases and methane production starts to increase

Question 12

advantages and disadvantages of using sediment slurries

Answer 12

+ large volumes allows subsampling at different time points so changes over time can be followed

• 3-D structure of sediment is distrubed
• surface sediment slurries generally include different redox layers (steep gradients cant be separated)

Question 13

advantages and disadvantages of sediment subcores

Answer 13

+ layers and gradients undisturbed

+ can be sampled at relatively high spatial resolutions

• each subcore allows only 1 sampling (no time course)
• time course experiments require a parallel number of cores