36 Nutrients and harmful algal blooms: general issues and examples from the northern gulf of mexico

Nutrient inputs have increased worldwide in both marine and freshwater ecosystems, stimulating algal growth. The resulting increased eutrophication can be manifested in different ecosystems as hypoxia/anoxia, loss of submerged aquatic vegetation, or Harmful Algal Blooms (HABs). HABs can significantly impact human and ecosystem health. It has been hypothesized that HABs are increasing worldwide, in part because of increasing nutrient inputs, but establishing a direct cause and effect link has been difficult for a variety of reasons. Two examples from the Louisiana coastal zone illustrate some of the difficulties and successes: annual Pseudo‐nitzschia spp. blooms on the shelf in the extended plume of the Mississippi River and sporadic, toxic cyanobacterial blooms in low salinity estuaries. The challenge is to determine if a HAB is caused by increased nutrient inputs and which nutrient(s) are the main culprit(s), to identify the source(s) of the increased nutrients, and to develop effective management strategies. The latter is often a social/political problem that must be well‐informed by the science.