An Investigation into How Mixotrophy and Deleterious Chemical Production Give Harmful Algae a Competitive Advantage in Systems Influenced by Anthropogenic Disturbance

Abstract

Harmful algae blooms (HABs)are an increasing problem world-wide. The increasing occurrence and magnitude of HABs has often been linked to anthropogenic disturbances, some of which include salinization, nutrient ratios skewed away from the Redfield ratio, and increased nutrient loading. The research presented here contributes to the impressive pre-existing collection of studies focusing on the HAB problem. In Chapter 1, it is shown that co-occurring disturbances can have synergistic effects on a mixotrophic HAB species, Prymnesium parvum. It was found that increased salinity, community composition change via removal of large zooplankton, and elevated P. parvum propagule pressure interact to influence the abundance of P. parvum cells, and that the effect is dependent on system specific salinity history. When P. parvum was inoculated into a plankton community at low density it increased most if zooplankton were removed and salinity was elevated. In Chapter 2, it was demonstrated that when the nitrogen to phosphorous ratio was skewed away from the Redfield ratio allelochemicals produced by P. parvum had acute mass mortality effects on multiple trophic levels. But, when nutrients were supplied at the Redfield ratio allelochemicals were ineffective and cell-to-organism contact was required for deleterious effects to occur. Importantly, only when the P. parvum population was at high densities did it appear to have negative effects on other organisms, suggesting that mass effects of allelochemicals may not contribute to bloom initiation. In Chapter 3, it was demonstrated that flexible nutrition could lead to population persistence of an otherwise non-competitive theoretical phytoplankton species, but only when the species was highly inedible to zooplankton grazers. Additionally, it was shown that as nutrient loading was increased, system stability decreased via the initiation of zooplankton- phytoplankton boom-bust cycles. However, the presence of a mixotrophic population stabilized the system by reducing the number of cycles that occurred. Notably, when nutrient loading was elevated community dynamics became complex and the mixotroph population density was unpredictable from year to year, sometimes reaching high densities. This finding demonstrates how a bloom of a mixotroph can get started with enrichment when no role of toxins is considered.