6.5.3 Conservative behaviour
Elements with conservative behaviour are characterized by vertical profiles
(similar to major ion profiles) that indicate essentially constant concentrations
with depth. These elements behave like the major ions, having long residence
times and being well mixed in seawater. These elements are not major compo-
nents of seawater, simply because their crustal abundances are very low compared
with the major ions resulting in correspondingly low input rates. Elements
showing this sort of behaviour form either simple cations or anions (e.g. caesium
(Cs+) or bromine (Br-) with low z/rratios and little interaction with water (see
Section 5.2)), or form complex oxyanions (see Section 5.2), for example molyb-
denum (Mo) and tungsten (W)—which exist in seawater as MoO 42 - and WO 42 -
respectively (Fig. 6.19). Conservative elements have little interaction with
biological cycles.6.5.4 Nutrient-like behaviour
As in continental waters (see Section 5.5.1), NO 3 - , DIP (dissolved inorganic
phosphorus) and silicate are usually considered to be the limiting nutrients for
biological production, although in some situations it has been suggested that
trace elements, particularly iron, may be limiting (Section 6.6). Excepting high-
latitude areas the oceans are so large and deep that they are effectively perma-
nently stratified. The production of biological material removes nutrients from
surface waters (Box 6.6). After death, this biological material sinks through the
water column, decomposing at depth to re-release the nutrients. The nutrients
are then slowly returned to surface waters by deep-ocean mixing processes and
diffusion. The net result is that the vertical profiles of nutrients are characterized
by low concentrations in surface waters (where biological utilization rates exceed
supply rates) and deep-water maxima, where decomposition rates exceed uptake
rates because of the absence of light (Fig. 6.20). Nitrogen and DIP are cycled218 Chapter Six
Table 6.8Comparison of total atmospheric and riverine inputs to the world’s oceans
(10^9 mol yr-^1 ). Based on Duce et al. (1991).
Element Riverine* Atmospheric
Nitrogen (ex N 2 ) 1500–3570 2140
Cadmium 0.0027 0.02–0.04
Copper 0.16 0.25–0.82
Nickel 0.19 0.37–0.48
Iron 19.7 580
Lead 0.01 0.43
Zinc 0.09 0.67–3.5
Total input—dissolved plus particulate. Estimates are based on data available in early 1990s and so include
significant amounts of material mobilized by human activity.
* Dissolved input only; particulate components are assumed to sediment out in estuaries and the coastal
zone.