Showing papers by "Xosé Anxelu G. Morán published in 2006"

Seasonal dynamics of picoplankton in shelf waters of the southern Bay of Biscay

Abstract: Seasonal variations in autotrophic and heterotrophic picoplankton community structure were studied by flow cytometry at 3 shelf stations in the central Cantabrian Sea (southern Bay of Biscay), from April 2002 to April 2003. A marked seasonality was found for autotrophs, with cyanobacteria (Synechococcus and Prochlorococcus) being more important in nutrient-depleted waters (June to October, 75 ± 3% of picophytoplanktonic cells) whereas picoeukaryotes clearly dom- inated in terms of biomass during the rest of the year (82 ± 2%). The absence of Prochlorococcus in spring and summer seems to be related to hydrology rather than to nutrient conditions. Under strati- fication the maximum abundance of cyanobacteria was found above the nutricline, in contrast to picoeukaryotes. Autotrophic groups cell size was strongly influenced by nutrient conditions while chlorophyll a fluorescence varied in parallel to irradiance. Heterotrophic bacteria were less variable than picophytoplankton and high nucleic acid content (HNA) cells dominated the community in win- ter and spring (64 ± 1%) but only at depth during the rest of the year. A similar response of low nucleic acid content (LNA) bacteria to environmental conditions suggests they are, at least partially, metabolically active, in contrast to the commonly held view that they are a homogeneous group of dead or dormant cells. A larger mean size of LNA compared with HNA bacteria was consistently observed in winter and spring. Regardless of variations in abundance and group composition, the great constancy of the picoplanktonic contribution to integrated algal biomass (17 ± 2%) over the seasonal cycle outlines the importance of picophytoplankton also in coastal zones.

Vertical distribution of phytoplankton biomass, production and growth in the Atlantic subtropical gyres

Abstract: Ninety-four stations were sampled in the Atlantic subtropical gyres during 10 cruises carried out between 1995 and 2001, mainly in boreal spring and autumn. Chlorophyll a (Chl-a) and primary production were measured during all cruises, and phytoplankton biomass was estimated in part of them. Picoplankton (o2mm) represented 460% of total Chl-a concentration measured at the surface, and their contribution to this variable increased with depth. Phytoplankton carbon concentrations were higher in the upper metres of the water column, whereas Chl-a showed a deep maximum (DCM). At each station, the water column was divided into the upper mixed layer (ML) and the DCM layer (DCML). The boundary between the two layers was calculated as the depth where Chl-a concentration was 50% of the maximum Chl-a concentration. On average DCML extends from 67 to 126 m depth. Carbon to Chl-a (C:Chl-a) ratios were used to estimate phytoplankton carbon content from Chl-a in order to obtain a large phytoplankton carbon dataset. Total C:Chl-a ratios averaged (7s.e.) 1037 7( n ¼ 22) in the ML and 247 4( n ¼ 12) in the DCML and were higher in larger cells than in picoplankton. Using these ratios and primary production measurements, we derived mean specific growth rates of 0.1770.01 d � 1 (n ¼ 173) in the ML and 0.2070.01 d � 1 (n ¼ 165) in the DCML although the differences were not significant (t-test, p40.05). Our results suggest a moderate contribution of the DCML (43%) to both phytoplankton biomass and primary production in the Atlantic subtropical gyres. r 2006 Elsevier Ltd. All rights reserved.

Response of Southern Ocean phytoplankton and bacterioplankton production to short-term experimental warming

Abstract: We examined the potential response of Southern Ocean pelagic ecosystems to warming through changes in total primary production (particulate plus dissolved 5 PPP + DPP) and bacterial production (BP), determined simultaneously at ambient temperature (21.4 to 0.4uC) and at 2uC in eight experiments performed near the Antarctic Peninsula in late spring 2002. Short (,6 h) time course experiments of radiocarbon uptake and photosynthesis–irradiance relationships consistently showed that a significant amount of photosynthate appeared as dissolved substances, with a mean 35% extracellular release (PER). Whereas PPP remained virtually unchanged (0.7 mg C m23 h21), DPP increased significantly at 2uC from 0.5 to 0.9 mg C m23 h21. The corresponding increase in PER (54% on average) was significantly and positively correlated with the temperature difference among treatments, suggesting that an increase in DPP could be expected with a temperature rise in the Southern Ocean. BP, estimated via [ 3 H]leucine incorporation, tended to increase at 2uC only at low absolute values, and this increment was inversely related to PPP. However, our results show that the estimated bacterial carbon demand (BCD) was generally well below concurrent DPP at both treatments (mean BCD:DPP ratios of 0.60 and 0.27 at ambient temperature and 2uC, respectively), indicating that temperature-related extra inputs of organic substrates were not fully and immediately processed by bacteria. To the extent that these results reflect general ecophysiological trends, warming of Southern Ocean surface waters could produce changes in plankton-mediated biogeochemical processes leading to a greater importance of dissolved organic matter fluxes. The crucial role of the oceans in the Earth’s climatic system has motivated efforts aimed at evaluating the effects of global warming on the structure and functioning of pelagic ecosystems and their associated biogeochemical fluxes (Sarmiento et al. 2004). One of the strongest evidences of recent warming has been found in the Southern Ocean (Gille 2002), and according to coupled ocean–atmospheric models, this ocean basin will continue to suffer a major temperature rise in the next decades (IPCC 2001). Because of its geographic location, future changes in the Southern Ocean will affect the other three major basins. The Southern Ocean is also one of the largest sinks of anthropogenic CO2 on Earth, and large increases in phytoplanktonic biomass and production have been recently predicted as a consequence of temperaturemediated major changes in stratification and growth season length (Sarmiento et al. 2004). Macronutrients are not used fully in many parts of the

Dissolved Organic Nitrogen Release and Bacterial Activity in the Upper Layers of the Atlantic Ocean

Abstract: The variability of the percentage of extracellular dissolved organic nitrogen (DON) release (PER), along with the relationship between DON release and bacterioplankton activity, was examined during five oceanographic cruises, carried out in the upwelling region of the NW Iberian Peninsula, the SW Bay of Biscay, and a latitudinal transect in the Atlantic Ocean (50°N–35°S). Rates of nitrogen uptake, DON release, and bacterial production were measured at 66 stations and sampled between August 1998 and October 2000. The percentage of DON release relative to the gross uptake of ammonium (PERNH4+) ranged from 3 to 46%, whereas that relative to total nitrogen (NH4+ + NO3− + urea) gross uptake (PERtotal) varied between 21 and 82%. The highest values for both PERNH4+ and PERtotal were found in oligotrophic oceanic waters ( 6 mg chlorophyll a m−3), such as the continental shelf off the NW Iberian Peninsula, PER held constant as nitrogen uptake increased. These results suggest the dominance of different processes controlling DON release in oceanic and neritic zones. DON release rates accounted for less than 15% of the variability observed in bacterial production rates, suggesting a weak response of bacterioplankton to phytoplankton on short time scales (hours). Furthermore, nitrogen budgets showed an excess of DON release in relation to bacterial requirements.