Water Fluxes and Residence Times
Interest in the exchange ofnonliving materials and organisms between estuarine ecosystems and the sea was initiated by the first quantitative metabolic studies on the high productivity of marsh dominated estuaries. These studies were first synthesized in simple energy budgets that were found to explain less than 50% of the productivity of estuarine ecosystems. Investigators speculated that the unaccounted-for energy must be exported from the estuarine ecosystem by tidal currents. This idea led to the 'outwelling hypothesis' that states that estuarine ecosystems produce much more organic material than can be utilized or stored by the system and that the excess is exported to the coastal ocean where it supports near coastal ocean productivity. While the energy budget or mass balance approach is a cheaper and quicker method of determining the direction of material fluxes, in recent years the direct measurement of material fluxes is favored because this approach provides statistically meaningful results.
Another aspect to the fluxes of materials in estuarine systems is the time the water mass remains in the system or residence time (also known as flushing time or turnover time). Residence time can provide essential information to resource managers on the retention and dispersal of toxins, the incubation of invasive species, and the carrying capacity of a system for benthic suspension feeders (Figure 4). Recent studies on the physics and geomorphology of water in estuarine tidal channels suggest that the residence time of water may vary greatly from place to place within some estuaries. Such variations have been used to explain growth variations in bivalves in different locations within the same estuary. Traditional estimates of an estuarine system's residence time can be computed from measurements of system volume, tidal prism, and water input to the system. The advent of fast computers and numerical models, however, now allows for much more modeling of these systems with the potential for more sophisticated spatial and temporal management strategies.
In riverine systems, river flow is the main physical cause of material and organismic transport from estuaries to the sea. Each of these systems are a unique and changing feature on the present landscape because rising sea level is drowning their basins and sediments are gradually filling their channels. For example in Chesapeake Bay, 35% of the particulate nitrogen and most of the phosphorus is buried in the sediments of the bay. Of the nitrogen in the bay water column, 31% was exported to the sea and 8.9% was removed from the system as commercial fish harvest. In general, the nutrients transported and exported by riverine estuaries are thought to be a significant source for generating new organic production in the coastal ocean. As many of these systems have dams or have them proposed, managers must take into account the direct and indirect effects of these structures on recreational and coastal fisheries.
In bar-built estuaries, tides are usually the major source of energy for the transport of materials into and out of the estuary. If the fastest currents are on the flooding tides, then the system tends to import suspended particulate material. In contrast, if ebbing tides have the fastest currents, then the system usually exports suspended particulate materials. The Wadden Sea of Northern Europe is a flood-dominated system and North Inlet in South Carolina is an ebb-dominated system.
In shallow, high-insolation, low-precipitation, warm systems, evaporation can dictate the direction of transport. This is the case in some small tropical systems where water loss due to evaporation is replaced by the influx of water and nutrients from the adjacent sea.
In addition to inanimate materials, the larval and adult stages of many organisms are exchanged between the estuary and the sea. Some organisms may be passively carried by estuarine currents while others may actively swim or take advantage of the direction of tidal flows to move across the estuary-ocean interface.
Primary producers, including phytoplankton and resuspended benthic microalge, depend on passive transport between estuaries and the sea. Most flux studies show that these organisms have a net seasonal or annual transport into the estuary from the coastal ocean. This import has been explained by passive filtration by estuarine wetlands and by active filtration by suspension-feeding animals within the estuary. Protozoans, bacteria, and viruses are also found in the estuarine water column and while they most certainly are passively transported by estuarine currents, the direction of their net flux is yet to be determined.
The exchange of invertebrate larvae between estuaries and the coastal ocean has been explained by two competing schools of thought, the passive and active hypotheses. In the passive hypothesis, the horizontal movements of larvae are mainly a function of current direction and velocity. The active transport school contends that invertebrate larvae swim both vertically and horizontally to take advantage of tidal currents. In one group that includes oysters, the early stage larvae stay high in the water column with later stages sinking to lower depths. This strategy allows downstream movement of early larvae with some exiting the estuary to the sea, while older larvae are entrained in inflowing bottom currents and effectively retained in the estuary. A second approach is used by larvae that migrate vertically in the water column in synchrony with tidal cycles. This strategy allows larvae to maximize upstream transport and retention. A third group has larvae that are immediately transported to the coastal ocean where they stay for weeks before returning into the estuary using wind and tidal currents. A final group uses the coastal ocean during their adult and larval life. In this case, the postlarvae enter the estuary maintaining their position by swimming against tidal currents.
Nekton organisms (fish, crabs, and shrimps) are mobile links between the various subsystems ofestuarine ecosystems as well as links between the estuary and the sea. These animals feed and accumulate biomass while in the estuary and then move back to the coastal ocean, thus exporting biomass and inorganic wastes.
While seasonal and latitudinal climatic effects on coastal and estuarine systems have long been documented, the impacts of global climate change (warming or cooling) on estuarine systems have only recently been quantified. Major storms, El Nino-Southern oscillation (ENSO) events, seismic sea waves, or tsunamis and sea level rise (SLR) are global effects that can significantly influence water and material fluxes in estuaries.
Hurricanes and major storms generally influence estuaries through storm surges and short-term increases in precipitation. These enormous pulsed fluxes of water can change the geomorphology of estuaries and their watersheds, massively resuspend sediments, and flush materials off the landscape and into the estuary. Tsunamis can be even larger than storm surges and can have similar impacts to even greater areas of the coastal ocean and estuaries. However, extensive marsh and mangrove wetlands common to estuaries can buffer these pulses of water and reduce the damage they can cause to the coastal landscape.
ENSO events only affect some estuaries. The effect is usually a drought or higher than average precipitation. For example in some South Carolina estuaries, ENSO-induced precipitation and upland runoff can depress salinity up to 75% for as much as 3 months.
SLR is an example of global change on both seasonal and annual time scales that directly influences estuarine systems. Seasonal changes in sea level are the result of air pressure changes at the water's surface and the expansion or contraction of water mass due to heating and cooling. In estuarine systems, these changes are reflected in the depth of the system, but more importantly in the area and time of exposure or submergence in the intertidal zone. SLR will gradually force the transgression of estuaries upslope along the coastal plain. Eventually, SLR will compete with human development for the coastal landscape.
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