Figure 4.42: Río de la Plata estuary, Ar-gentina/Uruguay, looking East. Water on the river is brown because of sediments carried from the Paraná and Uruguay rivers. The colour turns blue when approaching the South Atlantic Ocean.
The exact location of the colour change (which also implies a change from fresh to salt water) depends on winds and currents.
Figure 4.43: The Venetian Lagoon, Italy. No ideal, but charismatic example of a lagoon-type estuary, as only 11% is permanently covered by open water, while around 80% consists of tidal shallows and salt marshes.
Figure 4.44: A fjord-type estuary: Svalbard’s van Mijenfjorden, with the clear barrier island at its mouth. At this latitude, marine ecology is much simplified.
21Examples are the Severn (UK), Elbe (Germany), Chesapeake Bay (USA), Bahía Blanca (Argentina), Sydney Harbour (Australia) or Johor Strait (Sin-gapore). Note that not every river mouth is auto-matically an estuary, as it may not experience tidal mixing with seawater.
22Examples include most of the coast of New Jersey or Florida (USA), Venice (Italy), Lake Maracaibo (Venezuela), Keta Lagoon (Ghana) or Chilika Lake (India).
(Spartina spp.), which have worldwide distribution. They are often the first plants to take hold in a mudflat and begin its ecological succession into a salt marsh. Their shoots lift the main flow of the tide above the mud surface while their roots spread into the substrate and stabilize the sticky mud and carry oxygen into it so that other plants can establish themselves as well. Plants such as sea lavenders (Limonium spp.), plantains (Plantago spp.), and rushes (Juncus geradii) and grasses (Puccinellia spp., Festuca rubra) grow once the mud has been vegetated by the pioneer species.Woody plants (such as Halimione portulacoides), miniature mangroves if you wish, establish once the soil layer provides enough nutrients.
Salt marshes are quite photosynthetically active and are extremely productive habitats. They serve as depositories for a large amount of organic matter and are full of decomposition, which feeds a broad food chain of organisms from bacteria to mammals.
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23Fjord-type estuaries can be found along the coasts of Alaska, the Puget Sound region of western Washington state, British Columbia, eastern Canada, Greenland, Iceland, New Zealand, and Norway.
ened and widened existing river valleys so that they become U-shaped in cross-sections. At their mouths there are typically rocks, bars or sills of glacial deposits, which have the effects of modifying the estuarine circulation. “Fjord-type” estuar-ies are formed in deeply eroded valleys formed by glaciers (Fig. 4.44). These U-shaped estuaries typically have steep sides, rock bottoms, and underwater sills contoured by glacial movement. The estuary is shallowest at its mouth (in contrast to ordinary fjords), where terminal glacial moraines or rock bars form sills that restrict water flow. In the upper reaches of the estuary, the depth can exceed 300 m. The width-to-depth ratio is generally small. In estuaries with very shallow sills, tidal oscillations only affect the water down to the depth of the sill, and the waters deeper than that may remain stagnant for a very long time, so there is only an occasional exchange of the deep water of the estuary with the ocean.23
Of the thirty-two largest cities in the world in the early 1990s, twenty-two were located on estuaries. Unsurprisingly, many estuaries suffer degeneration from a variety of factors including soil erosion, deforestation, overgrazing, overfish-ing and the filloverfish-ing of wetlands. Eutrophication may lead to excessive nutrients from sewage and animal wastes; pollutants including heavy metals, polychlorinated biphenyls, radionu-clides and hydrocarbons from sewage inputs; and diking or damming for flood control or water diversion.
Water circulation The residence time of water in an estuary is dependent on the circulation within the estuary that is driven by density differences due to changes in salinity and temperature. Less dense freshwater floats over saline water, and warmer water floats above colder water (at temperatures greater than 4°C). As a result, near-surface and near-bottom waters can have different trajectories, resulting in different residence times.
Vertical mixing determines how much the salinity and tem-perature will change from the top to the bottom, profoundly affecting water circulation. Vertical mixing occurs at three levels: from the surface downward by wind forces, the bottom upward by boundary generated turbulence (estuarine and oceanic boundary mixing), and internally by turbulent mixing caused by the water currents which are driven by the tides, wind, and river inflow.
“Salt-wedge” estuaries are characterized by a sharp den-sity interface between the upper layer of freshwater and the bottom layer of saline water. River water dominates in this system, and tidal effects have a small role in the circulation patterns (e.g. the Mississippi estuary). The freshwater floats on top of the seawater and gradually thins as it moves sea-ward. The denser seawater moves along the bottom up the estuary forming a wedge shaped layer and becoming thinner
as it moves landward. As a velocity difference develops be-tween the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater.
“Partially stratified” estuaries are less dominated by river discharge (e.g the Thames). Turbulent mixing induced by the current creates a moderately stratified condition. Turbulent eddies mix the water column, creating a mass transfer of freshwater and seawater in both directions across the density boundary. Therefore, the interface separating the upper and lower water masses is replaced with a water column with a gradual increase in salinity from surface to bottom. A two layered flow still exists however, with the maximum salinity gradient at mid depth. Partially stratified estuaries are typi-cally shallow and wide, with a greater width to depth ratio than salt wedge estuaries.
In “vertically homogeneous” estuaries, tidal flow is greater relative to river discharge, resulting in a well mixed water column and the disappearance of the vertical salinity gradient.
The width to depth ratio of vertically homogeneous estuaries is large, with the limited depth creating enough vertical shearing on the sea floor to mix the water column completely.
If tidal currents at the mouth of an estuary are strong enough to create turbulent mixing, vertically homogeneous conditions often develop.
Challenging conditions for marine life Estuaries are incredibly dynamic systems, where temperature, salinity, turbidity, depth and flow all change daily in response to the tides. This dynamism makes estuaries highly productive habitats, but also make it difficult for many species to survive year-round.
As a result, temperate estuaries large and small experience strong seasonal variation in their fish communities. In winter, the fish community is dominated by hardy marine residents, and in summer a variety of marine and anadromous fishes move into and out of estuaries, capitalizing on their high productivity.
Two of the main challenges of estuarine life are the vari-ability in salinity and sedimentation. Many species of fish and invertebrates have various methods to control or conform to the shifts in salt concentrations and are termed osmocon-formers and osmoregulators. Many animals also burrow to avoid predation and to live in a more stable sedimental en-vironment. However, large numbers of bacteria are found within the sediment which has a very high oxygen demand.
This reduces the levels of oxygen within the sediment often resulting in partially anoxic conditions, which can be further exacerbated by limited water flux.
Phytoplankton are key primary producers in estuaries.
They move with the water bodies and can be flushed in and out with the tides. Their productivity is largely dependent
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upon the turbidity of the water. The main phytoplankton present is diatoms and dinoflagellates, which are abundant in the sediment. Still, the primary source of food for many organisms in estuaries, including bacteria, is detritus from the settlement of the sedimentation.
5
Human Effect
Fishermen in Sesimbra, Portugal
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1Jennings, S., Kaiser, M., and Reynolds, J. D. (2009).
Marine Fisheries Ecology. John Wiley & Sons, New York
2Naylor, R. L., Hardy, R. W., Buschmann, A. H., Bush, S. R., Cao, L., Klinger, D. H., Little, D. C., Lubchenco, J., Shumway, S. E., and Troell, M.
(2021). A 20-year retrospective review of global aquaculture. Nature, 591(78517851):551–563
3The 1982 United Nations Convention on the Law of the Sea.
4Ringius, L. (1997). Environmental NGOs and regime change: the case of ocean dumping of radioactive waste. European Journal of International Relations, 3(1):61–104
5“Fish”, in the context of fisheries, includes crustaceans, molluscs, echinoderms, turtles, whales and other non-fish marine animals.
Human activities affect every place on earth. As the human population size and our affluence grows, so does our impact on the natural world. We see it on land, where we live. We see it less when the ocean surface hides what is beneath as we fly over or float on it. And the coast, from which we watch the ocean waves break, overlooks only a tiny fraction of the actual biome. In this chapter we explore some of the effects human action has. We know of them, but we don’t see them; we see them, but we do not want to know of them.
Humans affect on marine life is direct, e.g. through fisheries and habitat destruction, as well as indirect, through pollution. Pollution encompasses a wide range of effects: nutrient and agrochemical input through rivers; human waste products including plastics; oil spillage; drifting fishing gear (“ghost nets”); introduction of chem-icals and heavy metals for oil and gas exploration; CO2-induced ocean acidification; climate change-induced coral bleaching; sound pollution through intense shipping traffic, ocean drilling, military (and much less but locally significant scientific) soundings.
As with many human effects on ecosystems, the rapid expan-sion of the human population and their increasing affluence has stretched many marine ecosystem beyond breaking point. Stocks of several fish species have been depleted, species driven (close) to extinction, habitats destroyed, food webs altered and organisms poisoned.1
Roughly a third of the world population depends for its survival on marine production. More and more open-ocean fishery is supple-mented by aquaculture.2Opportunities for win-win situations and symbiontic coexistence of humans and marine life are scarce; alter-ations of coastlines and destruction of fish nurseries (mangroves, coral reefs, sea grass meadows) are irreversible under continued use;
local dependence on the ocean in low-income countries without alternative.
Finally, the open ocean, beyond the 370 km (= 200 nautical miles) exclusive economic zone around a countries coast, is subject to in-ternational legislation,3but its enforcement is both logistically nigh impossible and politically highly problematic. As a consequence, fishing, resource exploration and ocean dumping (of radioactive waste, sludge, and any form of liquid waste) are tackled only by public shaming through environmental NGOs.4The Tragedy of the Commonsapplies to the oceans at a scale second only to that of the atmosphere.