T. Zohary et Rd. Robarts, EXPERIMENTAL-STUDY OF MICROBIAL P LIMITATION IN THE EASTERN MEDITERRANEAN, Limnology and oceanography, 43(3), 1998, pp. 387-395
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.