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- Mankind’s 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

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

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

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

Heterotrophic energy sources CPOM: Coarse Particulate Organic Matter (>1 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.

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)

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 (C:N), 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/normal), 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

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.

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