TheThe AquaticAquatic Food Web Food Web and and

thethe MicrobialMicrobial LoopLoop

Intensive System

Compound feed

Removal of waste products

Extensive System

Compound feed

Removal of waste products

Food Chain - community of organisms formed by trophic levels- stepwise system of trophic levels

I. First Trophic Level - Primary ProducersII. Second Trophic Level - Primary Consumers

(Herbivores)III. Third Trophic Level - Secondary Consumers

(Carnivores)IV. Fourth Trophic Level - Tertiary ConsumersV. ….VI. Highest trophic level - “Top”predator

Aquatic Food Chain

Pelagial

Benthal Compensation depth*

Littoral

Profundal

* Between cm and > 30 meters, dependent on season, weather, species, amount of phytoplankton, suspended particles

Biocoenoses - Compartements

Sediment

euphotic

aphotic

Primary Production

hv 6 CO2 + 6 H2O → C6H12O6 + 6 O2

Macronutrients: N, P, S, K, Mg, Ca, Na, Cl, (Si)

Trace elements: Fe, Mn, Cu, Zn, B, Si, Mo, V, Co

CD*

* Between cm and > 30 m, depending on season, weather, species, turbidity (amount of phytoplankton)

Photosynthesis

ChloroplastH2O

CO2

light

O2

(CH2O)

n CO2 + 2n H2A → (CH2O)n + n H2O+ 2n A

n CO2 + 2n H2O → (CH2O)n + n H2O+ n O2

• Conversion of CO2 to biomass depending on the availability of light

• Excluding feeding, photosynthesis is the main input of C-source and natural food for aquatic animals in aquaculture

• Primary producers: Macrophytes, algae, cyanobacteria, („purple bacteria“, green sulfur bacteria)

„Dark“ respiration in mitochondria

Light reaction (PSI and PSII)H2O O2

Dark reaction (Calvin cycle)CO2

carbohydrates(CH2O)

heat

heat

light

2 NADP+ 2 NADPH/H+ 3 ADP 3 ATP

Dependent on:1. climate (light and temperature) → annual PP in lakes decreases from

tropics to poles2. cultural (enrichment from catchment and man‘s activities)3. morphometric (size and shape)

Trophic status= Intensity of organic photoautotrophic production

temperate ultra-oligotrophic Ptot < 5 µg/Loligotrophic Ptot 5-10 µg/Lmesotrophic Ptot 10-30 µg/Leutrophic Ptot 30-100 µg/Lhypereutrophic Ptot > 100 µg/L

Phytoplankton (Redfield ratio) C:N:P = 106:16:1Peripyhton (Hillebrand & Sommer 1999) C:N:P = 119:17:1

Dependent on:1. climate (light and temperature)2. cultural (enrichment from catchment and man‘s activities)3. morphometric (size and shape)

Trophic status= Intensity of organic photoautotrophic production

temperate low → PRODUCTION → highshallow morphometric

eutrophic eutrophicDEPTHdeep oligotrophic morphometric

oligo-mesotrophic

temperate tropical

• Dimitic: circulation 2 times in spring and autum

• Warm monomictic: subtropical, 1 mixis in winter

• Oligomictic: sporadic mixis• Warm polymictic: frequent mixis due to

nocturnal cooling

Trophic status

in tropical lakes withtemperatures in hypolimnion > 20°C oxygen is alwaysdepleted

Photosynthesis

radiance

dept

h

PAR (photosynthetic active radiation) 380-740 nm (bacteria > 800 nm)

Zeu : Izeu = 0.01 I0‘

I0I0‘

Izeu

Photosynthesis

oligotroph mesotroph eutroph hypertroph

dept

h

gros

sP

rate

radiance [µE m-2 s-1]

P rate/ volume [mg C m-3 h-1]

Photosynthesis

oligotroph mesotroph eutroph hypertroph

dept

h

P rate/ volume [mg C m-3 h-1]

zeuzeu

zeu

zeu

Pmax

inhibitionsaturationlimitationradiance [µE m-2 s-1]

gros

sP

rate

Ik

Ik =20-300 µE m-2 s-1

Universal Phylogenetic Tree

Macrophyta

• Utilization of littoral production by benthic consumersmainly by DOM release and microbial decay, taken up as detritus with bacterial and fungal cells

• Direct usage only by few organism groups (snails, insectlarvae, beetles)

• Contribution of macrophytes to total primary prodcution depends on ratio of littoral : pelagial→ high in swallow tropical lakes

Phytoplankton / Algae• unicellular or filamentous eukaryotic organisms• green, blue-green, yellow-brown due to photosynthetic

pigments

Micrasterias sp.

Scenedesmus sp.Volvox sp.

Nietzschia sp.

Thalassiosira sp.Asteriolampra sp.

Euglena sp. Pediastrum sp.

Cyanobacteria• Prokaryotes, APP • Bluish pigment phycocyanin, also Chl a, red or pink forms

phycoerythrin (e.g. Red Sea: blooms of a reddish species of Oscillatoria, pink color of African flamingos from Spirulina)

• Chloroplast in plants: from symbiotic cyanobacterium, taken up by green algal ancestor of the plants in the Precambrian

• N fixation - convert N2 into organic nitrogen(cultivation of rice: floating fern Azolla distributed among rice paddies, cyanobacterium Anabaena in its leaves fixes N2 → inexpensive natural fertilizer for the rice plants)

Oscillatoria sp.

Cyanobacteria

• Blooms undesirable → many species produce populations toxic to humans and animals (microcystin, anatoxin)

• Species of Anabaena and Oscillatoria responsible for off-flavor of fish

Anabaena sp.

Chemotrophy

PP piscivorousplanktivorous

Primary consumersherbivorous

Secondary consumerscarnivorous

Pelagic Food Chain

(CH2O) + O2 → CO2+ H2O + energy

Metazoa (e.g. jelly fish)

Metazoa (e.g. Euphausia)

Algae, protozoa, metazoa

Algae, protozoa (Rotatoria)

Fungi, algae, protozoa (HNF)

Bacteria

Viruses

Organisms

0.5-10 * 1060.2-2 µmPicoplankton

> 2 cmMegaplankton

0.2-2 cmMakroplankton

0.2-2 mmMesoplankton

20-200 µmMicroplankton

2-20 µmNanoplankton

0.1-40 * 107> 0.2 µmFemtoplankton

~ N / mlSize

PlanktonPlankton: (small) organisms that float or drift in water body, i.e.

viruses, bacteria, fungi, algae, protozoa, metazoa

Nekton: own movement > current

Zooplankton

• Animal plankton• motile, movement is overpowered by currents• herbivor, carnivor or omnivores (phyto- and

zooplankton) • Freshwater: protozoa, rotifers, cladocerans,

copepods• Marine: jellyfish, salps, krill

copepodscladocerans

rotifers

Protozoa• Unicellular eukaryotes• Predators algae, bacteria, and microfungi• important food source for microinvertebrates• important ecological role in the transfer of bacterial and

algal production to successive trophic levels

Stalked ciliates

Free swimmers

Chemotrophy

Benthic Food Chain

Herbivorousanimals(larvae of insects, snails, grasscarp)

Carnivorousanimals(insects, turbellaria, crustacea, etc. Benthic fish

Primary consumers Secondary consumers

→ Interactions with pelagic food chain

Benthos

• organisms living in association with bottom sediments

• filter feeders (molluscs), consumers of detritus (shredder, tubifex), grazer

• includes oysters, clams, crabs, oligochaete worms (Tubificids),polychaete worms, small crustaceans, anemones, insectlarvae (Diptera), Gastropods (snails)

• The amount of total energy passed from one level to the next is decreased (heat loss, inefficiencies)

• The number of organisms at each successive level is reduced

• The total biomass decreases at each successive trophic level

The Aquatic Food Chain

Tota

l Bio

mas

s(g

)

Phyt

opla

nkto

n

Zoop

lank

ton

Fish

Fish

Fish

Food Chain and Energy Flow

10 000

1 000

100

10

1

Trophic Level1 2 3 4 5

Turnover ↔ Standing cropPhytoplankton > (multiple times)Zooplankton ~Higher trophic levels <

Naylor et al. 2001

„Feeding Fish to Fish“

Ecological pyramid of global aquaculture production in 1999 according to taxonomic group and trophic level (FAO 2001)

Aquaculture Production

Top down

Fish

Zooplankton

Algae, bacteria

Nutrients

Bottom up

Control Mechanisms

Syntheses:McQueen et al (1989): low levels by bottom up, high levels by top downPersson et al. (1988): alternated, number of levels & top level by bottom up

1. Daphnia suppresses protozoans, bacteria prevail as small freely dispersed rods and cocci

2. When copepods dominate, ciliates are preferentially consumed, HNF exert strong grazing pressure on planktonic bacteria, results in altered morphological and taxonomical bacterial composition with a high degree of grazing-resistance (e.g. filamentous forms and bacterial aggregates)

Interactions

1 2

Interactions

Influence of Competition

Primary Producers(100E)

herbivorous Zooplankton (20E)

Daphnia (2E)

Fish (0.2E)

Primary Producers(100E)

herbivorous Zooplankton (20E)

Daphnia (1E)

Fish (0.1E)

Copepods (1E)

The Aquatic Food Chain

• Interactions between pelagic and benthic food chain• Almost all organisms are eaten by more than one

predator• One animal - more than one level? → size selecting

filter feeders (Daphnia), diet overlap (omnivors)• Trophic level changes during ontogenesis, e.g.

planktivorous juveniles of piscivorous fish (pike, perch)

• Detritivors act on each level

„Food chains do not exist in real ecosystems“

Food Web• Interacting food chains• Defines feeding relationships among

organisms• Traces the flow of energy and the cycling

of materials (e.g. carbon)

• More “realistic” than simple food chains• More complicated

The Aquatic Food Web

Remineralisation

anorganicnutrients

CO2

PER, feces, sloppy feeding, lysis and autolysis, dead organisms

• PER (phytoplankton extracellular release) up to ~15% of fixed C

• Zooplankton/Daphnia: 18-100% of algal-C, ¼ as DOC, rest particulate feces

CO2O2

CO2O2CO2O2

CO2O2

Detritus = non-living organic matter, particulate (POM) and dissolved (DOM)

• important nutrient source for some organisms • in bottom sludge, anaerobic bacteria release low

molecular weight compounds, which bind to detritus• anaerobic decomposition is probably more desirable

in ponds because it does not consume O2 and its byproduct is not CO2

• unfortunately anaerobic decomposition is not that efficient

The Role of Detritus

Degradation of Organic Matter

POM

DOMleaching

Attachment / colonization of

microorganisms(biofilm)

Fragmentation byshredders (gammarus,

larvae of diptera, trichoptera)

ectoenzymes

mechanical

enhancement of surface

DOM

DOM : POM 10:1

DOM

POM

Cellulose - ß-1,4-linked glucose- up to 15 000 monomers- cell walls of plants, fungi

Xylan - polymers of various sugars, (ß 1-4)-linked/Hemicellulose - various branches (glucans, mannans, xylans...)

- 30 –100 monomers

Chitin - ß-1,4-linked N-Acetylglucosamine- parallel / antiparallel, linked to proteins and glucans- arthropoda, fungi, diatoms

Starch - up to 106 monomers glucose- amylose linear (α1-4), amylopectin branched (α1-4) (α1-6)- storage polymer of plants

Lignin - 18-30 % of wood- complex structure of phenylpropan derivates (C-O, C-C)- slow degradation, radicals (O2)

PolysaccharidesImportant Polymers

• Endo-(ß-1,4)-Glucosidasesinternal cleavage of (cellulose) molecules→ increase amount of free ends

• Exo-(ß-1,4)-Glucosidasessequential cleavage of oligomers from the ends of the chains→ release of cellotriose, cellobiose

• Dextrinases (ß-Glucosidases, Cellobiase)cleavage of oligomers→ release of dimers and monomers (glucose)

CellulasesDegradation of Polymers

Example: Cellulose („Cellulosome“)

Model for Detritus ProcessingAutochthonous

DOM/POMPolymers

Polynucleotides (DNA, RNA), polysaccharides, proteins, lipids, waxes,

lignins, polyphenols, humic matter

AllochthonousDOM/POM

OligomersNucleotides, sugars (DCCHO), peptides

(DCAA), lipids, polyphenols

DimersNucleotides, sugars, peptides, lipids

Monomers (UDOM)Amino acids (DFAA), sugars (DFCHO),

fatty acidsExtr

acel

lula

rrel

ease

of D

OM

by

aqua

ticor

gani

sms

Microbial biomass

Microbial loop processes

Higher food web processes

Microbialextracellularezym

aticdepolym

erizationprocesses

Microbialenzymesystems

Nucleotidases, Phosphatases, Glucosidases,

Proteases, Lipases, Ligninases,

Sulfatases, ...

Microbialcommunity

Algae, bacteria, fungi, viruses,

protozoa

After Münster 1991

control

growth

rapid uptake

endoactivity

exoactivity

Michaelis-Menten-Model

• 80-90% of DOM is polymeric• Depolymerization is rate-limiting step in nutrition of microheterotrophs

[S]

v

Km

vmax

Vmax/2 E + S ES E + Pk1

k2

k3

Bacteria• Numbers in different habitats:

Eutrophic freshwater systems1- 40 x 106

Coastal waters < 0.5 -10 x 106

Open ocean 0.01-2.5 x 106

Aquatic sediments 1-20 x 109

• In the ocean, bacteria are quite small, in sediments adsorbed onto sediment particles →difficult to quantify

• Most effective group producing extracellular enzymes

• Metabolically most diverse group:chemo-organo-heterotrophchemo-litho-autotrophphoto-litho-autotrophphoto-organo-heterotroph

Fungi spores

aquatic fungi on a dead leaf

leaf and fungi grazed by snails

• Dominate breakdown of leaves and allochthonous detritus(ligninases, polyphenoloxidases)

• Their activity increases palatability of the substrate to detritusfeeders

• Estimation: production of fungi is similar to that of bacteria, but so far only little is known about their taxonomy, biology and ecology (Bärlocher)

• Vegetative hyphae, spores

Secondary production (DOM to POM) by heterotrophic microorganisms is of great quantitative importance in most aquaculture situations

• microbes can attack organic substrates that can’t be utilized by animals

• microbes produce particulate food materials from dissolved organic material

• link between DOM and classical food chain, lead to fast turnover

• also competitors to primary producers for nutrients(esp. when limiting)

Secondary Production by Microbes

Plagioselmis prolongaBodo saliens

Heterotrophic nanoflagellates (HNF) 2-20µm• most important predators of bacteria• grazed by large flagellates and ciliates

Protozoa: HNF

Microbial Loop

Anorg. nutrients

Phototrophicpicoplankton

HNFCiliates

DOM

Algae hebivorousZooplankton Fish

Heterotrophicpicoplankton

carnivorousZooplankton

POM

grazingregeneration

origin and microbial processes

• Oxidation of OM (OC) to CO2(main consuming process of O2 in aquaculture ponds)

• Oxidation of NH4 to NO3 via NO2(also consumes large quantity of O2)

• Oxidation of reduced S-compounds (H2S, elemental S) to SO4 (low O2 demand in aquaculture)

• Conversion of CO2 to biomass by autotrophic bacteria(small amount of biomass produced in aquaculture facilities, compared to biomass by algae)

Aerobic Microbial Processes

• O2 depletion may lead to complete deoxygenation or anoxia in deeper layers of lakes or reservoirs, esp. in shallow lakes with high plant production, deoxygenation of sediment and water occurs frequently

• Can produce compounds that are toxic to cultured animals

• Reduction of NO3 and NO2 yields N2 gas or NH4, in aquaculture not welcomed due to the toxicity of NH4 and NO2, while N2 production is beneficial

• Reduction of oxidized S-compounds to H2S, toxic to most animals at even very low concentrations

• Consumption of OM without the reduction of O2, results in products which are not fully oxidized (alcohols, organic acids)

Anaerobic Microbial Processes

<CH2O>n CO2

O2 H2O

N2

NO3-

anaerobic respiration

lithotrophy

E0‘

-0.4

-0.3

-0.2

-0.1

0.0

+0.1

+0.2

+0.3

+0.4

+0.5

+0.6

+0.7

+0.8

NO2-

NH4+

NO3-

organotrophy

aerobic respiration

reduced respiratory chains⇒ less energy yield⇒ slow growth rates

Bacterial Respiration Processes

chemoorganoheterotrophy,anaerobicdenitrifying bacteriaChemolithoautotrophy,

aerobicnitrifying bacteria

NitrificationNO2

-

NO2-

NO3-

NH2-groups

NH2-groups

NH4+

NON2O

N2

N2

Denitrification

oxic

anoxic assimilation

ammonification

nitrogen fixation

assimilatorynitrate reduction

assimilationammonification

dissimilatory nitrate reduction

NH3

organic

Nitrogen Cycle

N-Fixation

Alternative Electron Acceptors

Sequence of electron acceptors in the sediment

(-0.15)- (-0.22)Methanogenesis-244CO2/CH4

Sulfur reduction-240S0/HS-

0.0-(-0.15)Sulfate reduction-218SO42-/HS-

0.3-0.1Iron reduction+150Fe3+/Fe2+

0.4-0.2Mangan reduction+390MnO2/Mn2+

Nitrate ammonification

+363NO3-/NH4

0.5-0.2Denitrification+751NO3-/N2

0.6-0.4Aerobic respiration+820O2/H2O

range E [V]E0‘ [mV]

Fermentation

• Chemo-organo-heterotroph metabolism

• Organic compounds serve as primary electron donorsand ultimate electron acceptor (disproportionation)

• ATP is produced via substrate level phosphorylationnot via membran potential and proton motive force

• Little energy yield: C only partially oxidized, ΔEhbetween substrate and product is small

→ Glucose respirated: 36 - 38 ATP

→ Glucose fermented: 2 - 4 ATP, 2 - 4 NADH

• Products: CO2, H2, ethanol, butanol, formiat, acetate, propionate, lactate, ...

Sediment

• „Solid material that settled down from a state of suspension“

• 3 major sources: detrital (erosion, catchment), biogenic, authigenic

• Complex environment (interstitial), strong gradients in chemical (Eh, pH) and microbial parameters

• Number of microorganims 3-4 magnitudes higher than in pelagial

• Sediment respiration as a measure of decomposition (BOD), related to trophic status, quantity and quality of available organic carbon and concentration of electron acceptors

• Absorption/desorption processes, crucial redox conditions at sediment-water-interface → influence overlying water

• P and NH3 may be released from sediments into the water (internalloading) → nutrient enrichment

• Bioturbation → heterogenity, deeper Eh gradient, less steep

Aqua“culture“ = manipulation of environment

Problem: direct use of resource that itself is vulnerable to water pollution (pond)

Manipulation

Polyculture • Culturing more than one species of organism in the same pond• Maximizing fish production by raising a combination of species with different

food habits, better utilization of available natural food

1 23

4

Plankton feederssilver carp (1), bighead carp (2),Nile tilapia, bluetilapia

Herbivoresgrass carp (3),rohu, guorami, Tilapia zillii

Bottom feeders (OM, clams, insects, worms, snails, bacteria): common carp (4), milkfish, Tilapia sp.

Piscivorous fish(control unwanted reproduction) catfish, snakeheads, cichlids,largemouth bass

Enhancement of Productivity

Input of organic material

• Manuilowa,1951: fish ponds of 28 ha, 6 t organic fertilizer→ numbers of zooplankton increased from 200 - 300 to

14 000 – 25 000/l

• Kusnezov,1955: Wolga-delta, bights for raising of carp and bream, by cutting reed

→ after 5-6 days fish dead

→ wind distributed reed on water surface → shading, decomposition let to oxygen depletion and fish mortality

→Take into account amount of O2needed for decay of OM

CO2 Balance

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO3

2-

pH < 6 CO2pH 7-10 HCO3

-

pH >10 CO32-

CO2 + H2O + CaCO3 ↔ Ca(HCO3)2

Study: Bangladesh

Catlaplanktonic feeder

Rohuperiphytic feeder

Kalbaush /orange-fin labeoopportunistic, bottom feeder

Experimental design• 12 ponds (mean depth 1.2 m) drained, aquatic vegetation in

embankment removed, limed, fertilized with cow dung

• different amounts of bamboo poles

• 10 days before stocking with fish

Azim et al. 2004. Aquaculture 232:441-453

Relationship between combined net fish yield and substrate density

Bangladesh, Azim et al. 2004

Mean values of water quality parameters in control, 50% substrate (S-50), 75% substrate (S-75) and 100% substrate (S-100) ponds

Comparison of yield parameters of 3 species in control, 50% substrate (S-50), 75% substrate (S-75),100% substrate (S-100) treatments

Bangladesh, Azim et al. 2004

Shading of pond led to higher numbers of zooplankton

• N-limitation in phytoplankton → increase C:N ratio of their grazers → negative effect on copepod growth and reproduction (Van Nieuwerburgh et al. 2004)

• P-limitation (Elser et al. 1998)

• Phytoplankton C:N:P 106:16:1 → C:N 6.625

• copepod Acartia sp. 48.3 ± 0.8% C, 12.4 ± 0.2% N → C:N ratio4.5 ± 0.1

• cladocerans Bosmina longispina maritima and Evadnenordmanni lower N content (9.3–10.8%) and higher C:N ratio of 5.1–5.7 (Walve and Larsson, 1999)

Study: Shading

Environmental Problems

WaterWaste and nutrient loadings (solids, N, P, chemicals, antibiotics, salinisation)Impacts on benthos and water column, on speciescomposition/diversity (tolerant species dominate), quality indices, stimulation of blooms, eutrophication, oxygen depletion

BiodiversityEscaped stocks → competition with/genetic contamination of localstocks, competition for feed and space, predators pressure on preyspecies, disease transmission→ directly or indirectly reduced biodiversity

Terrestrial environment (coastal areas)Salinisation of soils, excessive clearance of mangroves and protectivecover → degradation

Integrated agriculture-aquaculture systems use low levels of inputs→ less reliance on heavy feed and fertilizers, lower densities of farmed organisms → less chances of causing serious pollution and disease risks than intensive, feedlot-type systems

BUT: development of better domesticated breeds increases international demand → increased transfers of exotic breeds

Environmental Problems