Understanding the direct and indirect effects of algal toxins on marine copepods

Abstract

Copepods form an important link between phytoplankton and higher trophic levels. Several species of phytoplankton, including dinoflagellates of the genus Alexandrium, produce neurotoxins commonly known as paralytic shellfish toxins (PSTs). The toxins from harmful algae (HA) may impact copepod survival, eeding, and fitness by acting as a feeding deterrent and/or by causing physical incapacitation. However, copepods may be able to overcome these toxic effects and/or become tolerant to toxicity by partial metabolism. Published information on how HA affect survival, feeding and other physiological processes in opepods are difficult to compare due to the different concentrations of HA used as food, the level of toxins in the food, and the various responses measured on different copepod species from different locations. Very few experiments have examined how HA toxins influence the survival, feeding and fecundity of copepods within UK waters. This thesis aims to address this knowledge gap whilst also choosing organisms of wider geographical relevance. This study examined the effects of a toxin-producing dinoflagellate, Alexandrium catenella, on two physiologically different copepods: Acartia tonsa, a pelagic coastal copepod that is found in the UK and other coastal waters including Northern & Southern America and Australia, and Calanus helgolandicus, which is spread across the North East Atlantic with high numbers on the European shelf and in oceanic waters. In Chapter 3, short-term (24 h) survival and feeding experiments revealed that adult female A. tonsa can survive exposure to field-recorded bloom concentrations of toxic A. catenella. Survival only decreased when exposure levels exceed reported environmental concentrations by two orders of magnitude. The lethal median concentration (LC50) was 12.45 ng STX eq L−1. Ingestion rates were higher when offered A. catenella in the absence of alternative prey, potentially suggesting compensatory feeding. A. tonsa actively selected non-toxic Rhodomonas sp. over toxic A. catenella when offered a mixed diet. Chapter 4 demonstrated that the survival of female A. tonsa is not affected by prolonged (10 days) exposure to toxic A. catenella. However, additional feeding and egg production experiments suggested that whilst A. tonsa can obtain enough energy from ingesting toxic A. catenella to survive, it suffers reproductive impairment when feeding on this prey alone. In Chapter 5, C. helgolandicus showed a decrease in feeding rate when feeding on toxic A. catenella compared to when feeding on the non-toxic congener, Alexandrium tamarense. On the other hand, the egg production and hatching success rates were not affected by the relative abundance of toxic A. catenella and non-toxic A. tamarense in diet, suggesting they may have used biomass reserves to sustain egg production. Body toxin analysis of C. helgolandicus showed they may bioaccumulate toxins in their bodies; however, the retention efficiency was very low. Full toxin profiles for A. catenella, including 8 to 12 PSTs, are presented in all experiments. This study furthers our understanding of PST-producing HA-copepod interactions, and how they may be affected by the increased frequency and magnitude of HA blooms.

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