Soft Shores - Estuarine

1. Soft Shores - Estuarine MR2505 Lecture 5

2. Soft Shores - Estuarine • Estuary: defined as a partially enclosed body of water • Coastal environment in contrast to marine • Salt and freshwater environment • A range of habitats grading from riverine to marine • Sheltered from ‘sea’ waves • Tidal (ebb and flow) (currents) • Erosional and Depositional environment • Sediments: mud, sand, silt

3. Soft Shores - Estuarine • Many different examples of estuaries e.g. deep and shallow flooded river valleys, fjords • Dynamic • Temporal-spatial environment • Varying salinity/fresh water mixture

4. There are four main types of estuary: • Coastal Plain Estuaries. These are typically wide and shallow estuariesformed by the flooding of pre-existing valleys at the end of the last ice-age. • Bar-Built Estuaries. These are quite widespread around the UK, they have a sediment bar across the mouth of a partially drowned river valley. • Complex Estuaries. Quite rare in the UK, these are formed by various influences such as sea level change, erosion and geological constraints from hard rock outcrops. • Ria Estuaries. Drowned river valleys with estuarine features restricted to the upper reaches.

6. In estuarine environments sediment that becomes too heavy to be ‘transported’ will ‘settle’ and be deposited • What is an estuary? • Characteristics provide a clue to the ‘depositional’ environment • What are these characteristics? How would define an estuary? • Consider the Ythan estuary • Mudflats and sandflats with small amounts of saltmarsh • Tidal regime: flood and ebb tides • Water velocity & flow • Sediment sequence (where?) • What are the processes active? Stokes Law? • Problem is that time period over which ‘settling’ takes place is ‘too short’ • What other processes are active? • Fresh water and salt water mixes: clay particles flocculate (stick together) • Large areas of inter-tidal mud flats form • Natural coastal defence

8. Soft Shores - Estuarine • Sediment trap - sea and river • Mixing - lateral and vertical salinity differences • Tidal effects • Currents

9. Soft Shores - Estuarine • Mudflats are defined as "sedimentary intertidal habitats created by deposition in low energy coastal environments, particularly estuaries and other sheltered areas". • Sediments generally consist of silts and clays with a high organic content. • Physical processes active e.g. erosion and deposition, link mudflats and other coastal habitats such as saltmarshes and maritime cliffs. • Mudflats frequently occur as part of the natural sequence of habitats between the sub-littoral zone and vegetated saltmarshes.

10. Soft Shores - Estuarine • Where and when deposited, how, suspended, mobile • Affected by sediment, shape and salinity • Habitat structure related to tidal amplitude • Micro, meso, macro-tidal • Different types of habitat

11. Soft Shores - Estuarine • Mud, sand, salt marsh, mangrove (and hard substrata) • Rocky shores as well within estuaries although most concentration on soft sediments • Dynamic nature of environment and salinity affect fauna and flora • Observations suggest species numbers decline with distance from sea

12. Soft Shores - Estuarine • Stability of sediment affects establishment of dense or diverse communities • Some movement of sediments close to sea and associated with currents • Soft sediments prone to erosion and may be washed out • Distribution of turbidity maximum may affect density layers, lack of oxygen and anoxia

13. Soft Shores - Estuarine • Dependent upon climate • Tolerance of brackish water affects distribution of organisms in an estuary • Stenohaline and Euryhaline • Most benthic animals and plants are Euryhaline species of marine origin

14. Soft Shores - Estuarine • Affects the way estuarine animals live when it changes • Use substratum to escape when salinity changes • Challenge varies for mobile and static animals • Different sediments have different fauna

15. Soft Shores - Estuarine • Water flow, oxygen, nutrients, food and larval settlement • Biological interactions as well as physical • Food supply • Deposit feeders • Detritus supply from rivers and sea

16. Soft Shores - Estuarine • Suspended sediment may clog filtering mechanisms • Competition can be important within estuaries • Behaviour • Predators

17. Soft Shores - Estuarine • Suspended sediment may clog filtering mechanisms • Competition can be important within estuaries • Behaviour • Predators • Complex – many factors involved in the distribution of the benthos • Framework approach (physical and biological) • Historical approach – old versus new to explain distribution and ‘richness’

18. Soft Shores - Estuarine • Life in the water column • Species diversity of phytoplankton and zooplankton decreases with distance from sea • Neritic waters (coastal) have different planktonic species to ocean • Salinity is one explanation • Other factors are turbidity and light climate, benthic stage • Phytoplankton growth in estuaries varies between estuaries and their hydrographic regime e.g. mixing/stratification and distribution of nutrients

19. Soft Shores - Estuarine • Zooplankton: estuarine existence as a result of the flow of water or circulation within the estuary • Densities often low – slow life cycle but fast removal seaward • Active movement (vertical rise and sinking to take advantage of currents at different depths) and passive distribution (via drift) allows them to survive • Large populations of phytoplanton support large populations of zooplankton • Also seasonal migrational planktivorous fish (different examples around the World)

20. Soft Shores - Estuarine • Estuaries are also nurseries for fish and crustaceans • These are not planktivores but live a demersal existence feeding on the benthos • Frequent and infrequent visitors (saline water and fresh water) • Estuarine dependent/opportunist, marine stragglers,riverine, and migrant species • Proportions of each can vary dependent upon season • Fish distribution is dependent upon: physical and chemical characteristics of environment

21. Soft Shores - Estuarine • Therefore salinity tolerance! • Temperature (warm shallow, cool deep water) • Substratum • Food supply • Refuge from predators • Factors define micro-habitats

22. Soft Shores - Estuarine • Estuarine Food Webs • Simple way of viewing a complex system • Ythan Estuary (Hall and Raffaelli) produced a 92 species, 409 links food web based on 30 year research programme

23. Soft Shores - Estuarine • Sandy beaches • Few signs of life at the surface, most within the sediment • High diversity of organisms (not a high biomass) • Physical features of the sandy shore determine the species that live there

24. Soft Shores - Estuarine • Wide range of beaches: sediments and sediment size, profiles, exposure • Some evidence of ‘zonation’ (low, middle and upper shore) – tidal height • Particle size e.g. speed with which it dries out • Adaptations to life on a sandy shore • Relate to instability of sand and need for rapid response to changes

25. Soft Shores - Estuarine • Adapted to survive in the surf zone • Surfing and Burrowing • Dangerous location but very productive • Suspension feeders • Also orienteering and rhythmic behaviour • Move up and down in the sediments

26. Soft Shores - Estuarine • Biological factors • Competition/predators/scavengers • Relationships/associations between organisms (mutualism, commensalism, parasitism)

27. Soft Shores - Estuarine • Mudflats • Sheltered areas • Less mobile than sand (but also contain sand) • Often low salinity • Flatter than sand areas • Tidal currents responsible for movements

28. Soft Shores - Estuarine • Creek system – drainage • Coarse sediments in creeks • Cohesive sediments • Flocculation of clay due to salinity • Cohesiveness allows organisms to create and maintain burrows • Prevents erosion • But stable by comparison to sand

29. Soft Shores - Estuarine • Immense diversity of organisms, and high biomass (cf. sandy beaches) • Angiosperms and Algae • Epifauna (surface of sediment) • Crabs and snails • Predators and Grazers invade: fish (Mullet, Flounders, Eels), Crustaceans (crabs), birds (waders (Dunlin), crows, Shelduck, geese)

30. Soft Shores - Estuarine • Infauna: bivalve molluscs, crustaceans, polychaete worms, burrowing anemones (macro- and meiofauna e.g. nematodes, copepods) • Number of species declines from saline (marine) mudflats to those at head of estuary • Adaptations: daylight and exposure e.g. diatoms an photosynthetic flagellates migrate to surface – exhibit migratory rhythms • Seagrasses: underwater meadows (low tidal to sub-littoral) – colonize mud and sand with rhizomes – fed on by geese

31. Soft Shores - Estuarine • Seagrass offers a firm substratum, increased surface area – more diverse and abundant fauna – sheltered habitat • Mud is not permeable like sand and so oxygen is scarce • Accumulation of organic material leads to bacteria and further loss of oxygen • Also sulphides • Adaptations to acquire oxygen: water currents, ability to breathe air; detoxification

32. Soft Shores - Estuarine • Invertebrate fauna of mudflats provides a substantial food source for wading or shore - birds especially in winter and also during staging posts • Feeding on estuarine flats is essential for these bird communities

33. Soft Shores - Estuarine • Distribution of organisms on mudflats is determined by: • Particle size composition • Period of water coverage • Salinity • Drainage • Organic content of sediment • Competition, Predation • Mobility of species