Nitrogen (N) is highly variable in either forms or concentrations in coastal and estuarine environments, and dinoflagellates such as Alexandrium tamarense are bound to expose to different forms of N in high concentrations due to either horizontal dispersion or diel vertical migration. N supply in pulses was simulated by exposing A. tamarense to sudden increase in N concentrations of three different sources; nitrate, ammonium and urea, and examined the physiological, toxic and optical characteristics of A. tamarense coming into contact with these N sources. The present study showed that when the cells were exposed to different N environments and concentrations, they exhibited acclimative strategy by regulating their cellular materials which was associated with growth. The ability of Alexandrium to utilize various forms of N sources is an advantage for survival and maintenance of high growth rates in competitive coastal ecosystems. The ability to take up, adapt and regulate cellular contents depending on the N source suggests that N sources may affect the dynamic of dinoflagellate bloom in the coastal ecosystem.

The cellular toxicity was dependent on the N source utilized and the variability in toxicity was different among N source. The present study concludes that nutritional status of cells is the controlling factor for toxin synthesis. Differences in toxicity and toxin profile due to the forms of N source utilized suggest that when field population of Alexandrium cells coming into contact with elevated N, the level of toxicity and the degree of deleterious impact on the coastal ecosystem is dependent on the N utilized. Populations utilizing ammonium could be more toxic than those growing on nitrate or urea. The ability to adapt and acclimate to different N environment allows Alexandrium species to maintain its population for prolong period, thus posing a threat in contaminating shellfish. Although toxicity and growth of Alexandrium are known to vary geographically, the nutrient-toxicity relationship could provide a better understanding of the bloom dynamics of Alexandrium. This can aid in the predicting of Alexandrium blooms and is essential for the mitigation of harmful dinoflagellates in coastal ecosystems.

Varying patterns of the chlorophyll a-specific absorption due to physiological changes could affect the accuracy of bio-optical models such as primary production. Therefore, variability in the absorption characteristic caused by environmental conditions should be incorporated into models. Monitoring and predicting HABs is central to ameliorate their negative impact on human health and the economies of local communities. The optical approach using a combination of absorption coefficients at three wavelengths examined in the present study, showed feasibility in detecting A. tamarense. Variable environmental conditions do not seem to affect the absorption ratios dramatically and these ratios could be an adequate biomarker for dinoflagellates such as A. tamarense and could be used to detect the presence of harmful taxa. In addition, these ratios could detect the presence of A. tamarense from diluted samples. If the absorption coefficients at various wavelengths could be extracted from remote sensing reliably, it would be feasible to monitor HABs or even detect the presence of harmful taxa in coastal areas on a large scale basis. Moreover, this absorption ratio technique may enhance the capability for detecting harmful species.

Variability in growth, physiological, toxic and optical characteristics of A. tamarense is dependent on the N source utilized and concentrations. Among the three N sources, ammonium has been shown to induce highest growth rate and cellular toxicity in A. tamarense. The ability to adapt and acclimate to different N environments suggests that field population of Alexandrium could utilize multiple N sources for ensuring uninterrupted growth and proliferation and could maintain high cellular toxicity. The information on the physiological responses of cells to different N sources observed in the present study could provide a better understanding of the bloom dynamics of Alexandrium. Understanding the dynamics of bloom can assist in the predicting of Alexandrium blooms and is essential for the mitigation of harmful dinoflagellates in coastal and estuarine ecosystems. Various approaches has been developed over the last decade, however, there is no single approach that can give prior warning of the initiation of a bloom. Although, the present absorption ratio approach has not been tested in the field, its sensitivity in distinguishing diluted samples showed its potential in detecting harmful dinoflagellates and it could provide a means in interpreting field populations. The technique proposed could assist in expanding the capabilities for detecting harmful taxa.