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Content archived on 2024-05-21

Cycling of phosphorus in the mediterranean


Problem to be solved
The Mediterranean in general and the eastern basin in particular is the largest body of water in the world that is thought to be phosphorus limited. However all the evidence at present for this conclusion is indirect and is based on chemical measurements and a limited number of bottle experiments. The Mediterranean is also well known to be an extremely vulnerable marine ecosystem especially as regards t o nutrient inputs. As a result of its unusual anti-estuarine circulation, it exports nutrients into the North Atlantic making it highly oligotrophic. Relatively small changes in the fluxes of nutrients have a disproportionate effect on the marine ecosystem structure in the region. Although the Barcelona treaty and subsequent EC regulations severely restrict the nature and amount of waste that can be disposed of within the Mediterranean, anthropogenic inputs of nutrients continue to reach the basin in large amounts. These nutrients come via the atmosphere, river discharges, dredge spoils as well as directly via activities such as mariculture that is an expanding industry in the region. In order to reduce the impact on the marine ecosystem it is necessary to understand the P cycling in detail.
Scientific objectives and approach
The aim of CYCLOPS is to confirm directly that P is the growth-limiting nutrient in the Eastern Mediterranean using a novel field strategy that avoids artefacts of bottle experiments. Detailed measurements will be made to determine how small additions of phosphate change the microbial ecosystem structure and increase biomass and productivity. In the first year, we will carry out a set of mesocosm experiments involving the addition of P, N, and Si to define the changes that we will expect to see during the field experiments. In year 2 we will carry out a cruise in which phosphate and an inert SF6 tracer will be dispersed in a 4 km 2 patch of surface water within the stable structure of the Cyprus eddy. The total amount being added to this small area, which is designed to mimic natural processes, is similar to a natural dust event and less than 20% of the natural winter turnover. Our novel techniques for P, tracer and microbiological assays are sensitive enough to track these small P additions thereby ensuring our experiments are reversible, localized and have negligible ecosystem effects.
This is ideal for both
(1) biogeochemical budgeting of P and
(2) compliance with the Barcelona convention and all other relevant EC regulations and directives regarding nutrient inputs to the Mediterranean. Between this experiment and a second enrichment experiment, we will carry out 2 short cruises to determine the natural state and seasonal variability of the system. In the second field experiment we will add phosphate 3 times successively to the same patch of water to determine by detailed biological and chemical measurements the change in biomass, ecosystem structure and phosphorus speciation.
Expected impacts
The scientific understanding developed during this study will be used to develop a quantitative understanding of P cycle that will be used to calibrate nutrient & ecosystem models including the Mediterranean Forecasting system, a model currently being developed to provide a real-time oceanographic predictions across the basin. The results of this project will be used to update directives and regulations to further protect the Mediterranean from ecosystem damage. New technologies will be developed to solve environmental problems. For example, the new technology of non-polluting intensive mariculture systems will be targeted to remove P from the effluent as well as the present N, if we can show that P is the critical nutrient in the region.
The CYCLOPS addition experiment, in which we added phosphate fertiliser to a large (4 x 4 km) patch of the eastern Mediterranean and followed in detail the biogeochemical responses, was the first non-Fe addition Lagrangian experiment anywhere in the world. Such experiments require a high degree of technical sophistication that is only possible within the best oceanographic institutions. This expertise is now available within a cohort of European scientists who specialise in working within the Mediterranean. The eastern Mediterranean is the most oligotrophic body of water anywhere in the world. In order to carry out the CYCLOPS programme successfully it was necessary to develop new analytical methods, particularly the new nanomolar dissolved nutrient methods and the alkaline phosphatase procedure, and to modify existing techniques (e.g. primary productivity, chlorophyll, DOP, etc) for use in this system. Again these methods and expertise are now available to the community for further studies on this unique environmentally sensitive ecosystem.

As a result of the CYCLOPS experiments we now have a much more sophisticated and detailed understanding of the base of the food chain in the Eastern Mediterranean and how it operates. We concluded that nutrient limitation in the system is seasonal, with the system being conventionally phosphorus limited during the winter bloom, which lasts from November until March. During summer (March - October) we showed that the nutrient limitation on productivity was different for different parts of the food chain. Bacteria show a simple phosphorus limitation, while phytoplankton are nitrogen and phosphorus co-limited. This conclusion is contrary to the usual conventional wisdom regarding nutrient limitation, in which all parts of the microbial ecosystem are considered to be nutrient limited together. We also found that microbial grazers were very important in the system and it seems that rather large grazers were able to increase in numbers and crop much smaller organisms than is usual in other systems. The system is thus extremely efficient at recycling nutrients on a very fast time scale. We have hypothesised that this may be the reason why the amount of fish caught within the system seems to be more than can be supported by the measured primary productivity. Our study demonstrated the difference between the microbial ecosystem in the nearshore and that which operates offshore. In the nearshore system there is a reasonable population of eukaryote (relatively large) phytoplankton such as diatoms which, when given a pulse of nutrients, are able to reproduce and grow. Thus if there is a nutrient source, such as upwelling or pollutant such as a river or mariculture system, the phytoplankton which will grow are diatoms and other relatively large phytoplankton. This was demonstrated by the massive diatom blooms that used to occur as a result of the annual Nile flood. By contrast in the offshore regions there are only nano- and picoplankton, and bacteria. As a result when a pulse of nutrients is added, the microbial response is very different, causing changes only in the lowest part of the food chain initially. We have developed an ecosystem model that is able to describe quantitatively these interactions and has been offered to the Mediterranean forecasting community for use in their basin scale models.

On a more regional scale, the CYCLOPS programme has been able to explain the reason why the Eastern Mediterranean in phosphate limited. A total nutrient budget has been carried out for the basin as a whole. We have shown that there is a far larger input of bioavailable N than P. The ratio is greater than 50:1 compared to the conventional Redfield ratio of 16:1. Although similar skewed input ratios are also found in other parts of the world's oceans, it is the unique circulation of the eastern Mediterranean that causes the basin to be the largest basin the world that is unequivocally P limited. In other oceans of the world there is enough organic matter exported to the deep water and shelves to enable extensive denitrification to occur. This causes any excess N in the system to be vented as gaseous nitrogen. This does not occur in the eastern Mediterranean thus preserving this high N/P ratio in the system. This is because almost all of the nutrients supplied to the system are exported through the straits of Sicily by the unique anti-estuarine circulation. The main input of nutrients to the system is from atmospheric supply, both natural Saharan dust and also pollutant supply from Europe. This supplies 60% of the N and 30% of the P. This atmospheric supply is thus regionally very important. However it is hypothesised that when pollutants are added to the system, they may affect local ecosystems but on a basin-wide long-term scale they will simply be carried out of the basin by the intermediate water flow over the straits of Sicily.

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Woodhouse Lane
United Kingdom

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