Dynamics and Ecological Implications of Ostreopsis siamensis Blooms in northeastern New Zealand

Abstract

Blooms of the benthic dinoflagellate, Ostreopsis siamensis have become a concern on shallow reefs in northern New Zealand since their discovery in the 1990s. This species occurs in dense concentrations on seaweeds producing a thick mucus biofilm, and is of concern due to its ability to produce palytoxin analogues. The aim of this investigation was to ascertain the response of O. siamensis to environmental and seasonal variables and the impacts upon ecological communities. I utilised observational and experimental techniques to ascertain its effect and explored a number of abiotic factors which have been hypothesised to influence O. siamensis bloom dynamics along the coastline of Leigh, Auckland over the 2016-2017 season. Previous studies have shown O. siamensis blooms peak in summer when temperatures are warmest, but this study provides the first demonstration that cells persist at low concentrations throughout winter and elevated concentrations can also occur in spring when water temperatures are ~16oC. Consistent with previous studies, large wave events dispersed cells, but this study also suggested blooms may be dispersed in shallow water during spring tides and that blooms will be less severe at sites with greater freshwater input. This likely explains why blooms have not been reported from inner parts of the Hauraki Gulf. This investigation suggests that despite the high concentrations of O. siamensis cells observed on the large brown seaweed Carpophyllum plumosum, the abundance and species composition of small mobile invertebrates (epifauna) on the seaweed did not show a negative response. Experimental exposure of epifauna to O. siamensis however, suggested that some species may be vulnerable to being entrapped in the biofilm, which results in increased mortality. This suggests that the effect of O. siamensis is likely to have a structural impact on epifauna rather than an intoxicating effect. The health of sea urchins was found to be worst at sites with the highest concentrations of O. siamensis, but this did not translate into reduced grazing rates on kelp. This suggests that during the early stage of bloom, O. siamensis was not having a negative effect on grazing rate. While these results demonstrated limited effects on epifauna and sea urchin grazing rates, the severity and occurrence of these effects will likely vary between years depending on the intensity and longevity of blooms. The results clearly indicate that the intensity and longevity of blooms will vary among sites and be determined by complex relationships between water motion, temperature and salinity. Further research should focus on assessing the ecological impact of O. siamensis at multiple stages of the bloom and investigating how the intensity and longevity of blooms may be affected by future environmental changes.