Within Lake - Reducing internal nutrient load

Background science

Even if a lake has only been subjected to low levels of diffuse nutrient sources entering the lake over a long period of time (e.g. 100 years) there may be high levels of nutrients in the bottom sediments. When these nutrients are released from the sediments, i.e. under anoxic conditions or due to sediment resuspension or microbial processes, they become available for phytoplankton and plant growth. These nutrient releases can lead to the lake becoming eutrophic. The pool of nutrients that is reactivated from the bottom sediments can prevent the trophic state from improving, even when external nutrient loads may have been reduced for several years. Consequently, reducing the internal nutrient load in some lakes may be important to improve the trophic state.

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Methods

Nutrient stabilisation / sediment capping

Chemical flocculants and other adsorptive compounds may provide a useful means of removing nutrients from lake water or preventing release of nutrients from the bottom sediments. The nutrients most commonly limiting primary production are nitrogen and phosphorus. Dissolved inorganic forms of nitrogen (NO3- and NH4+) and phosphorus (PO43-) are immediately available for plant uptake and therefore control of these species provides an opportunity for direct impacts on phytoplankton production and biomass. The ionic charge on these nutrient species is also a key factor in the selection of appropriate flocculant or adsorptive materials.

Flocculants have often been spread onto the surface of the lake by boat or by air, or applied as a solution. These compounds include, for example:

• alum - which is relatively stable under most environmental conditions but has potential to be toxic to aquatic organisms.
• iron compounds (e.g. ferric chloride) - but which may readily liberate phosphorus under anoxic conditions.

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Dredging

Dredging may be possible in cases when the internal nutrient load needs to be reduced, recreational activities are impaired due to sediment build-up, for macrophyte control or to remove toxic substances present in the lake bed. Sediment removal for reduction of the internal nutrient load is only recommended in lakes where all possible efforts have been made to reduce external nutrient inputs, and where, because of cycling of nutrients from internal sources, there has been no improvement in water quality.

A single lake dredging may not necessarily provide a permanent solution to eutrophication. Due to sedimentation and ongoing external nutrient inputs, dredging will most likely have to be repeated on a regular basis or augmented with other biomanipulation techniques such as fish removal.

There are several factors that must be considered before deciding to dredge:

• The amount of sediment to be removed.
• The characteristics of the newly exposed sediment.
• The possibility to release toxic substances present in the sediment.
• Resuspension of sediments during dredging, which may be rich in nutrients.
• Longevity of the dredging treatment.
• Potential for anoxia where dredging is irregular and deep holes have been created.
• Disposal of the dredged sediments.