Friends of Rietvlei
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Productivity in Estuaries
Estuaries are probably one of the most complex and interesting ecosystems on earth. They are those unique areas that form the interface between marine and freshwater environments. In the past people thought of estuaries as simply a combination of marine and freshwater ecosystems, it is only relatively recently that ecologists have realized that estuaries form a unique ecosystem quite different from either the marine or freshwater systems they separate.
Estuaries have four main characteristics that together contribute to their uniqueness:
Estuaries are highly productive systems;
Estuaries are multiple producers;
Estuaries are nett exporters of nutrients; and
Estuaries have a natural tendency towards eutrophication.
High productivity
Estuaries rank with coral reefs and tropical rain forests as the most productive natural ecosystems on earth. The following table compares the gross primary productivity of several systems. There are several ways of measuring gross primary productivity, here we use kcal/m/year - this is a measurement of the amount of energy produced (kilocalories) in a square meter of surface area per year.
Ecosystem kcal/m/year
Estuaries: 20 000
Coral Reefs: 20 000
Tropical forests: 20 000
Moist temperate forests: 8 000
Upwelling zones in the sea: 6 000
Grasslands & natural pastures: 2 500
Cultivated lands with little or no energy subsidy: 3 000
Fuel subsidised & mechanised agriculture: 12 000
The process of primary production is, in very simplified terms, the conversion of elements like oxygen, hydrogen, carbon, nitrogen, etc into amino acids (the basics of proteins, vitamins, etc) and carbohydrates (sugars, fats, etc) which can in turn be utilized in the food chain - in other words nutrients. Plants use energy from sunlight to combine and convert these elements, the nutrients thus produced enter the food chain through organisms feeding on the plants, or in the process of plant material decomposing (rotting) which releases nutrients into the soil or surrounding water where it is also taken up in a food chain. Bacteria and a few other benthic organisms can produce nutrients through chemical reactions in the absence of sunlight or oxygen.
A typical estuary normally has a meandering channel where the major currents flow, this is surrounded by sandbanks, mudflats, saltmarshes and reedbeds. Primary production occurs in all these areas, but it is minimal in the current channel and on the sandbanks. Most primary production in an estuary takes place in the following three areas:
Reedbeds are mostly situated in the upper reaches of an estuary, although they also occur lower down in areas where seawater do not penetrate too often. Reedbeds are not only important for filtering water moving down into the estuary, but the reeds are primary producers like any other plants. Nutrients from them are released into the system, but most will actually build up in old plant material which is then flushed down into the estuary (and out to sea) releasing nutrients as the plant material decomposes. Salt Marsh
Spartina saltmarsh

Reedbed
Reedbeds & mudflat
Salt marshes are the most productive areas and responsible for a major portion of nutrients produced in an estuary. The most dominant plant in saltmarshes is Spartina spp, a short stubby grass with a very rapid growth rate and turnover. Spartina can survive in freshwater and seawater, and as it thrives in shallow areas which is regularly flushed by tides, nutrients produced by the plants is rapidly released into the rest of the system. Spartina beds also host a vast number of bacteria and other organisms consuming plant material and supporting small invertebrates that form the basis of the food chain.
The third production area are mudflats, often adjacent to Spartina beds and sometimes forming part of salt marshes. In the mud flats we find that production is mostly through the activity of benthic organisms (see below).
Multiple producers
There are three types of producers that power (ie provide energy to) our natural world:
Macrophytes - mainly terrestrial plants like trees, shrubs and grasses that convert energy from the sun into nutrients through the process of photosynthesis. Aquatic macrophytes in this group include reeds, grasses and a few other plant species like Potamogeton growing under water.
Phytoplankton - aquatic algal forms free-floating in the surface layers of oceans and lakes where they photosynthesize to convert the sun's energy into nutrients; amongst these are green and blue-green algae.
Benthic organisms - includes types living in or on the bottom sediments of oceans, lakes and rivers. They mostly convert energy by anaerobic means to nutrients (ie not using oxygen) and includes diatoms, benthic algae, dinoflaggelates and bacteria.
Macrophytes and phytoplankton are only productive during the day, many of them also cease (or drastically decrease) production during cold winter months. Benthic organisms are not dependent on light and can produce throughout the year.
Most ecosystems have only one or at most two of these producers - terrestrial systems have only macrophytes to produce nutrients. In the open sea phytoplankton is the most important primary producer. Estuaries, however, have all three types of producers - this is the main reason for the very high annual productivity, the processes of primary production can go on day and night and virtually throughout the year.
Nett exporters of nutrients
The high productivity, together with the fact that they are multiple producers, mean that estuaries produce more nutrients than the system itself can utilize. The result is that:
The enormous amount of nutrients produced in estuaries support a vibrant food chain within the estuary. Although there are not many animal species living in the relatively harsh conditions in an estuary (see the Estuarine Fish page), there are large numbers of these animals present.
However, even this food chain cannot utilize all the nutrients and a considerable percentage is exported to the sea by tidal action.
The exported nutrients often form the basis of adjacent inshore food chains, particularly along sandy shores where primary production is lower than on rocky shores. In many parts of the world coastal fisheries are dependent on nearby estuaries for the primary source of nutrients to sustain the fishery.
Natural tendency towards eutrophication
The production of nutrients occur in the estuary itself; it is its own production system. Contrary to common belief, rivers in general do not fertilize estuaries, and there is not much contribution of nutrients from the sea.
Although estuaries export nutrients to the sea, they are also nutrient traps in that nutrients build up in tidal pools, deep water areas, salt marshes and adjacent wetlands. Normal tides, even spring tidal cycles, do not flush estuaries adequately to remove all the excess nutrients; it is up to major floods to flush out the estuary and remove built-up nutrient material.
The result of nutrient levels building up in an estuary is eutrophication, in other words an enrichment of the estuary itself. Eutrophication is the cause of algal blooms, as well as the distinctive smells of salt marshes and wetlands. Eutrophic conditions in an estuary or portions of an estuary is normal, and is not a negative factor. Without eutrophic conditions, estuaries would not be the highly productive systems they are and would not have excess nutrients to export.
While eutrophic conditions in an estuary is a desirable state, it does sometimes happen that this develops into hypertrophic conditions. Under such conditions nutrient production rises to such high levels that the system virtually "chokes up" - too much nutrients leading to even greater algal blooms and more over-production resulting in oxygen depletion and thus die-offs of oxygen-dependent organisms. This is most noticeable when fish die off in large numbers. Read about the fish die-off in December 2006.
Hypertrophic conditions can also be caused by pollution of the sytem when high levels of nitrogen and phosphates enter the system. This can come from stormwater drains, agricultural drainage, and even just from surface run-off in built-up areas. Damming of rivers has made the situation worse by preventing floods which would normally flush the estuary; now global warming with its effects of lower rainfall is also adding to the problem in many parts of the world.
Our Water Quality section contains specific data about nutrient and pollutant levels in Rietvlei and the Diep River estuary.
Written by Niel van Wyk. Based on talk given to Friends of Rietvlei in 2007.
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