Biogeochemical nutrient cycles and nutrient management strategies

Nutrient loading by riverine input into estuarine systems has increased by 6-50 times for the N load from pristine conditions to present, whereas a 18 -180 times increase has been observed in the P load. Reductions in the rati o of N to P delivery has also occurred with time. In a review of nutrient l imitation in estuarine systems, it is shown that many estuarine systems dis play P limitation in the spring, switching to N limitation in the summer wi th some estuaries displaying dissolved silicate limitation of the spring di atom bloom. Historical and recent changes in nutrient loading and their eff ect on nutrient limitation have intensified the debate on the control of nu trient delivery to estuaries from both agricultural and point sources, and as to what nutrient (N or P) should be managed for in estuarine systems. It is hypothesized that potential reductions in P may help oxygen depletion e specially in deep estuaries and reduce fast growing macrophytes such as Ulv a sp., although P reductions probably will have little effect on summer chl orophyll concentrations, an important recreational management goal. Reducti ons in N loading should reduce summer chlorophyll concentrations and improv e the conditions for submerged aquatic vegetation and thus improve ecosyste m functioning. Finally, if only P reductions are pursued, that is if we are able to reduce P such that it is limiting year around in estuarine systems , it is likely that the export of N from estuarine systems would increase t o the bordering N-limited marine systems, thus only exporting the problem o f enhanced production with eutrophication.