Lecture 2: Biogeochemistry in freshwaters

Question 1

What is the aquatic medium composed of?

Answer 1

Particulate matter >45nm
Dissolved matter <45nm
Water matrix

Question 2

What characterises humic substances in lakes?

Answer 2

Yellow/black colouring, high molecular weight and refractory.

Question 3

How do humic substances contribute to aquatic ecosystems?

Answer 3

Formed from the decomposition of organic terrestrial matter, they absorb a large amount of UV radiation and constitute 40-60% DOM of many aquatic ecosystems.

Question 4

What would be expected of areas where humic substances were present in great quantities?

Answer 4

Acidic, low O2, low light, unfavourable bottom strata so low productivity and diversity expected.

Question 5

What might counterbalance the negative expectations of humic ecosystems?

Answer 5

Humic substances are a major source of organic carbon for heterotrophs, so potentially buffer against anthropogenic influence.

Question 6

What are the two sources of freshwater organic carbon?

Answer 6

Allochthonous - organisms and sediments originating in another system / littoral, dissolved and particulate detrital carbon.
Autochthonous - originate in system

Question 7

Moving from DOC to biota to lakes, what two things decrease, and what increases?

Answer 7

Increasing lability, decreasing pool size and turnover time.

Question 8

What contributes to DOC at the bottom of the lake model?

Answer 8

Photochemical transformations, Exoenzymes and inorganic nutrients.

Question 9

Benthic algae, phytoplankton and macrophytes convert light into DOM by what processes?

Answer 9

Secretion and autolysis

Question 10

How is DOM converted into POM?

Answer 10

Via physical flocculation

Question 11

What takes up POM?

Answer 11

Predators

Question 12

What defines a heterotrophic system?

Answer 12

When community / bacterial respiration exceeds primary production (Consume more organic carbon than they produce via photosynthesis)

Question 13

What does sustained net heterotrophy imply?

Answer 13

Organic carbon from outside the system is being respired within it.

Question 14

Partial pressure of CO2 is nearly always greater in the surface water of lakes than the atmosphere, what does this imply?

Answer 14

Lakes are nearly always sources of CO2 to the atmosphere

Question 15

Describe the abiotic pathway for photo-degradation of DOM compared to the biotic pathway.

Answer 15

Abiotic = sunlight on DOM, which is converted straight to CO2.
Biotic = sunlight on DOM, converted to low molecular weight DOM, which via bacteria is converted to CO2

Question 16

There is a trade off between positive and negative effects of radiation in aquatic systems - what does this depend on?

Answer 16

the colour of the lake. Darker lakes = more absorption of CO2 at surface and so organic matter breaks down at surface. More DOC = brown, so UV absorbed, protecting aquatic organisms.

Question 17

What might the effect of melting permafrost be on DOC?

Answer 17

Might release large amount of DOC into the landscape, or allow greater water movement through mineral soils, therefore increasing DOC retention through sorption.

Question 18

What does DOC affect in aquatic ecosystems?

Answer 18

Microbial metabolism, light, climate, acidity and primary production, as well as thermal structure and mixing depth of lakes due to its dark colour.

Question 19

How does DOC regulate ecosystem production?

Answer 19

Its absorpitive properties impede photosynthesis.

Question 20

What internal loss processes affect DOC?

Answer 20

Mineralization, sedimentation and dilution due to precip at the surface.

Question 21

Old models of inland aquatic ecosystems assumed them to be passive in their effect on global Carbon balance. Describe new theories on this.

Answer 21

Inland waters are active, store terrestrially derived carbon in sediments and lose CO2 to the atmosphere and ocean. Act simultaneously as net sources of CO2 to the atmosphere and net carbon sinks to sediments.