Global Harmful Algal Blooms

Rationale. The GEOHAB programme adopted the comparative approach (Anderson et al. 2005) from the cellular to the ecosystem level. This approach is based on the view that the ecology and oceanography of HABs can best be understood through study of the causative organisms and affected systems in relation to comparable organisms and systems. Important physical processes occur over a wide range of scales. Similarly, relevant biological processes occur at subcellular and cellular levels, as well as at the population, community, and ecosystem levels including, for example, symbiosis, parasitism, allelopathy, and viral infections (e.g., Chambouvet et al. 2008; Ianora et al. 2011). Because HABs are natural phenomena, application of the comparative approach can also provide further insights in the understanding of plankton ecosystem dynamics. Improved generalizations about the causes and consequences of HABs would be particularly useful in management and mitigation of their effects. GlobalHAB will continue to use the comparative approach for studies within and between different aquatic systems (coastal, brackish, open oceanic and freshwater environments). 

Case studies reported by GEOHAB (e.g., GEOHAB 2005, GEOHAB 2010) and elsewhere (e.g., Pitcher et al. 2017) illustrate some of the advances achieved by the application of the comparative approach. Ongoing and future research on HABs can progress by applying this method with the suitable experimental design to carry out comparisons in a scientific way (Underwood 1992). Some examples are briefly presented here, but not described in detail. The comparative approach can contribute to understand similarities and differences concerning CFP incidence and Gambierdiscus dynamics in the main affected areas, the Pacific Ocean and the Caribbean Sea. Comparing the dynamics of the main benthic HAB taxa, Ostreopsis and Gambierdiscus, can also shed light on poorly known ecological aspects related to the benthic life strategies and facilitate the design of effective management tools in each case. This information would also support the development of trait-based approaches for both comparative systems and modeling by identifying commonalities across the two genera. An interesting comparison could also be addressed on the mechanisms of human intoxication via sea spray associated with the benthic Ostreopsis (Vila et al. 2016) versus the planktonic Karenia blooms (Pierce et al. 2005). 

Another example of blooms suitable for the comparative approach are the blooms of Pseudochattonella spp. (Dictyochophyceae) that have caused fish mortalities in northern Europe, as well as in Japan (e.g., Okaichi 1997) and in South America. In Scandinavia, blooms started in 1998 (Granéli et al. 1993; Dahl et al. 2005) and the organisms are likely to have been introduced to the area. Initially, blooms occurred in early summer, but since 2001 blooms have happened immediately after or together with the spring diatom bloom, usually in March. In 2016, blooms of Pseudochattonella caused mortalities of 20% of the total Chilean salmon production in a few days (Clément et al. 2016) and subsequent exceptional blooms of Alexandrium catenella devastated shellfish production (Hernández et al. 2016). By comparing the blooms in different geographic areas, an increased understanding of why blooms occur and their effects will be achieved. 

Tools for managing blooms of biotoxin-producing dinoflagellates in shellfish production areas can also be obtained by application of the comparative approach. For instance, Dinophysis spp. that produces okadaic acid and other diarrhetic toxins are found in areas with contrasting environmental conditions such as the west coast of the Iberian Peninsula, Scandinavia and the east coast of North America. During the past decade, understanding of Dinophysis ecology has increased significantly through laboratory work (e.g., Reguera et al. 2012). Applying this new knowledge in field studies in different environments is likely to result in a better understanding of bloom development and how toxin content per cell varies. Similarly, Alexandrium, producer of paralytic toxins, is another key genus suitable for the comparative approach. It has a worldwide distribution and is found in very different environments and climates, including the Mediterranean Sea region, and Chilean and Scandinavian fjords (Anderson et al. 2012). Furthermore, in the case of Alexandrium and Dinophysis, comparisons at the level of single species could clarify taxonomy and physiology questions such as the ones indicated in the Biogeography and Biodiversity and Adaptive Strategies themes, respectively. 

Finally, toxin-producing filamentous cyanobacteria such as Nodularia occur in contrasting areas such as the Baltic Sea (Kahru and Elmgren 2014), river mouths in Australia (Huber 1984) and in lakes in Brazil (Karlson et al. 2012), where nutrient conditions and the capability of nitrogen fixation are key forcing factors. By studying conditions leading to blooms at different latitude, salinities, etc., an increased understanding of bloom formation and effects of climate change on cyanobacteria blooms may be achieved.