Estuaries - Benthos and the importance of salinity

Question 1

What estuarine input ‘fuels’ the estuarine system?

Answer 1

Detritus

Question 2

What organisms feed on detritus?

Answer 2

Detritus is eaten by deposit feeders and detrivores. Deposit feeders feed on the detritus suspended in the water column and that rests on the sediment bed.

Question 3

How do deposit feeders feed on detritus?

Answer 3

They have the ability to ingest sand and mud, take in all the organic matter and squirt out “cleaned grains”.

Question 4

What is the green sheen often seen on the surface of estuaries?

Answer 4

This is plankton that’s risen to the water surface.

Question 5

What are phytoplankton?

Answer 5

These are microscopic forms of algae that photosynthesise.

Question 6

Give the Latin names of 1 common copepod and 3 common mysids (zooplankton) found in UK estuaries and their average size.

Answer 6

Eurytemora affinis (1-2mm)

Neomysis integer

Schistomysis spiritus

Mesopodopsis slabberi (mysids are 1 - 1.5cm)

Question 7

Give the common and latin names of two common polychaete worms found in UK estuaries.

Answer 7

Common ragworm - Hediste diversicola

King rag - Hediste virens

Question 8

What are the general characterisitcs of estuarine polychaetes?

Answer 8

Lug worms and rag worms are very abundant but not very diverse.

Rag worms are predators but also scavengers (deposit feeders)

Question 9

Where does H. diversicola and H. virens live within the estuary and why?

Answer 9

H. diversicola lives in the upper estuary and H. virens lives in the lower estuary. Zonation occurs because the king rag has high levels of aggression to allow itself to live in the most productive parts of the estuary. This is an adaptive behaviour to increase success.

Question 10

Describe the characteristics of an estuarine lug worm and give the latin name of a common species found in UK estuaries.

Answer 10

Arenicola marina

Lug worms are unable to burrow in sediment more that 80 nanometres in diameter.

They are deposit feeders and feed by drawing water down the tail shaft and consume the organic material.

They will only live in medium to fine sands/muddy sands. not in soft mud because it’s hard to maintain their burrows.

Question 11

How is the Macoma balthica (mollusc) well adapted to estuaries which makes them so common?

Answer 11

Macoma balthica has two modes of feeding.

It has 2 siphons that draw water in across the gill and out the exhalent siphon. They’re common because they have two modes of feeding and feed on suspended particles or settled particles on the bed once the tide goes out.

Question 12

What are the adaptations of hydrobia and burrows and their size?

Answer 12

Hydrobia are 2-4mm and crawl on the surface of the estuarine bed and feed on diatoms.

They can produce a mucus to trap particles in water and feed on them.

Question 13

What technique have plankton have developed in regards to hydrobia to avoid being affected by strong currents?

Answer 13

Plankton will attach themselves to hydrobia shell.

Question 14

Give the latin name of the common shrimp and their diet.

Answer 14

Crangon crangon feed on juvenile flatfish when fully grown.

Question 15

Why is there such a high diversity of estuarine fish?

Answer 15

There is a mixture of migratory, freshwater stragglers, estuarine fish, marine stragglers and marine estuarine dependant fish.

Question 16

What are the most estuarine-dependant marine teleosts?

Answer 16

Flounder

Sprat

Herring

Bass

Poor cod

Bib

Whiting

Thin-lipped mullet

Sea snail

Question 17

Where in an estuary would you find the most diversity of fish?

Answer 17

Highest number of species are found in the more marine areas of the estuaries.

Question 18

What are the issues with the ‘Remane diagram’?

Answer 18

1. Location: Original model is based on one location, rivers entering baltic waters, not strictly an estuarine site.

The model is based on benthic microfauna only, never defined ‘brackish-water’ fauna

important to note that there is limited research into these areas prior to the 1950’s.

2. The model doesn’t examine the whole estuarine community

Macrophytes and algae dominate at the more saline reaches (25-30psu) but at higher salinities (>30psu) start to decline.

Seaweeds in the Thames is still common at lower salinities (5 species in upper estuary <5psu)

3. Brackish water is not a unique medium with its own well defined highly adapted fauna. Estuarine organisms do not exist.

As noted by (Barnes, 1989) True estuarine species are found over a range of salinities (not restricted to brackish water. Plus distribution varies from estuary to estuary.

Question 19

Describe the general trend of diversity within estuarine seagrass beds.

Answer 19

There is no classic gradual decline in species richness. Instead there are zones of ‘stability’ separated by a sharp discontinuity. Even at low salinity sites there’s still on average 40% diversity.

It’s thought that there are other factors other than salinity affecting the species, such as the stability of the sediment.

Question 20

What is meant by the term estuarine resident?

Answer 20

This means a species will breed in estuaries and some can be restricted to them.

Question 21

Draw the proposed new model for estuarine diversity along the salinity continuum, from fresh to hypersaline conditions by Whitfield.

Answer 21

See diagram

Question 22

Why is there a low diversity of benthic organisms in terms of the “stability - time hypothesis”?

Answer 22

(Sanders, 1969) did a widespread survey of benthic diversity and proposed the “stabilty - time hypothesis” This means that the more stable an environment is, the longer organisms have to adapt leading to greater diversity.

Sanders proposed that estuaries contained physically controlled communities, fluctuating and unstable conditions due to estuaries being recent developments and being ephemeral.

The hypothesis is represented as a ‘stress gradient’.

Question 23

Draw a diagram of (sanders, 1969) stability - time hypothesis)

Answer 23

see diagram

Question 24

Does salinity tolerance determine estuarine distribution?

Answer 24

It’s impossible to test the theory, but there are other factors which change in an estuary. Such as the osmotic concentration of solutes, dissolved gases and the density and viscosity.

However, Stenohaline species cannot tolerate low salinities which suggests exclusion, whereas Euryhaline species can tolerate a wide range of salinities due to the fact they can osmo-regulate. This leads to a ‘potential niche’ reduced to a ‘realised niche’.

Question 25

Draw a diagram that expresses the effects of the variables that affect distribution of benthic species in estuaries (Little, 2001).

Answer 25

see diagram