Global Harmful Algal Blooms

Rationale. This Theme constituted Programme Element 4 in GEOHAB (GEOHAB 2001), which was implemented by the CRP on Eutrophic Systems (GEOHAB 2006). Important progress in research and management was achieved through the implementation of GEOHAB (e.g., see recent review by Glibert and Burford 2017), but the specific objectives identified at the beginning of that programme are still relevant and applicable in GlobalHAB. Coastal nutrient eutrophication is often considered to be primarily responsible for the increased number of high-biomass HAB occurrences in marine, brackish and freshwater habitats (Heisler et al. 2008, Glibert and Burford 2017 and references therein). Increases in HABs associated with eutrophication or nutrient loading have been widely reported (e.g., Anderson et al. 2002, Heisler et al. 2008). Although a simple link can be established because nutrient inputs yield changes in algal biomass (Ryther and Dunstan 1971), the effect of nutrients on specific harmful species is still poorly understood. Variations in nutrient supply can impact biodiversity and the success of HAB species depends on the intersection of the physiological adaptations of the harmful algal population, environmental conditions, and interactions with co-occurring organisms, as summarized by Glibert and Burford (2017) (and references therein). However, in some cases, HABs are not directly driven by anthropogenic nutrients (Gowen et al. 2012). Nutrient loading is most frequently associated with high-biomass blooms, rather than to low-biomass biotoxin-producing blooms. However, even high-biomass HABs can occur in areas with low anthropogenic nutrient additions (Davidson et al. 2014). For example, toxic HABs of genera such as Alexandrium can occur in regions that do not experience significant anthropogenic nutrient additions, and the most toxic HABs occur in the most pristine areas in the world, such as Patagonia and Alaska. 

 

Alexandrium minutum bloom in a tropical coastal zone. Photo: P.T. Lim.

 

Fundamental questions remain unresolved in relation to understanding how the fluxes of both inorganic and organic nutrients from anthropogenic and natural sources promote high- and low-biomass HABs, and how nutrient fluxes are connected to the drivers of climate change and other environmental stressors. Specific studies are required to understand the role of nutrients in the promotion of particular species, and how changes in nutrient dynamics are linked to the onset, maintenance and decay (potentially leading to hypoxia and/anoxia, Anderson et al. 2002) of HABs, and the modulation of toxin production. These questions are fundamental for better understanding of the response of planktonic (e.g., Pseudo-nitzschia, Alexandrium) and benthic (e.g. Gambierdiscus, Ostreopsis) HAB taxa to nutrient availability. In the case of BHABs, the limited available data do not allow drawing a clear causal relationship between inorganic nutrient supply and benthic blooms (GEOHAB 2012). The nutrients required to reach bloom concentrations are likely acquired directly from nutrient-rich sediments, or directly from the macroalgal surfaces (Litaker et al. 2010 and references therein). Furthermore, it is hypothesized (but not tested) that nutrient inputs, which favor the growth of macroalgae (Lapointe et al. 2010), could also promote blooms. Finally, the role of nutrient availability o

n toxicity requires detailed studies (Chinain et al. 1999; Vanucci et al. 2012). Understanding the link of between the supply and availability of different and availability of different forms of nutrients with the occurrence of HABs and their harmful effects, is fundamental to design management strategies.