Summary and overview of the CYCLOPS P addition Lagrangian experiment in the Eastern Mediterranean

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Abstract

CYCLOPS was a European Framework 5 program to further our understanding of phosphorus cycling in the Eastern Mediterranean. The core of CYCLOPS was a Lagrangian experiment in which buffered phosphoric acid was added to a <4×4 km patch of water together with SF6 as the inert tracer. The patch was followed for nine days in total. Results obtained prior to the experiment showed that the system was typically ultra-oligotrophic and P-starved with DON:DOP, PON:POP and DIN:DIP all having ratios greatly in excess of 16:1 in surface waters. To our surprise, we found that although the added phosphate was rapidly taken up by the microbial biota, there was a small but significant decrease in chlorophyll a and no increase in primary production, together with an increase in heterotrophic bacterial activity, ciliate numbers and in the gut fullness and egg numbers in the zooplankton community. A microcosm experiment carried out using within-patch and out-of-patch water showed that the phytoplankton community were N and P co-limited while the bacteria and micrograzers were P-limited. Thus this system tends to N and P co-limitation of phytoplankton productivity in summer possibly caused by bioavailable DIN being converted into non-bioavailable forms of DON.

On the basis of the data collected within the programme it was concluded that this behavior could be explained by three non-mutually exclusive processes described as (1) trophic by-pass in which the added phosphate gets directly to the grazing part of the predatory food chain from the heterotrophic bacteria bypassing the phytoplankton compartment phosphate, (2) trophic tunnelling in which phosphate is rapidly taken up by both phytoplankton and bacteria via rapid luxury consumption. This causes an immediate change in the phosphorus content but not the abundance of the prey organisms. The added P then “reappears” as responses at the predator level much more rapidly than expected, and (3) mixotrophic by-pass in which inorganic nutrients, including the added P, are taken up by mixotrophic ciliates directly, bypassing the phytoplankton. For details of the results of this study and the processes described, the readers are referred to the relevant papers within this volume.

The implications of these results for nutrient cycling in the Eastern Mediterranean are discussed. In particular it is noted that the efficient and rapid grazing observed in this study might explain why the system although impacted by anthropogenic nutrient input has shown little or no measurable change in microbial productivity since added nutrients are rapidly transferred out of the photic zone via the by-pass and tunnelling processes and are exported from the basin. It is also suggested that fish productivity is higher than has been suggested by conventional food chain models due to this grazing. Two possible reasons for the unusual P-starved nature of the basin are presented.

Introduction

This manuscript presents an overview of the CYCLOPS program and in particular a synthesis of the results of the P-addition experiment. Here we describe briefly the initial background and aims of the project. The location of the study area and the conditions in the surface waters prior to the experiment are presented, followed by a brief description of the logistics of the Lagrangian experiment. The sampling and methods of all the experiments are summarized. After describing the physical conditions in the Cyprus Eddy during the experiment, the biological changes caused by addition are summarized together with those chemical changes that are not presented elsewhere in this volume. A synthesis of the overall meaning of all the CYCLOPS results is then given. This is followed by a broad overview of the implications of our new understandings of the controls on nutrient cycling in the Eastern Mediterranean for environmental studies in the Eastern Mediterranean and elsewhere.

Section snippets

The logical basis for the CYCLOPS experiment

The Mediterranean in general and the eastern basin in particular is one of the largest bodies of water in the world that is thought to be phosphorus-limited (Krom et al., 1991). This conclusion was based originally on the molar ratio of nitrate:phosphate in the deep water of the Eastern Mediterranean, which is 29:1 (Kress and Herut, 2001; Krom et al., 1991, Krom et al., 2005) and far higher than that in the Western Mediterranean (23:1) and the Eastern North Atlantic (16:1). In the Eastern

The Eastern Mediterranean as a vulnerable ecosystem

The Mediterranean in general and the Eastern Mediterranean in particular is ultra-oligotrophic with the lowest nutrient content in the world found in its deep waters as well as nutrients below conventional analytical detection limits in its surface waters. This is caused by its unusual anti-estuarine circulation. Nutrient-depleted surface waters flow into the basin at the straits of Gibraltar and Sicily, while intermediate waters containing significant amounts of dissolved inorganic nutrients

Description of the CYCLOPS program

In May 2000 a microcosm experiment was carried out in the National Institute of Oceanography, Haifa, Israel. The purpose of this experiment was to test analytical methodologies for use in the ultra-oligotrophic conditions found in the South-East Levantine basin and to conduct a preliminary, controlled examination of the response of Eastern Mediterranean surface water to nutrient additions. In this experiment nitrate, phosphate and glucose were added to microcosm bottles and the biological and

Practical description of the May 2002 P-addition experiment

The P-addition experiment was undertaken on 17–27 May 2002 from the R/V Aegaeo in the center of the Cyprus Eddy (see Fig. 1). The phosphate solution was prepared prior to sailing on the quay at Limassol, Cyprus, by adding concentrated phosphoric acid to 500 kg of sodium bicarbonate in nutrient-depleted seawater in 6×3000-l tanks. The final mean concentration of ∼2.2 M dissolved inorganic orthophosphate was confirmed by direct measurement. SF6 saturation was achieved using an in-line saturation

Sampling and methods

Details of all the casts taken and the measurements made in each cast are given in Table 1, Table 2. A full listing of this information together with all the measurements made is available on the CYCLOPS website (www.earth.leeds.ac.uk/cyclops). The chemical and biological parameters determined during this study are given in Table 2. All of the methods for the data presented elsewhere in this volume are given in the relevant papers. The details of methods given below are only for the chemical

Physical oceanography of the Cyprus Eddy during the CYCLOPS experiment

The CYCLOPS addition experiment was carried out as close as was possible to the center of the Cyprus warm-core Eddy. This was done for practical reasons. The Eastern Mediterranean is highly dynamic. The CYCLOPS experiment was limited by having to raise P concentration significantly and the only way to do this practically was to start with a small patch. By carrying out the experiment in the center of a warm-core eddy we made it easier to constrain and thus follow the patch over time.

The Cyprus

Understanding the response of the microbial community to the P-addition

It is suggested that there are three, not mutually exclusive, mechanisms that could explain the observed increase in bacterial activity and positive predator responses, together with a decrease in phytoplankton biomass. One could be termed a “trophic bypass” mechanism. In this process, the added phosphate gets directly to the grazing part of the predatory food chain from the heterotrophic bacteria bypassing the phytoplankton compartment (Fig. 12). The second process is a “trophic tunneling”

Acknowledgements

This project was funded by the EU through project EVK3-CT-1999-00009 “CYCLOPS”. We wish to thank the crew of R/V Aegaeo for endurance and assistance, particularly the captain and first officer, Konstantinos Chandros and Theodoros Kanakaris for their enduring patience and the ships bosun Vassilis Panagiotopoulos who was consistently interested in what we were doing. His prayers to the sunset were memorable. The second cruise upon which most of this project was based took place from Limassol. We

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