Figure 11.2 is a simplified diagrammatic representation of the principal eutrophication kinetics and water column-sediment interactions included in the primary production and sediment nutrient flux submodels of eutrophication models. Specifically, the kinetics shown in Figure 11.2 come from the model Row Column Aesop (RCA), a generalized, three-dimensional, time-varying water quality modeling program which has been described in detail (Di Toro, Lowe, and Fitzpatrick,
2000; HydroQual, 1999b). Following an overview of the features of hydrodynamic submodels, brief descriptions of the key features of primary production and sediment nutrient flux submodels as shown in Figure 11.2 are presented.
a. Hydrodynamic transport. An example of a hydro-dynamic transport submodel is ECOM3D which has been described in detail (Blumberg and Mellor, 1980 and 1987). ECOM3D and other similar hydro-dynamic sub-models provide advection and dispersion terms and water temperatures needed for primary production and sediment nutrient flux submodels.
b. Available light. The light that algae can use for growth is modeled as a function of four dependencies: incident solar radiation, the pho-toperiod or fraction of daylight, the depth of the water column, and light extinction or attenuation. One modeling framework frequently used is an extension of a light curve analysis formulated by Steele (1962). The available light is important to primary production in the lower Hudson River Estuary.
c. Algal growth. Phytoplankton growth is modeled for two functional groups or assemblages: winter diatoms and summer flagellates. The reasonphyto-plankton are considered as two assemblages rather than as individual species is the growth rate of an individual population of phytoplankton in a natural environment as a complicated function of the species present and their differing reactions to solar radiation, temperature, and the balance between nutrient requirements and nutrient availability. The complex and often conflicting data pertinent to this problem have been reviewed exhaustively (Rhee, 1973; Hutchinson, 1967; Strickland, 1965; Lund, 1965; Raymont, 1963). The available information is not sufficiently detailed at present to specify the growth kinetics for individual algal species in a natural environment, but we can divide the assemblages into distinct functional groups, namely diatoms and flagellates.
d. Nutrient and organic carbon cycling. Five phosphorus, six nitrogen, two silica, and six organic carbon principal forms are included in the nutrient and carbon formulations in RCA as schematically shown in Figure 11.2. Starting with the phosphorus forms, inorganic phosphorus is utilized by phy-toplankton for growth and is returned to various organic and inorganic forms via respiration and predation. A fraction of the phosphorus released during phytoplankton respiration and predation is in the inorganic form and is readily available for uptake by other viable phytoplankton. The remaining fraction released is in the dissolved and particulate organic forms. The organic phosphorus must undergo a mineralization or bacterial decomposition into inorganic phosphorus before it can be used by other viable phytoplankton.
During algal respiration and death, a fraction of the algal cellular nitrogen is returned to the inorganic pool in the form of ammonia. The remaining fraction is recycled to the dissolved and particu-late organic nitrogen pools. Organic nitrogen undergoes a bacterial decomposition, the end product of which is ammonia. Ammonia nitrogen, in the presence of nitrifying bacteria and oxygen, is converted to nitrite nitrogen and subsequently nitrate nitrogen (nitrification). Both ammonia and nitrate are available for uptake and use in cell growth by phytoplankton; however, for physiologicalreasons, the preferred form is ammonia.
Two silica forms are considered. Available silica is dissolved and is utilized by diatoms during growth for their cell structure. Unavailable or particulate biogenic silica is produced from diatom respiration and diatom grazing by zooplankton. Particulate biogenic silica undergoes mineralization to available silica or settles to the sediment from the water column.
Pools of dissolved and particulate organic carbon are established on the basis of timescale for oxidation or decomposition. Zooplankton consume algae and take up and redistribute algal carbon to the organic carbon pools via grazing, assimilation, respiration, and excretion. Since zooplankton are not directly included in the kinetics, the redistribution of algal carbon to the organic carbon pools by zooplankton is simulated by empirical distribution coefficients. An additional term, representing the excretion of dissolved organic carbon by phytoplankton during photosynthesis, is included in the model. This algal exudate is very reactive. The decomposition of organic carbon is assumed to be temperature and bacterial biomassmediated. Sincebacterialbiomassisnotdirectlyin-cluded within the model framework, phytoplank-tonbiomassis usedas a surrogate variable. An additional loss mechanism of particulate organic matter is that due to filtration by benthic bivalves. This loss is handled in the model kinetics by increasing the deposition of nonalgal particulate organic carbon from the water column to the sediment.
e. Sediment dynamics. The mass balance equations of the sediment submodel account for changes in particulate organic matter (carbon, nitrogen, phosphorus, and silica) in the sediments due to deposition from the overlying water column, sedimentation, and decay or diagenesis. The decay of particulate organic matter follows firstorder kinetics as described by Berner (1964, 1974, and 1980). The end products of diagenesis or decay of the particulate organic matter include ammonia nitrogen, dissolved inorganic phosphorus, and dissolved inorganic silica. These end products can undergo additional biological, chemical, andphysical processing within the sediment layer such as nitrification, sorption, and exchange with the overlying water column. Of particular importance to the overlying water column is the calculation of sediment oxygen demand (SOD). A more complete development of the sediment submodel theory is presented elsewhere (Di Toro and Fitzpatrick, 1993).
f. Dissolved oxygen balance. The dissolved oxygen balance includes both sources and sinks. Algal growth has two sources: the production of dissolved oxygen from photosynthetic carbon fixation and an additional source of oxygen from algal growth when nitrate rather than ammonia is utilized. Atmospheric reaeration is another source of dissolved oxygen. Sinks include algal respiration, nitrification, and oxidation of carbonaceous material.
g. Primary production. Primary production, an in-directmeasureofthedepthintegratedalgalgrowth rate, is calculated in RCA. Both source and sink terms from the dissolved oxygen balance, photo-synthetic carbon fixation and algal respiration, are used to calculate primary production in oxygen units.
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