Estuaries and Lagoons


The largest estuaries are drowned river valleys flooded by changing sea levels 7000 years ago, when sea levels rose by as much as 150 metres. They flooded the coastline, leaving behind deep offshore submerged canyons and shallower coastal estuaries. The flooding created a mix of landscapes behind, so some of the larger estuaries are surrounded by a complex set of waterways, embayments, mudflats, sandbanks islands and sheltered embayments. Sydney Harbour, Moreton Bay, and the Derwent are noted for this.

Estuaries are water bodies in some way affected by the mixing of salt and fresh water. Estuaries and lagoons are by definition mainly about salinity and this physical feature has a lot to do with deciding the biodiversity of an estuary. In the upper reaches of a river this influence can be minimal but as the river water flows downstream it encounters a denser layer of salt water and it begins to flow over the top of it. This is the “salt wedge” and this mixing zone will move up and down a river depending on rainfall and the tides. A swimmer or diver can see this layer called a halocline and it appears as a shimmer effect near the surface.

The ocean has salinity measured at about 35 grams per litre, but a few marine animals have adapted to low salinity and can cope with 25 g/L, provided that the water temperature is moderate. Some more primitive animals like mussels can survive in 5g/L. Plants are primarily affected by available sunlight, so areas of cloudy water represent the major challenges. Rich blooms of plankton can attract fish, but if they bloom is great numbers as the nutrient levels and temperature rise, it can cause summer fish kills. This low oxygen environment means the algae can also be dominated by toxic blue green algae, this irritates the skin of swimmers and poisons marine life.

Although Australian estuaries are flushed by relatively small rivers by world standards, these estuaries are still charged with nutrients washing from the land and they support a huge range of biodiversity. Even small coastal lagoons without major rivers tend to accumulate vegetation debris and the sediments they create cause mangroves and salt marsh to sprout on what otherwise might be barren sand. In coastal lagoons and lakes the freshwater outflows are unpredictable. Smaller creeks entering the sea near a sandy beach often have insufficient outflows of fresh water to keep pace with the accumulation of sand. The mouth becomes blocked and the water body becomes less saline, or in the case of smaller bodies in arid areas, even more saline as the water evaporates. Decaying matter can deoxygenate a smaller lagoon and a burning sun can raise the temperature causing algal blooms and cyclical fish kills.

Estuaries have complex habitats to brackish and muddy bottoms, mudflats, rocky shores and eventually, the coarse sands and saline waters of the ocean as the river reaches the sea. Port Phillip Bay was created by the flooding of the Yarra river valley but the water spread out across a flat plain surrounding the river and gradually filled it with silt over a huge area. The water body is so large that the salinity rarely varies from the open sea.  The inflows and the outflows have been enough to keep the river mouth and channel open through the narrow former river gorge that we now call the Rip. This is a coastal embayment, noted for having animals similar to the open sea inside a sheltered lagoon. The bottom is mostly sand near the sea and more nutrient laden sediments lie in the back of the bay where the flushing effect of the tide is weakest.

The Swan estuary is another example of a permanently open barrier estuary. However, its shallow sand bar has long since been removed by mechanical dredging and it behaves now more like a sheltered embayment. In the Blackwood River WA the river changes seasonally and turns it into a freshwater river in winter, but it totally dries out in summer and becomes the same salinity as the sea. This creates very challenging conditions for species trying to live there year round. Other bodies that can close up for long periods and become saline coastal lagoons include Lake Conjola in NSW and the Peel Harvey estuary in WA. Chances are there is a smaller closed lagoon near your favourite beach spot.

Sea grass beds and salt marsh forms in these shallow lagoons and they then attract bird life and fish. After a long dry spell a wide range of species get established, but floods tend to change all this. They flush the narrower sections of river systems of pollutants and anoxic waters. The animals right down to the plankton head for the seabed where they hope to find refuge from the rush of fresh water at the surface. Most are swept away or killed by the low salinity. The species evolved to live in estuaries will breed quickly and survive in enough numbers to quickly repopulate the riverbanks. A pulse of nutrient-laden water out of the rivers is a bonanza for marine animals waiting offshore for just such an easy feed. The species more mobile animals adapted to high salinity quickly swim back into the rivers.

Basically, no two estuaries or lagoons are the same and even within the estuaries there are very substantial changes in the environment that provide many micro habitats for a range of species.

Larger estuaries are particularly popular places for human development. Estuaries often have access to fresh water, deep navigable channels for shipping and good protection from the weather. They were early focal points for European settlement. They have often become urbanised and industrialised, often with negative impacts for the environment. Places like Sydney Harbour and the Derwent are heavily modified by human behaviour. Excessive nutrient in waterways is a common problem after rainfall events, especially in areas where humans are fertilising fields and discharging sewerage into restricted waterways. Even the rainwater off our roads has enough oil and other nutrient particles in it to smother the area around outfall pipes with smelly algae. Bacteria can still thrive in very adverse conditions and loves these nutrients, but it strips oxygen from the water as it eats, killing other marine life. Land clearing has also increased sediments loads, swamping popular riverside beaches and rocky foreshores with sediment. These mud banks are often laced with pollutants including a legacy of heavy metals from our unregulated industrial plants of the 1960s and 70s. As a consequence of these changes large areas of our major estuaries are smothered in mats of anoxic and contaminated sediments that support little life other than a few relatively primitive worms and other simple invertebrates. The sludge can be seen on the riverbanks of places like the Derwent above the Tasman Bridge, the backwaters and rivers feeding Sydney harbour, and in upper Port Phillip Bay around the mouth of the Yarra.

We can also love smaller lagoons to death. Soon people build homes near placid lagoons with soothing views of trees and bird life. They clear the trees, pollute the water and begin to complain when lengthy sanding up of the lagoon mouth which naturally causes closure of waterways, bad smells and cyclical fish kills. The end result is that even some small lagoons get artificially manipulated by humans.

Due to natural factors and human disturbance, at any time there may be unoccupied ‘niches’ awaiting exploitation. As sea levels rise and fall quite quickly in a geological time scale, estuaries are often dried out or drowned every 10000 years, not enough time for a full range of species to develop that are well adapted to these fluctuating conditions. There is generally an ecological niche somewhere that can be filled by a new species. Large ocean going vessels dock in the harbours and discharge ballast water. These contain exotic species and at least 150 species have now become established in Australia. Because local predators are not adapted to deal with them, the end result is often a population explosion that strips the bottom of many native species. A survey of the Derwent at Lindisfarne found NO native animals in its transacts, all were imported species from the northern hemisphere. Pacific sea stars have denuded the bottom of invertebrates and affected the survival of the endemic spotted hand fish. The introduction of live oysters from New Zealand in the 1900s probably saw an end to the Derwent sea star, when the NZ cushion star larvae was also imported in the crates.

Although estuaries tend to have low species numbers the quantity of life is exceptional, 3 grams of plankton is produced in a square metre of sea water, only mangroves, forests and grassy paddocks produce more (4 grams). It will support three times the invertebrates of an open coast and ten times the quantity of fish. Some fish species are totally dependent on estuarine waters for part of their lifecycle and would die completely without access to estuaries for their eggs and larvae. Estuary dependant fish are estimate to make up 60% of the NSW fish catch by weight. This also attracts birds to shallow mudflats and there is so much food that many species migrate to southern Tasmania from as far away as the Bering Straits.