Modelling mixotrophy in harmful algal blooms: More or less the sum of the parts? Aditee Mitra, 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: Mixotroph Dysfunctional model Kleptochloroplast Switching Predator–prey Phototroph Heterotroph Harmful algal bloom Model skill Mixotrophs are significant components of planktonic food webs, are frequently associated with harmful algal bloom events, and thus warrant inclusion in coastal ecosystem models. There are, however, insufficient quantitative data to support the construction and testing of simple empirical descriptions of mixotrophs. Here, a complex mixotroph model based upon phenomenological understanding (Flynn and Mitra, 2009) was used to generate control “realities” against which to compare contrasting simple descriptions of mixotrophy using a Turing Test approach. The simplest description, adding together phototrophic and heterotrophic functions gave the worst output. The best model tested, in keeping with the evolution of these organisms, used phototrophy as a nutritional supplement mechanism for heterotrophy. However, none of the simple models described kleptochloroplasty — an important process in some harmful bloom species. None of the simple models correctly matched the balance of phototrophy and heterotrophy (grazing); while fits to bulk parameters (biomass, nutrients) could be acceptable, rate processes were often completely in error. This is of particular concern because of the difficulty in determining rate processes. A generalised implication is that a fit to bulk data gives no assurance that the model structure is not dangerously dysfunctional; determining model skill should include locating and removing structural dysfunctionality. © 2010 Elsevier B.V. All rights reserved. 1. Introduction 1.1. Role of mixotrophy Organisms that possess the capability of combining phototrophy and heterotrophy are termed mixotrophs. Depending on light, nutrient and prey availability, mixotrophs display varying proportions of phototrophic and heterotrophic activity. Mixotrophs thus occupy a unique niche affecting trophic levels both below and above them with the potential to change the dynamics of the system. Mixotrophy can, therefore, affect biogeochemical cycling of nutrients. Mixotrophs do not form a unique group but occur under different physiological guises, amongst different species ranging over a variety of taxonomic groups. From an evolutionary point of view phagotrophy in eukaryote microbes is believed to be the primitive state from whence pure phototrophic protists evolved (Raven, 1997; Raven et al., 2009). Within planktonic organisms, mixotrophy is a common phenomenon in marine as well as freshwater systems (Jones, 1997; Raven, 1997; Stoecker, 1998; Jones, 2000). Indeed, mixotroph populations can be responsible for ecologically catastrophic events such as harmful algal blooms (Kempton et al., 2002; Vaqué et al., 2006; Burkholder et al., 2008). In the presence of abundant light, nutrients or prey, strict autotrophs and/or heterotrophs dominate. Mixotrophy comes into play in mature systems. In such systems mixotrophs act as conduits for energy and elements from different parts of the food web. Thus, in post-autumn bloom when there is low light as well as low food availability, mixotrophs photosynthesising and engulfing bacteria can channel energy to higher trophic levels (e.g., Myung et al., 2006). This activity also improves the C:N:P ratio of the mixotrophs resulting in these becoming nutritionally replete food for the higher trophic levels (so-called seston upgrading; Ptacnik et al., 2004; Weithoff and Wacker, 2007). In the post spring bloom period, when autotrophs become increasingly reliant upon regenerated nutrients, mixotrophs including HAB species are advantaged through their ability to consume other organisms. When confronted with unfavourable conditions, mixotrophs possess an advantageous survival strategy, thriving in conditions where food and/ or light limits growth of their non-mixotrophic competitors. Therefore, one could expect occurrences of “ideal mixotrophs” in nature capable of balancing autotrophy and phagotrophy to maintain a high growth rate under varying environmental conditions. However, there is no evidence of occurrence of such organisms in reality. Indeed, mixotrophic organisms typically have lower growth rates compared to dedicated autotrophs or heterotrophs (Raven, 1997; Stoecker, 1998). This reflects the compro- mises required to operate two nutritional modes within one cell type, and is suggestive of a complex regulatory interaction between the processes, rather than them being simply additive. As we shall see, this has impli- cations for modelling mixotrophic activity. Journal of Marine Systems 83 (2010) 158–169 ⁎ Corresponding author. Tel.: +44 1792 295726; fax: +44 1792 295955. E-mail address: k.j.flynn@swansea.ac.uk (K.J. Flynn). 0924-7963/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2010.04.006 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys