Water Column Sampling Techniques

Question 1

Water Surface Techniques

Answer 1

Surface water samples may be collected by hand from shore or small boats or using the ship’s underway pumping system
Rarely use divers, but useful for precise depths and sediment cores
Depth profiles usually use bottles on a CTD rosette

Question 2

Water bottles

Answer 2

Rapid exchange with surrounding water
Reliable closures
Resistant to corrosion, no contamination
Easy to handle

Question 3

Bottles on a hydroline

Answer 3

Thin cable deployed from winch
Bottles attached at set intervals
Bottles closed using ‘messengers’

Question 4

Hydroline bottles pro and cons

Answer 4

Simple
No knowledge of water column structure
Line unlikely to be vertical

Question 5

Rosette bottles

Answer 5

Can be closed individually by signal from ship
Other equipment can be attached to rosette
Obtain a preview of water column
Can sample specific features
Large volumes obtained by closing more than 1 bottle

Question 6

Sample processing

Answer 6

For analysis of chemical constituents, water samples are usually filtered.
Sources of particles:
- biology
- dust
- rivers and resuspension
Particle concs may range from g/L in turbid estuaries to a few ug/L at intermediate water depths

Question 7

Filtering

Answer 7

0.2um filter usually used to separate dissolved phase from particulate phase
Ideal filter:
- high mechanical strength
- uniform particle cut off
- no contamination/adsorption

Question 8

Sample storage

Answer 8

need to prevent changes in composition of dissolved phase by biota.
For macronutrients usually freeze
- cannot store samples of ammonia

Question 9

Trace metals storage

Answer 9

Acidify to pH ~2

Question 10

DOC/DON/DOP storage

Answer 10

Typically freeze or add mercuric chloride

Question 11

In situ collection particles

Answer 11

In situ filtration:
- allows filtration of several 100L of seawater
- rapid deployment and recovery
- Biased to smaller particles
Sediment traps:
- better at collecting rarer larger particles

Question 12

Sediment traps

Answer 12

Cones or tubes
Mouth size few cm to ~1m
Can be fixed (Eulerian) or drift (langrangian)
Array of collectors that can be switched at intervals to achieve time series data
Collectors usually contain a poison to preserve samples

Question 13

Advantage of sediment traps

Answer 13

Direct flux measurements
Time series possible by rotation
Large amount of material
Allows collection of larger particles

Question 14

Limitations sediment traps

Answer 14

Over or under sample
Swimmers and potential for microbial alteration of collected material
Expensive
Poor at collecting small particles

Question 15

Advantages in situ pumps

Answer 15

Rapid deployment
Fine particles
Large mass

Question 16

Limitations In situ pumps

Answer 16

Can’t time series
difficult to estimate fluxes
Large particles may be missed