Food webs in streams: Energy and matter flow

1
Food webs in streams Energy and matter flow

• Lecture Outcomes
• Name and describe a variety of stream organisms,
  their adaptations to feeding and their role in
  energy flow in streams
• Describe the various sources of energy in stream
  systems
• Compare and contrast the processing of different
  organic matter fractions (DOM, CPOM and FPOM and
  primary production in stream food webs)
• Topics for week 7
• Group 1- Mankinds utilisation of running waters
• Group 2-Adaptations of organisms to lotic
  habitats
• Group 3-River regulation/Dam construction
• Group 4- Biodiversity in running waters
• Group 5-Acidification- causes and consequences

2

• Plecoptera
• Stoneflies. About 36 species in British Isles.
  Larval stage characterised by two long tails.
  Herbivore/carnivore. Live for one two three years
• Odonata
• Dragonflies (Anisoptera) and damselflies
  (Zygoptera). About 38 species, 2 found in
  fast-flowing streams. Internal gills via anus!
  Extendible mandibles
• Ephemeroptera
• Mayflies ca. 50 species. Occupy wide range of
  habitats, but species have particular
  requirements. Three tails and feather like gills.
  Adults do not feed.
• Hemiptera (True bugs)
• Suborder Heteroptera. Pondskaters and
  waterstriders/ waterboatmen. Piercing mouthparts.
• Megaloptera
• Alderflies. 3 species. Predators
• Trichoptera
• Caddisflies. 200 sp. Most live in transportable
  cases. 45 species are caseless caddis. Construct
  silk nets to trap food or silk galleries attached
  to rocks. Free-living predators.
• Lepidoptera (moths and butterflies)
• Diptera
• Flies. about 6600 species in B. Isles. Craneflies
  (Tipulidae), mosquitos and midges (Chironomidae)
• Coleoptera (beetles)
• e.g. Gyrinidae Whirligig beetles- adults
  surface prey

3

• Organisms and food webs require energy
• Autotrophy grows on inorganic nutrients CO2
  as carbon source
• Heterotrophy requires organic nutrients
  organic carbon source
• At any one trophic level there are energy losses
  to the next trophic level due to efficiency of
  consumption, assimilation and production
• However, we can also describe the flow of energy
  between trophic levels, and different
  compartments of ecosystems
• When comparing other ecosystems to the stream
  ecosystems, we see a pronounced reliance on
  imports of organic matter to the stream
• Autochthonous- o.m. from within stream primary
  production
• Allochthonous- o.m. from outside stream system

4

• Autotrophs and primary production
• periphyton (epiphytic microbes), algae,
  bryophytes (moss) and macrophytes (flowering
  plants)
• Primary production can be limited by
• Light (diel variation, seasonal variation,
  shading by trees, turbidity)
• Flow rate (influences turbidity)
• Temperature
• Grazing
• Nutrient availability

5

• Heterotrophic energy sources
• CPOM Coarse Particulate Organic Matter (gt1 mm)
• needles and leaves (important input) death of
  stream macrophytes woody debris plant and
  animal parts
• availability of CPOM to stream is highly variable
  in time and space
• FPOM Fine Particulate Organic Matter (0.5 ?m to
  5mm)
• Decay of CPOM (important input), faeces of
  consumers, microbial uptake of DOM, flocculation
  and adsorption of dissolved organic matter,
  sloughing of algae, sloughing of organic layers,
  litter and soil, stream bank and channel.
• Dissolved Organic Matter (less than 0.5 ?m)
• This is the largest pool of organic carbon in
  running waters. About 10- 25 - identifiable
  molecules remainder comprised of general
  categories such as fulvic and humic acids of
  little biol. importance
• Groundwater (important input), leachate from
  terrestrial detritus (important input),
  throughfall, extracellular release and leachate
  from both algae and macrophytes excreted by
  consumers, and released by bacterial
  decomposition.

6

• How are these energy sources (DOM, FPOM, CPOM)
  incorporated and utilised ?
• 1. Microbial Loop
• 2. DOM food web
• 3. CPOM food web
• 4. FPOM food web
• DOM (largest pool of organic carbon)
• uptake and assimilation into microbial biomass
• abiotic process of flocculation and adsorption ?
  FPOM
• may form aggregates around bubbles ? FPOM
• e.g. waterfalls 66
• DOM (contd) Microbial Loop (plays a role in
  the incorporation of DOM into microbial biomass
  on benthic layers)
• gelatinous polysaccharide matrix secreted by
  microbes forms organic biofilm on benthic
  surfaces
• binds algae, bacteria, fungi, detrital particles,
  exudates, enzymes nad metabolic products
• can be major transformers of energy and matter
• extent of contribution to consumer food webs can
  be important (but is site-dependent)

7

• CPOM
• e.g. needles, leaves, macrophytes, twigs,
  branches, berries, dead animals etc
• most representative and researched topic ? leaves
• Breakdown rate of leaves (6weeks to 6 months)
  largely controlled by
• substrate type (CN), CPOM size, feeding
  activity, environmental factors
• breakdown rate largely controlled by above
  factors, there are three important phases in a
  sequence of events in decomposition process
• Rapid leaching
• Microbial colonisation and decomposition
• Mechanical and biological fragmentation
• Prefer leaves that have been conditioned by
  microbial colonisation (autoclave/antibiotic/norma
  l), and are more nutritious (but dependent on
  fungi)
• Mechanism of benefit
• jam on a cracker 60 vs 20 assim. efficiency
• microbial catalysis makes leaf more digestible
• But
• ingested microbial biomass- 10 that of leaf
• 70-90 of growth from leaf matrix
• probably depends on fungus, leaf and detritivore

8

• Shredders within-guild variation in feeding
• some caddis all parts of leaf
• some stonefly avoid venation ? mesophyll,
  cuticle and epidermal cells
• snails/Gammarus softer tissues
• larger crustaceans tear and engulf larger leaf
  bits
• FPOM
• e.g. large fraction (??) of decay of CPOM ?
  FPOM, production of faeces, flocculation and
  adsorption of DOM
• Relatively little is known about the fate of
  FPOM, although the qualitative pathway is known
• Production of shredder faecal material correlated
  with collector ingestion.
• E.g. caddisfly (S) ? 50 input of blackfly (FC)
• Blackfly (Simulium) can compact fine particles
  into larger faeces.

9

• Summary
• Lotic environments rely greatly on inputs of
  solid and dissolved organic matter
  (allochthanous) from the catchment.
• Organic matter subdivided into DOM, FPOM and
  CPOM, and can vary greatly by type and size (from
  dissolved nutrients through organic particles to
  dead organisms).
• There are specialised organisms and trophic
  pathways that utilise allochthanous matter as
  heterotrophic energy sources (e.g. leaf litter)
• Classification of invertebrate consumers of
  streams has been useful for description and
  analysis. River size, hydrology and vegetation
  significantly influence which pathways dominate.
  Although these functional groups are working
  conveniences, they serve as very useful general
  descriptors.
• NEXT WEEK Floods and disturbances