2. Nutrient dynamics
The paragraphs below provide an overview of the close interaction between nutrient loads and sediment in lake ecosystems. Nutrient loads on a lake ecosystem (mainly dissolved phosphate, PO4 3-) play an important role in the dynamics of aquatic ecosystems. Lakes in the clear water state with an abundance of aquatic macrophytes can withstand high levels of phosphate whilst maintaining clear water. This is due to the fact that phosphate is mainly stored in the sediment and vegetation and therefore not accessible for algal growth. The maximum level of phosphate loading at which the system remains in a clear state is called critical phosphate loading. When a shallow lake turns to a turbid state, phosphorus will mainly be stored in fish and algae. Both can have an important negative influence on water transparency and can cause a flux of soluble phosphorus from the bed sediment to the water column, through resuspension, excretion and release from the sediment as a result of anoxia.
Internal and external loading
There are two main nutrient sources found in lake ecosystems: internal (from the lake sediment) and external (from incoming waters). External nutrient loads have increased over the last century, largely resulting from fertiliser inputs from agriculture and other human activities in the watershed. An important internal source for high nutrient loading are the remains of algae, plants and aquatic wildlife. High external nutrient loadings do not always have to result in a high concentration of nutrients in the water column, however. Sediments can act as a buffer and are able to store large amounts of phosphorus, even up to 90% of the total amount of phosphorus present in the system. In the case of internal loading, the phosphorus stored in the sediment is released to the water column.
Decomposition and nutrients
The organic material accumulated in the bed can be an important source of minerals. Dead and slowly decomposing plants, algae and other organic material in the water column sink to the lake bed. There it forms an organic layer called detritus. As bacteria decompose these organic components, soluble nutrients like nitrogen and phosphorus become available. Through mineralisation, organic phosphate and nitrogen are converted into phosphate and nitrate. These nutrients are stored in the sediment and can gradually diffuse to the water column. Both algae and macrophytes are able to use these nutrients, thus closing this part of the nutrient cycle.
Nutrients and macrophytes
In most shallow eutrophic lakes, macrophytes reduce the availability of nutrients in the water column as they take up nutrients from the water for their growth. Macrophytes are especially efficient in reducing the nutrient availability if the nutrient concentration in the water column is relatively high. This for instance is the case in lakes that are recovering from the turbid algae-dominated state (Kufel 2002). Macrophyte beds can reduce nutrient availability further, as these beds can trap suspended particles from the water and prevent their resuspension (Madsen et al. 2001).
In lakes with low nutrient concentrations in the water column, macrophytes have been found to act as a nutrient source. In temperate lakes a large part of the macrophyte biomass dies at the end of the growing season. Decomposition of this plant material can result in a temporary nutrient release. During the growing season, decomposition of old plant parts (leaf turnover) and nutrient leaching from the plant can form a flux of nutrients from the sediment into the water column in these oligotrophic systems (Graneli and Solander 1988).
Nutrients and algae
Large densities of algae occur only in situations with large quantities of nutrients in the water column. As the algae grow and multiply to ever larger concentrations, phosphorus levels in the water column will drop. At a certain point, as the gradient grows, the balance between nutrients stored in sediments and those in the water column is disrupted. As a result, there will be a flux of phosphorus from the sediment into the water column until the equilibrium across the sediment-water barrier is restored again. This keeps the system in a turbid and highly productive state.