Local differences in phytoplankton-bacterioplankton coupling in the coastal upwelling off Galicia (NW Spain)

E Teira, V Hernando-Morales, A Fernández, S Martínez-García, XA Álvarez-Salgado, A Bode, MM Varela
2015 Marine Ecology Progress Series  
17 18 33 release and bacterial production (BP) was found off Vigo, which could be related to the 34 quality of the DOC produced by actively growing phytoplankton. By contrast, DOC 35 release and BP rates were decoupled off A Coruña, likely due to unaccounted DOC 36 associated to indirect trophic processes or to the low availability of freshly produced 37 exudates associated to overflow or photoinhibition mechanisms. 38 39 2005, Lopéz-Sandoval et al. 2011). On the other hand, some other studies
more » ... some other studies suggest that 56 part of the recently fixed photosynthetic carbon might be indirectly released in dissolved 57 form as a consequence of trophic processes, such as grazing or cell lysis, ending in cell 58 breakage (Nagata 2000, Teira et al. 2001a, 2001b). The dominant process implicated in 59 DOC production may affect the quality and quantity of the released material and 60 influence the bacterioplankton ability to use this DOC, and the degree of coupling 61 between bacteria and phytoplankton. A recent review by Fouilland & Mostajir (2010) 62 clearly questions the immediate and direct dependency of bacteria on phytoplankton 63 exudates for carbon acquisition in aquatic systems, based on the relatively small fraction 64 4 of bacterial carbon demand (BCD) met by concurrent dissolved primary production. 65 According to Fouilland et al. (2014) such a lack of direct coupling would occur under 66 strong grazing pressure conditions, as bacteria would primarily consume indirectly 67 released DOC. 68 The upwelling system off NW Spain is characterized by the episodic enrichment 69 of inorganic nutrients into the coastal zone during spring and summer, with amplified 70 effects due to the presence of the large coastal embayments present in that area, coined 71 as "rías" (e.g. Álvarez Salgado et al. 2011). Different effects of upwelling have been 72 observed between the Rías Baixas, located to the south of Cape Fisterra, and the rías 73 located to the north of Cape Fisterra and in the southern Bay of Biscay. In the former, 74 the large size (>2.5 km 3 ), V-shape and NE-SW orientation of the rías favours the net 75 influx of upwelled water in response to the dominant northerly winds. The relatively 76 warm (>13ºC) Eastern North Atlantic Central Water (ENACW) of subtropical origin 77 upwells preferentially in this area, and the upwelling dynamics cause an export of the 78 organic matter produced inside the rías to adjacent shelf areas. This leads to a very 79 efficient utilisation of the upwelled nutrients. In the latest (to the North of Cape 80 Fisterra), colder (<13ºC) and nutrient-richer ENACW of subpolar origin also usually 81 upwells, but the N-S orientation of the rías favour upwelling under less intense and 82 frequent easterly winds. This, together with a much smaller size of these embayments 83 and narrowness of the shelf do not allow an efficient utilization of the upwelled 84 nutrients (Álvarez-Salgado et al. 2011). As a consequence, higher levels of primary 85 production have been reported for shelf waters off the Rías Baixas than those off the 86 other rias during the productive period (e.g. Bode et al. 1994). Differences in 87 phytoplankton community structure have been also reported between both areas along 88 the seasonal cycle; with a lower contribution of nano-and pico-phytoplankton in the 89 104 105 Material and Methods 106 Sampling. Seawater samples were collected at two shelf stations off A Coruña 107 (E2) and Vigo (E3) (Galicia, northwest Spain) at approximately monthly intervals from E3 could not be sampled because of stormy weather. At each sampling 110 date, vertical profiles of temperature, salinity, chlorophyll-fluorescence and 111 photosynthetically available irradiance (PAR) were obtained with a SBE-25 CTD 112 equipped with a Seapoint in situ fluorometer and a Licor spherical PAR sensor. Water 113 from 7 fixed levels was sampled with 5 l Niskin bottles that were attached to a CTD 114 6 rosette sampler (A Coruña) or to the hydrographic wire (Vigo). The maximum sampling 115 depths at E2 and E3 were 77 and 97m respectively. 116 Aliquots for inorganic nutrients determination (ammonium, nitrite, nitrate, 117 phosphate and silicate) were collected at all sampled depths in polyethylene bottles and 118 frozen at -20ºC until analysis by standard colorimetric methods with a Bran-Luebbe 119 segmented flow analyser. Chlorophyll-a (chl-a) concentration was determined at all 120 depths from acetonic extracts of plankton retained by GF/F filters and measured by the 121 fluorimetric method (Parsons et al. 1984). 122 Dissolved organic carbon (DOC) concentration, particulate organic carbon and 123 nitrogen (POC and PON) concentrations, dissolved organic matter fluorescence, 124 particulate and dissolved primary production, bacterial biomass, bacterial production, 125 and microbial plankton community and bacterial respiration were determined at two 126 depths corresponding approximately to the optical depths of 100 and 1% of surface 127 PAR (E 0 ) ( Table 1 ). The mean depth at 1% E 0 was 35m in E2 and 40m in E3. The 128 closest to the 1% E 0 available sampling depth in E3 was always 50 m, where the PAR 129 averaged 0.5% of E 0 , whereas at 30m depth PAR averaged 5% of E 0 . For simplicity we 130 will refer to 100 and 1% E 0 depth for both sites. 131 Dissolved organic matter. Water for the analysis of dissolved organic carbon 132 (DOC) was filtered through 0.2 µm filters (Pall, Supor membrane Disc Filter) in an all-133 glass filtration system under positive pressure of N 2 , collected into pre-combusted 134 (450ºC, 12 h) 10 ml glass ampoules, and acidified with H 3 PO 4 to pH < 2 before heat 135 sealing. Samples were measured in a Shimadzu TOC-V analyser (Pt-catalyst). 136 Fluorescence measurements were performed at a constant room temperature of 25ºC in 137 a 1cm quartz fluorescence cell in a Perkin Elmer LS 55 luminescence spectrometer. 138 Fluorescence intensity was measured at four fixed excitation/emission wavelengths of 157 incubated ashore for 2-3 h in an incubator which simulated the irradiance experienced 158 by the cells at the original sampling depths. Experimental incubations started always at 159 around 14:00 h local time. Bottles were maintained at surface seawater temperature 160 during incubation. Two 5ml subsamples were drawn from each bottle and filtered 161 through 0.2m polycarbonate membrane filters using low vacuum pressure (<7 kPa). 162 Labelled dissolved inorganic carbon was removed by acidifying the filters and filtrates 163 with 100 l of 50% HCl for 12 h. The filtrates were maintained in open scintillation 164 235 variance by ranks. It tests the null hypothesis that 3 or more groups all come from the 236 same distribution. It uses the ranks of data and is therefore resistant to outliers. The 237 Mann-Whitney significance test was applied a posteriori to analyse the differences 238 between every pair of groups. aquatic ecosystems, Oxford University Press, London, p 1-18 Williams PJ le B, Yentsch CS (1976) An examination of photosynthetic production, excretion of photosynthetic products, and heterotrophic utilisation of dissolved organic compounds with reference to results from a coastal subtropical sea. Mar Biol 35: 31-40 Wood AM, Van Valen LM (1990) Paradox lost? On the release of energy-rich compounds by phytoplankton. Mar Microb Food Webs 4: 103-116 Zlotnik I, Dubinsky Z (1989) The effect of light and the temperature on DOC excretion by phytoplankton. Limnol Oceanogr 34: 831-839
doi:10.3354/meps11228 fatcat:m6mdiqtahnemtaijf6ygp54rh4