Pulp and paper technical guidance: criteria and guidance for pronounced eutrophication, chapter 3

Eutrophication Process

Eutrophication, or nutrient enrichment, is a process of overfertilization of a water body by nutrients that results in the production of more organic matter than the self-purification reactions of that water body can overcome (Chambers et al. 2001). Often, pulp and paper mill effluents can be significant external sources of both dissolved and particulate organic matter in aquatic systems, and can lead to increased eutrophication in the receiving environment.

In the case of mild and moderate eutrophication, the typical response is an increase in the abundance and number of benthic invertebrate taxa (taxon richness) relative to the reference conditions. With pronounced eutrophication numbers of opportunistic fauna increase significantly, while a decrease in dissolved oxygen concentrations in the sediments or overlying water produces a gradual disappearance of more sensitive fauna (including larger taxa) (Hellawell, 1986).

Hyper- or severe eutrophication is observed when the abundance and taxon richness of benthic organisms decline; at this stage negative impacts on fish stocks and plant life are usually observed. Hypoxic or anoxic conditions can develop when oxygen is consumed by decomposing organic matter. If currents are weak and the organic matter is not being flushed from the area, these conditions may generate potentially toxic reduced compounds such as methane, ammonia and hydrogen sulphide (Pearson and Rosenberg, 1978).