Washed to the Sea Trivia Quiz

Answer: They all surround an estuary

Many of the world's largest and most populous cities lie on estuaries. Although there are many factors associated with this, two of the most important requirements for any developing city are the need for a reliable freshwater supply and easy access to a port for local and international shipping trade and the arrival of supplies.

For example, Edo, meaning 'estuary', was the original name for Tokyo; the estuarine Tokyo Bay is the mixing zone where the Sumida and Arakawa rivers flow into the Pacific Ocean.

Answer: A section of waterway where fresh river water and saline marine water meet and mix.

At its most simple but still largely correct definition, an estuary is the aquatic mixing zone where a freshwater river meets the ocean or other marine environment. Sea water intrudes up the river valley driven by tidal movement while the freshwater from the river is unidirectional, heading out to sea.

There is a range of estuarine types, and associated definitions, that account for unusual circumstances and unique geographical features but a useful generic definition is 'An estuary is a narrow, semi-enclosed coastal body of water which has a free connection with the open sea, at least intermittently, and within which the salinity of the water is measurably different from the salinity in the open ocean.'

Answer: Yes

One of the key features of an estuary is the meeting of freshwater from the river and salt water from the ocean (or other marine environment). Under normal circumstances, the incoming sea water, driven by tidal movement, goes under the freshwater from the river as the salt water is more dense due to the presence of the dissolved salts, including sodium chloride.

Although the density difference seems small (about 2.5% - temperature dependent), this is easily sufficient to create two largely non-mixing layers. Thus a vertical profile taken anywhere in the estuary will have fresh(er) water at the surface of the estuary and much saltier water - perhaps even sea water - at the bottom of the estuary.

Answer: All of these

In some locations around the world, (unpolluted) river water is predominantly clear whereas in other locations (e.g. much of Australia, the Huang He 'Yellow' River in China, many rivers in sub-Saharan Africa and the US southwest), the rivers appear brown, muddy or 'murky'. Geologically old soils are often highly weathered with small soil particle sizes. Soils are denser than water and hence will sink unless the particle size drops below about 1 micron (1 micrometre) in diameter. Many clays are smaller in size than this nominal cut-off and hence when these clays are washed into the river by rain fall or perhaps erosion, they remain suspended in the water and produce that 'muddy' look.

The faster the water velocity, the more power a stream has to keep particles from settling. Higher water velocities can scour more particles from the sediment and may cause erosion of banks, both of which result in more particles in the water. The dissolved salt content also plays an important role and will be explained in a later question! But briefly, the higher the dissolved salt content, the lower the number of suspended particles.

The presence of these suspended soil particles also has effects on the effectiveness of visual predators in the water, as well as restricting how far sunlight penetrates into the water column. This latter effect can greatly diminish the amount of photosynthesis by plants in the water way as sunlight is essential for photosynthesis. Less photosynthesis means a reduced food supply for those biota that feed on such plants.

Answer: Brownian Motion

Named after Scottish Botanist, Robert Brown, Brownian Motion is the random movement of small particles in water (and other fluids) caused by collisions with other particles including water molecules. These random motions and hence collisions are driven by temperature and micro-scale temperature fluctuations. The smaller the particle, the faster the velocity (speed). These random collisions are not a force acting in a particular direction, but on average will keep a sub-micron particle suspended in the water column. For particles larger in diameter than about a micron, the effect of gravity overcomes Brownian Motion effects and particles will sink.

The Stokes-Einstein Equation IS related to spherical particles in fluids such as water but refers to the diffusion rate of such particles rather than what keeps such particles suspended.

Answer: They have clumped together and settled down to the bottom of the river.

When the small particles clump together, they form aggregates larger than 1 micron in size and they can then settle to the bottom as gravity is more important than Brownian Motion. So why don't they clump together in freshwater? It is due to the surface charge on these particles. The outer layer of a typical riverine particle is made up of an organic matter coating. This organic matter is largely from decaying animal and plant detritus and contains a number of different types of functional groups. From the perspective of surface charge, the most important functional group is the carboxylic acids (vinegar/acetic acid is a carboxylic acid). At the pH of typical river water (circumneutral, so 6-8), these carboxylic acids lose a proton (or H+) and become negatively charged. There are a very large number of such groups on the organic matter coating of the particles, thus giving the particles a significant negative surface charge. The important point then is that these particles then repel each other, preventing the particles from clumping or aggregating.

However, salt water contains a large concentration of cations (positively charged ions) such as sodium, potassium, calcium and magnesium. These cations - especially the Ca2+ and Mg2+ - are attracted to the negatively charged particles and effectively neutralize that surface charge. Therefore particles can now clump together and settle from the water column, thereby causing the water to clear up and lose that murky appearance from the particles.

Answer: Clear

As noted in the previous question, the cations in the salt water overcome the electrostatic repulsion between particles and allow them to aggregate and settle. The highest concentration of these cations is in the salt water layer of the estuary. Although the actual cation concentrations in seawater may appear low: Sodium (Na+) is around 10.5 g per litre (g/L), Magnesium (Mg2+) is 1.2 g/L and Calcium (Ca2+) is around 0.4 g/L, these are easily enough to cause effective and relatively rapid water clarification.

When water from the bottom layer of a muddy estuary is sampled (e.g. using a Niskin bottle), students are often surprised that this water is almost always crystal clear.

Answer: Many fewer species in estuaries

Typically the number of species found in estuaries is only 10-20% of the number found further upstream in the freshwater and out in the open ocean. The major reason for this paucity in estuaries is the nature of the estuary itself and in particular the fact that salinity can vary from nearly freshwater to nearly seawater and can change rapidly.

Species evolve to live in either freshwater or marine and those in the latter need special osmoregulatory processes at the cellular level to cope with the high salt content. In estuaries, the salinity varies across the whole range from fresh to marine and few species can cope with this fluctuating environment.

(This question does presume that the number of species found in the freshwater river isn't severely restricted by excessive pollution).

Answer: True

If the riverine flow of freshwater, usually from large storm events on the surrounding land, is sufficiently high, this water can flush all of the salt water from the estuary and wash it to the sea. Normally, the energy in the freshwater layer is insufficient to overcome the density difference with the salt water layer and the latter remains largely intact, just moving backwards and forwards with the tides.

However, with very large freshwater flows, the energy of that surface layer is high enough to break down the density difference and cause complete mixing from top to bottom. Ongoing high freshwater flows then push this mixed water column out to sea and the estuary becomes largely freshwater. Once the high flow dissipates, the salt water moves back into the estuary, sliding under the freshwater due to tidal movement.

Answer: Water soluble pollutants

As seen in earlier questions, particles can be trapped in estuaries as they aggregate and settle to the bottom. Hence pollutants that are bound to these particles are then found in the sediments of the estuary. Pollutants that are hydrophobic ('water-hating') such as many organic molecules or metals in cationic form (e.g. lead, copper, nickel, cadmium - all in the +2 oxidation state) bound to the negatively-charged organic matter coating on the particles are removed from the water flowing out to sea.

Whether a contaminant is radioactive or not does not determine its fate; instead that fate is based on whether it is water soluble or hydrophobic. Water soluble pollutants are not strongly attracted to particles and hence travel with the water and are "washed to the sea".