L4 - Deposit Feeders Flashcards
- Organisms that feed on organic matter in the sediments
- Dominate macrofauna in finer-grained (muddy) sediments
- Many phyla represented – highly diverse feeding guild
- Significant ecosystem process
- Annelids (primarily polychaetes)
- Molluscs (bivalves & gastropods)
- Arthropods (crustaceans)
- Echinoderms
Food from sediment
- Plant and macroalgae detritus (coastal/shelf only)
- Benthic microalgae (coastal only)
- Organic detritus (dead phytoplankton & animals, faecal pellets)
- Bacteria
- In deeper environments quality and quantity of food available to deposit-feeders is low
- Major paradigm
- Bulk sediment properties very low in particulate organic matter (POM) - typically < a few %
- Much of the organic matter has low nutritional value
- POM C:N ratios > 20, living tissue ~ 7
- How can deposit-feeders can grow rapidly on this poor food source?
Microbial stripping
- Early hypothesis: For POM to be nutritional for deposit-feeders it must be decomposed by microbes and converted into microbial tissue
- Decomposition:
- Fragmentation increases surface area:volume ratios
- Leaching
- Microbial decay increases N content
- However in many habitats heterotrophic bacteria abundance in sediments is not high enough meet all the energy demands of many species
Microbial gardening
- A form of microbial stripping
- Deposit-feeder activities enhance microbial productivity (reduce competition, free up resources)
- N absorbed by bacteria from water is made available to deposit-feeders
Microbial gardening
• Even with microbial gardening bacteria abundance in sediments is still not that high but may be very important for specific fatty acids, amino acids & vitamins
Feeding strategies
- Process large volumes of sediment – typically one body weight day-1
- Even at low nutritional value if you consume enough you will survive
- Particle selection
- Many species will select smaller particles (higher surface area:volume ratio) which are associated with higher food quality (i.e. more bacteria) especially tentaculate feeders
- Fluidising sediment sorts particles
- Many particles are rejected for ingestion
- Modify gut retention times depending on particle quality – intracellular digestion retains nutritious particles
- Specialist enzymes to detach microbes from sediments
- Still much to learn due to identifying what exactly many deposit-feeders are eating and how to assess food quality
Ecological significance – chemical gradients
• Burrows alter the amount of oxidised surface area influencing nutrient cycles in sediment
Burrows oxygenated sediment, speeds up break down of inorganic materials
Ecological significance – chemical gradients
Intricate network system left behind by burrowing crab
oxygenates network altering nutrience cycle and creating habitats
Ecological significance – chemical gradients
Sub-surface deposit-feeders: Arenicola marina (lug worm)
(Feeds head town tail up to avoid predation from birds)
Ecological significance – sediment transport
• Alter grain size at surface, oxidise sediment at depth enhancing microbial activity
Ecological significance – sediment transport
- Benthic algae
- Primary producre
- Migrates through sediment
- Produces carbohydrates which bind algae together and creates more stable less errosive susceptible environments.
Ecological significance – sediment transport
• High densities of deposit-feeder affect sediment stability, erosion rates
Ecological significance – primary production
Cinerarium – heart urchin
- Common sub-tidal species
- Bulldozes top 5 cm of sediment
- Homogenizes sediment
- Complex feedback with bacteria & benthic algae
Ecological significance – primary production
Ecological significance – primary production
Macrofaunal abundance by feeding type in Long Island Sound (Sanders 1958)
- Trophic group amensalism – Rhodes & Young (1970)
- Reworking of sediment by deposit-feeders destabilises sediment making it stressful to suspension-feeders (clogs gills, reduces food quality)
- Modified by Woodin (1976) to include adult/larval interactions
