Monitoring methods and bloom dynamic studies of the toxic dinoflagellate genus Alexandrium
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Records of annual monitoring of the dinoflagellate genus Alexandrium in Cork Harbour, Ireland date back to the 1990's. Intense bloom densities in this area have caused major economical loss and impacted negatively on the ecosystem. It has been established that the noxious species Alexandrium minutum is responsible for the toxic events on the south coast of Ireland, leading to outbreaks of paralytic shellfish poisoning (PSP). PSP is a potentially lethal syndrome in humans caused by the ingestion of potent neurotoxins collectively known as saxitoxins, which are vectored through shellfish. A reliable method of prediction for harmful algal blooms (HABs) in this region could potentially lead to bloom mitigation methods and help reduce the severity of their impact. As we still know relatively little regarding the behavior and life cycle patterns of A. minutum, further investigation into its sexual and asexual reproductive phases, and how they impact upon the species' population dynamics may assist its predictive ability. During the course of this study, A. minutum blooms in the North Channel area of Cork Harbour were monitored during the summer periods of 2011 and 2012. A time series data set of annual A. minutum bloom densities between the years 2004 and 2010 is also presented. Hypnozygote studies carried out in situ found a mandatory cell maturation time of the order of months to precede successful excystment, with temperature acting as a significant influencing factor. Bloom magnitude was a found to be independent of the cyst density measured the previous winter, suggesting hypnozygote cells provide the bloom inoculum and environmental factors determine bloom magnitude. Encystment levels within the population were also assessed through the deployment of sediment traps. It was determined that regardless of bloom intensity, an average of 2.5% of the proportion of A. minutum vegetative cells encyst annually within the North Channel. A cyst dynamics model simulating the annual cyst input to the sediment between 2003 and 2012 found intense blooms (>100,000 cells L-1) are not necessary to maintain cyst stocks. A simple ecological model was used to simulate bloom initiation based on the environmental parameters of irradiance, temperature and tidal dilution, while sexual reproductive stages were used to simulate bloom termination. Tidal dilution was found to exert a substantial level of control over bloom development, a process thought to require 10-14 days. It was evident that maximum bloom development followed the lowest spring tide in June. This result may contribute towards developing a reliable method of A. minutum bloom prediction. Aside from the role physiological changes play in A. minutum population dynamics, an attempt was made to determine the level of influence parasitism exerted on the functional ecology of the population. Both parasitic marine genera Amoebophrya and Parvilucifera were indentified in the North Channel. While Amoebophrya was evidently infecting various dinoflagellate host species, the host to Parvilucifera was not confirmed. No parasitic infection was observed within the A. minutum population. This work constitutes the first report of infections on dinoflagellates in Irish coastal waters and creates a platform for future studies. As observations of dinoflagellate population dynamics in the natural environment require an accurate and consistent method of monitoring, this study also researched the comparability of the various methods commonly employed for the monitoring of HAB events, and determined the potential of more automated methods. The FlowCAM showed its ability at successfully detecting individual species of interest and provided an efficient analysis of the phytoplankton community structure. No significant difference was found between it and the use of fluorescent in situ hybridisation techniques (FISH). The FlowCAM is suggested as a valuable contributor to consistent plankton records as part of ongoing monitoring efforts
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