Quantification of Life-stage Transition Behaviors in Harmful Algae and their Implications for Pelagic and Benthic Distributions

Abstract

Harmful algal blooms (HABs) occur when accumulations of algae or algal toxins have adverse impacts on aquatic ecosystems, public health and/or coastal resources. Many of the behavioral and physiological functional traits that regulate HAB dynamics remain poorly understood. Improved understanding of these traits and innovative technologies to detect HAB cells <italic>in situ</italic> are important for assessing future bloom scenarios and establishing appropriate HAB mitigation and management strategies. Many HAB-forming species exhibit a dual-stage life history, alternating between pelagic and benthic life stages. Rates of transition between pelagic and benthic habitats can regulate cell dispersal and contribute to the timing and severity of HABs. Yet, life stage transitions are among the least understood aspects of HAB dynamics. The focus of this research was to characterize and quantify behaviors of harmful motile marine algae during life-stage transitions and to assess their influence on pelagic and benthic population distributions. Two HAB species that commonly occur in the Salish Sea, the fish killing raphidophyte, <italic>Heterosigma akashiwo</italic>, and the toxic dinoflagellate, <italic>Alexandrium catenella</italic>, were the focus of these studies. A combination of field, laboratory and modeling methods were used to elucidate the role of life-stage transitions in bloom dynamics. The primary objectives were to (1) examine benthic emergence characteristics of naturally occurring cysts of <italic>A. catenella</italic> (Chapter 2); (2) quantify strain-specific swimming characteristics during life stage transitions of <italic>H. akashiwo</italic> (Chapters 3 & 4); (3) assess how specific behavioral and physiological traits regulate vertical fluxes and population distributions of <italic>H. akashiwo</italic> (Chapters 3 & 4); and (4) develop a low cost, field-deployable sensor to detect and characterize algal benthic emergence <italic>in situ</italic> (Chapter 5). The research outcomes indicated that important algal physiological and behavioral traits, including cell survivorship during transitions between life stages, internal regulation of benthic emergence rates, transitional swimming behaviors and post-transition specific growth rates, govern vertical distributions of some harmful algae and the timescales over which HABs form and dissipate. These traits were shown to be largely species and strain dependent, resulting in both species- and region-specific benthic emergence and growth strategies. Results demonstrate that behavioral and physiological traits expressed during life-stage transitions play critical roles in regulating distributions of harmful algal populations. Further, <italic>in situ</italic> detection and monitoring of benthic emergence and other behavioral traits of harmful algae will improve mechanistic understanding of HAB formation and enhance our capacity for successful bloom prediction.