Processes to be Represented in Lake Ecosystem Models

The basis of all biogeochemical and ecological lake models are mass balances of nutrients, dissolved oxygen, and organic compounds - particularly aquatic organisms - in the lake. Figure 1 shows a graphical representation of biological processes in a lake ecosystem that build, at a certain level of simplification or refinement, the basis of most lake ecosystem transformation process models.

By primary production, nutrients are converted into phytoplankton biomass (in shallow lakes, periphyton may also considerably contribute to primary production). This process requires light and produces dissolved oxygen. Herbivorous zooplankton grazes on phytoplank-ton. Carnivorous zooplankton feeds on herbivorous zooplankton. Omnivorous zooplankton feeds on herbivorous zooplankton and on phytoplankton. All or some of these plankton classes serve as food for planktivorous fish which again are the food source for carnivorous fish. All these grazing and predation activities require dissolved oxygen and lead to release of particulate organic material (fecal pellets and remainings from sloppy feeding), dissolved organic matter (released from broken cells), and nutrients. Death of all organisms transforms them into particulate organic matter. Furthermore, respiration of organisms transforms biomass into nutrients. Particulate organic matter is hydrolyzed to dissolved organic

Figure 1 Overview of important biological processes in the pelagic zone of surfacewaters. Gray ovals represent state variables (chemical compounds or organisms), and arrows represent transformation processes. The following processes are considered: 1, growth of phytoplankton (primary production); 2, growth of herbivorous zooplankton; 3, growth of omnivorous or carnivorous zooplankton; 4, growth of planktivorous fish; 5, growth of carnivorous fish; 6, respiration; 7, release of dissolved organic matter during death, sloppy feeding, and exudation; 8, death; 9, hydrolysis; 10, mineralization. Small arrows indicate oxygen consumption or production.

Figure 1 Overview of important biological processes in the pelagic zone of surfacewaters. Gray ovals represent state variables (chemical compounds or organisms), and arrows represent transformation processes. The following processes are considered: 1, growth of phytoplankton (primary production); 2, growth of herbivorous zooplankton; 3, growth of omnivorous or carnivorous zooplankton; 4, growth of planktivorous fish; 5, growth of carnivorous fish; 6, respiration; 7, release of dissolved organic matter during death, sloppy feeding, and exudation; 8, death; 9, hydrolysis; 10, mineralization. Small arrows indicate oxygen consumption or production.

substances which are mineralized into nutrients. These last two processes are of particular importance in the sediment of the lake. In the presence of dissolved oxygen, mineralization is accompanied by dissolved oxygen consumption. In deeper sediment layers, where all dissolved oxygen diffusing into the sediment from the water column is used up, mineralization requires reducing nitrate, manganese oxide, iron hydroxide, or sulfate. Finally, mineralization is also possible by methanogenesis.

Transport processes lead to partial spatial separation of these transformation processes. Figure 2 gives an overview of the most important transport processes in a lake or reservoir. Depending on the density of the inflow and on stratification of the lake, the inflow enters the lake at a certain depth (with some entrainment of water from the layers above). As the outflow is not at the same level

Exchange of radiation, heat, momentum, and gases

Inflow

Exchange of radiation, heat, momentum, and gases

Inflow

Outflow

Figure 2 Important transport and exchange processes in a lake or reservoir.

Outflow

Figure 2 Important transport and exchange processes in a lake or reservoir.

(at the surface for natural lakes and close to the bottom for many reservoirs), this leads to vertical advection of (part of) the water column. In addition, the water column is mixed by turbulent diffusion. During periods of stratification (usually caused by warmer and less dense water layers laying above colder and denser layers), horizontal mixing is usually much faster than vertical mixing. Radiation, heat, momentum, dissolved oxygen, carbon dioxide, and molecular nitrogen are exchanged over the lake surface. Due to their mobility, fish, zooplankton, and some phytoplankton species move actively through the water column. Particulate substances are deposited at the surface of the sediment due to sedimentation. Dissolved substances are transported within the sediment porewater and between porewater and lake water by molecular diffusion.

The interaction of transformation and transport processes discussed separately in the previous paragraphs (Figures 1 and 2) often leads to the following typical spatial separation of processes in a lake: primary production of phytoplankton takes place in the upper layer of the lake, where sufficient light is available. This process consumes nutrients. Nutrients are delivered by the inflow and produced by respiration or mineralization either in the epilimnion or in the depth of the lake from where they diffuse to the surface layer. Zooplankton can actively move through the water column. Herbivorous zooplankton feeds on phytoplankton in the surface layer of the lake. Fish dominantly feed on plankton in the surface layer in the pelagial and in the littoral as the light allows them to catch their food. Particulate organic material produced by the organisms is usually sedimenting through the water column much quicker than mineralization takes place. For this reason a large fraction of particulate organic material reaches the sediment where mineralization processes consume dissolved oxygen, nitrate, and other compounds that can be used for the oxidation of organic substances. Due to the small diffusivities in the sediment and often also in the hypolimnion, this leads to large gradients of dissolved oxygen, nitrate, and mineralization products in the sediment and the hypolimnion of the lake. In shallow lakes, benthic organisms and Macrophytes can contribute to substance turnover in a similar way as described here for the pelagial.

A lake ecosystem model should represent the main physical, chemical, and biological processes of the most important substances in the lake and represent the biological communities building the ecosystem.

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