Estuarine And Coastal Habitats Of Plankton

Estuary processes determine the fate of nutrients discharged from river catchments.

These processes include:

• physical dynamics (such as rainfall, water residence times and tidal flushing), catchment effects (including nutrient and sediment run-off)

• biological function (such as primary production by algae, whether they are benthic, phytoplankton or macro-algae and seagrass)

• biogeochemistry (where bacteria may shift nutrients, such as nitrogen or phosphates, from the sediment or into the air)

• factors such as secondary and tertiary production.

Traditionally, an estuary is defined in terms of the limit of penetration of oceanic salt, which moves upstream under the influence of the ocean tide. In this sense, a commonly used definition is that of Pritchard (1952), who defined an estuary as 'a semi-enclosed coastal body of water that has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage'. However, this definition does not include lakes and lagoons that are often not influenced by tides.

A broader definition would take into account the diversity and spatial variability of estuarine fauna and flora. Collett and Hutchings (1977) define estuaries as the tidal portions of river mouths, bays and coastal lagoons, irrespective of whether they are dominated by hypersaline, marine or freshwater conditions. Included in this definition are inter-tidal wetlands - where water levels can vary in response to the tidal levels of the adjacent waterway -together with perched freshwater swamps, as well as coastal lagoons that are intermittently connected to the ocean.

The tidal range undergoes a regular fortnightly cycle, increasing to a maximum over a week (spring tides) and then decreasing to a minimum over the following week (neap tides), because of the monthly orbit of the moon around the earth. Solstice tides, or king tides occur in June and December of each year, when the sun is directly over the Tropics of Cancer and Capricorn, respectively.

The characteristics of tides vary across spatial scales. For example, on the south east coast of Australia, tides are generally semi-diurnal with high and low tides occurring about twice a day. These tides have diurnal inequality where the height of two consecutive tides varies (Figure 2.8). Tides elsewhere have different characteristics: for example, many regions in Western Australia experience one tidal cycle each day (a diurnal tide).

Inside the estuary, the timing and dynamics of tidal currents become more complicated. Meanders around topography can slow tidal movement upstream, such that peak tides upstream occur hours after peak tides on the coast. The tidal limit of an estuary is the region of an estuary where there are no discernable changes to water levels as a result of tidal movement. The salinity limit is where there are no measurable changes to salinity over tidal cycles. The tidal limits and saline limits are often different, with tidal limits generally being further upstream.

Figure 2.8 Progression of the tides within a day, and over a lunar month. The upper line shows tidal fluctuation on the open coast, while the lower line shows the damped tide inside a nearby coastal lagoon. (NSW DECC.)

Flood and ebb tides have different velocities, which can result in more water moving upstream into estuaries at flood tides than leaving at low tides. This can change the flow regimes of these systems (Figure 2.8).

The shapes of estuaries can influence the behaviour of tidal movement. In some estuaries with long thin channels upstream of a wide embayment near the ocean, the change of shape can force the upstream tidal range to be greater than that downstream. Alternatively, tidal movement becomes attenuated rapidly in estuaries with thin channels connecting them to the ocean, but which have wide reaches upstream. Influencing the depth or width of estuaries through dredging activities or by seawall construction can affect their hydrology.

Run-off from the land can vertically stratify the estuary, with less dense, brackish, turbid water on top and denser, salty, clear, oceanic water beneath. This salty layer is sometimes termed 'the salt wedge' and can penetrate many kilometres upstream, along the bottom (see Section 2.7). When there has been no recent downpour, one can place two floats in the estuary -one with a drogue near the surface and the other with a drogue just off the bottom - and observe the surface float move downstream and the bottom one move upstream.

In the coastal ocean, the surface waters are warmed by the sun and, along with wind mixing and some fresh water, to create a surface mixed layer that may be 2 to 50 m deep. The layer may completely disappear during the winter storms, or become very shallow during hot calm days. The temperature boundary between the two layers is known as a thermocline. Other similar boundaries include haloclines (by salinity), pycnoclines (by density), or nutriclines (by nutrients). At the temperature boundary, phytoplankton find the best of light and nutrient conditions and frequently bloom - forming a sub-surface chlorophyll maximum.

Even a wind- and tidally mixed estuary is remarkably structured into different planktonic habitats. The most obvious is where the 'estuarine plume' of brown brackish water meets the clear blue ocean water. Within a matter of minutes, or metres, you could be sampling completely different water (Figure 2.9a). If you are not aware of this change, then your 'replicate' samples will be very different - making any comparisons very difficult. The estuarine plume is usually less dense by nature of lower salinity (even fractionally less), and is also identified by colour, and by being warmer in summer and cooler in winter than the ocean. An estuarine plume is usually quite shallow - less than a few metres deep (Figure 2.9b) - such that in the wake of a ship cutting across the plume one can see the clear ocean water churned up from beneath.

Where the 'brown meets the blue', there is a convergence where the denser ocean water wedges underneath the estuarine plume, leaving any buoyant material from either side trapped at the surface as an oily looking line of water, mixed with flotsam. This line is known as a slick, or a 'linear oceanographic feature' (Kingsford 1990). Not only are these slicks evident near the estuary mouth on the ebb tide, they are evident on the flood tide, often as a 'V-shaped' front (Figure 2.10). This is because the ocean water is retarded by the shore line, while the ocean water in the central channel can push further upstream. Both ebb tide and flood tide fronts are favourite haunts of seagulls and pelicans.

Other convergence lines are evident behind islands and headlands (for example, Suthers et al. 2004). It is thought that pre-settlement fish and invertebrates may be concentrated in these slicks, which are often moved onto reefs or seagrass beds as the tide turns. In this oceanographic way, some areas characteristically receive more young prawns and fish than other parts of estuaries and deserve to be protected (or rehabilitated). It is important to note that tidal wakes and eddies exist for up to 6 hours of a sinusoidal varying current, while the wake of an oceanic island can last for weeks (for example, Heywood et al. 1990; Suthers et al. 2006).

Islands in shallow water (less than 40 m deep) have different oceano-graphic processes to deep oceanic islands. The wakes of shallow islands a) 18.5 p—=

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Figure 2.9 a) Example of temperature-salinity (T-S) signatures. The importance of concurrent physical data when collecting plankton is shown in this T-S diagram from within the estuary (1 km from shipping terminal) to the coastal ocean (6 km). At each station, the sampling depth is inferred from least dense (shallow, top left) to most dense (deeper, bottom right). The brackish estuarine plume is evident in the less dense water at stations 1 and 2.5 km. A distinctive estuarine plume front was visible at the surface near Station 5 km (after Kingsford and Suthers 1996). b) Vertical section plot of salinity, from the estuary (left) into the coastal sea (right), showing the surface plume of low salinity water. Arrowed stations are those used in (a) above.

Figure 2.10 Estuarine and coastal habitats: a) A landscape view of an estuarine V-front, as the flood tide is retarded along the channel edge and the saltier (denser) coastal water wedges beneath the estuarine water. b) An estuarine plume front showing the ebb tide flow of brackish (less-dense) water flowing on top of coastal water, which has a coastal flow deflecting the plume. c) A topographic front generated in the lee of a headland or island. d) A vertical section of an estuarine plume front, showing the convergence and sinking along the thermocline or halocline (dashed line) front creating a slick of buoyant material (foam, flotsam). e) Vertical stratification showing a thermocline (dashed line), an internal wave, the breakdown of stratification in shallow water and the potential for upwelling or downwelling. f) T-S signature of a water mass determined from a series of temperature and salinity measurements (line of dots). The depth or distance down-estuary are implied from the least dense (top left) to most dense (bottom right). The dominant types of plankton and water mass associated with particular T-S characteristics are indicated.

Figure 2.10 Estuarine and coastal habitats: a) A landscape view of an estuarine V-front, as the flood tide is retarded along the channel edge and the saltier (denser) coastal water wedges beneath the estuarine water. b) An estuarine plume front showing the ebb tide flow of brackish (less-dense) water flowing on top of coastal water, which has a coastal flow deflecting the plume. c) A topographic front generated in the lee of a headland or island. d) A vertical section of an estuarine plume front, showing the convergence and sinking along the thermocline or halocline (dashed line) front creating a slick of buoyant material (foam, flotsam). e) Vertical stratification showing a thermocline (dashed line), an internal wave, the breakdown of stratification in shallow water and the potential for upwelling or downwelling. f) T-S signature of a water mass determined from a series of temperature and salinity measurements (line of dots). The depth or distance down-estuary are implied from the least dense (top left) to most dense (bottom right). The dominant types of plankton and water mass associated with particular T-S characteristics are indicated.

may bring deep or benthic plankton near to the surface by eddy pumping (similar to stirring in a tea cup) (Wolanski et al. 1996), or by the tidal current scouring around the sides of an island and bringing material to the surface (Suthers et al. 2004). Whatever the mechanism, while often complex, the wakes are often obvious from the slightly turbid plumes shown in remote sensing. They can also be seen from aircraft flying above them.

On a calm sunny morning in coastal waters, one may see rows of slicks, 100-200 m apart and parallel to the shore. These are generated by internal waves, which are waves moving along the thermocline (similar to the familiar air-water waves). These waves are created by sudden tide changes or currents at particular submarine cliffs. At the leading edge of each wave is a slight downwelling, which traps any buoyant particles such as oils and, possibly, plankton.

The key to sampling a variable estuarine environment is to always record temperature and salinity with a calibrated electronic meter. Talk to fishers about the local tides and typical currents. Spend some time looking at the waterway with drift objects, such as oranges, to appreciate the individual traits and the appropriate spatial and temporal scales before making any comparisons.

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