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The Aquatic Food Web and the Microbial Loop

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The Aquatic Food Web and the Microbial Loop TheThe AquaticAquatic FoodFood WebWeb andand thethe MicrobialMicrobial LoopLoop Intensive System Compound feed Removal of waste products Extensive System Compound feed Removal of waste products Aquatic Food Chain Food Chain - community of organisms formed by trophic levels - stepwise system of trophic levels I. First Trophic Level - Primary Producers II. Second Trophic Level - Primary Consumers (Herbivores) III. Third Trophic Level - Secondary Consumers (Carnivores) IV. Fourth Trophic Level - Tertiary Consumers V. …. VI. Highest trophic level - “Top”predator Biocoenoses - Compartements Littoral Pelagial euphotic Benthal Compensation depth* Profundal aphotic Sediment * Between cm and > 30 meters, dependent on season, weather, species, amount of phytoplankton, suspended particles Primary Production hv 6 CO2 + 6 H2O → C6H12O6 + 6 O2 CD* Macronutrients: N, P, S, K, Mg, Ca, Na, Cl, (Si) Trace elements: Fe, Mn, Cu, Zn, B, Si, Mo, V, Co * Between cm and > 30 m, depending on season, weather, species, turbidity (amount of phytoplankton) Photosynthesis • 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) light O H2O 2 Chloroplast CO2 (CH2O) n CO2 + 2n H2A → (CH2O)n + n H2O+ 2n A n CO2 + 2n H2O → (CH2O)n + n H2O+ n O2 light heat Light reaction (PSI and PSII) H O 2 O2 2 NADP+ 2 NADPH/H+ 3 ADP 3 ATP heat Dark reaction (Calvin cycle) carbohydrates CO2 (CH2O) „Dark“ respiration in mitochondria Trophic status = Intensity of organic photoautotrophic production Dependent on: 1. climate (light and temperature) → annual PP in lakes decreases from tropics to poles 2. cultural (enrichment from catchment and man‘s activities) 3. morphometric (size and shape) temperate ultra-oligotrophic Ptot < 5 µg/L oligotrophic Ptot 5-10 µg/L mesotrophic Ptot 10-30 µg/L eutrophic Ptot 30-100 µg/L hypereutrophic Ptot > 100 µg/L Phytoplankton (Redfield ratio) C:N:P = 106:16:1 Peripyhton (Hillebrand & Sommer 1999) C:N:P = 119:17:1 Trophic status = Intensity of organic photoautotrophic production Dependent on: 1. climate (light and temperature) 2. cultural (enrichment from catchment and man‘s activities) 3. morphometric (size and shape) temperate low → PRODUCTION → high shallow morphometric eutrophic eutrophic DEPTH deep oligotrophic morphometric oligo-mesotrophic Trophic status temperate tropical in tropical lakes with temperatures in hypolimnion > 20°C oxygen is always depleted • 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 Photosynthesis I0 radiance ‘ I0 Izeu depth PAR (photosynthetic active radiation) 380-740 nm (bacteria > 800 nm) ‘ Zeu :Izeu = 0.01 I0 Photosynthesis P rate/ volume [mg C m-3 h-1] depth oligotroph mesotroph eutroph hypertroph gross rate P radiance [µE m-2 s-1] Photosynthesis P rate/ volume [mg C m-3 h-1] zeu depth zeu zeu zeu oligotroph mesotroph eutroph hypertroph Pmax -2 -1 Ik =20-300 µE m s I gross rate P k radiance [µE m-2 s-1] limitation saturation inhibition Universal Phylogenetic Tree Macrophyta • Contribution of macrophytes to total primary prodcution depends on ratio of littoral : pelagial → high in swallow tropical lakes • Utilization of littoral production by benthic consumers mainly by DOM release and microbial decay, taken up as detritus with bacterial and fungal cells • Direct usage only by few organism groups (snails, insect larvae, beetles) Phytoplankton / Algae • unicellular or filamentous eukaryotic organisms • green, blue-green, yellow-brown due to photosynthetic pigments Micrasterias sp. Euglena sp. Pediastrum sp. Nietzschia sp. Scenedesmus sp. Thalassiosira sp. Volvox sp. Asteriolampra 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) Cyanobacteria Oscillatoria sp. Anabaena sp. • Blooms undesirable → many species produce populations toxic to humans and animals (microcystin, anatoxin) • Species of Anabaena and Oscillatoria responsible for off- flavor of fish Chemotrophy Pelagic Food Chain PP planktivorous piscivorous Primary consumers Secondary consumers herbivorous carnivorous (CH2O) + O2 → CO2+ H2O + energy Plankton Plankton: (small) organisms that float or drift in water body, i.e. viruses, bacteria, fungi, algae, protozoa, metazoa Size Organisms ~ N / ml Femtoplankton > 0.2 µm Viruses 0.1-40 * 107 Picoplankton 0.2-2 µm Bacteria 0.5-10 * 106 Nanoplankton 2-20 µm Fungi, algae, protozoa (HNF) Microplankton 20-200 µm Algae, protozoa (Rotatoria) Mesoplankton 0.2-2 mm Algae, protozoa, metazoa Makroplankton 0.2-2 cm Metazoa (e.g. Euphausia) Megaplankton > 2 cm Metazoa (e.g. jelly fish) Nekton: own movement > current Zooplankton • Animal plankton rotifers • motile, movement is overpowered by currents • herbivor, carnivor or omnivores (phyto- and zooplankton) • Freshwater: protozoa, rotifers, cladocerans, copepods • Marine: jellyfish, salps, krill copepods cladocerans 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 Primary consumers Secondary consumers Herbivorous Carnivorous animals animals (larvae of (insects, insects, turbellaria, Benthic fish snails, grass crustacea, etc. carp) → 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, insect larvae (Diptera), Gastropods (snails) The Aquatic Food Chain • 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 Higher trophic levels < Zooplankton ~ Phytoplankton (multiple times) > Turnover Total Biomass (g) 10 000 1 000 ↔ 100 10 Standing crop Food ChainandEnergyFlow 1 1 25 4 3 Phytoplankton Zooplankton Trophic Level Fish Fish Fish „Feeding Fish to Fish“ Naylor et al. 2001 Aquaculture Production Ecological pyramid of global aquaculture production in 1999 according to taxonomic group and trophic level (FAO 2001) Control Mechanisms Fish Zooplankton Top down Bottom up Algae, bacteria Nutrients Syntheses: McQueen et al (1989): low levels by bottom up, high levels by top down Persson et al. (1988): alternated, number of levels & top level by bottom up Interactions 1 2 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 Influence of Competition Fish (0.2E) Fish (0.1E) Daphnia (2E) Daphnia (1E) Copepods (1E) herbivorous Zooplankton herbivorous Zooplankton (20E) (20E) Primary Producers Primary Producers (100E) (100E) The Aquatic Food Chain „Food chains do not exist in real ecosystems“ • 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 The Aquatic Food Web 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 Remineralisation O2 CO2 O CO O CO CO2 2 2 O2 CO2 2 2 anorganic nutrients 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 The Role of Detritus 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 Degradation of Organic Matter leaching DOM POM Attachment / ectoenzymes colonization of DOM microorganisms (biofilm) Fragmentation by mechanical shredders (gammarus, DOM larvae of diptera, enhancement of POM trichoptera) surface DOM : POM 10:1 ImportantPolysaccharides Polymers Cellulose
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