Benthic SOES2017

Rachel Carson - "most stupendous snowfall the earth has ever seen"

Steady accumulation of marine sediments forming layer on the seafloor over millions of years

Why is Sediment thickness greater in the Atlantic than the Pacific?

Major rivers drain into the Atlantic, carrying more sediment

Rank these environments from lowest to highest sedimentation rates:

Deep ocean

Continental margins

Bays/deltas

Deep ocean (0.5-1.0 cm/1000 yrs)

Continental margins (10-50 cm/1000 yrs)

Bays/deltas (>500 cm/1000 yrs)

5 phases of sediment composition

Mineral (solid particles)

Water (interstitial fluid)

Vital (living organisms)

Organic (dead biological material)

Gas (e.g., methane, CO₂)

Sediment thickness

Sedimentation rate x Accumulation Time

4 Main sources of Marine Sediments

Lithogenous

Biogenous

Hydrogenous

Cosmogenous

Lithogenous

Weathered rock (e.g clay/sand)

Biogenous

biological debris (e.g shells/plankton)

Hydrogenous

Precipitated from seawater (e.g manganese nodules)

Cosmogenous

extraterrestrial (e.g meteorite dust)

Convert 0.25mm to the Krumbein pH Scale

φ = -log₂(particle size in mm) log₂(0.25) = 2φ (medium sand).

Classify a sediment that has

Mz = 4.2φ / IGSD = 1.3/ IGS = -0.4

Silt/clay / Poorly sorted / Coarse skewed

Flocculation

clay particles clump due to electrostatic forces, forming larger aggregates that settle faster.

Why are sediments considered 4D bioreactors?

They vary spatially (3D) + temporally (4D) in composition, hosting microbial/invertebrate communities that drive biogeochemical cycles

How does human activity disrupt sediment dynamics?

Damming rivers → Reduces sediment supply to coasts, causing erosion.

Bottom trawling → Resuspends sediments, smothers benthic habitats.

How do sediment layers preserve the earths history?

Strata made by sequential deposition

Coastal erosion reveals these layers

What do sediment profile images reveal about recent seafloor history?

Biogenic features (burrows, tubes) and colour gradients show biogeochemical activity in the top few cm

3 ecosystem services provided by marine sediments

Carbon sequestration

Nutrient cycling

Habitat for Benthic organisms

How do intertidal sediments change during tidal cycles?

Move between saturated (high water/gas content) and exposed (compacted, mineral-dominated) states, changing biogeochemical processes.

Why is laser diffraction preferred over sieving for particle analysis?

Faster and more precise for fine particles and automated

Sieving is standard for gravel and sand

3 modes of particle transport in water

Suspension (fine particles carried by the flow)

Saltation (bouncing along the bed)

Traction (rolling of large grains)

How does dredging affect sediment ecosystems

Resuspends toxins, destroys benthic habitats

What determines whether an area accumulates sediment or erodes

Balance of Wave energy, particle size and cohesion.

If sedimentation rate = 20 cm/1000 yrs and seafloor age = 5 Myr, calculate thickness.

20 cm × 5,000 = 1,000 meters.

How might bottom trawling skew sediment analysis?

Resuspends fines, mixes layers → obscures historical records + smothers organisms.

5 factors that determine sampling choice

Target species

Practical constraints

Study Question

Budget

Historical Practices

Why sampling must start with clear objectives

ensures equipment matches needs

Forbes (Flawed Conclusion)

No life below 420m

(inefficent dredges failed to correctly sample)

Trawls

Beam/otter designs, semi-quantitative, damage prone

Dredges

Modified teeth, species specific, high seafloor impact

When would you choose a grab over a corer?

For quantitative surface macrofauna in soft sediments

Why are box corers ideal for biogeochemistry

Preserve sediment-water interface, allow multi-sub sampling

How multi corers address pseudoreplication

Distribute individual cores among researchers to ensure independence

Nematodes

Meiofauna (63-500 µm)

Clams

Macrofauna (500 µm-3 cm)

Deep-sea amphipods

Megafauna (>3 cm)

Why use 1% Rose Bengal Stain?

Colours Organisms pink to sort easily under magnification

2 Stage sieving process for macrofauna

Coarse sieve (5mm-1cm) removes debris

Fine sieve (500µm) retains target fauna

3 Insights from Sediment Profile Imaging

Biogenic Structures

Redox Layers

In situ organism-sediment interactions

Limitations of AUV seafloor surveys

High image volume needs intensive processing.

3 elements that prevent wasted resources

Clear aims/ testable hypothesis

Method-data alignment

Appropriate replication

Why does a 15min trawl take 60mins ?

deployment recovery and deck processing

Design a Method to study abyssal nutrient cycling

Multicorer- Undisturbed vertical profiles

SPI- time series bioturbation

AUV- Landscape scale context

3 challenges unique to intertidal sampling

Tidal time windows

Mudflat equipment transport

Patchy fauna distributions

How is sampling efficiency quantified for trawls ?

Area swept = Tow distance x Net Span

How sieve sizes differ for macrofauna

Shallow: 500µm

Deep-sea: 300µm

Causes of grab sample failures

Bow wave

Hard substrates

Operator Error

Aristotle

empirical observation first

Plato

reliance on rational thought alone

18th/19th century insight

Species occur in specific environmental zones

Which bidirectional relationship did Mobius identify

Fauna influence sediment whilst being controlled by it

How did Pruvot redefine homogenous seafloor

Into distinct zones with unique sediment-fauna associations over small spatial scales

Which dichotomy did Southern (1915) introduce

MIcrolithic (infauna) vs Macrolithic (epifauna)

Petersens Quantitative grabs

like airships sampling random copenhagen streets

(unreliable for community analysis)

2 Metrics used for Petersens defining species

Constancy: Present in >50% samples

Dominance: High abundance/biomass

Petersen and Thorson 7 communities

Macoma

Tellina

Venus

Abra

Amphiura

Ophiura

Amphipod

Macoma

Shallow waters and estuaries

10-60m

Tellina

Shallow waters and exposed beaches

0-10m

Venus

Sandy bottoms in open seas

7-40m

Abra

sheltered or estuarine, mixed to muddy bottoms

5-30m

Amphiura

soft-bottomed communities

15-100m

Ophiura

soft fine muds

100-300m

Amphipod

estuarine/ brackish water

0-10m

Jones Zoogeographic refinements, (emphasized 3 environmental filters)

Temp

Salinity

Sediment Type

1st Order species

50% presence + 5% total biomass

Thorsons explanation for global community similarities

Genera with ecologically equivalent species replace each other in similar habitats

Flaw when testing petersens communities with computers

communities are only reproducible using presence/ absence data, not abundance/biomass

What requirement did Mills add to assemblage definitions

Must be separable by ecological survey

Community (Stroud 2015)

all interacting taxa

Assemblage (Stroud 2015)

community subset

Thorsons theory was

supported in warm waters

contradicted in cold waters

SPI for Benthic surveys

Only sediment profile

non invasive

limited to soft sediments

AUV for benthic surveys

Landscape scale mosaics

Needs colour correction

Hard substrates possible

How would you test Thorson's parallel communities today?

Global metabarcoding + environmental DNA across latitudinal gradients.

Why might Petersen have overemphasized Macoma balthica?

Ubiquitous species easier to sample - reflects grab limitations.

What 21st-century tools could resolve assemblage debates?

Machine learning classification of ROV imagery + autonomous sensor networks.

Why focus on sediments in benthic studies

cover most of the seafloor

host diverse communities with unique adaptions

Hard substrates

2D sessile species

High variability

Low species richness

Soft Sediments

3D mobile infauna

Stable environment

High biodiversity

Adaption types

Ecological (Phenotypic)

Evolutionary (Genotypic)

Acclimation

Short-term single stressor response

acclimatization

multi-stressor adjustment in nature

Microphytobenthos Diurnal VM

Surface in day for photosynthesis

Burrow 4mm at night to avoid grazers

Endogenous rhythm persists in constant light

Surf diatom strategy

Lose mucus coat (float to surface)

carried shoreward by currents

Regain mucus and adhere to sand

Bulia Snails

enlarged foot, surf using waves to carrion

Donax clams

wedge shaped shell, rapid burrowing to avoid desiccation

Emerita crabs

Modified legs, filter feed

Burrows in muds (cohesive)

U shaped, many openings

need active ventilation

Burrows in sands (non-cohesive)

J shaped, water penetrates pore spaces

Why mud dwellers actively ventilate burrows

small sediment particles prevent passive water exchange

diffusion isn't sufficient alone for gas exchange

4 Cyclic behaviours

Diurnal, Tidal, Lunar, Seasonal

How Macoma Balthica responds to crangon crangon shrimp

deep burrowing to avoid predation

3 species with flexible feeding strategies

Pseudopolydora kempi (deposit/suspension switch)

Hediste diversicolor (omnivory/scavenging)

Streblospio benedicti (resource tracking)

What was revolutionary about Petersens Approach?

First quantitative classification of benthic communities using dominant species

Thorsons "football team" analogy for global patterns

Ecologically equivalent species fill similar roles in similar habitats

Why might behavioral plasticity reduce fitness?

Energy reallocated to foraging/predator avoidance may limit growth/reproduction

How does Donax serra exhibit local adaptation?

Cold-region clams are rounder/flatter than warm-region wedge-shaped clams due to environmental pressures, not genetics.

Benthic- Pelagic Coupling

dynamic exchange of energy, mass and nutrients at sediment-water interface

Why is B-P coupling significant?

critical for food webs, nutrient cycling and carbon budgets

3 Stages of Classical coupling

Deposition (organic matter sinks to seafloor)

Mineralisation (decomposition within sediments)

Release (Nutrients return to water column)