|Paper title||Optical detection of harmful algal blooms in the Belgian coastal zone: A cautionary tale of chlorophyll c₃|
|Form of presentation||Poster|
Phaeocystis globosa is a nuisance haptophyte species that forms annual blooms in the southern North Sea and other coastal waters. At high biomass concentration, these are considered harmful algal blooms due to their deleterious impact on the local ecosystems and economy, and are considered an indicator for eutrophication. In the last two decades, methods have been developed for the optical detection and quantification of these blooms, with potential applications for autonomous in situ or remote observations. However, recent experimental evidence suggests that the interpretation of the optical signal and its exclusive association with P. globosa may not be accurate. Specifically, in the North Sea, blooms of P. globosa are synchronous with those of the diatom Pseudo-nitzschia delicatissima, that are found growing over and inside the P. globosa colonies. P. delicatissima is another toxic harmful bloom-forming species with similar pigmentation and optical signature to P. globosa.
In this study, we combine new and published measurements of pigment composition and inherent optical properties from pure cultures of several algal and cyanobacterial groups, together with environmental spectroscopy data, to identify the pigments generating the optical signals captured by two established algorithms: (1) The classification tree based on the positions of the maxima and minima of the second derivative of the water-leaving reflectance data; and (2) the Chlorophyll c3 (Chl c3) concentration estimation with a reflectance exponential baseline height. We further evaluate the association of those pigments and optical signals with P. globosa.
Our results show that the interpretation of the pigment(s) generating the optical signals captured by both algorithms were incorrect and that the published methods are not specific to P. globosa, even in the context of the phytoplankton assemblage of the southern North Sea. The positions of the maxima and minima in the second derivation of the water-leaving reflectance are defined by the relative concentrations of total Chl c and photoprotective carotenoids (PPC), and not Chl c3 and total carotenoids, as previously suggested. Similarly, the exponential baseline height captures the signal of total Chl c concentration, and cannot isolate the signal from Chl c3 due to the large overlap in the Soret band center position in the Chl c family. Additionally, the position of the minima and maxima of the second derivative can be affected by the presence of Chl b and environmental conditions influencing PPC concentration.
More fundamentally, we found that the optical and pigment signatures of Phaeocystis species are part of a broad pigmentation trend across unrelated taxonomic groups, related to chlorophyll c3 presence. Based on a large database of pigmentation patterns from pure cultures, we observed that the presence and amount of Chl c3 is positively correlated with the concentration of total Chl c and negatively correlated with PPC concentration. This has important consequences for the interpretation of pigment and optical data, particularly in environments where multiple species with similar pigmentation pattern co-occur, as observed in the southern North Sea during P. globosa blooms.
The available information on the relative contribution of cell biomass and pigments to the total pool from diatoms containing the Chl c3 and P. globosa suggests that it is not possible to unequivocally assert that the signal is generated by P. globosa. This is a consequence of year to year variation in the relative cellular biomass of these species during the bloom, the progressive colonization of P. globosa by P. delicatissima along the bloom development, and the low pigment to cellular biomass from P. globosa when compared to the diatoms.
We therefore propose and validated an algorithm to estimate the fraction of Chl c3 from the total Chl c pool, as it carries information on the presence of this pigment and the relative dominance of species presenting the pigmentation pattern of high total Chl c and low PPC. In the southern North Sea, this pigmentation pattern is only observed in P. globosa, P. delicatissima and Rhizosolenia species, the first two being HAB species and dominating the biomass and pigment signal. The Chl c3 fraction in the southern North Sea therefore can be interpreted as an indication of the the relative dominance of HAB species. The new algorithm suffers minimal influence from co-occurring pigments (e.g., Chl b, other forms of Chl c, carotenoids) and can be applied to absorption or reflectance data, with potential for application to the next generation of aquatic space-borne hyperspectral missions. We further elaborate general recommendations for the future development of algorithms for phytoplankton assemblage composition, considering the biology, ecology, optical signal and interpretation.