Estuaries: Background and Definitions

1. Estuaries: Background and Definitions Professor Mike Elliott, Institute of Estuarine & Coastal Studies, University of Hull, University of Hull, HU6 7RX, UK; Mike.Elliott@hull.ac.uk

2. There are a number of processes that occur in estuaries but are absent from either river or marine ecosystems. Reid and Wood (1976) suggested that there are three main factors involved: • usually there are two opposing current systems which vary according to relative magnitudes of river flow and tidal regime. These water currents exert considerable effects upon sedimentation, water mixing and other physical features within an estuary. • the mixing of sea- and fresh-water produces a chemical environment, represented mainly by salinity change, unlike that of either the marine or river ecosystem. • the changing salinities in association with tidal and riverine movements necessitate physiological adjustment by all inhabitants of the estuary.

7. Estuarine Definitions: 3 important components that need to be incorporated into the definition of an estuary: coastal containment, the mixing of fresh water with sea water and the presence of an estuarine biota. Hence the following global definition: “An estuary is a semi-enclosed coastal body of water which is connected to the sea either permanently or periodically, has a salinity that is different from that of the adjacent open ocean due to freshwater inputs, and includes a characteristic biota”. This definition encompasses those estuaries that close and are therefore not tidal, as well as those that are sometimes hyperhaline. It also includes coastal systems (e.g. certain estuarine lagoons and bays) that do not have inflowing rivers but where groundwater inputs provide estuarine conditions that support an estuarine biota. (Whitfield & Elliott 2011, Treatise on Estuarine and Coastal Science)

12. Stylised examples of (a) a classical northern hemisphere macrotidal estuary with a longitudinal salinity gradient and (b-e) SWAustralian Estuaries, which typically comprise a short and narrow entrance channel, wide central basin and the lower reaches of their tributary river(s). (b) Estuary permanently open to the sea with a longitudinal salinity gradient; (c) estuary permanently open to the sea with an overall longitudinal salinity gradient but with a region of hypersalinity; (d) estuary seasonally closed to the sea by a sand bar across its mouth (dotted box)but remaining hyposaline, and (e) estuary normally closed by a sand bar at its mouth and which is markedly hypersaline. (Noted often in Australian and South African Estuaries)

13. Tidal Wave 1) energy dissipated with progress up estuary due to bed friction (thus lead to decrease in wave height); 2) energy may be reflected from narrowing sides back into centre; 3) asymmetry of tidal wave with progression up estuary (i.e. short flood, long ebb); Tidal Asymmetry Leading to: (i) differential velocities between ebb and flood tides; (ii) more sediment deposited on ebb tide than flood tide; (iii) upper part of estuarine channels become net sediment traps.

14. Estuarine Circulation Types • Highly stratified, salt-wedge estuaries, e.g. Mississippi, N. Esk. • Fjords, e.g. W. Coast sea lochs. • Partially mixed, e.g. Mersey, Forth. • Homogenous, fully mixed, e.g. Solway, Delaware. • A. Salt-wedge Estuaries (highly stratified) • small tidal range, thus very little mixing; high freshwater inflow, not too • much mixing, up and down stream migration of salt-wedge, little bed-load • transported in from the sea but some suspended load may be brought in • from the sea; much freshwater bed and suspended load; may produce • sand bars at wedge tip or delta at mouth (dominated by fluvial sands).

15. B. Partially-mixed Estuaries Large tidal range, small river input, greater mixing than A but still some vertical salinity difference; more marine sediment (bed-load and suspended load) input; coarse sediment bedload deposited at mouth but fine sediments transported inland; suspended load deposited at upstream limit of saltwater flow. C. Well-mixed (homogeneous) Estuaries Wide estuaries (> 0.5 km) in relation to depth; strong tidal currents, weak river flows; no vertical salinity gradient but possibly lateral one; Coriolis force leading to fw and sw going to their right (in N hemisphere); some lateral mixing; marine sediments brought into estuary and deposited in right bank (going inland) and vice versa for fluvial sediments.

16. Classification and Distribution of Estuarine Organisms River limnetic Head oligohaline Upper mixohaline Middle true estuarine Lower euryhalinemarine Mouth stenohalinemarine migrants

17. Regions of an Estuary • Head - f.w. enter, river currents predominate; tidal but very limited • (if any) salt penetration. TFA and start of FSI; salinity <5psu. • Sediments becoming finer downstream. • Upper reaches - FSI, mixing of f.w. and s.w. Minimal currents especially • at high tide, leading to turbidity maximum. Mud deposition, • Salinity 5 - 18 psu. • Middle reaches - currents due to tides and bottom topography. • Principally mud deposits but sandier where currents faster. • Salinity 18-25psu. • Lower reaches - faster currents due to tides. Principally sand deposits • but muddier where currents weaken. Salinity 25-30psu. • Mouth - strong tidal currents. Clean sand or rocky shores. Salinity • >30psu similar to adjacent sea.