Lack of P-limitation of phytoplankton and heterotrophic prokaryotes in surface waters of three anticyclonic eddies in the stratified Mediterranean Sea

Abstract

Abstract. We investigated the identity of the limiting nutrient of the pelagic microbial food web in the Mediterranean Sea using nutrient manipulated microcosms during summer 2008. Experiments were carried out with surface waters at the center of anticyclonic eddies in the Western Basin, the Ionian Basin, and the Levantine Basin. In situ, the ratio of N to P was always higher in both dissolved and particulate organic fractions compared to the Redfield ratio, suggesting a relative P-starvation. In each experiment, four different treatments in triplicates (addition of ammonium, phosphate, a combination of both, and the unamended control) were employed and chemical and biological parameters monitored throughout a 3-4 day incubation. Temporal changes of turnover time of phosphate and ATP, and alkaline phosphatase activity during the incubation suggested that the phytoplankton and heterotrophic prokaryotes (Hprok) communities were not P-limited at the sites. Furthermore, Correspondence to: T. Tanaka (tsuneo.tanaka@obs-vlfr.fr) statistical comparison among treatments at the end of the incubation did not support a hypothesis of P-limitation at the three study sites. In contrast, primary production was consistently limited by N, and Hprok growth was not limited by N nor P in the Western Basin, but N-limited in the Ionian Basin, and N and P co-limited in the Levantine Basin. Our results demonstrated the gap between biogeochemical features (an apparent P-starved status) and biological responses (no apparent P-limitation). We question the general notion that Mediterranean surface waters are limited by P alone during the stratified period.

Ecology of aerobic anoxygenic phototrophic bacteria along an oligotrophic gradient in the Mediterranean Sea

“…On the basis of our nutrient-addition experiments, we cannot speculate about P-limitation of AAP bacteria since growth of AAP and total prokaryotes were not stimulated by P addition, although higher heterotrophic bacterial activity was observed. Consistent with our results, Tanaka et al (2011) did not find any indication of P-alone limitation although a clear P-starved status in the Mediterranean basins was observed. Moreover, we observed that net growth of AAP bacteria was enhanced by glucose and nitrogen additions in the eastern basin whereas net growth of total prokaryotes was not stimulated by these additions.…”

Processes and patterns of oceanic nutrient limitation

“…Rather, once stratified oligotrophic conditions are well established, N limitation or NP co-limitation seems to occur 47,48,59 . The higher bioavailability of organic phosphorus compounds 41,48 (see Supplementary Information), acclimatization mechanisms for coping with phosphorus stress 25 and/or the selection for higher N:P stoichiometry under low nutrient conditions 11 could explain the tendency towards nitrogen limitation.…”

“…3), increases in phytoplankton biomass and productivity often result from the addition of iron alone, in both bottle and in situ experiments 53,54 . In certain oligotrophic regions, including the eastern Mediterranean 20,48 and the subtropical North Atlantic 25,41,57,58 , bioavailable forms of phosphorus can become severely depleted alongside nitrogen (Figs 1c and 3), and microbial populations frequently show evidence of phosphorus stress (Box 1) 20,25,57,58 . Even in these systems, however, addition of phosphorus alone does not typically result in increased autotrophic activity or biomass 20,47,48,59 .…”

“…In certain oligotrophic regions, including the eastern Mediterranean 20,48 and the subtropical North Atlantic 25,41,57,58 , bioavailable forms of phosphorus can become severely depleted alongside nitrogen (Figs 1c and 3), and microbial populations frequently show evidence of phosphorus stress (Box 1) 20,25,57,58 . Even in these systems, however, addition of phosphorus alone does not typically result in increased autotrophic activity or biomass 20,47,48,59 . Rather, once stratified oligotrophic conditions are well established, N limitation or NP co-limitation seems to occur 47,48,59 .…”

“…S2) 41,47,48 . A significant proportion of the subsurface pools of many nutrients results from remineralization of organic matter produced in the surface layer ( Fig.…”