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

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.