(15)_Soft_sediment_intertidal

Soft Sediment and Intertidal Zones

• Overview of sandy shores and mud flats in intertidal ecosystems

Sandy Shores

Characteristics of Sandy Shores

• Exposed beaches: Most organisms buried.

• Protected sand flats: More organisms at the surface; higher diversity.

• Few seaweeds; no sessile animals.

Wave Action Effects

• Wave action influences particle size and slope:

  • Heavy wave action: Coarse sediments – found on exposed beaches.

  • Little wave action: Fine sediments – found on sand flats (Nybakken & Bertness Fig. 6.38).

Water Retention in Sandy Shores

• Coarse sand beaches: Drain well; dry quickly; host fewer organisms.

• Fine sand beaches: Retain more water (capillary action); support more organisms; sediment suitable for burrowing.

Key Characteristics of Sandy Habitats

• Sand buffers against significant temperature and salinity changes.

• Sand acts as a barrier against UV light and desiccation.

• Oxygen availability may be limited in fine-grained sands.

Organism Adaptations in Soft Intertidal Zones

• Organisms may burrow deeper in intertidal zones (up to 50 cm).

• Common adaptations include heavy shells and longer siphons for filter-feeding.

• Faster burrowing observed in organisms on exposed beaches (Levinton Fig. 16.26).

Tidal Influences on Organisms

• Swash riding: Organisms periodically leave sediment, ride waves, and reburrow for feeding and refuge from predation (Levinton Fig. 16.9).

• Example species: Emerita sp., Donax sp.

Sandy Beach and Flat Communities

Primary Producers

• Limited primary producers

  • Exposed sand beaches: Mostly benthic diatoms.

  • Protected flats: Diverse diatoms, dinoflagellates, cyanobacteria often form surface film.

Dominant Macrofauna and Meiofauna

• Dominant macrofauna: Bivalves, annelids, crustaceans; primarily suspension and detritus feeders, scavengers.

• Meiofauna: Microscopic organisms in sediment (62 – 500 µm), include ciliates, flatworms, nematodes; serve as food for macrofauna (Levinton Fig. 15.4).

Zonation in Exposed Beaches

• Zonation not clearly defined due to tidal migrations.

• Supralittoral fringe: Burrowing scavengers (air-breathing crustaceans).

• Midlittoral zone: Highly variable.

• Infralittoral fringe (surf zone): Highest aquatic diversity.

Sand Flat Community Structure

• Dominated by annelids (surface and burrowing deposit feeders).

• Includes various amphipods, bivalves as surface burrowers and suspension feeders.

• Predators include moon snails, whelks, crabs, birds, and fish.

Variability in Sand Flats

• Communities highly variable temporally and spatially due to predation and biological disturbances.

• Many infaunal species in sand flats have opportunistic life history traits.

Impact of Predation in Sand Flats

• Significant impacts from predators through direct predation and habitat disturbance.

• E.g., Moon snail (Polinices duplicatus) preys on soft-shelled clams while disturbing substrate, affecting other infauna.

Mud Flats

Characteristics of Mud Flats

• Form in protected areas, fine-grained sediments accumulate organic matter.

• Located in bays, lagoons, estuaries; more stable than sand substrates.

• Often develop anaerobic conditions due to poor water exchange.

Redox Potential Discontinuity (RPD)

• RPD: Zone of rapid chemical change.

• Below RPD: Anaerobic conditions; organic compounds decomposed by anaerobic bacteria.

• Above RPD: Aerobic decomposition; compounds oxidized.

• Chemoautotrophic bacteria inhabit the RPD, contributing to primary production (Levinton Fig. 15.5, 15.6).

Adaptations in Mud Flats

• Organisms often have permanent/semi-permanent burrows, with visible openings.

• Thinner shells are common as adaptations to anaerobic conditions.

• Organisms utilize oxygenated surface water for respiration, often possessing hemoglobin or similar pigments.

Organisms in Mud Flats

• Support various seaweeds, diatoms, and seagrasses; notable for substantial primary productivity.

• Large microbial populations include heterotrophic bacteria and chemoautotrophic bacteria in RPD crucial for nutrient cycling and food sources.

• Macrofauna similar to sand-dwelling species but generally larger and more abundant.

Trophic Structure of Mud Flats

• Most energy derived from detritus; abundant organic matter and productivity sustain food webs.

• Dominant feeding types: deposit and suspension feeders consuming primarily detritus; few herbivores as plant matter quickly becomes detritus colonized by bacteria (Levinton Fig. 15.8).

Shorebird Interactions

• Shorebirds exhibit varied migration patterns, morphology, and habitat specificity.

• Examples include Ruddy Turnstone, Marbled Godwit, Northern Phalarope.

Importance of Bird Predation

• Wintering shorebirds have a significant impact on infaunal communities.

• English mud flats: Shorebirds consume up to 90% of Hydrobia snail population and 80% of nereid annelids.

• On the East Coast of the US, 50-70% of invertebrate populations are affected.

• Recovery of invertebrate populations observed with seasonal bird migrations.