Sandy and Muddy Shores

  • Created by: rosieevie
  • Created on: 29-05-17 18:26

The Sedimentary Environment

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
1 of 18

Oxygen and Sediment Chemistry

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

2 of 18

Organic Content in Sands

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

3 of 18

Biodiversity - Plants and Algae

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

4 of 18

Biodiversity - Macrofauna (>0.5/1mm)

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

5 of 18

Biodiversity - Meiofauna (

  • 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

6 of 18

The Fossil Record

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

7 of 18

Macrofauna Diversity/Biomass vs Particle Size Grad

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

8 of 18

Feeding Modes and Burrowing in Sediments

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
9 of 18

The Infaunal Fossil Record

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

10 of 18

How Organisms Modify Their Environment

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
11 of 18

Sediment Stabilisation by Benthic Microalgae

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

12 of 18

Sediment Stabilisation by Tube Worms

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

13 of 18

Changing Bed Roughness

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

14 of 18

Food Web Dynamics - Sandy Beaches

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
15 of 18

Food Web Dynamics - Mudflats

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

16 of 18

Anthropogenic Impacts

Climate change

Ocean acidification

Habitat alteration - beach replenishment schemes



Invasive species

Freshwater Inflow

17 of 18

Freshwater Inflow Case Study - Colorado River

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
18 of 18


No comments have yet been made

Similar Marine Science resources:

See all Marine Science resources »See all Sandy and Muddy Shores resources »