Ž . Journal of Marine Systems 25 2000 431–446 www.elsevier.nlrlocaterjmarsys From biogeochemical to ecological models of marine microplankton Paul Tett a, ) , Hilary Wilson a,b a Department of Biological Sciences, Napier UniÕersity, 10 Colinton Road, Edinburgh EH10 5NT, UK b School of Ocean Sciences, UniÕersity of Wales, Bangor, Ireland Received 15 December 1998; accepted 17 June 1999 Abstract Models must simplify the complexity of real marine pelagic ecosystems. How much simplicity is needed? A series of Ž . increasing numbers of state variables is used to illuminate this issue and to illustrate biogeochemical element-conserving Ž . and ecological semi-freely dynamically interacting models of the marine microplankton, defined as all organisms less than w 200 mm. The models are those of Riley Riley, 1946. Factors controlling phytoplankton populations on Georges Bank, J. x Ž . Marine Res., 6, 54–73. for phytoplankton, a nitrogen-conserving version of Riley, the microplankton model MP of Tett wTett, P., 1990. A three layer vertical and microbiological processes model for shelf seas. Proudman Oceanographic x Ž . Laboratory, Report 14, 85 pp. with a constant ratio h of microheterotrophs to total microplankton, a related model AH Ž . with Lotka–Volterra dynamics for interacting autotrophs and heterotrophs, and a simple microbial loop model ML . It is Ž . argued that models must be at least biogeochemical. The biogeochemical model MP simulated changes during a microcosm experiment better than the simplest ecological–biogeochemical model AH, which is, as here parameterised, an unstable system. ML, with protozoans able to switch food sources, is more stable than AH, and satisfactorily simulates the seasonal cycle of chlorophyll in the North-East Atlantic. MP performed well when forced with a time-series of h taken from ML. Time-varying h is one way of providing the smallest amount of microbial ‘ecology’ needed in biogeochemical pelagic models without introducing instability. q 2000 Elsevier Science B.V. All rights reserved. Keywords: ecosystem-stability; microbial-loop; microplankton; model 1. Introduction Models of biological–physical interactions in the Ž sea must take account of physical transports which ) Corresponding author. Tel.: q 44-131-455-2633; fax: q 44- 131-455-2291. Ž . E-mail address: p.tett@napier.ac.uk P. Tett . . conserve the total quantity of transported variables and of non-conservative biological or chemical pro- Ž . cesses which convert one variable into another . Ž . Fransz et al. 1991 review progress with such mod- els. The development of computing power has in the present decade begun to allow the coupling of mod- els of biological processes with increasingly realistic Ž three-dimensional physical models Sarmiento et al., 1993; Aksnes et al., 1995; Gregoire et al., 1998, . Luytens et al., 1999 . Although horizontal transports 0924-7963r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0924-7963 00 00032-4