More than propagule pressure: Successful invading algae have physiological adaptations suitable to anthropogenically changing nutrient environments

Abstract

While increased propagule pressure may increase the possibility of success of an invading species, success is also a function of the environment to which the organisms have been introduced and the physiological strategies of the invading organisms and how those strategies relate to those of the native fauna or flora. Using examples of altered nutrients in aquatic environments, the effects of environment on the success of harmful algae are herein considered. Many harmful algae are introduced species, especially via ballast water. While increasing numbers and frequency of harmful algal blooms in eutrophic waters may be comparatively easy to understand—more nutrients, more biomass, more opportunities for species to proliferate—one of the more perplexing issues is why some harmful algal bloom species seem to become more prevalent under conditions of disproportionate nutrient supply. Focusing here specifically on the case of excess nitrogen loading relative to that of phosphorus, there are a number of key adaptive strategies that may allow certain types of harmful algal blooms to thrive. Successful invaders may be smaller in cell size, thereby reducing phosphorus cellular demands, and they may have nutrient substitution or acquisition strategies allowing them to use a source of nutrient not otherwise available to competitors. They may also create local environmental changes, such as increases in pH that detrimentally affect competitors while altering sediment biogeochemical processes increasing efflux of the otherwise limiting nutrient. Finally, their growth and cellular elemental stoichiometry also affect food quality for grazers, including toxin production, which has been shown in numerous species to increase when the cells are not grown under nutrient-balanced conditions. Through their detrimental effects on grazers, they may further destabilize ecosystem environments increasing susceptibility for additional ecosystem changes, including creating environmental conditions suitable for additional invasions to occur.

Keywords:

• harmful algae
• harmful algal blooms
• nutrient ratios
• stoichiometry
• P limitation

Acknowledgements

The author wishes to thank the organizing committee of Marine Invasive Species: Management of ballast water and other vectors conference for the invitation to give this keynote lecture. This is a contribution of the GEOHAB Core Research Project on HABs and Eutrophication. It is contribution number 4931 from the University of Maryland Center for Environmental Science.