Using cell size to represent phytoplankton diversity in studies of nitrogen dynamics in the southern Benguela
Phytoplankton are a key component in the functioning of marine ecosystems and play a central role in the cycling of nitrogen and other elements. Metrics that can adequately represent the biogeochemical processes associated with phytoplankton diversity are needed in order to make use of remote sensing and modeling platforms. A single-value size proxy, effective diameter (Deff ), represents the mean volume to surface area ratio across the nano and micro plankton size fraction (2-200µm) in the southern Benguela, but has yet to be tested regarding its biogeochemical relevance. Cell size imposes overarching constraints on phytoplankton metabolism; there are therefore strong grounds for evaluating the usefulness of the metric (Deff ) in studies of nitrogen dynamics in diverse, natural assemblages. Three case studies were used to explore the nitrogen dynamics in naturally occurring assemblages and to evaluate the relationships between Deff and the uptake of the different sources of nitrogen. Two of the case studies comprised high biomass, harmful algal blooms observed off Lamberts Bay during an upwelling/downwelling cycle. The third case study used bi-monthly sampling over a full year in Saldanha Bay. The Lamberts Bay case studies involved blooms occasionally dominated by HAB-forming species: a mixotrophic ciliate, Myrionecta rubra, and a dinoflagellate, Prorocentrum triestinum. The nitrogen uptake rates followed the well observed pattern of high nitrate uptake by large cells and regenerated nitrogen uptake by small cells. Myrionecta rubra had a wide range of nitrate (O₃⁻ ) uptake rates (0.02-0.3 µmol N L⁻¹ h⁻¹). Prorocentrum triestinum showed slower rates of O₃⁻ uptake (0.01-0.2 µmol N L⁻¹ h⁻¹) and dominated in low O₃⁻ , stratified conditions. Diatoms were the most efficient utilisers of O₃⁻ and total nitrogen in these cases. The effective diameter was significantly related to the uptake rates of ammonium (NH₄⁺ ) (r=-0.54, p<0.005) and urea (r=-0.59, p<0.005), but not O₃⁻ (r=0.27, p=0.11). This was attributed to some instances of bi-modality in observed size distributions as well as potentially specialist nutrient uptake strategies employed by diatoms. The year-round data from Saldanha Bay indicated the system was diatom-dominated and was used to assess 1 how well Deff could represent the nitrogen uptake strategies employed by the diverse diatom assemblages. The Saldanha Bay system has O₃⁻ limited surface waters during summer, and light-limited bottom waters during winter. No significant relationship was found between Deff and the mass-specific uptake rates of the different nitrogen species in this data set. This was attributed to the complex shapes of the size distributions and the comparatively low biomass observed. Uptake kinetic experiments revealed high variability for maximum uptake rates (Vmax) and half saturation values (Ks) for both O₃⁻ and NH₄⁺ . For O₃⁻ : Vmax ranged 0.007-0.17 µmol N L⁻¹ h⁻¹, and Ks ranged between 0.2-42.5 µmol N L⁻¹. For NH₄⁺ Vmax was observed between 0.02-2.7 µmol N L⁻¹ h⁻¹; and Ks values ranged 0.1- 14.02 µmol N L⁻¹. Variability was observed in association with the availability of the ambient sources of nitrogen, but some variation was accounted for by the presence of different diatom species. From these three case studies it was concluded that the single-value size proxy was an adequate metric to quantify the uptake of regenerated nitrogen in scenarios of high biomass algal blooms. Such blooms are a pervasive feature in the southern Benguela Ecosystem. For lower biomass blooms, however, Deff did not adequately represent the nutrient dynamics of diverse diatomdominated assemblages. The variable shape of the size spectrum is an important factor in determining the rates of nutrient uptake and, in cases of bi- or multi-modality, this information could be lost when represented by a single descriptor such as Deff . It was subsequently hypothesised that size spectra could be used to accurately represent the nitrogen dynamics in diverse phytoplankton assemblages. This was tested by comparing the observed uptake rates of the three case studies to estimated uptake rates based on size spectra. Observed particle size distributions were used to estimate the uptake of O₃⁻ and NH₄⁺ , based on theoretical relationships to calculate size-dependent values of Vmax and Ks. Michaelis-Menten models were applied to measured ambient nutrient concentrations and particle size distributions, generating size-integrated estimates of O₃⁻ , NH₄⁺ and total N uptake rates. The variability in the estimated uptake rates was similar to that of the measured values. It was thus concluded that the representation of phytoplankton diversity by size spectra allowed modification of model parameters, such that improved estimates of uptake rates of O₃⁻ and NH₄⁺ could be obtained for a dynamic eutrophic environment.
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