Ecology of Coastal Marine Sediments

Comprehensive practice questions covering the physical, chemical, and biological aspects of coastal marine sediment ecology, including sediment properties, biogeochemical cycling, biodiversity, and human impacts.

How is the arithmetic phi ($\emptyset$) scale defined in sedimentary geology?

It is defined as the $-\log_2$ of the particle size in $mm$, so that a unit change represents a factor of two change in grain size.

According to the Wentworth scale, what is the particle diameter range for silt?

$0.0039\,mm$ to $0.0625\,mm$ (specifically between phi scales $4$ and $8$).

Define sediment porosity ($\emptyset$) and its mathematical expression.

It is a measure of the void space between grains ($V_P$) presented as a ratio of the bulk volume ($V_T$); the formula is $\emptyset = V_P / V_T$.

What is permeability ($K$) in the context of marine sediments?

An indication of how easily water can move through pore spaces, expressed in $m^2$, measured by the rate at which water passes through a core under constant pressure.

What threshold of silt/clay content typically defines the transition from non-cohesive to cohesive sediments?

A silt/clay content greater than approximately $10\%$, which results in individual grains sticking together due to electromagnetic forces.

What is the Reynolds number ($Re$) and what does it assess?

A dimensionless number used to assess whether flow is turbulent or laminar by comparing factors generating turbulence (speed and length scale) to those dampening it (viscosity).

Define bed shear stress ($\tau$) and its standard units.

The force exerted at the seabed determining particle deposition and resuspension, measured in $N\,m^{-2}$ (or $N\,m^{-2}\,s^1$ in some contexts).

What is the Diffusion Boundary Layer (DBL)?

The oxygen gradient in the near-bed water, typically < 1\,mm thick, regulated by sediment oxygen demand and Benthic Boundary Layer dynamics.

What are the common C:N ratios for phytoplankton/bacteria versus macrophyte detritus?

Phytoplankton and bacteria typically have ratios $< 10$, while macrophyte detritus typically has ratios $> 20$.

What substance do diatoms excrete to bind sediment and aid vertical migration?

Extracellular polysaccharides (EPS), a sticky mucus-like substance.

What does the Redox Potential Discontinuity (RPD) represent?

The depth in the sediment where free oxygen no longer exists and the environment transitions from aerobic to anaerobic.

According to the Hjulström curve, how does the force required to erode a cohesive bed change with grain size?

Counter-intuitively, the force required to erode a cohesive bed increases with decreasing grain size due to inter-particle electromagnetic forces.

List the sequence of terminal electron acceptors used by anaerobic bacteria beneath the RPD from highest to lowest energy yield.

Nitrate, manganese oxide, iron oxides, sulphate, and finally carbon dioxide.

Provide the chemical reaction and free energy yield ($\Delta G^\circ$) for Oxic respiration.

$CH_2O + O_2 \rightarrow CO_2 + H_2O$ with a yield of $-479\,kJ\,mol^{-1}$.

What is anaerobic ammonium oxidation (anammox)?

A pathway in the anoxic zone where nitrite and ammonium are converted directly to nitrogen gas ($N_2$).

Define the size thresholds for Meiofauna.

Animals that pass through a $0.5\,mm$ mesh sieve but are retained on a mesh larger than $0.062\,mm$ or $0.031\,mm$.

What is the difference between active and passive suspension feeders?

Passive feeders rely on ambient currents to bring particles to their appendages; active feeders (like bivalves or sponges) expend energy to generate their own water currents.

What is 'skimming flow' in benthic ecology?

A phenomenon induced by high densities of structural elements (like tube worms or seagrass) where the benthic boundary is raised to the element tops, creating a low-flow and low-turbulence zone at the sediment surface.

Define alpha, beta, and gamma diversity in a spatial context.

Alpha diversity occurs at a specific point or site; Gamma diversity is the total diversity across a region; Beta diversity is the variability or turnover between sites.

State the Arrhenius power law regarding species richness.

Species richness increases as a function of the area over which samples are collected.

What characterizes the 'Insurance Hypothesis'?

The idea that maintaining a high number of rare species ensures the maintenance of ecosystem functions and resilience if current dominant species fail due to environmental change.

What are 'tipping points' or 'regime shifts'?

Non-linear changes in ecosystem state, often from a valued to a less valued one, where removal of the original stressor does not always result in recovery due to hysteresis.

Define 'functional functional redundancy'.

A situation where multiple species share similar roles in particular ecological functions, suggesting that if one species is impacted, the function can still persist.

How much of the atmospheric $CO_2$ emitted by humans was absorbed by the ocean between the start of the industrial revolution and the mid-1990s?

Approximately $30\%$, absorbing about $1\,M$ tonnes of $CO_2$ per hour.

What is 'bioremediation' in soft-sediment habitats?

The practice of directly or indirectly enhancing organisms in an ecosystem for their specific role in contaminant breakdown or removal from reactive environments.