Detritus chains, Decomposers, Microbial loop

Wetzel 489-525, 731-783

CO2

Littorial flora, Phytoplankton, Bacteria

Heterotrophic organisms

Detritus

CO2

Allochthonous DOC, POC

Outflow DOC, POC

The organic carbon cycle

Detritus

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).

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

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

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.

Detritus food chain (Con.)

Carbohydrate

lipid

Hydrolysissimple sugars

Fatty acids

fermentation Alcohol,

CO2 , CH4

Protein Amino acids NH3 NO2+NO3

proteolysis deamination nitrification

N2

denitrification

Factors affecting decomposition:

• Quality and quantity of organic matters

• Physical parameters: temperature, stratification, basin parameter, size of particles

• Chemical parameters: oxygen level

Detritus food chain (con.)Table 17-13

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.

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

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.

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.

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 4.1

Fine particulate organic matter

620 32.7

Dissolved organic matter 1200 63.2

Subtotal

1897 100.0

Total annual budget of Carbon fluxes for Lawrence Lake, Michigan. (table 23-16) Components   g C m-2 yr-1 PercentageInputs Autochthonous Phytoplankton 43.4 19.1 Submersed macrophytes 87.9 38.8 Epiphytic algae 37.9 16.8 Algal secretion and autolysis 14.7 6.5 Littoral plant secretion 5.5 2.5 Heterotrophy 2.8 1.2 Dark CO2 fixation 7.1 3.1 Allochthonous stream and groundwater DOC 21 9.3 Stream 4.1 1.8 Shoreline litter POC 0.01 0

226.4 100Outputs Respiration Benthic respiration 117.5 54.6 Bacterial respiration of DOC 20.6 9.6 Bacterial respiration of POC 8.6 4 Algal respiration 13 6.1 Permanent sedimentation 14.8 6.9 Coprecipitation of DOC with CaCO3 2 1Outflow dissoved 35.8 16.5 particulate 2.8 1.3

215.1 100

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.

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)

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.

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

The role of bacteria

• Decomposers

• Food

• Fix nitrogen

Traditional view of food web

Phytoplankton

Fish

Zooplankton

nutrients

Microbial loop—Stone & Weisburd, 1992

Phytoplankton

Fish

nutrients

Zoop. Bacteria

Protozoa

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

Life at small size scales1. Picoplankton (less than 2 m) . In the North Atlantic, 60% of primary

production . In oligotrophic seas, 80-90% of primary

production2. 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.

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.

Control of bacterioplankton by biota (Fig 17-6)

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.

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.