Sandy and Muddy Shores

Rocky shores are in 2D environment - inhabited by sessile and mobile epifauna

Shoft shores are 3D - inhabited by mobile epifauna and infauna = live within or on top of sediment

Porosity - volume of pore space between particles

• Small particles e.g.clay/silt - reduced porosity

Permeability - rate of percolation of water through sediment

Low porosity = low permability

Water content - related to particle size, beach profile and water table height

• Dilatant sands - when pressure applied, sand becomes dry and hard packed = difficult to burrow
• Thixotropic sands - high clay content become wetter = easily penetrated when agitated = easy to burrow in to
• Mud - do not drain = saturated with water = soft and easy to burrow in to

Heterotrophic bacteria - decompose organic material at surface where oxygen is abundant

Oxygen consumption at surface - deprives deeper layer of oxygen = anaerobic

= Less oxygen the deeper in sand you go = anaerobic bacteria dominate and and smells sulfury

Depth of oxygenated layer varies - grain size determines permeability

• Larger sand grains = deeper oxygen layer
• Exposed shores - larger gains of sand so larger oxygen layers

Burrowing animals generate respiratory current w/in burrows 

Others extend long siphons into oxygenated areas = sheltered from predation but also breathe e.g. soft-shelled clams

Redox discontinuity layer - transition layer between oxygen rich and poor layers

Coarse sands in high turbulence environments = low organic content 

Fine sands/muds = high organic content

Amount and quality - effect on sediment's oxygen concentration and biogeochemical processes

Less macroalgae - need hard substratum for holdfast

Sometimes blooms of ephemeral green algae on mudflats

Brown algae sometimes attached to pebbles

Benthic diatoms often present - biofilms on sand/mud

Spartina plants (cord-grass) - main saltmarsh plants

Deposit feeders - amphipods, lungworms

Predators - shore crabs, anenome, ragworm

Detritovores - heart urchin

Filter feeders - soft shelled clams, cockles

Grazers - hydrobia snail

Macoma balthica bivalve - changes from deposity feeder on sheltered shores to filter feeder on exposed shores

Infauna - very mobile and make deep burrows

Ratios of types of fayna vary on sediment type - microfauna common in fine sand, scarcer in mud where meiofauna and macrofauna dominate

• Nematodes
• Polychaetes - temporary - grow into adult macrofauna
• Ostracods
• Ciliates
• Gastrotrichs
• Harpacticoids

1 million species of meiofauna

Over a million individuals found per square metre

Types of fauna inversely related in terms of biomass and abundance

Exceptional fossil record of species on all continents, sometimes at high elevations

Date back to 3.5 billion years - 1st evidence of living organisms - Stromatolites (cementation of sediment grains by biofilms of cyanobacteria)

Diversity dramatically changed over history of Earth - mass extinction evens

Certain bivalves common in muddy shore environments throughout Phanerozoic

Zonation schemes related to hydrodynamics (water table and wave action) 

Distinct lack of vertical zonation up a shore

Sediments buffer physical stresses (temp fluctuations/desiccation) and organisms are mobile - burrow into intertidal region

Progressive addition of species from exposed to sheltered conditions but loss of those unable to tolerate a reducing environment

Species richness, abundance and biomass increase w/ decreasing exposure and increasing sediment stability

Vertical distribution w/in a sediment - some organisms prefer shallower oxygenated sands but because of interspecific competition = reside in deeper sands

Deposit feeders - ingest sediments and derive nutrition from extracting detritus/organic material in form of bacteria, protozoans, diatoms, fungi, meiofauna

Surface deposit feeders - feed on surface, rich in benthic microalgae and bacteria

Head-down deposit feeders - consume particles at depth and defecate at surface

Filter feeding bivalves characteristic of sandier sediments

Burrowing animals - hydro-mechanical or simple digging mechanisms

• Thixotrophic sands required - sediment is less resistant to concentrated shear force
• Not only invertebrates - eels, rays and walruses do too

Many examples of fossilised burrows (trace fossils) - created by diagenesis of burrowed sediments - sediments change into each other

Few examples of infaunal mode in Cambrian - more common in Ordovician = extensive and deep burrows

After late Permian extinction, benthic animals increasingly infaunal - consequence of increased durophagy in Mesozoic Marine Revolution

Infaunalisation large scale in Mesozoic period

• Lungworms - originate in Triassic - burrow >30cm in sand
• Bivalves diversify post late Permian

Many taxa moved from shallow waters to deep ocean and other safe places (underwater caves) over evolutionary history - direct response to late Permian extinction and diversification of durophagous predation during MMR

Presence and activity of flora/fauna modify sediment physics and chemistry by acting as:

Biostabilisers e.g. diatoms

• Increase cohesiveness
• Make sediment surface smoother
• Form protective layer over surface

Bioturbators e.g. whales, crabs, walruses

• Make sediment surface rougher
• Regrade sediment particle structure
• Reduce sediment strength
• Oxygenate sediment
• Modify geochemistry profiles
• Exclude filter feeders - cannot cope with moving sediments

Microalgae - live on top 1-2mm of sediments = biofilms

Secrete mucus (EPS) in order to migrate

EPS increases cohesiveness of sediments - reduces bed roughness

Effect greatest in spring/summer on upper shores - increased photosynthesis

High sand mason (tube worm) diversity = stabilisation - skimming flow and protecting bed from turbulence 

Presence of few tube worms = complete destabilisation - promote bed scour through wake turbulence - water gets trapped between worms

Animals changing bed roughness include hydrobia, lung worms and sand masons

Head down deposit feeders (bamboo worm) - vertically rework sediments = smaller particles excreted at top

= Sediment size distribution lower at top

Bamboo worms = conveyor-belt deposit feeders

Lower species diversity = simple food webs because:

• Subjected to heavy wave action or occur w/in sheltered bays
• Costal geomorphologists clasify beaches using slope, particle size and wave action into range of morphodynamic states ranging from reflective to dissipative
• Primary production low and dependent on imported surf-zone phytoplankton - drawn into beach in swash
• Few deposit feeders - most are filter feeders

Higher species diversity

In upper reaches of estuaries and very sheltered regions

Contribute up to 50% of SA in some estuaries

Shallow aerobic surface and deep anaerobic black smelly layer

Primary production - dominanted by benthic diatoms - macroalgae uncommon

Microbial communities more dominant 

Benthic invertebrates dominanted by grazers and deposit feeders

Climate change

Ocean acidification

Habitat alteration - beach replenishment schemes

Harvesting 

Pollution

Invasive species

Freshwater Inflow

14 main dams = no freshwater reached Gulf of California since 1960

Prior to dams - mud flat habitats and tidal range of 10m

In Colorado delter 2 clam types found = brackish Mulina clams and marine cortezi clams

• Since dams - diversity reduced and numbers of Mulina clams decreased
• Affected predators e.g. snails/crabs

Fish and porpoises dependent on freshwater inflow too

• Totaba fish size and number decreased due to dam and overfishing
• Dam blocks fish from moving upstream to lay eggs in upper Delta
• Stoped larval and jevenille Totaba from travelling to brackish water for early growth
• Now Totoaba have:
  • Slow growth rates and smaller sizes
  • Reduced population growth rate = critically endangered
• Valuable to illegal market - swim bladders
• Methods used to catch wish affected porpoises
• Solutions = pulse flows and preventing illegal fishing