Within the past 50 years, eutrophication—the over-enrichment of water by nutrients such as nitrogen phosphorus—has emerged as one of the leading causes of water quality impairment. The two most acute symptoms of eutrophication are hypoxia (or oxygen depletion) and harmful algal blooms, which among other things can destroy aquatic life in affected areas.
Fig 1. The eutrophication process and subsequent formation of sea-bottom hypoxia in coastal waters (click here for larger version). Image Credit: Pew Trusts
Fig 2. This divided body of water shows the remarkable difference between mesotrophic (moderately enriched) (upper basin) and eutrophic water (lower basin).
Image Credit: Fisheries and Oceans Canada
Before nutrients—nitrogen in particular—are delivered to coastal ecosystems, they pass through a variety of terrestrial and freshwater ecosystems, causing other environmental problems such as freshwater quality impairments, acid rain, the formation of greenhouse gases, shifts in community food webs, and a loss of biodiversity.
Once nutrients reach coastal systems, they can trigger a number of responses within the ecosystem. The initial impacts of nutrient increases are the excessive growth of phytoplankton, microalgae (e.g., epiphytes and microphytes), and macroalgae (i.e., seaweed). These, in turn, can lead to other impacts such as: loss of subaquatic vegetation, change in species composition, coral reef damage, low dissolved oxygen, and the formation of dead zones (oxygen-depleted waters) that can lead to ecosystem collapse.
