Relationship between primary producers and bacteria in an oligotrophic sea-the Mediterranean and biogeochemical implications

Abstract

The proverbial blue colour of the Mediterranean reflects some of the most extreme oligotrophic waters in the world. Sea-surface Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satelhte data show the relatively clear, pigment poor, surface waters of the Mediterranean with a generally increasing oligotrophy eastward, apparent even from space. Integrated over depth, however, the east and west Mediterranean show similar amounts of phytoplankton and bacterial biomass. By contrast, primary production and bacterial production are 2 to 3 times lower in the eastern Mediterranean than in the west. However, the relationship between bacterial production and primary production in the east and west are significantly different. While bacterial production is hrectly proportional to primary production in the east, in the west it increases as approximately the square root of primary production. This suggests that the bacteria in the west are relatively decoupled from local contemporaneous primary production. In contrast, the gradient of close to 1 in the log bacterial production versus log primary production relationship in the east suggests less temporal decoupling and, therefore, less seasonal accumulation of DOC. In addition, the constant proportionahty between bacterial and primary production of 0.22, whlch, if all primary products are respired, gives an estimated geometric mean bacteria growth efficiency of 22% (95% confidence limits of 17 and 29%) for data in the eastern Mediterranean. Our data suggest that the degree of bacteria-phytoplankton coupling has an important effect on apparent trends between bacterial and phytoplankton production in high frequency data. The combination of low primary production and bacterial dominance of secondary production in the east is also of significance as it could account for the low fisheries production, the low vertical flux of material and low biomass of benthic organisms in the region.

Accumulation of DOC in Low Phosphate Low Chlorophyll (LPLC) area: is it related to higher production under high N:P ratio?

“…It implies that heterotrophic bacteria may benefit from slow growing nutrient limited phytoplankton with high exudation rates, especially if we consider them more competitive for nutrients and if we assume P.E.R to be a reliable source of DOC for heterotrophic bacteria. Another consequence is the more direct coupling between primary production and bacterial production under P-limited conditions, a feature that has been shown experimentally when comparing the eastern and the western basin (Turley et al, 2000). Although model results could be improved especially concerning carbon dynamics, overall results are quite consistent with our current knowledge of the carbon cycle in the surface layer of the Mediterranean sea.…”

Thaliacean distribution and abundance in the northern part of the Levantine Sea (Crete and Cyprus) during the eastern Mediterranean climatic transient, and a comparison with the western Mediterranean basin

“…Counts of P. atlanticum based on 0.500 mm meshes varied between rare findings and up to 187 colonies 1,000 m -3 according to season . Possible seasonal variations included and corroborating previous faunistic studies (Godeaux 1999, and references therein), the thaliacean fauna is richer in species and specimens as compared to the eastern Mediterranean, where phytoplankton and bacterial production is up to three times less than in the western Mediterranean Sea (Turley et al 2000). Notwithstanding, the seasonal fluctuations of salps and doliolids in the SE Aegean Sea (Siokou-Frangou and Papathanassiou 1989) seem to fit the respective pattern in the western Mediterranean Sea.…”

Plankton in the open Mediterranean Sea: a review

“…Oligotrophy seems to be mainly due to the very low concentration of inorganic phosphorus, which is assumed to limit primary production (Berland et al, 1980;Thingstad andRassoulzadegan, 1995, 1999;Thingstad et al, 2005). Additional features of the MS are i) the decreasing west-east gradient in chl a concentration, as shown by color remote sensing (D'Ortenzio and Ribera d 'Alcalá, 2009;Barale et al, 2008) as well as by in situ data (Turley et al, 2000;Christaki et al, 2001), ii) a high diversity compared to its surface and volume (Bianchi and Morri, 2000), and iii) a relatively high number of bioprovinces (sensu Longhurst, 2006), with boundary definition mostly based on the distribution of the benthos and the necton (Bianchi, 2007). The MS is also a site of intense anthropic activity dating back to at least 5000 years BP, the impact of which on the marine environment has still to be clearly assessed and quantified.…”