The general objective of MEDIPELAGOS is to study the nutrient limitation of the pelagic Northwestern Mediterranean ecosystem. Specifically, the project consists of determining:
1) the nutrient(s) (N, P) which limit primary production;
2) the pathways of nutrient cycling within the pelagic food web; and
3) the effect of external nutrient pulses.
The hypothesis will be examined that a hydrographically stable ecosystem will promote a 'microbial-loop'-based food web, while nutrient-pulsed systems (e.g. fronts) favour the development of a 'classic' food web which produces sedimenting material.
Apart from traditional indicators for N/P limitation (N/P - ratios of dissolved nutrients, increase in particulate -N/P following short-term incubation with phosphate, alkaline phosphatase activity, and turn-over time for orthophosphate), a newly reported technique based upon the induction of DNA synthesis following release of N limitation will be developed further for the study of P limitation.
The focus of the nutrient cycling studies will be on phosphorus using radiolabeled phosphorus, and size- and chemical fractionation. The effect of nutrient pulses will be investigated using both a field (cruise) and laboratory based (microcosms) approach. Field sampling will follow time series of physico-chemical variables, biomass of the main groups of planktonic producers and consumers, and changes in the relative importance of grazing through the 'classical' or the 'microbial-loop' food webs. Microcosm experiments will examine the effect of nutrient ratios and frequency of nutrient inputs on the response of physiological indicators based on the biochemical composition of planktonic components.
Existing process models describing nutrient cycling and the effect of nutrient pulses will be developed further.
Funding SchemeCSC - Cost-sharing contracts