Tuesday, December 20, 2011

Limnology -Lake Horowhenua

Dr Maxx Gibbs


Pg 24

3 Limnology

Nutrients and sediments from the land accumulate in lakes and are stored in the lake sediments as a legacy from past land-use practices. Nitrogen (N) is continously mineralised and released from the sediment in the form of ammonium (NH4-N) which can be converted to nitrate (NO3-N) by nitrifying bacteria or sebsequently to nitrogen gas (N2) by denitrigying bacteria. The N2 gas is lost from the lake and the amount of N in the lake gradually reduces. Phosphorus (P) is mineralised to phosphate in the sediment but is retained in the sediment bound to iron and manganese oxides while the overlying lake wter contains oxygen (aerobic conditions). The bound P can be released from the sediments in the form of dissolved reactive phosphorus (DRP) when dissolved oxygen concentrations in the lake fall to zero anoxic. This consumption of oxygen for decomposition is referred to as sediment oxygen demand.

The N and P released from the sediment are readily used by algae for growth. If there is a surplus of N ( i.e., P - limitation to algal growth ), the dominant algal species will most likely be diatoms and green algae. If there is a surplus of P ( N - limitation to algal growth ), the dominant algal species will most likely be cyanobacteria ( blue-green algae ) which can use (fix) atmospheric nitrogen for growth. When the algae die they return the N and P in their cells to the sediment where it can be recycled again, augmenting the new N and P entering the lake from the catchment.

Nutrients released from the lake sediments are referred to as the internal nutrient load while nutrients entering the lake from the catchment via streams, ground water and direct discharge are external loads.

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