Benthic ecosystems

Question 1

What are the three main types of material a benthic environment is made of?

Answer 1

1. bedrock
2. gravely sand
3. mud

Question 2

What dictates what organisms can exist in a benthic environment?

Answer 2

Sediment type

Question 3

In terms if structure how is a benthic environment discussed?

Answer 3

It is discussed in terms of surface area. They are basically a 2D structure

Question 4

Most benthic animals are infaunal. What is meant by infaunal?

Answer 4

Infauna usually construct tubes or burrows and are commonly found in deeper and sub-tidal waters. Clams, tube worms, and burrowing crabs are infaunal animals

Question 5

What are the most important characteristics in a benthic environment?

Answer 5

Supply of water, oxygen, nutrients and stability of sediment

Question 6

What sedimnet changes occur with depth?

Answer 6

Redox, suphide, iron complexes, methane and ammonia levels

Question 7

What is a redox potential?

Answer 7

A measure that reflects the balance between oxidation and reduction processes.

Question 8

What is the redox potential of the benthic surface and what does this show?

Answer 8

Benthic surface has a positive redox potential which shows oxygen is present.

Question 9

What is the redox potential with increased depth and what does this show?

Answer 9

With depth redox decreases and becomes negative showing a reduced environment.

Question 10

What are the three main colours of sediment and what does this show?

Answer 10

Yellow/brown - oxidised sediment

Black - reduced sediment

Grey - in between redox potential discontinuity zone

Question 11

What is the redox potential discontinuity? (RPD)

Answer 11

This is a region of rapid change in potential.

Question 12

What is the RPD like in sand and mud?

Answer 12

In sand RPD is very deep and wide

In mud RPD 1-2mm and abrupt

Question 13

What is found in the RPD zone?

Answer 13

Only get bacteria and meiofauna (those that contain symbiotic bacteria)

Question 14

What is the Wentworth scale?

Answer 14

This is a method for determining sediment particle size.

Phi units

Phi - -log2 particle diameter (nanometres)

Question 15

What is meant by ‘well sorted sediments’?

Answer 15

grains of similar size

Question 16

What particles bind sediments together making mud cohesive?

Answer 16

Clay particles

Question 17

Give the characteristics of well packed sand (Dilitant sand)

Answer 17

High proportion of fine grains, low permeability, dense, firm

Question 18

Give the characteristics of poorly packed sand (Thixotropic sand)

Answer 18

Few fine grains, high permeability, less dense, unstable

Question 19

What is bioturbation?

Answer 19

the disturbance of sedimentary deposits by living organisms. This can be through burrowing or re-working sediments.

Re-working of sediment increases the water content from ~40% ~ 70%

Question 20

give example species that use bioturbation.

Answer 20

E.g lug worms can increase oxygen and nutrients on sediment.

E.g Bamboo worms can change particle grading which increases organic matter.

E.g Macoma (bivalve) produces a layer of faecal pellets up to 1 cm deep. Can re-work 600 cm3 sediment per day.

E.g Nucula (bivalve) re-works sediment up to 5 times a day.

E.g Cerastoderma (bivalve) produces 100,000 tonnes material each year as faeces of pseudofaeces.

Question 21

What species disturb sediment and why?

Answer 21

So the filter feeders and others cannot co-exist.

E.g Amphipods and Lug worms Corophium only exist where Arenicola densities are low.

E.g Amphipod Pontoporeia kills juvenile Macoma (bivalve) bu constant burrowing

Question 22

How can sediment become more stable?

Answer 22

Micro-organisms (bacteria and diatoms) can make the sediment more stable by excreting mucilage (extracellular polymeric substance). This sticks particles together making the sediment more stable.

This is then enhanced as higher plants like seagrass and mangroves grow and stabalises the sediments further by roots.