Читать книгу Fish and Fisheries in Estuaries - Группа авторов - Страница 54

Residence time (and its corollary the flushing rate) is a key metric that provides a measure of how long a passive particle might be retained in an estuary. An estuary is flushed both by river discharge and the tides as well as by internal density‐driven currents, wind‐driven currents, storm events and potentially Coriolis effects (Wolanski & Elliott 2015). Estimates of residence time applied to estuaries worldwide suggest that, except for the largest estuaries (e.g. Baltic Sea, Chesapeake Bay), the residence time of a well‐mixed estuary is seldom more than a week or two, and often it is a few days for small estuaries (Uncles et al. 2002). Hence, pelagic eggs and larval fishes of many species that spawn in the estuary could be swiftly exported seaward during their preflexion stage, before reaching sizes and developmental stages that would enable them to swim to settlement nurseries (Strydom & Wooldridge 2005).

Many estuaries are stratified and experience oceanic inflow along the bottom and riverine outflow to the ocean near the surface. The stratified LOICZ model (Swaney et al. 2011) can be applied to calculate residence time in these estuaries. Additionally, residence time has been calculated for many partially well‐mixed estuaries worldwide from field measurements of the water budgets, based on a salinity balance. In these estuaries, residence time increases with the mean spring tidal range and decreases with the distance from the estuary mouth to the tidal limit (Uncles et al. 2002).

Some water particles (hence also passive pelagic eggs and many preflexion stage larvae) that exit an estuary on the ebbing tide re‐enter the estuary at a later time on a rising tide (Chant et al. 2000, Strydom & Wooldridge 2005). The exposure time is defined as the time spent in the estuary and its seaward plume until a particle never re‐enters the estuary. The ratio between the number of particles re‐entering and the number of particles leaving is the return coefficient r, and it is <1. The return coefficient for passive particles can be estimated from measurements of the volume, salt and temperature fluxes across the estuary mouth (MacDonald 2006). Once exported from the estuary, a particle will only re‐enter if the coastal currents do not carry it away from the mouth. Thus, flushing of an estuary depends in part on the currents in the coastal ocean. Further, the flushing of water and eggs or larvae of fishes from an estuary is generally accelerated during river floods and oceanographic events such as upwelling, the passage of an oceanic eddy and storms (de Castro et al. 2006, Hinata 2006). A caveat, of course, is that the fish eggs and larvae truly are passive.

Stratification of estuarine waters with an upper low‐salinity or freshwater layer and a deeper saline layer may contribute to the probability of pelagic eggs or passive larvae being exported. Stratification can be a permanent feature throughout the year in some estuaries if freshwater run‐off is perennial and large (e.g. most fjords); it can be very short‐lived (i.e. a few days) if the freshwater discharge is large but short‐lived or episodic. In most estuaries, stratification changes seasonally with varying river flows (Vijith et al. 2009). Stratification nearly always diminishes the residence time of water in the surface layer. Accordingly, eggs or passive larvae residing near the surface are expected to be swiftly exported to the sea, despite density stratification and up‐estuary flow near bottom. In some cases, even estuarine‐resident larvae can be flushed from the upper reaches towards the ocean during high river flow through an estuary (Strydom et al. 2002).

The residence time of water in many estuaries is usually short (i.e. high flushing rate) compared to the pelagic larval duration (PLD) of eggs and preflexion larval stages in many fishes. Larvae are more likely to remain trapped in the estuary during all or part of their PLD in two types of estuaries, namely inverse estuaries and lagoons. In inverse estuaries in the tropics and semi‐tropics during the dry season, evaporation greatly exceeds freshwater inflow (Potter et al. 2010). A salinity maximum zone is formed and both oceanic and riverine water flow near the surface towards this zone (Wolanski 1986). There, this water downwells and spreads along the bottom both seaward and landward. The residence time in such estuaries can be several months and passive larval fishes can remain in or ingress to the estuary without a need for specific behaviours to prevent seaward export. Long tidal creeks in the tropics during the dry season also present a special case of a very long residence time of surface waters because these creeks behave as evaporation ponds, a process that is accelerated in the presence of extensive fringing saltmarshes or mangrove swamps (Wolanski & Elliott 2015). In such cases, ocean water is advected landward at the surface to replenish the water loss by evaporation. The high‐salinity water produced by evaporation downwells in the upper reaches of tidal creeks and is exported seaward along the bottom.

Another unique type of estuary that is more likely to retain fish larvae due to various entrapment mechanisms is the intermittently open–closed estuary type, i.e. lagoons, which are blocked at the mouth by a sand bar. The mouths are narrow and shallow, and flushing is often restricted to the rainy season during opening events. In lagoons, the narrow mouth measurably inhibits the propagation of the tides to the inner parts of these systems and the residence time is long, typically a few months between opening events (Whitfield et al. 2008, Newton et al. 2014, Whitfield 2019). Despite the narrow mouth and connecting channel restrictions in this type of estuary, ingress of larvae and early‐stage juveniles from the surf zone is robust and effective (Hall et al. 1987).

In other estuaries, fish larvae may remain confined in an estuary due to various entrainment mechanisms. In one example, larvae originate from eggs spawned in water that is trapped in estuarine embayments. Embayments can extend residence time in an estuary if the volume of water moving at the flood tide into the lateral embayment is small compared to the water volume in the embayment (Okubo 1973, Wolanski & Ridd 1986). One or two eddies may exist in the embayment depending on its length (Uijttewaal & Booij 2000, Valle‐Levinson & Moraga‐Opazo 2006 ). A shear layer separates the fast‐flowing tidal currents in the main body of the estuary from the eddy, or eddies, in the embayments. If the prevailing current in the estuary is small (<0.2 m s⁻¹), water in the embayment is nearly completely trapped. However, if the prevailing current is strong (>0.4 m s⁻¹), the free shear layer has energetic eddies imbedded within it (Wolanski 1994) and upwelling in the eddies, with large vertical velocities (up to 0.2 m s⁻¹).

Eddy processes within estuaries may also act to retain and locally entrap larvae. Transient eddies may form that behave as lateral embayments, existing only at certain phases of the tide. When they form on the ebbing tide, larvae are retained and are not exported seaward (King & Wolanski 1996). Eddies that may retain larvae are also shed by headlands, islands, shoals and man‐made structures along the banks of estuaries. The flow field in the lee of such obstacles is variable and related to an island wake effect (Wolanski et al. 1984, Wolanski et al. 1996). In slow flows, no eddies form. However, as flow increases an eddy or an eddy pair is generated. For swifter flows, meanders develop but the water (and eggs and larvae) remains trapped in the lateral embayment. For higher flows, typically between 0.2 and 0.4 m s⁻¹, instabilities develop, e.g. meanders and rolling vortices that enhance mixing between waters in the main body of the estuary and water inside the embayment (Uijttewaal & Booij 2000). This destabilisation process can flush larvae from the embayment, terminating their entrapment and exporting them seaward.

Another transient physical process that may retain fish larvae in a stratified estuary is the buoyant freshwater plume being pushed into fringing tidal wetlands at flood tides. There, the fish eggs and larvae are isolated from the tidal currents in the main estuary. At ebbing tides, this water, with entrained eggs and larvae, returns to the estuary forming a brackish water plume adjacent to and along the banks, which is made evident by a small‐scale oceanographic front at the edge of the plume and parallel to the banks (Wolanski 1992).

Yet another process that can reduce export of eggs and larvae is asymmetry in estuarine structure, which can take many forms. In an estuary cross‐section, the tidal currents are generally stronger in deeper than in shallow waters because of bottom friction. As a result, on flood tides, the incoming high‐salinity water travels faster in the deeper sections of the estuary where it is denser than the surrounding brackish, estuarine water on the shallower sides. This dense water sinks and spreads laterally, entraining water from near the estuary’s banks to form a mid‐channel axial convergence. Floating material is aggregated along the convergence line, and fish and crab larvae are often found sheltered below this canopy (Kruger & Strydom 2011). Such convergence lines are transient features that turn on and off with the changing tides. In other systems, a front is formed when high‐salinity coastal water entering the estuary on a flooding tide meets outflowing brackish estuarine waters. This intruding, denser, saline water sinks under the estuarine water and is another transient feature that can be used by fish larvae to promote retention. On a falling tide, the process is reversed, with axial convergence near the bottom and axial divergence near the surface.

Turbulence also may modulate estuary flushing rates. Tidal and mean flows generate eddies, and thus turbulent mixing that ranges at scales typically from tens of metres to centimetres (Pope 2003). Macroscale eddies are generated by wind on the water surface, by unstable shear flows within the water column and by bed friction over the substratum and lateral edges of the estuary. The size of these eddies is constrained by the spatial dimensions of the estuaries, i.e. their depth and width, and/or the thickness of the stratified water layers. Turbulent kinetic energy is transferred from the largest to the smallest scales in a cascade that dissipates energy. In estuarine systems with varying salinity and temperature, a fraction of that energy is used to vertically mix the water, converting the kinetic energy into potential energy. This process is most important in controlling the distribution of small organisms that serve as prey for fish in early‐life stages, e.g. zooplankton and their prey that may occur in microlayers near micro‐density interfaces.

To remain in the estuary, early‐stage larvae may rely on passive entrainment into physical features such as localised plumes and eddies, salt fronts and turbidity maxima described above to avoid being dispersed and then exported seaward by the net currents in the estuary. The entrainment process is governed by the local horizontal turbulence, which is parameterised by the horizontal diffusion coefficient K _x . In open waters, K _x increases with increasing size L of the diffusing patch (cloud), i.e. also of the size of the turbulent eddies that mix the cloud (Okubo 1971). However, in estuaries, especially deltaic systems with a complex bathymetry comprising headlands and shoals, macro‐turbulence is generated at the scale of the bathymetric variations and thus the value of K _x may be much larger (Andutta et al. 2011, Hrycik et al. 2013). Such mixing often creates readily visible streaky features at the surface. High winds also generate streakiness by creating a Langmuir circulation, which is observed as parallel streaks or lines of foam with associated floating debris. The lines arise from wind‐driven vortices at the water surface aligned with the wind that create zones of upwelling and divergence on one side of each vortex and a zone of downwelling and convergence on the other side (Akan et al. 2013). Floating seaweed and debris, jellyfish and plankton accumulate in Langmuir convergence zones. These convergence features may aggregate eggs and larvae of fishes but are not expected to extend their residence time in the estuary. Somewhat similarly, convergence zones may appear at frontal regions in estuaries, for example where two water bodies of different densities meet.