|The significance of observed nutrient concentrations in the discussion about nitrogen and phosphorus as limiting nutrients for the primary carbon flux in coastal water ecosystems|
Söderström, J. (1996). The significance of observed nutrient concentrations in the discussion about nitrogen and phosphorus as limiting nutrients for the primary carbon flux in coastal water ecosystems. Sarsia 81: 81-96
In: Sarsia. University of Bergen. Universitetsforlaget: Bergen. ISSN 0036-4827, more
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Decisions about which of N or P limits the production in the marine environment, based on concentration ratios or enrichment experiments, have as a prerequisite that concentrations mirror nutrient availability. This is an assumption that lacks a theoretical background. With the aid of a simple hydraulic model it is pointed out that concentrations in a steady state flux system, at a certain input, depend on the mechanisms that regulate the transports from the system. In a coastal water these mechanisms for the nutrient flux are water exchange and biological activities. Because the biological activities are not the same for nitrogen and phosphorus, the resulting N : P concentration ratios can not be expected to represent the ratios for the availabilities of the nutrients. Conclusions about N as the limiting nutrient, which are based on concentration ratios, or concentration dependent bioassays, can therefore not be accepted. Examples from the monitoring of coastal waters in Sweden, show that, in accordance with the Redfield concept and the Smith & Hollibaugh concept of carbon controlled nitrogen cycling, the same concentrations of inorganic nutrients appear in areas with similar ecosystem structure, but different nutrient supplies and different primary productions. Concentrations depend more on ecosystem structure than on flux size. Some observations from the Swedish west coast on C : N : P ratios in particles are described. The conclusion is that flux calculations involving Redfield ratios are meaningful tools in the assessment of the relative role of different nutrients in the flux of energy and matter in coastal ecosystems. An example is described, which explains the - seemingly incompatible - simultaneous occurrence of supply observations showing nitrogen surplus, and concentration-enrichment observations showing nitrogen deficiency. With concentration ratios and enrichment experiments ruled out as methods for determining which of N and P limits plankton production in coastal waters, no valid arguments are left for N as the limiting nutrient in Swedish and Danish waters. Deliveries of nutrients to the Baltic Sea, and the Swedish and Danish coastal waters of Kattegat and Skagerrak, have a pronounced surplus of nitrogen in relation to the Redfield ratio for plankton. Evidently phosphorus is initially the limiting nutrient. This fact, together with the greatly improved knowledge in the last decades about the processes in the marine nitrogen cycle, makes it evident that the expensive measures against nitrogen output, imposed by Swedish and Danish authorities in order to diminish eutrophication in the coastal waters, are doomed to failure.