1.1 Introduction
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, āWhat constitutes a bloomā¦has regional, seasonal, and species-specific aspects; it is not simply a biomass issue.ā¦The salient criterion to use in defining whether a āharmfulā 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.ā 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,
Anthropogenic seedings are not, in themselves, bloom stimulation events; they are only the first phase of a multi-phase process. A newly vectored, non-indigenous species is initially pioneering: it must either find an open niche or displace a niche occupant as its first step towards successful accommodation within the community.ā¦Until colonization is achieved, alien species introduced into water masses that have been modified by cultural nutrient enrichment, water mass conditioning by aquaculture, or climatological disturbances, will not bloom. Successful colonization alone is not decisive, it usually must be accompanied at some point, or coincide with habitat disturbance ā a pre-condition for many HAB occurrences. (p. 292)
Changes in environmental conditions supportive of the increasing global occurrence of HAB are predominantly anthropogenic in nature, such as changes in nutrient loads resulting from expanding human population and associated nutrient pollution from agriculture and animal operations, alterations due to human changes in fishing pressure or aquaculture development, and/or large-scale changes in flow from major water diversion projects. However, changes in environmental conditions may also be due to interactions between trophic and biogeochemical changes that occur once new species become established, or to altered abiotic parameters or physical dynamics, such as temperature and stratification that are caused by climatic changes (e.g., Sunda et al., 2006; Glibert et al., 2011; Glibert, 2015; Wells et al., 2016). The complex set of adaptive strategies associated with different species will lead to some species being more or less successful in contrasting environmental conditions (e.g., Margalef, 1978; Collos, 1986; Glibert and Burkholder, 2011; Glibert, 2015, 2016). The growth of some species can alter the biological and biogeochemical environment, in some cases changing the environment favorably for their own further growth, or for growth of other harmful species. No amount of pressure from an altered rate of species introductions will ensure success of that species in a new environment unless conditions are suitable for its growth (e.g., Smayda, 2002; Glibert, 2015). The success of HAB lies at the intersection of the physiological adaptations of the harmful algal species and/or strain (population), the environmental conditions, interaction with co-occurring organisms (both biogeochemically and trophodynamically), and physical dynamics that alter abiotic conditions and/or aggregate or disperse cells (or can alter abiotic conditions in a favorable or unfavorable manner), in turn promoting or inhibiting their growth. āStrainā is mentioned here because it is well established that there can be high intraspecific variation (strain differences) within a given harmful algal species in a wide array of traits ranging from morphology, reproductive characteristics, and nutritional preferences to toxicity (Burkholder et al., 2005; Burkholder and Glibert 2006, and references therein).
As stated by Wells et al. (2016, p. 69) in their review of HAB and climate change, for HAB to be successful, it depends on the āspecies āgetting thereāā¦ābeing thereā as indigenous speciesā¦and āstaying thereā.ā The same is true for nutrients and related environmental conditions. They must āget there,ā often from anthropogenic sources; they must ābe thereā; and they must āstay there,ā often through physical dynamics, changes in trophodynam...
