Noctiluca scintillans ,a large heterotrophic dinoflagellate, has been present in New South Wales coastal waters since 1860. Noctiluca was a minor component of the phytoplankton up until the last decade, but since then, Noctiluca blooms appear to have increased in frequency and intensity, with the majority of blooms recorded in recent years. There has been public concern that this increase in Noctiluca blooms may be a result of chronic sewage discharge from three deepwater ocean outfalls in the coastal waters surrounding Sydney. Such concern is consistent with the debate that Noctiluca maybe a coastal eutrophication indicator species. Over a one year period (1997-1998) at a long-term monitoring station off Sydney, Noctiluca was present year round with peak abundances coinciding with episodic slope water intrusions (and subsequent upwelling) during spring and summer. Extensive red tides of Noctiluca often succeeded diatom blooms (specifically Thalassiosira spp.) which were initiated by these intrusions.  80-90% of Noctiluca cells contained food particles and there was a positive linear relationship between abundance and the proportion of cells containing food. Prevalent food items within the vacuoles of Noctiluca were diatoms. Water samples collected north of Sydney also showed that highest numbers of Noctiluca occurred in areas predisposed to upwelling. Upwelling is likely to be the mechanism that promotes population growth of Noctiluca along the coast of N.S.W. Despite these findings, it is still difficult to disregard the direct or indirect effects of anthropogenic nutrients on the growth of Noctiluca. A long term assessment of physico-chemical dynamics in the water column off Sydney showed that there was no real change in phytoplankton biomass nor in uplifting/upwelling frequency in the last decade, yet the recent year round presence of Noctiluca is unprecedented for this region. A shift in the dominant diatom genera to Thalassiosira (believed to be the optimal food source of Noctiluca) concomitant with a rise in temperature and reduction in nitrate and phosphate levels may be the cause for the increase in Noctiluca numbers during this sampling year. The presence of a strong ENSO signal in 1997-1998 was shown in companion studies to dominate physico-chemical conditions, and thereby mask any anthropogenic effects. Future investigations need to incorporate this influence to resolve and partition climatic and anthropogenic signals.

Pseudo-nitzschia abundance is extremely high in the northern Gulf of Mexico in the plume of the Mississippi River, especially when river flow and nutrient inputs are high. Five sediment cores were collected to determine if, and to what extent, Pseudo-nitzschia valves preserved in the sediment in order to reconstruct changes in Pseudo-nitzschia abundance in the past century. Pseudo-nitzschia increased in relative and absolute abundance in all five sediment cores, most markedly in the sediment layers corresponding to the time period between the 1950s and 1970s as determined by lead-210 sedimentation rate estimates. Previous research has demonstrated that a dramatic increase of fertilizer use in the United States occurred at this time, which has resulted in the development of eutrophication and hypoxia in the coastal waters of the northern Gulf of Mexico. Scanning electron microscope analysis of the fine structure of Pseudo-nitzschia and other lightly-silicified diatom valves preserved in the sediment indicates that the increase in Pseudo-nitzschia abundance appears to reflect a response to eutrophication rather than valve dissolution processes. This study provides evidence for a direct link between coastal eutrophication and harmful algal blooms.

 

Abstracts:

On the long-term response of harmful algal blooms to the evolution of eutrophication off the Bulgarian Black sea coast: Are the recent changes a sign of recovery of the ecosystem? — The uncertainties

 

Snejana Moncheva, Institute of Oceanology, BAS, Varna, 9000, P.O. Box 152, Bulgaria Based on the level of eutrophication of the Black sea coastal ecosystem the 1954 - 1998 period could be subdivided into: a relatively pristine period (60-70ies), a period of an intensive anthropogenic pressure (the 80-ies), and the 90-ies a period of relaxation, related to the collapsing economy and agricultural production and the concurrent reduction of the land-based nutrients load to the basin. In the present paper the long-term trends (1954-1998) of phytoplankton blooms (species involved, frequency and timing) are discussed in relation to the evolution of anthropogenic eutrophication and the variability of temperature, sun activity and total zooplankton biomass. The main peculiarities of the current period - an increase in the diversity of red-tide species; a shift in their taxonomic composition, a decrease of blooms frequency and maximum densities attained; a reduction of summer blooms on the account of an increase in winter-spring and autumn events, coincide well with the decrease of nutrients level and the shift of their ratios especially in summer. The MDS plot of phytoplankton blooms and the environmental matrix reveal similar clustering discriminating between 70ies-90ies and 80ies, the 90-ies being more close to the 70-ies. The results suggest that the recent changes of phytoplankton blooms could be considered a sign of recovery as a response to the relative improvement of the chemical parameters of the coastal zone. The main uncertainties are still the maintained capacity of the ecosystem to produce high biomass, the concerted environmental changes and similar alterations reported for other regions of the World Ocean, suggesting a possible global climatic signal of influence too.