Do external resource ratios matter? Implications for modelling eutrophication events and controlling harmful algal blooms Kevin J. Flynn ⁎ Institute of Environmental Sustainability, Department of Pure and Applied Ecology, Swansea University, Swansea SA2 8PP, UK abstract article info Article history: Received 15 September 2009 Received in revised form 31 March 2010 Accepted 6 April 2010 Available online 24 April 2010 Keywords: Phytoplankton N:P resource ratio Competition P-stress Limiting resources Light limitation Relationships between nutrient N:P ratio and P-limitation in phytoplankton are explored using a multi- nutrient photoacclimative quota-based model. The relationship depends on concentrations of input and residual nutrients, and also on variable phytoplankton C:N:P stoichiometry. In reality, usually only the residual nutrient concentrations and their ratios are known. However, the total amount of nutrient present in the system affects biomass growth potential through self-shading, and thence the potential for variation in organismal N:P. The critical external N:P resource ratio above which P becomes limiting increases as residual concentrations of nutrients increase to saturate transport kinetics; oligotrophic waters require a lower nutrient N:P to avoid P-limitation than do eutrophic waters. In eutrophic systems, which may support harmful algal blooms (HABs), and/or in systems in which light is rapidly attenuated (sediment loading, gelbstoff), P-limitation may not develop even in high resource N:P situations due to light limitation. This is more likely in high washout systems, where phytoplankton growth rates must remain elevated. The only diagnostics for nutrient stress are cellular functions (C-fixation, C:N:P), and the only nutrient parameters of consequence are concentrations and not ratios of them. Control of resource ratios alone should not be considered as a tool for mitigating HABs. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The ratio of nutrients (N:P, P:Si, and N:Si) supporting phytoplank- ton growth has long interested aquatic science, primarily as factors affecting succession. There is a rich history of practical and theoretical research on the topic, much building from work in the 1970's. Two lines of research were explored at that time, the one principally ascribed to the work of Tilman (Tilman, 1977, 1982) considered the importance of the external resource ratio (e.g., Si:P) as a factor affecting competition and succession. The second originated from the work of Droop (1974), and developed by others (e.g., Mykelstad, 1977; Rhee, 1978; Rhee and Gotham, 1980; Turpin, 1986), considered the role in competition of the internal resource ratio (more often referred to as nutrient quota ratios, e.g., N:P). In many of these works, for both internal and external resources, deviations of the ratio around that described by the Redfield ratio is considered significant; this is because that ratio is typically considered to be the “optimal” ratio for phytoplankton growth. In fact there appears no physiological basis upon which to assume the importance of such a fixed ratio (Geider and La Roche, 2002). For both of these subject lines (external and internal ratios) the resource ratio at which growth is equally limited by two nutrients is identified as having particular significance as a switch point, of particular importance in defining the competitive advantage of one species over another when growing in an environment with different resource availabilities. Throughout this work this critical ratio will be identified as ext R crit or int R crit for external or internal resources respectively. Interest in the topic has been expanded with the realisation that the ratio as, and if, reflected in internal cellular N:P also affects the value of phytoplankton as food organisms by virtue of the stoichiometric disparity between predators and their prey (e.g., Urabe, 1993). The impact of this disparity can be exacerbated by other processes, such as the accumulation of noxious compounds (Mitra and Flynn, 2005; Pohnert et al., 2007). The concepts of ext R crit or int R crit have driven extensive theoretical discussion, supported by modelling. In the context of phytoplankton, Tilman (1977) used both a Monod and an internal-stores (Droop- quota) type of model, reporting that they gave similar results. The work considered P and Si limitations; co-existence and competition between organisms could be explained across a gradient of nutrient ratios. To date this work has been cited approximately 500 times, generating a mass of observational, experimental and theoretical studies. The work was developed by Tilman in various outputs, perhaps most notably in Tilman (1982). Although the original theory, Journal of Marine Systems 83 (2010) 170–180 ⁎ Tel.: +44 1792 295726; fax: +44 1792 295955. E-mail address: k.j.flynn@swansea.ac.uk. 0924-7963/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2010.04.007 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys