Simply stated, eutrophication results when excess nutrients like Nitrogen (N) and Phosphorous (P) enter waterways and are not flushed out of the system. The old adage is true, "everything in moderation." Excess nutrients in themselves are not bad, however they can contribute to the deterioration of water quality in an estuarine system.
Excess nutrients are often consumed by phytoplankton species. This
consumption leads to an exponential growth of the phytoplankton or something
that is called an "algal bloom." Some phytoplankton species are harmless,
however some are not. In Japan, red tides are caused by a toxic phytoplankton
species that grow exponentially. These red tides can obliterate the oyster
mariculture system and affect the local fishery economy.
Even non-harmful phytoplankton species contribute to the problem caused by eutrophication. When the algal bloom dies, some portion of the biomass is consumed, however, a large portion falls to the seafloor. Some of this biomass will be consumed by the benthic fauna. During the summer, however, density stratification results from the increased solar radiation. This stratification partitions the water column. Benthic fauna still consume this biomass and use oxygen in the process.
What often results is "dead" bottom layer that contains little oxygen and even fewer benthic species.
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