EXPERIMENTAL-STUDY OF MICROBIAL P LIMITATION IN THE EASTERN MEDITERRANEAN

In this study we experimentally tested the hypothesis that phosphorus was the primary nutrient limiting phytoplankton and bacterial growth i n the eastern Mediterranean Sea, and examined the spatial variability in P Limitation during winter. Complementary measurements were employe d using water sampled during January 1995 from nine pelagic stations e ast of the Straits of Sicily. Ambient concentrations of inorganic P (P -i) in the upper 50 m of the water column in seven of the stations wer e 20-40 nM. The upper limit of bioavailable P ranged from 6 to 18 nM, suggesting severe P shortage. Orthophosphate turnover time ranged from 2 to 7 h in those P-i-depleted waters. In nutrient-enrichment bioassa ys using subsurface water from the Ionian and Levantine basins, P addi tion caused significant increases in bacterial activity, bacterial num bers, and chlorophyll a relative to unenriched controls. The addition of NH4+ + Fe + EDTA did not have these effects. In a similar bioassay using Cretan water, microbial growth was obtained even in the unenrich ed controls, suggesting that other factors (e.g. grazing, light) were influential. Higher ambient P-i concentrations were encountered in the Cretan Sea (90 nM) and in the core of the Rhodes gyre (210 nM), where our sampling coincided with a convective mixing event. In those stati ons, P sufficiency was indicated. We concluded that in the pelagic wat ers of the eastern Mediterranean in winter, P was the primary limiting nutrient when other factors (such as light or grazing) did not contro l microbial biomass or activity. In ultra-oligotrophic waters, a delic ate and dynamic balance differentiates between times when the microbia l populations are nutrient limited and times when growth becomes limit ed by other factors. We caution that the interpretation of data obtain ed using conventional methods that were developed and tested in more e nriched systems may not be valid in ultra-oligotrophic systems.