Biogeosciences Plankton in the open Mediterranean Sea: a review
Yüklə 0,96 Mb. Pdf görüntüsü
|
- Bu səhifədəki naviqasiya:
- Fig. 12.
1558 I. Siokou-Frangou et al.: Mediterranean plankton Table 3. Site, sampling data depths and variables measured and source of the studies considered in this review. S: surface, INT: integrated data, chl a: chl a concentration, BA: bacterial abundance, VA: Viral abundance, BP: bacterial production, VBR: ratio of viral abundance respect to bacterial abundance, VBM: viral mortality on bacteria, n: number of data. Location
Date Depth
n Variables References (m)
WMS NW MS
June 1995 5–200 (S, INT) 42 chl a, BA, VA, VBM, VBR Guixa-Boixereu et al. (1999b) NW MS
June 1999 5–200 (S, INT) 10 BA, VA, VBM, VBR Weinbauer et al. (2003) Alboran Sea October and November 2004 1–200 (S, INT) 6 BA, VA, BP, VBR Magagnini et al. (2007) W MS
October and November 2004 1–200 (S, INT) 16 BA, VA, BP, VBR Magagnini et al. (2007) Tyrrhenian Sea October and November 2004 1–200 (S, INT) 11 BA, VA, BP, VBR Magagnini et al. (2007) Straits of Sicily October and November 2004 1–200 (S, INT) 15 BA, VA, BP, VBR Magagnini et al. (2007) EMS
Adriatic Sea May 91–November 1992 0,5 (S) chl a, BA, VA, VBR Weinbauer et al. (2003) January–February 2001 1–1200 (S) 6 BA, VA, BP, VBR Corinaldesi et al. (2003) April–May 2003 (S) BA, VA, BP, VBR Bongiorni et al. (2005) Ionian Sea October and November 2004 1–200 (S, INT) 19 BA, VA, BP, VBR Magagnini et al. (2007) 4.1 Viruses The net effect of viruses with regard to the pelagic food web is the transformation of particulate organic matter (the host) into more viruses, and returning biomass into the pools of dissolved and colloidal organic matter – “the viral shunt”. Studies on viruses in the open MS are scarce, even less than in other marine regions. To date, most Mediterranean work has addressed viral control on bacterial biomass rather than the characterization of the viral community. The studies have revealed viral abundance in the surface waters which vary between 0.08±0.01×10 7 and 1.6±4.8×10 7 viruses ml − 1
while lower values occur in deeper waters (Fig. 12, Table 3). In the MS as in other marine areas, viral abundance in- creases from oligotrophic to more eutrophic waters. Existing data (Table 3) also suggest that, while viral abundance cor- relates with chl a concentration (n=46, r=0.409, p<0.05), a tighter relationship exists between viral and bacterial abun- dance (n=46, r=0.549, p<0.01) implying that bacteria are more probable virus hosts than phytoplankton cells. Con- sidering the data set for bacterial abundances and bacterial production (BP), we found that viral abundance were re- lated to both variables (n=24, r=0.520, p<0.05 and n=24, r= 0.421, p<0.05, respectively). The low correlation be- tween viral and bacterial abundance partly reflects the fact that the virus to bacteria abundance ratio (VBR) in the upper 200 m layer of the MS varies between 5 and 50 (Fig. 12). The wide range of this ratio suggests that viruses may be associated to different types of host organisms, and/or that viral concentrations vary over short times, causing different sampling events to reflect different phases of infection and release from host cells. Comparing the WMS and EMS, viral and bacterial abun- dance appears to be more tightly coupled in the west than in the east (Fig. 13 and Table 4). However, these trends
ferent Mediterranean sites (A), average integrated values (1–200 m), that were normalized in each case dividing them by the maximal considered depth (B). Bars are SD of the mean. NWM: NW-MS, ALB: Alboran, WM: WMS, THY: Tyrrhenian, SICH: Straits of Sicily, ADR: Adriatic, ION: Ionian (B). have to be taken cautiously, because the number of samples from the EMS is relatively limited, and differences between slopes are not statistically significant (t -student, t value
= 2.3;
p= 0.17). Viral infection accounts for less than 20% of bac- terial mortality in the Catalan Sea thus being definitely less important than mortality due to grazing by protists (Guixa- Boixereu et al., 1999a,b). However, virus-induced mortal- ity can occasionally prevail over grazing by heterotrophic Biogeosciences, 7, 1543–1586, 2010 www.biogeosciences.net/7/1543/2010/ I. Siokou-Frangou et al.: Mediterranean plankton 1559
Fig. 13. Relationship between bacterial and viral abundance (log transformed), taken at depths between 5 and 200 m, for WMS and EMS waters. nanoflagellates, for example at higher bacterial abundance in coastal waters (Weinbauer and Peduzzi, 1994; Boras et al., 2009). In a gradient from eutrophic to oligotrophic waters in the Adriatic Sea, viral production was higher in eutrophic areas and viral decay rates were not balanced by viral pro- duction rates over short time scales (Bongiorni et al., 2005). Alonso et al. (2002) characterized 26 bacteriophages of the viral community found in the Alboran Sea. Most of them belonged to two of the three tailed families of the order Cau- dovirales; phages were grouped in 11 classes on the basis of protein patterns and their size ranged between 30 nm and > 100 nm. Different morphotypes of bacteria hosted viruses of different sizes. Thus, virus between 30 and 60 nm mainly infected rods (74%) and spirillae bacteria (100%), while viruses between 60 to 110 nm were mostly found inside cocci (65.5%).
4.2 Bacteria The first study in the open MS examined the ultra- oligotrophic waters of the Levantine Sea (Zohary and Robarts, 1992) revealing that bacterial abundance, at 3×10 8
− 1 , was clustered around the lower threshold of the world ocean value (Cho and Azam, 1990). In the MS, while bacterial concentations are quite stable and bacterial production is low (Table 5) there are important variability aspects to consider: (i) the west-east gradient of decreasing bacterial production (Christaki et al., 2001; Van Wambeke et al., 2000, 2002), and (ii) the enhanced metabolic activities and production related to specific hydrologic discontinuities, such as currents, eddies and frontal areas (Fern´andez et al., 1994; Moran et al., 2001; Van Wambeke et al., 2004; Zer- voudaki et al., 2007). Interestingly, the slopes of log-log lin- ear regressions for bacterial biomass and bacterial production obtained for the WMS and EMS (Fig. 14a) are not signif- icantly different (t value
= − 0.22; p=0.85) with both slopes being smaller than 0.4, thus suggesting top-down control on bacteria (Billen et al., 1990; Ducklow, 1992). Table 4. Number (n) of data used to find out the relationships be- tween different variables in the Western Mediterranean (WMS) and the Eastern Mediterranean (EMS). BA: bacterial abundance; BP: Bacterial production; VA: viral abundance; PP: primary production; HNF: heterotrophic nanoflagellates abundance; Cil: ciliate abun- dances; Chl: chl a concentration. Variables WMS
EMS Source
(n) (n)
BA-VA 42 0 Guixa-Boixereu et al. (1999a) 38 19 Magagnini et al. (2007) 0 6 Weinbauer et al. (2003), Corinaldesi et al. (2003) Bongiorni et al. (2005) 10 0 Weinbauer et al. (2003) BA-BP
0 174
Christaki et al. (2003) 8 0 Vaqu´e et al. (2001) 0 13 Robarts et al. (1996) 13 18 Van Wambeke et al. (2002) 26 50 Christaki et al. (2001) 0 91 Van Wambeke et al. (2000) BP-PP
48 29 Turley et al. (2000) 22 24 Christaki et al. (2002) 26 0 Pedr´os-Ali´o et al. (1999) HNF-BA 12 0 Christaki et al. (1996), Christaki et al. (1998) 36 45
8 0 Vaqu´e et al. (2001) 0 48 Siokou-Frangou et al. (2002) Cil-Chl 20 42 Pitta et al. (2001) 8 0 Vaqu´e et al. (2001) 79 0 Dolan and Marras´e (1995) Following the general pattern of increasing oligotrophy eastward, bacterial production is several times lower in the eastern than in the western basin (Turley et al., 2000; Van Wambeke et al., 2000, 2002). However, the relation- ship between bacterial production and primary production is quite similar in the EMS and WMS. Expanding on the data set from Turley et al. (2000) (Table 4), plots of log BP and log primary production (PP) for the WMS and the EMS display similar positive slopes (t value = −
0.22; p=0.87) (Fig. 14b). This significant positive relationship between BP and PP sug- gests that primary production is an important source of DOC fuelling bacterioplankton. A crucial factor that might limit bacterial production in the MS is the availability of inorganic nutrients, especially phos- phorus. Nutrient control on bacterial production, as well as on bacterial adaptations to cope with the oligotrophy of the open MS, has been experimentally approached in a number of studies. During a Lagrangian experiment, phosphate ad- dition to ultra-oligotrophic surface waters of the Levantine Sea caused an unexpected ecosystem response: a decline in www.biogeosciences.net/7/1543/2010/ Biogeosciences, 7, 1543–1586, 2010 Yüklə 0,96 Mb. Dostları ilə paylaş:
|