The Eutrophication Process and Modeling Representation


Ineutrophicationofmarine systems, nutrientloads contribute to excessive near-surface phytoplank-ton growth, resulting in algae blooms, which are both a source and sink of dissolved oxygen as the algae grow, settle, respire, die, and decompose. If vertical stratification of the water column is present due to variations in salinity and temperature, vertical oxygen transfer from the water surface is diminished. Algae become a net sink of dissolved oxygen in the lower layers of the water column, leading to hypoxia and possibly anoxia in extreme situations. Either condition canbe lethal to marine life.

components of eutrophication modeling

Modern eutrophication models include three linked submodels: hydrodynamic, primary production, and sediment nutrient flux. The hydrodynamic sub-model calculates the water circulation and transport pattern that is passed to the primary production submodel and accounts for the movement and residence time of nutrients and algae through the water column as well as the vertical structure or stratification of the water column. The primary production submodel calculates the algal dynamics as a function of nutrient loadings, temperature, and light. The sediment nutrient flux submodel accounts for the exchange between the water column and sediment of nutrients and organic matter and diagenesis. Diagenesis is the process of chemical changes which take place in the sediment after materials are deposited from the water column and before decomposed or mineralized materials are returned to the water column by physical exchange mechanisms. Comprehensive interactive sediment models use fluxes of algae and detrital particulate organic matter from the water column to compute the fluxes of nutrients and oxygen demand resulting from the mineralization of organic matter (Di Toro and Fitzpatrick, 1993; Di Toro, 2001).

Inclusion of sediment interactions within eu-trophication models necessitates the simulation of organic matter mineralization over time scales longer than one year. Over such a time scale, loadings of nutrients and carbon may vary dramatically as do physical conditions. For the models to accurately capture the dynamics of the algal populations, it is necessary to incorporate the variability in loadings and physical conditions. In the case of a large system such as the Hudson River Estuary, loadings include headwater tributary inputs, runoff from the land, effluent from large industries, wastewater treatment plants, and combined sewer overflows, deposition from the atmosphere, and oceanic inputs. The variability of these loadings is probably hourly, but the major signal may be captured on a daily basis. Similarly, the variability in physical conditions occurs with high frequency that must be incorporated into eutroph-ication models. The factors influencing light conditions are appropriately specified at time scales of one day to one hour.

In general, nutrients include nitrogen, phosphorus, and, for diatomaceous phytoplankton, silica. In the saline lower Hudson and other typical marine environments, observed nutrient concentrations and ratios of inorganic nutrients indicate that nitrogen is the nutrient that limits phytoplank-ton growth. For this reason, marine eutrophica-tionmodelsaddressmanagementofnitrogenloads but all nutrients are included in the computations for completeness. Additionally, discharges of organic carbon can also adversely affect concentrations of dissolved oxygen and are included in modeling calculations. Management actions which reduce organic carbon loads are also therefore considered.

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