1. Detritus chains, Decomposers, Microbial loop
Wetzel ,

2. The organic carbon cycle
Allochthonous DOC, POC
Outflow DOC, POC
CO2
Littorial flora, Phytoplankton, Bacteria
Heterotrophic organisms
Detritus
CO2
Detritus

3. Detritus is all dead organic matter, distinguishable from living organic and inorganic matter.
. POM constitutes only about 10% of the total detrital organic matter. DOM is about 90% (see Figure 17-9).

4. Detritus (Con.) Nonhumic substances:
Carbohydrates, proteins, peptides, amino acids, fats, waxes, resins, pigments, and other low-molecular-weight organic substances.
Labile, easily to be utilized and degraded by microogranisms, exibit rapid flux rates; low instantaneous concentrations in water
Humic substances (80% of the organic matter):
Humic acids, fulvic acids, humin; the formation of humic substances occurs during the degradation of higher aquatic and terrestrial plant material (celluloses, hemicelluloses, and lignin) by fungi and bacteria.
Loss: photolysis, microbial decompositon, aggregation and sedimentation, and outflows

5. Sources:
. Autochthonous– dead organic carbon is synthesized and cycling within the system.
. Allochthonous– inputs of dead organic carbon from sources external to the ecosystem that enter and cycle in the system

6. Detritus food chain
Is any route by which chemical energy contained within detrital organic carbon becomes available to the biota.
Includes the cycling of detrital organic carbon, both dissolved and particulate, to the biota by direct heterotrophy.
Emphasizes the actual trophic linkage between the non-living detritus and living organisms and recognizes the metabolic activities of bacteria attached to detrital substrates as a trophic transfer.

7. Detritus food chain (Con.)
fermentation
Hydrolysis
Alcohol,
CO2 , CH4
simple sugars
Fatty acids
Carbohydrate
lipid
proteolysis
deamination
nitrification
Protein
Amino acids
NH3
NO2+NO3
denitrification N2

8. Factors affecting decomposition:
Quality and quantity of organic matters
Physical parameters: temperature, stratification, basin parameter, size of particles
Chemical parameters: oxygen level

9. Detritus food chain (con.)
Table 17-13

10. Oligotrophic lakes
1) organic inputs are small
2) organic matter is exposed during sedimentation to oxic conditions for long distances
3) degradation of sedimenting organic matter is relatively complete
4) organic sediment accumulation is slow
Eutrophic lakes
1) Massive inputs of organic matter
2) sedimentation is rapid
3) less volume of aerobic water
4) rapid accumulation of organic matter in anaerobic hypolimnia and sediments.

11. In streams or rivers Fig. 23-2, Fig. 8-8

14. In rivers
The role of animals in comparison to microflora in decomposition of POM varies widely, but is certainly small.
DOM (>90%) drives the metabolism of streams and rivers.

15. In lakes
The decomposition of carbon by microflora of the littoral and the sediments, probably dominates in most lakes of the world since a vast majority of lakes are small to very small.
In large lakes or in hypereutrophic lakes, the inputs from littoral sources are proportionately low, and animals may assume a somewhat greater importance in the degradation of POC.
Decomposition of DOC is almost completely by bacteria and fungi and by physical photolysis.

16. Annual Organic Carbon budget estimates in a 500-m segment of the Kogesawa River, Uratakao, Japan (table 23-19)
Organic matter kg C yr-1
Percent (%)
Inputs
Primary production
100
5.1
Litterfall into stream 90
4.6
Lateral movement to stream 38
1.9
Fine particulate organic matter
480
24.5
Dissolved organic matter
1170
59.8
Groundwater inflow, DOC 80
4.1
Subtotals
1958
100.0
Outputs
Community respiration 77
620
32.7
1200
63.2
Subtotal
1897

17. Total annual budget of Carbon fluxes for Lawrence Lake, Michigan
Total annual budget of Carbon fluxes for Lawrence Lake, Michigan. (table 23-16)
Components   g C m-2 yr-1 Percentage
Inputs
Autochthonous
Phytoplankton
Submersed macrophytes
Epiphytic algae
Algal secretion and autolysis
Littoral plant secretion
Heterotrophy
Dark CO2 fixation
Allochthonous
stream and groundwater DOC
Stream
Shoreline litter POC
Outputs
Respiration
Benthic respiration
Bacterial respiration of DOC
Bacterial respiration of POC
Algal respiration
Permanent sedimentation
Coprecipitation of DOC with CaCO3 2 1
Outflow
dissoved
particulate

18. Detritus food chain (conclusions)
The central pool of organic carbon is DOC
Detrital metabolism occurs principally in the benthic region (lakes and rivers) and the pelagic area during sedimentation (lakes).
Most organic detritus is metabolized by the bacteria and the fungi.
Mineralizing rate is influenced by the geomorphology of the ecosystem and the type of organic matter.
Organic matter entering rivers and lakes from the landscape is mostly in dissolved form and is chemically modified by the flora and microflora of streams and wetlands before reaching the lake.

19. Decomposers Abundance and distribution
Aquatic bacteria are typically nutrient limited the numbers and biomass of bacteria increase with increasing productivity and concentrations of inorganic and organic compounds in lakes. In eutrophic lake we usually expect a higher bacteria biomass than oligotrophic lake. Bacteria are high in epilimnion, decreases in hypolimnion and increases sharply at water and sediment interface, but decline again below the interface. (table 17-1)

20. Decomposer (con.)
Seasonal: Genereally lower during winter than during summer. This is because autochthonous and allochthonous orgaic matters are low in low temperature. Bacteria biomass generally increases with organic matter loading. A close relation often exists between seasonal changes in biomass of phytoplankton and bacteria. Often bacteria increase lag behind pulses of phytoplankton 5-10 days. The reason probably is that phytoplankton organic matter which facilitating the development of bacteria.

21. Seasonal changes in bacterial numbers and production (Fig. 17-2)

22. The role of bacteria
Decomposers
Food
Fix nitrogen

23. Traditional view of food web
Fish
Zooplankton
Phytoplankton
nutrients

24. Microbial loop—Stone & Weisburd, 1992
Fish
Protozoa
Zoop.
Bacteria
Phytoplankton
nutrients

25. The microbial loop is simply a model of the pathways of carbon and nutrient cycling through microbial components of pelagic aquatic communities. (Wetzel, pp409)

26. Life at small size scales
Picoplankton (less than 2 m)
. In the North Atlantic, 60% of primary production
. In oligotrophic seas, 80-90% of primary production
Bacteria
. Consume 20-60% of primary production
. Three major ways: faeces and sloppy feeding of ZP, release of exudates from algae, hydrolysis of organic particles from bacteria.

27. Life at small size scales (con.)
3. Virus
4. Protozoa (dominant bacterivores)
. Microflagellates and protozoan ciliates
. High intrinsic growth rates
. Active bacterivores.
. Graze on picophytoplankton
. Mineralize nutrients efficiently.

28. Control of bacterioplankton by biota (Fig 17-6)

29. Functions of Microbes
Small, high surface area to biomass ratios, permitting a more intimate contact with the environment, a greater uptake potential for nutrients and a more rapid turnover of nutrients and organic matter than larger organisms
Slow sinking rates and tend to remain in the upper waters of lakes and oceans for long periods before settling out to greater depths. So the nutrients enters the microbial loop have a greater likelihood of remaining in the photic zone longer than those incorporated directly into larger metazoans with faster sinking rates.

30. Conclutions (microbial loop)
The microbial loop is a model of pathways of carbon and nutrient cycling through microbial components of pelagic aquatic communities.
Protistan zooplankton are the most important microbial consumers and have major functions in organic carbon utilization and nutrient recycling.