Causes of Harmful Algal Blooms

Much has been written about the underlying causes of harmful algal blooms (HAB), the complex interplay of factors that lead to their proliferation, and the unique set(s) of factors contributing to blooms of different species of algae. In general, the overarching causes that have received much attention in the literature include degradation of water quality and increasing eutrophication; increasing aquaculture operations; transport of harmful species via ballast water or shellfish seeding, leading to new introductions; and climate change (e.g., Hallegraeff and Bolch, 1992; Hallegraeff, 1993; Anderson et al., 2002; Glibert et al., 2005, 2014a; Heisler et al., 2008; Wells et al., 2016; and references therein). This chapter reviews these complexities while highlighting the key role of changes in nutrients; estuarine/marine microalgal species are emphasized, and information is also included on some freshwater HAB. While some have suggested that increased monitoring or surveillance has led to a perception of an increase in HAB, there is now compelling evidence from many regions showing conclusively that increases in HAB proliferations are real, not sampling artifacts (Heisler et al., 2008). What is a HAB? In his seminal paper, Smayda (1997a, p. 1135) stated, “What constitutes a bloom . . . has regional, seasonal, and speciesspecific aspects; it is not simply a biomass issue. . . . The salient criterion to use in defining whether a ‘harmful’ species is in bloom and the distinctive feature of such blooms lie not in the level of abundance, but whether its occurrence has harmful consequences.” Since the publication of that paper, biomass criteria for a few HAB species have been defined, but more generally HAB continue to be defined in terms of the extent to which they cause harmful events (fish kills), toxic events (shellfish and finfish poisoning), ecosystem disruption (nutritional and/or prey-size mismatches, such as picocyanobacterial blooms), or large biomass events (hypoxia or anoxia). In all cases, for a HAB to occur, the HAB species must be present and its biomass relative to other species in the assemblage changes, although the HAB species does not need to be dominant or in high abundance to elicit some of these effects. In general, the factors that promote HAB can be reduced to two: changes in the rate of introductions of species to new areas and changes in local conditions leading to conditions more conducive to the growth of individual species. Environmental changes can be subtle and not all factors may change together, leading in some cases to situations where one factor may seem to be favorable, but growth is impaired due to a change in another factor. The success of an introduced species in a new environment is not ensured; instead, there must be a match of environmental factors and the species capable of exploiting the environment. As Smayda (2002) also wrote,