Lecture 2: Biogeochemistry in freshwaters Flashcards
(21 cards)
What is the aquatic medium composed of?
Particulate matter >45nm
Dissolved matter <45nm
Water matrix
What characterises humic substances in lakes?
Yellow/black colouring, high molecular weight and refractory.
How do humic substances contribute to aquatic ecosystems?
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.
What would be expected of areas where humic substances were present in great quantities?
Acidic, low O2, low light, unfavourable bottom strata so low productivity and diversity expected.
What might counterbalance the negative expectations of humic ecosystems?
Humic substances are a major source of organic carbon for heterotrophs, so potentially buffer against anthropogenic influence.
What are the two sources of freshwater organic carbon?
Allochthonous - organisms and sediments originating in another system / littoral, dissolved and particulate detrital carbon.
Autochthonous - originate in system
Moving from DOC to biota to lakes, what two things decrease, and what increases?
Increasing lability, decreasing pool size and turnover time.
What contributes to DOC at the bottom of the lake model?
Photochemical transformations, Exoenzymes and inorganic nutrients.
Benthic algae, phytoplankton and macrophytes convert light into DOM by what processes?
Secretion and autolysis
How is DOM converted into POM?
Via physical flocculation
What takes up POM?
Predators
What defines a heterotrophic system?
When community / bacterial respiration exceeds primary production (Consume more organic carbon than they produce via photosynthesis)
What does sustained net heterotrophy imply?
Organic carbon from outside the system is being respired within it.
Partial pressure of CO2 is nearly always greater in the surface water of lakes than the atmosphere, what does this imply?
Lakes are nearly always sources of CO2 to the atmosphere
Describe the abiotic pathway for photo-degradation of DOM compared to the biotic pathway.
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
There is a trade off between positive and negative effects of radiation in aquatic systems - what does this depend on?
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.
What might the effect of melting permafrost be on DOC?
Might release large amount of DOC into the landscape, or allow greater water movement through mineral soils, therefore increasing DOC retention through sorption.
What does DOC affect in aquatic ecosystems?
Microbial metabolism, light, climate, acidity and primary production, as well as thermal structure and mixing depth of lakes due to its dark colour.
How does DOC regulate ecosystem production?
Its absorpitive properties impede photosynthesis.
What internal loss processes affect DOC?
Mineralization, sedimentation and dilution due to precip at the surface.
Old models of inland aquatic ecosystems assumed them to be passive in their effect on global Carbon balance. Describe new theories on this.
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.
