Iron, nitrogen, phosphorus and zinc cycling and consequences for primary productivity in the oceans

INTRODUCTION

Primary productivity in the ocean amounts to the net assimilation of CO₂ equivalent to about 50 Pg (petagram, i.e. 10¹⁵ g) C year^-1, while on land this is approximately 60 Pg C year^-1 (Field et al., 1998). Almost all of this primary productivity involves photosynthesis, and in the ocean it occurs only in the top few hundred metres, even in waters with the smallest light attenuation (Falkowski & Raven, 1997). About 1 Pg C of marine primary productivity involves benthic organisms, i.e. those growing on the substratum (Field et al., 1998), in the very small fraction of the ocean which is close enough to the surface to permit adequate photosynthetically active radiation (PAR) to allow photolithotrophic growth. This depth at which photosynthetic growth is just possible varies in time and space, and defines the bottom of the euphotic zone (Falkowski & Raven, 1997). The remaining ∼49 Pg C is assimilated by phytoplankton in the water column (Field et al., 1998). This chapter will concentrate on the planktonic realm, while acknowledging the importance of marine benthic primary producers and their interactions with micro-organisms (e.g. Dudley et al., 2001; Raven et al., 2002; Raven & Taylor, 2003; Cooke et al., 2004; Walker et al., 2004).

The global net primary productivity of the oceans is less than that on land, despite about 70 % of the Earth being covered in ocean and primary productivity over considerable areas of land being limited by water supply.