7. Nutrients

• Introduction
• Effect of an increase in nutrients
• Estuary pollutants
• Nutrients in Lake Illawarra
• Reducing nutrient loads in Lake Illawarra

Introduction

Information Sheet No. 6 examined sedimentation in Lake Illawarra. This newsletter examines the impact of excessive nutrients in the Lake.

Effect of an increase in nutrients

Nutrients (in particular phosphorus and nitrogen) along with water and carbon dioxide are the essential building blocks for plant growth. Sunlight is also required to join the elements together. This process is called photosynthesis and it occurs in aquatic plants as well as in land plants.

High nutrient levels can occur naturally, although sewage and urban stormwater are the main sources of excessive nutrients in many NSW estuaries. The over-fertilisation of crops and pasture can also create large nutrient reserves which gradually seep into estuaries. Forest areas tend to have relatively low nutrient outputs.

Most NSW estuaries in their natural state are low in nutrients. A small increase in nutrient levels therefore does not necessarily damage aquatic plants or the ecosystem. However, an oversupply of nutrients can cause excessive growth particularly of algae (blooms).

Algal blooms can become so dense that they shade the plants under them. The lack of sunlight weakens and can kill algae and seagrasses alike. At night, living algae use up oxygen in the water. Dead and decaying plant and animal matter also uses up oxygen. Lack of oxygen can result in fish kills. A buildup of organic material on the estuary bed can result in formation of a fine black ooze in which very little will live and is odorous when disturbed or exposed.

Note blooms do not always occur when there are high nutrient levels in the water. Other factors may be more important. For example, suspended solids can block sunlight and slow plant growth enough to prevent a bloom.

Estuary pollutants

There are a large number of potential estuary pollutants. In some NSW estuaries, runoff from acid sulphate soils is a major problem. Soils which contain pyrites produce an acid after exposure to the air. This occurs when swampy ground containing pyrites is drained or when the soil is used for reclamation above the water table. Acid waters kill aquatic plants and animals and damage steel and concrete structures.

Contamination by toxic chemicals can also be a problem. Heavy metals such as lead, mercury and tin, or compounds such as pesticides and petroleum products can be washed into estuaries. The presence of these chemicals is usually associated with smelting, refining, chemical production, etc., or with farming practices such as sheep dipping or crop spraying.

The presence of organisms which are harmful to the health of humans or animals is also a form of pollution. The main source of these pollutants is sewage. Generally the level of harmful organisms is very small and hence their presence is difficult to detect. To overcome this problem water is tested for a very common, harmless microscopic bacteria known as E. Coli which lives in the gut of all animals. The presence of a large number of these bacteria is used as an indicator of the possible presence of harmful organisms. Levels are usually highest after rain when the sewerage system has overflowed, septic tanks have spilled and animal faeces have been washed into stormwater drains.

Nutrients in Lake Illawarra

Nutrient levels in Lake Illawarra continue to rise due to the increasing urban and industrial expansion within its catchment. Pollution created in these areas reach the lake through stormwater drains, creeks, sewage systems and direct flow into the lake.

Present high nutrient levels in Lake Illawarra are due to a variety of factors including those below:

1. The increased rate of infilling of the lake by sediments, which carry nutrients and pollutants to the lake. Land clearing for urban development and agriculture allow soil, fertilisers, animal faeces and other waste products to be washed into the lake
2. The unstable nature of the lakes entrance. This restricts water flow through the lake. Nutrients which would normally be flushed through to the ocean are settling in the lake
3. Major development within the catchment. Runoff from industries and urban streets bring large amounts of pollutants to the lake

Because of these factors Lake Illawarra is highly susceptible to nutrient problems such as recurrent algal blooms and fish kills, particularly when the entrance has been closed for some time and little flushing of the lake occurs.

The main nutrients which are monitored as indicators of nutrient levels in the lake are Phosphates and Nitrates. Concentrations above 5 - 15 ug/L of phosphates and 10 - 100 ug/L of nitrates are known to result in water quality problems for the lake. Nuisance algal growths regularly occur in Lake Illawarra and have been reported to have been present on a frequent basis since the early 1970s.

Key findings of a Lake Nutrient Dynamics study with respect to existing water quality and ecological conditions in the lake are as follows:

• the lake is strongly nitrogen limited;
• on an annual basis, sediment nutrient release is the dominant process affecting lake nutrient levels. Consequently, sediment nutrient release (generally uniform across the lake) controls baseline nutrient levels;
• catchment runoff events cause rapid influxes of available nutrients to the lake which may be sufficient (in association with the elevated baseline levels due to sediment nutrient release) to trigger algal blooms. The available nutrients are rapidly incorporated into algal and seagrass populations, from where they progressively migrate (in a combined or unavailable form) into the lake sediments;
• seagrass beds in the lake are a significant available nutrient 'sink';
• the lake contains phytoplankton levels which regularly approach or exceed 'eutrophic' levels;
• the levels of phytoplankton are approaching recognised limits at which light limitation can significantly affect seagrass. Should these phytoplankton limits be exceeded, and seagrass losses occur, a `feedback' process could result whereby the increased nutrient levels (due to lesser seagrass scavenging of available nutrients) cause enhanced phytoplankton growth, which causes greater seagrass loss, exacerbating the process.

Reducing nutrient loads in Lake Illawarra

The level of nutrients entering Lake Illawarra can be reduced by:

Planning controls including:

• prohibiting the destruction of wetland areas which fringe estuaries as these help to trap and recycle nutrients,
• legislating to reduce phosphorus levels in detergents and the other significant sources of nutrients in sewage,
• restricting the use of fertilisers, particularly those containing orthophosphate to the minimum level required,
• maintaining a buffer to watercourses to limit nutrient runoff,
• prohibiting the deposition or flushing of animal faeces and other nutrient-rich products into stormwater drains.

Physical works such as:

• stabilising the entrance to improve lake flushing;
• dredging to remove nutrient rich sediments from within the lake;
• diverting sewage overflows away from the lake;
• constructing settling ponds and wetlands on stormwater drains to recycle nutrients;
• revegetating banks of creeks and drains to filter overland runoff.

Community education

• educating the community on how to limit nutrient outputs (such as the disposal of pet faeces, car washing techniques, use of garden fertilisers, etc.),
• educating the community on fertiliser use to optimise benefits and minimise runoff into rivers and estuaries.

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