Experimental study of microbial P limitation in the eastern Mediterranean

Abstract

In this study we experimentally tested the hypothesis that phosphorus was the primary nutrient limiting phytoplankton and bacterial growth in the eastern Mediterranean Sea, and examined the spatial variability in P limitation during winter. Complementary measurements were employed using water sampled during January 1995 from nine pelagic stations east of the Straits of Sicily. Ambient concentrations of inorganic P (P,) in the upper 50 m of the water column in seven of the stations were 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,-depleted waters. In nutrient-enrichment bioassays using subsurface water from the Ionian and Levantine basins, P addition caused significant increases in bacterial activity, bacterial numbers, and chlorophyll a relative to unenriched controls. The addition of NH,' + Fe + EDTA did not have these effects. In a similar bioassay using Cretan water, microbial growth was obtained even in the unemriched controls, suggesting that other factors (e.g. grazing, light) were influential. Higher ambient P, 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 stations, P sufticiency was indicated. We concluded that in the pelagic waters of the eastern Mediterranean in winter, P was the primary limiting nutrient when other factors (such as light or grazing) did not control microbial biomass or activity. In ultra-oligotrophic waters, a delicate and dynamic balance differentiates between times when the microbial populations are nutrient limited and times when growth becomes limited by other factors. We caution that the interpretation of data obtained using conventional methods that were developed and tested in more enriched systems may not be valid in ultra-oligotrophic systems.

The N:Si:P molar ratio in the Strait of Gibraltar

“…The phytoplankton and bacterial activities significantly increase the N:Si:P ratio in the surface and deep waters at the eastern entrance of the Strait. Data from both the northwestern [Thingstad et al, 1998] and the eastern Mediterranean [Zohary and Robarts, 1998] indicate that the growth rate not only of phytoplankton, but also of heterotrophic bacteria, is phosphoruslimited during stratification.…”

Bottom up effects on bacterioplankton growth and composition during summer-autumn transition in the open NW Mediterranean Sea

“…Such relationship was not found on this data set, probably because of the low range of activities measured here (range of V max 0.6-2.9 nmol l −1 h −1 , range of T T 9-47 h), compared to 0.6-12.6 nmol l −1 h −1 and 14-786 h for the same parameters, respectively, in Van Wambeke et al (2002). P limitation conditions also occurred at different areas in Mediterranean Sea and at different periods of the year; in summer, winter, spring (Zohary and Robarts, 1998;Sala et al, 2002;Van Wambeke et al, 2002;Pinhassi et al, 2006). The frequent enhancement of growth by P addition alone suggests that the nutrient pool otherwise satisfied growth requirements at least during the 24 h incubation period.…”

“…In the Mediterranean Sea, the availability of P can limit photosynthesis, nitrogen uptake and heterotrophic bacterial production (Fiala et al, 1976;Thingstad et al, 1998;Moutin and Raimbault, 2002). Notably, P Limitation of heterotrophic bacterial production has been demonstrated on different places (Thingstad et al, 1998;Zohary and Robarts, 1998;Van Wambeke et al, 2002). Transition between P and labile carbon limitation has been shown along vertical profiles (Sala et al, 2002;Van Wambeke et al, 2002), or seasonally in a given place (Pinhassi et al, Published by Copernicus Publications on behalf of the European Geosciences Union.…”

Comportamiento de Nutrientes en la Pluma del Río Ebro

“…La composición elemental del bioseston (primariamente fitoplancton, zooplancton, bacterias,...) engloba una gran variedad de elementos (Hecky y Kilham, 1988;Vollenweider et al, 1996): Tradicionalmente el crecimiento y la producción de las poblaciones de plancton (bacterias y algas) de los ecosistemas litorales se ha considerado que está limitado por el nitrógeno (Conley, 2000;Graneli, 1987;Le Pape et al, 1996;L'Helguen et al, 1996;Sik-Lee et al, 1996;Thompson, 1998;Vollenweider et al, 1996;Zohary y Robarts, 1998 …”

“…Tradicionalmente el crecimiento y la producción de las poblaciones de plancton (bacterias y algas) de los ecosistemas litorales se ha considerado que está limitado por el nitrógeno (Conley, 2000;Graneli, 1987;Le Pape et al, 1996;L'Helguen et al, 1996;Sik-Lee et al, 1996;Thompson, 1998;Vollenweider et al, 1996;Zohary y Robarts, 1998). En cambio, otros estudios indican que es el fósforo el nutriente limitante, como parece ocurrir a lo largo de la costa este de Estados Unidos (en la bahía de Chesapeake y en el río Hudson) (Myers y Iverson, 1981;Taft y Taylor, 1975), en grandes áreas a lo largo de la costa de China (Harrison et al, 1990), en el mar de los Sargazos (Cotner et al, 1997), en la pluma del estuario del Garona en la bahía de Vizcaya (Herbland et al, 1998), y en el mar Mediterráneo.…”