Nutrient cycling in the south east Levantine basin of the eastern Mediterranean: Results from a phosphorus starved system

The south east Levantine basin of the eastern Mediterranean is a uniquely P starved system with a nitrate:phosphate ratio in the deep water of 25–28:1, a PON:POP ratio of 27–32:1 and a DON:DOP UV ratio of ∼100:1 (which probably represents a DON:DOP TOTAL of ∼50:1). The C:N:P ratio of nutrients accumulated in the deep water from decomposed organic matter was 106:8.5–10.8:0.34–0.43 similar to the measured ratios for dissolved and particulate organic matter and much higher than the Redfield ratio. It is concluded that the P limitation of the eastern Mediterranean is due to the lack of P within the system and not in the preferential removal of P relative to N. Results from the first extensive deployment of on-board nanomolar nutrient measurements in this low nutrient low chlorophyll system showed that free ammonia (50–80 nM) was present in the surface waters while nitrate was less than 10 nM, confirming the results obtained elsewhere in the CYCLOPS addition experiment results that grazing/nutrient recycling is a dominant process in this system. The total DIN:DIP ratio in the nutrient depleted waters above the chlorophyll maximum was predominantly greater than 16:1, suggesting that the system has not switched to N limitation. A primary nitrite maximum was observed immediately below the chlorophyll maximum at the top of the nutricline, which was similar to those found previously in oligotrophic ocean locations. Where nanomolar technology was deployed through the nutricline, it was found that the phosphocline started at the same depth as the nutricline for nitrate and silicate, a conclusion that would not have been made if only conventional micromolar technology had been available. An intercomparison of nutrient procedures suggested that freezing samples is acceptable for samples with a concentration above 20 nM (DIP) and 400 nM (nitrate and nitrite), which represent most of the previously published data from intermediate and deep waters from the Levantine basin. However for concentrations lower than this, which in practice means within the photic zone in the eastern Mediterranean, it is highly desirable to use on-board measurements and ideally the new nanomolar techniques.