Changes in phytoplankton ecophysiology across a coastal upwelling front

The abundance, taxonomic composition and patterns of macromolecular synthesis of phytoplankton were determined across an upwelling-induced thermal front in the central Cantabrian Sea (southern Bay of Biscay) during July 1993. Enhanced levels of phytoplankton biomass, diatom abundance and photosynthetic rate were measured on the coastal side of the front. Relative carbon (C) incorporation into proteins increased noticeably on the oceanic side, taking values of up to 64%, whereas changes in the relative C incorporation into lipids and low-molecular-weight metabolites followed an opposite trend. Phytoplankton cells on the oceanic side of the front were adapted to the prevailing growth-limiting conditions by maintaining the synthesis of functionally essential molecules—proteins—rather than the synthesis of storage compounds. As a result, the carbon to nitrogen uptake ratio varied from ~5.7 in offshore waters to 8.0 in the nearshore region. Our results suggest that the taxonomic and physiological changes in phytoplankton assemblages as a response to upwelling may result in an increase in the synthesis of organic C relative to the upward flux of nitrate. During coastal upwelling events, phytoplankton cells experience an increase in biomass-specific nutrient uptake rates (e.g. Wilkerson and Dugdale, 1987) and display changes in their patterns of carbon (C) allocation into different biochemical pools (e.g. Barlow, 1984). Moreover, the specific synthesis rates of proteins and polysaccharides have been proved to show up to a 5-fold increase after an advective pulse (Marafi6n et al., 1995). These changes may partially be due to variations in the relative abundance of species (Zimmerman et al., 1987). A combined approach is therefore needed where both taxonomic composition and the physiological state of phytoplankton assemblages are taken into account. Sambrotto etai, (1993) reported elevated C to nitrogen (N) consumption ratios in continental shelf regions (southeast Bering Sea and Gerlache Strait), as well as along sections in the North Atlantic, suggesting that the excess loss of dissolved inorganic C was due to biological processes that recycle N more efficiently than C. In this regard, the C-partitioning technique is particularly useful, as it allows the estimation of phytoplankton N uptake from the measured C incorporation into the protein fraction (DiTullio and Laws, 1983). If upwelling induces a shift in the patterns of carbon partitioning among biomolecules, it may be hypothesized that phytoplankton in an upwelled water mass could show significant differences in the C:N uptake ratio as compared with microalgae in non-upwelled waters. During July 1993, we monitored the development of a coastal upwelling pulse, taking advantage of the existence of a sharp, well-defined thermal front. We analysed the effects of upwelling on the taxonomic composition of microalgae and their patterns of macromolecular synthesis by comparing the phytoplankton © Oxford University Press 1999 E.Maranon and E.Feraindez assemblages on both sides of the upwelling front. The variations in the C:N uptake ratio across the upwelling front were assessed and their potential implications for the biogeochemical cycling of C and N in upwelling systems evaluated. Sampling was conducted in coastal and offshore waters of the central Cantabrian Sea (southern Bay of Biscay) on 13 July 1993. Fifteen surface (2-3 m depth) water samples were taken at ~1 km intervals along a transect running from 43°43.34'N, 6°09.28'W to 43°35.34'N, 6°08.77'W. In addition, the vertical distribution of the variables under study was determined at six points along the transect and at two stations located near the coastal (station A, 43°35.68'N, 6°08.60'W) and oceanic (station B, 43°42.00'N, 6°09.00'W) ends of the transect. Surface distribution and vertical profiles of temperature and salinity were obtained with a SeaBird CTD probe. Vertical profiles of photosynthetically active radiation (400-700 nm; PAR) were determined with a Li-Cor 4ir underwater sensor. Seawater samples were collected with Niskin bottles for the determination of nutrient concentration, chlorophyll a concentration, taxonomic composition of microplankton, and particulate organic C and N concentrations. The analytical methods followed were the same as in Fernandez et al. (1994). Cell counts were transformed to C biomass as in Holligan etal. (1984). Seawater for C incorporation experiments was screened through a 200 urn sieve upon collection from the Niskin bottles. Experiments were carried out under natural light conditions in an incubator equipped with a set of neutral-density plastic screens which provided a range of irradiance levels (2,8,20,40,60 and 100% of incident irradiance). Each sample was incubated under an irradiance level similar to that experienced by the cells at the sampling depth. Temperature was kept at 17 ± 1°C by means of recirculating water. Experiments started early in the morning (-7:00 a.m.). Duplicate 70 ml polycarbonate bottles were filled with seawater from each sample, inoculated with 370 kBq (10 u-Ci) of NaHCO3 and incubated during 24 h under a natural lightdark cycle. At the end of the incubation, samples were filtered (<100 mm Hg) through Whatman GF/F filters, which were subsequently stored at -20°C until further analysis. The patterns of C incorporation into different photosynthetic end-products were determined as in Ferndndez etal. (1994). This technique allows separation of cellular material into four fractions: proteins, polysaccharides and nucleic acids, lipids and low-molecular-weight metabolites (LMWM). Total carbon incorporation was calculated as the sum of the C activity in each fraction. In previous experiments, no significant differences were found between total C incorporation in non-fractionated samples and the sum of the C incorporated into the four fractions (r-test, P > 0.2, n = 10). The C:N uptake ratio was calculated stoichiometrically from the percentages of C allocation into each fraction by assuming that (i) the average molar C:N ratio is 3.8 for proteins and 3.7 for nucleic acids (Laws, 1991), (ii) the free amino acid-N to protein-N ratio was 0.05 (Dortch et al., 1984), (iii) the amount of radioactive carbon present as free nucleotids was negligible (<0.5% of total incorporated carbon) and (iv) 4% of total C fixed was incorporated into nucleic acids (Fraga and P6rez, 1990). The vertical distribution of temperature along the transect revealed the effects of upwelling, as shown by the outcrop of isotherms towards the coast (Figure 1 A).