Bacterioplankton abundance and biomass stimulated ...

Continental Shelf Research 164 (2018) 28–36

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Bacterioplankton abundance and biomass stimulated by water masses intrusions over the Southern Brazilian Shelf (between 25°57'S and 29°24'S)

T

⁎

Maria Luiza S. Fontesa,b, , Alexandre Berrib, Melissa Carvalhoa, Alessandra L.O. Fonsecab, Regina V. Antônioa,c, Andrea S. Freirea a

Departamento de Ecologia e Zoologia, Centro de Ciências Biológicas, Campus Universitário, s.n. Universidade Federal de Santa Catarina, Florianópolis 88040-970, Santa Catarina, Brazil b PPGOCEANO, Coordenadoria Especial da Oceanografia, Centro de Ciencias Fisicas e Matematicas. Campus Universitário, s.n. Universidade Federal de Santa Catarina, Florianópolis 88040-970, Santa Catarina, Brazil c Campus Araranguá, Rodovia Governador Jorge Lacerda, 3201, km 35.4. Universidade Federal de Santa Catarina, Araranguá 88906-072, Santa Catarina, Brazil

A B S T R A C T

In order to understand the distribution of bacterioplankton along the continental shelf off the Santa Catarina coast (25°57'S and 29°24'S) and whether water masses were affecting this distribution, we measured abundance, biovolume, and biomass of bacterioplankton, and the physico-chemical characteristics of the water masses in three transects in two distinct seasons: summer and winter of 2010. During the austral summer, high continental runoffs from the Itajaí River and Babitonga Bay mix over the shelf with Tropical Water (TW) and originate the summer Subtropical Shelf Water (STSW). Bacterial biomass reached an average of 2.5–5.0 μgC L−1 in summer, with values observed in the southernmost transect (Araranguá) being one order of magnitude higher when compared to the others, where nutrient-rich South Atlantic Central Water (SACW) intrusions are often near the coast. On the contrary, in the austral winter, the Plata Plume Water (PPW) spread on the first 40 m of water positively influenced bacterial abundance and biomass, which were on average ten times higher than those observed in the summer (abundance of up to 2.5 × 109 cells L−1, and biomass up to 60 μgC L−1). As a conclusion, PPW, indicated by the colder waters, high silicate and Kd values, and high concentrations of particulate organic carbon have a positive effect on the abundance and biomass of bacterioplankton, whereas the intrusions of SACW, and local freshwater runoffs have secondary effects on bacterial parameters. This is the first description of the influence of PPW intrusions over the dynamics of bacterioplankton, considering two contrasting environmental periods, and, consequently, to carbon budget studies in the Southern Brazilian Shelf (SBS). Taking into account both abundance and biomass of bacterioplankton and their association with water masses in the SBS, our results provide fundamental information for further biogeochemical studies.

1. Introduction Bacterioplankton (Bacteria and Archaea) are the major constituents of the picoplankton in oligotrophic marine waters (Biddanda et al., 2001; Cotner and Biddanda, 2002; Finkel et al., 2009), and the main agents of ocean biogeochemical cycles (Kirchman, 1994; Weber and Deutsch, 2010). As a consequence, their responses are closely related to physico-chemical characteristics of the water masses (such as temperature, salinity, nutrients and organic matter) (Carlson et al., 2009; Fortunato and Crump, 2011; Fuhrman and Steele, 2008; Giovannoni and Vergin, 2012; Morris et al., 2005; Treusch et al., 2009). Each water mass typically contains its specific bacteria Agogué et al. (2011);

Hewson et al. (2006); Samo et al. (2012); Seymour et al. (2012); however, the same water mass can have bacterioplankton distributed in patches if spatial scales are of several kilometers or more (Hewson et al., 2006). Thus, the distribution of bacterioplankton and its relationship with hydrological regimes on the continental shelves around the globe, where a myriad of micro-, meso-scale events are present, remains understudied. The Southern Brazilian Shelf (SBS) – extending from 22° to 34°S along the Southwestern Atlantic - is amongst the most important fishing grounds along the 8.000-km Brazilian shelf. Local fisheries production is regulated by coastal upwelling of South Atlantic Central Water (SACW) in the summer and by the La Plata River plume water in the

⁎ Corresponding author at: Departamento de Ecologia e Zoologia, Centro de Ciências Biológicas, Campus Universitário, s.n. Universidade Federal de Santa Catarina, Florianópolis 88040-970, Santa Catarina, Brazil. E-mail address: [email protected] (M.L.S. Fontes).

https://doi.org/10.1016/j.csr.2018.05.003 Received 19 January 2017; Received in revised form 3 October 2017; Accepted 9 May 2018 Available online 05 June 2018 0278-4343/ © 2018 Elsevier Ltd. All rights reserved.


M.L.S. Fontes et al.

Monthly average composites for March and August 2010 of sea surface temperature (SST) and satellite-derived particulate organic carbon (POC) (with 4-km resolution) were obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard the Aqua satellite. POC concentration (mg m−3) was derived from the 490 nm-based model of Stramski et al. (2008), which is the standard MODIS algorithm. Vertical profiles of temperature, salinity and density were obtained with a conductivity-temperature-depth (CTD) profiler (mini STD/CTD sensor data model SD201) at all stations, following standard CTD processing techniques for derived parameters. CTD data were used to prepare temperature-salinity (TS) diagrams with depth, and cross-shore sections using Ocean Data View ODV 4.5.3. Water masses were identified based on the thermohaline intervals proposed by Piola et al., (2000, 2008a) and Moller et al. (2008). Water samples were collected with 5 L Van Dorn bottles at least in three selected depths: surface, base of the euphotic layer according to the Secchi disk depth, and 20 m above the bottom, to determine nutrients, chlorophyll a (Chl-a), and bacterioplankton parameters. At shallow stations, water samples were collected at surface and near bottom. Secchi disk measurements also were used to calculate the coefficient of light attenuation (Kd = 1.7/depth Secchi disk) and the average depth of the euphotic zone (3 times Secchi depth) in summer and winter. One liter aliquots were filtered through AP-40 Millipore filters (diameter of 47 mm and porosity of 0.7 µm) and immediately frozen (−18 °C) in polyethylene flasks (the filtrates) and Falcon tubes (the filters) for further nutrient and chlorophyll a analysis, respectively. For bacterioplankton, fifty mL was fixed with 2% paraformaldehyde (final concentration), followed by immediate filtration (on the vessel) of two mL aliquots (replicate) through 0.2 µm dark polycarbonate membrane filters and staining with DAPI (4′,6-diamidino-2-phenylindole) (0.5 mg mL−1 final concentration) for 15 min (Porter and Feig, 1980). Filters were mounted on microscope slides and stored in −20 °C until microscopic analyses in the laboratory.

winter (Acha et al., 2004; Piola et al., 2008b), demonstrating the importance of seasonality on the fisheries stocks. Eukaryotic multicellular plankton (zooplankton and ichthyoplankton) are often used as water masses indicators, where each water mass off the SBS (Plata Plume Water - PPW, Subtropical Shelf Water - STSW, Southern Atlantic Central Water - SACW and Tropical Water - TW) showed its own species assemblages (Brandão et al., 2015; Lopes et al., 2006; De Macedo-Soares et al., 2014, Piola et al., 2005; Zhu et al., 2013; Muelbert et al., 2008). Eichler et al. (2008) studied the distribution of benthic foraminifera in association with water masses in the SBS and found estuarine species related to PPW in winter, while in summer, deep water species were related to SACW. Brandini and co-authors (2014) reported that the abundance of microphytoplankton was positively correlated with SACW intrusions on the continental shelf, whereas flagellates predominated in the oligotrophic TW, and phytoflagellates were associated with the PPW (Brandini and Fernandes, 1996). Otherwise, studies of the dynamics of bacterioplankton along the Brazilian shelf are still scarce and are restricted to coastal areas, such as estuaries along the coast of Rio de Janeiro and São Paulo (Barrera-Alba et al., 2009; Gregoracci et al., 2012; Vieira et al., 2008). Considering the influence of water masses on the distribution of larger planktonic organisms along the SBS, we hypothesized that bacterioplankton abundance and biomass are also regulated by the seasonality of water masses and their predominant constituents. 2. Material & methods 2.1. Study area The Southern Brazilian Shelf (SBS) is the most productive coastal area under the influence of the Brazil Current, and is classified as Class II ecosystem, with moderately high productivity (150–300 gC m−2 year−1) (Heileman and Gasalla, 2008). The seasonal distribution of water masses over the SBS is a result of synoptic events driven by dominant wind features throughout the year (Campos et al., 2013; Moller et al., 2008; Piola et al., 2008a, 2008b). For example: in winter, freshwater discharges of Río de La Plata (Plata Plume Water - PPW) added to the Patos Lagoon plume flow northwards are a result of constant and intense SW winds (Campos et al., 2013; Moller et al., 2008; Piola et al., 2008a) which will influence the biological productivity (Ciotti et al., 1995). In summer, NE winds lead to the retreat of the freshwater plume, and induce coastal upwelling of the South Atlantic Central Water (SACW) (Campos et al., 2013; Moller et al., 2008; Piola et al., 2008a), which stimulates local productivity, particularly at Cape Santa Marta (28°S) (Vasconcellos and Gasalla, 2001). Additionally, processes onshore differ from those offshore, where inner shelf is predominantly driven by local winds and coastal waters, and the middle and outer shelves are more influenced by the Brazil Current (Palma and Matano, 2009; Pereira et al., 2009). The whole SBS is often divided in major latitudinal regions, and the continental shelf off Santa Catarina state (~26–30°S) is considered to be in the transition of tropical and temperate environments (Acha et al., 2004). Major water masses have been described for this section of SBS, with the northern-central shelf of Santa Catarina state being influenced by the runoffs of continental waters originated from Babitonga Bay and Tijucas river, while the southern part of the shelf is distinguish by frequent upwelling events in the Cape Santa Marta (Fig. 1).

2.3. Laboratory analyses In the laboratory, Chl-a was extracted from cells concentrated in the filters with acetone 90% (v/v) for 18 h at 4 °C in the dark. The absorbance of the extracts was read on a spectrophotometer, and the determination of Chl-a was performed using SCOR-UNESCO (1966) equations. Nitrate plus nitrite (nitrate hereafter), ammonium, phosphate and silicate analysis was performed by spectrophotometric methods (Micronal, Model AJX-1600) according to the protocols described in Grassholff et al. (1999). NP ratio was calculated as the molar ratio between the sum of total dissolved inorganic nitrogen and phosphate. Nutrient concentrations for each sampling site were related to the correspondent water mass present in the area. Silicate was used as indicator of La Plata River plume influence as for other freshwater discharges (Braga et al., 2008). Filters previously mounted on slides for microscopic counting and individual cell measurements were analyzed in an Olympus Bx40-FL epifluorescence microscope (Olympus, Tokio, Japan), under 1000× magnification with immersion oil. Total bacterial cells (autotrophic and heterotrophic were not differentiated) were counted under the excitation of UV light using as many random fields of view as microscopereplicates as needed in order to maintain the coefficient of variation < 30%. Bacterial biovolume was calculated, in μm3, using the algorithm suggested by Massana et al. (1997), where major and minor axis were measured and bacterial biomass (in fg C cell−1) was estimated using the algorithm based on biovolume: B = 120 × V0.72, where B = biomass, V = biovolume and 120 = conversion factor for carbon (fg C μm−3) (Norland, 1993), followed by multiplication between the carbon content per cell by the number of cells per volume.

2.2. Sampling and data analyses Two oceanographic cruises were carried out on board the research vessel Soloncy Moura (CEPSUL/ICMBio) in 2010: from February 26 to March15 - summer, and from August 10–27 – winter. On each cruise, three cross-shelf sections off the Santa Catarina coast (latitude between 26° and 30°S) were performed with oceanographic stations located on the 20, 50, 100 and 200 m isobaths (< 50 km apart) (Fig. 1). 29



Fig. 1. Distribution of the oceanographic stations along three cross-shelf sections in the Santa Catarina coast, at the South Brazil Shelf: Babitonga Bay (26°S), Florianópolis (27.6°S), and Araranguá (29°S). Isobaths are shown in the color bar.

Fig. 2. Temperature-Salinity (TS) diagrams for March/summer (A), and August/winter (B) cruises. Depths (m) of TS points are shown in gray-scale color bar. Water masses are as follows: Plata Plume Water (PPW), Subtropical Shelf Water (STSW), South Atlantic Central Water (SACW), and Tropical Water (TW). This classification was based on the thermohaline intervals of Piola et al., (2000, 2008a) and Moller et al. (2008).

2.4. Statistical analyses

6.1.16.

One-way analysis of variance (ANOVA) was used to test differences of bacterial abundance and biomass among all water masses (total N = 72). Data were log-transformed prior to analysis, and all ANOVA assumptions were tested (Zar, 1999). Multiple regression analysis of bacterial abundance versus non-redundant abiotic variables was done in Statistica (StatSoft Inc. OK, US). Redundancy analysis (RDA) was performed for bacterial and abiotic data using PRIMER-E Ltd version

3. Results 3.1. Physico-chemical characteristics of water masses TS diagrams showed a strong mixture of water masses in March, with a wide distribution of the Subtropical Shelf Water (STSW) and the intrusion of South Atlantic Central Water (SACW) in the internal 30



Fig. 3. Monthly average compositions of satellite-derived sea surface temperature (°C), with a 4 km spatial resolution in March 2010 (A), and in August 2010 (B).

Fig. 4. Monthly average compositions of satellite-derived surface particulate organic carbon (POC) concentration (mg m−3) derived from the 490 nm-based model of Stramski (Stramski et al., 2008), with a 4 km spatial resolution: POC in March 2010 (A), and POC in August 2010 (B).

Table 1 Mean ± SD (minimum-maximum) concentrations of dissolved inorganic nutrients (µM) and of chlorophyll a (µgL−1) and coefficient of light attenuation (Kd, m−1) and NP measured in each water mass on the continental shelf off Santa Catarina during March and August 2010 cruises. Abbreviations of water masses: South Atlantic Central Water (SACW), Subtropical Shelf Water (STSW), Plata Plume Water (PPW). N = number of samples per water mass in each cruise. ND = not detected by the method.

March STSW (N = 21) SACW (N = 7) August PPW (N = 21) STSW (N = 23)

Kd (m−1)

Silicate (µM)

Phosphate (µM)

Nitrate (µM)

Ammonium (µM)

NP

Chlorophyll a (µg L−1)

0.31 ± 0.22 (0.09–0.85) 0.12 ± 0.01 (0.09–0.14)

3.76 ± 2.98 (0.45–10.57) 3.47 ± 2.18 (0.97–7.23)

0.43 ± 0.19 (0.13–0.83) 0.77 ± 0.56 (0.33–1.94)

1.29 ± 0.77 (0.26–2.60) 5.28 ± 4.10 (0.49–12.70)

7.71 ± 3.51 (0.96–13.12) 5.52 ± 2.63 (2.15–9.16)

24 ± 11 (9–51) 16 ± 7 (7–40)

1.12 ± 0.61 (0.14–3.06) 0.60 ± 0.45 (0.07–1.42)

0.41 ± 0.10 (0.24–0.57) 0.24 ± 0.09 (0.10–0.57)

9.29 ± 2.90 (5.83–17.62) 7.06 ± 4.38 (1.66–16.94)

0.52 ± 0.11 (0.33–0.77) 0.46 ± 0.19 (0.17–0.94)

1.32 ± 0.72 (0.56–3.19) 2.68 ± 2.46 (0.58–9.05)

0.63 ± 0.50 (0.07–1.91) 0.64 ± 0.66 (0.11–2.50)

5±3 (1–16) 8±6 (2–56)

0.58 ± 0.41 (ND−2.27) 0.27 ± 0.24 (ND−1.19)

31



Fig. 5. Bacterial abundance (cells L−1) and biomass (μgC L−1) at Babitonga −26°S (A, B), Florianópolis – 27.6°S (C, D) and Araranguá − 29°S (E, F) transects in March (summer) 2010. Abbreviations of water masses: South Atlantic Central Water (SACW), Subtropical Shelf Water (STSW), Tropical Water (TW), Plata Plume Water (PPW).

3.2. Bacterioplankton distribution

platform (Fig. 2). In August, the temperature of STSW decreased and the Plata Plume Water (PPW) was present in the top 50 m of the water column (Fig. 2). Low temperatures of the STSW in August (winter) are a product of the mixture between both PPW and Tropical Water (TW), while warm temperatures of the STSW in March was a product of mixture between TW and plumes of Itajaí River and Babitonga Bay. The STSW was shallower in summer, reaching the 50 m depth, whereas in winter, it deepened, reaching depths of up to 200 m, with the PPW on top of it. SST images showed warmer waters (> 24 °C) in March (summer), while a cold-water plume (< 18 °C) extended northwards in August (winter), reaching up to 25°S (Fig. 3). Estimates of particulate organic carbon (POC) based on satellite showed values < 300 mg. m−3 throughout the shelf, with high values near coastal freshwater discharges in March (Fig. 4A), whereas POC > 300 mg. m−3 was well distributed along the continental shelf in August, with a few outer shelf peaks of POC > 800 mg m−3 (Fig. 4B). SACW was nitrate and phosphate-rich compared to the other water masses, while STSW was ammonium and chlorophyll a-rich in summer; PPW had the highest silicate concentrations and light attenuation coefficient (Kd) values (Table 1). Statistical results showed that nitrate increased with depth (positively correlation with water depth (r = 0.70, p = 0.00)), ammonium with salinity (positively correlated with salinity (r = 0.50, p = 0.000)); and silicate was inversely correlated with salinity (r = −0.52, p = 0.000) and temperature (r = −0.34, p = 0.001). Furthermore, nitrate and phosphate reached maximum values of 12.7 and 1.94 µM, respectively, in the SACW.

3.2.1. Summer In general, bacterial abundance and biomass decreased fro the coast towards the shelf break (Fig. 5). However, cross-shelf distributions differed when comparing transects: e.g. at the Babitonga transect, bacterial abundance and biomass were higher in the bottom waters of 50 and 100 m isobaths; whereas at Florianópolis and Araranguá transects a cross-shelf bimodal (high values near the coast and again at the shelf break) distribution was observed. Along the Babitonga transect, a maximum in bacterial abundance and biomass was observed in the bottom waters, 8–9 × 108 cells L−1 and 22.0 μg C L−1, respectively; whilst the values in the shallow waters were only 1–3 × 108 cells L−1 and 2.5–5.0 μg C L−1 (Fig. 5 A-B). At Florianópolis transect, the maximum values of abundance and biomass reached up to 5–6 × 108 cells L−1 and 7.5–10.0 μg C L−1, respectively. These maxima were found in the coastal bottom waters under the influence of SACW near the coast, and at 200 m isobaths in the 100 m depth, also under the influence of the SACW (Fig. 5 C-D). At southernmost transect, Araranguá, the maximum values were once again observed as a bimodal distribution, in coastal waters up to 40 km away from the coast under the influence of SACW upwelling and in the frontal zone formed between STSW and TW at the 200 m isobaths, with values of 5.5–6.5 × 108 cells L−1 and 10.0–17.5 μg C L−1 (Fig. 5 E-F). 3.2.2. Winter In general, both bacterial abundance and biomass in August were ca. 10 times higher than those values found in March (Fig. 6). At Babitonga transect, maximum bacterial abundance and biomass values occurred in the subsurface waters at the 100-m isobath, with values of 32



Fig. 6. Bacterial abundance (cells L−1) and biomass (μgC L−1) at Babitonga −26°S (A, B), Florianópolis – 27.6°S (C, D) and Araranguá −29°S (E, F) transects in August (winter) 2010. Abbreviations of water masses: South Atlantic Central Water (SACW), Subtropical Shelf Water (STSW), Tropical Water (TW), Plata Plume Water (PPW).

Fig. 7. Average ± standard error (SE) of bacterial abundance (A, B), and biomass (C, D) estimated for each water mass, obtained from all samples collected in March and August 2010. Abbreviations of water masses: South Atlantic Central Water (SACW), Subtropical Shelf Water (STSW), Plata Plume Water (PPW). *Statistical difference among groups per sector (p < 0.05). n = 72.

33



the STSW in winter is still small compared to the abundance and biomass of bacteria found in the PPW (Fig. 7). 3.3. Relationship between bacterioplakton and the environment The Redundancy Analysis (RDA) plot showed a temporal distinction of bacterioplankton samples through its dbRDA1 - first axis: 55.7. %, while particular features within each water mass was shown in its second axis (42.2%). Chlorophyll a and temperature were closely related to bacterioplankton in the STSW in summer; whereas ammonium and salinity were related to bacteria in the SACW and deep STSW, while silicate and Kd were correlated to bacterioplankton in the PPW (Fig. 8). Distance-based linear model, distLM, showed that seven variables (chlorophyll a, nitrate, ammonium, temperature, salinity, Kd and silicate) explained 62% of total variability in bacterioplankton (data not shown) - among them: ammonium and nitrate were negatively correlated with bacterial abundances, whereas silicate and Kd were negatively related (shown in the multiple regression analysis) (Table 2). 4. Discussion Fig. 8. Redundancy Analysis (RDA) ordination diagram of the six main explanatory variables of the biomass and abundance of bacterioplankton in the water masses. Temp = temperature; NO3- = nitrate; NH4+ = ammonium; Sil = silicate; Kd = light attenuation coefficient; Chl = Chorophyll a. Abbreviations for water masses in this figure: South Atlantic Central Water (SACs in summer, SACw in winter), Subtropical Shelf Water (STs in summer, STw in winter), and Plata Plume Water (PPs in summer, PPw in winter); s = summer, w = winter.

Abundance and biomass of bacterioplankton are influenced by the seasonal patterns of water mass distribution along the Southern Brazilian Shelf (SBS). In austral summer (March), the coastal intrusion of SACW in the proximity of Araranguá transect appears be the cause of doubling the bacterial biomass compared to the other transects, bringing nutrients to the upper waters. This upwelling in the euphotic zone is due to the intensification of northerly and northeasterly winds and, consequently, of surface currents (Campos et al., 2013; Piola et al., 2008a). The SACW intrusion over the SBS is a common event, especially around the Santa Marta Cape region between 27° and 29°S (Campos et al., 2013; Matsuura, 1986; Moller et al., 2008), and it has also been described to reach the coastal waters off the Itajaí River estuary (Brandini et al., 2014; Pereira et al., 2009), which is located between the Florianópolis and Babitonga transects. In winter, SACW intrusion over the SBS is prevented by the weakening of the N-NE winds and the surface currents, and by the intensification of southerly winds which push the upper water layers towards the coast (Brandini, 1990; Ciotti et al., 1995), resulting in the northward flow of continental discharges of two largest plumes over the SBS, Plata plume water (PPW) and Patos Lagoon-Mirim system plume (Campos et al., 2013; Moller et al., 2008; Piola et al., 2008a). These plumes are integrated during the northward flux and result on the enrichment of oligotrophic coastal waters with terrigenous material (Nagai et al., 2014). This northward flux of PPW is a common feature in August, when PPW can be detected as far as in latitudes of 25°S (Souza and Robinson, 2004). PPW is a silicate and organic matter-rich water mass (Ciotti et al., 1995), where dissolved and particulate organic material is abundant. Continental discharges over the continental shelf are positively correlated with POC concentrations (Piola et al., 2005, Zhu et al., 2013). The satellite data showed here corroborates with previous assumptions that PPW is an important reservoir of particulate organic carbon (POC) in the surface waters on the SBS in winter (Ciotti et al., 1995). This increasing concentration of POC in August appears to be the main driver for bacterioplankton biomass stimulation. Additionally, lower Kd, colder waters and high concentration of silicate associated with the intrusions of PPW over the SBS explained most of the variability in bacterial abundance and biomass. POC images showed additional high levels near local small estuaries such as Babitonga Bay, Itajaí -Açu River estuary where annual average discharge are of 350 m3 s−1 (Schettini, 2002). These estuaries have much lower discharge rates compared to La Plata estuary (annual discharge averages 24,000 m3 s−1 - Piola et al., 2005), but locally they contribute with significant amounts of POC to the coastal waters (Campos et al., 1996; Souza & Robinson, 2004). PPW is important to structure food webs, and consequently, to the

Table 2 Stepwise multiple linear regression analysis for bacterial abundance. R = 0.690, R2 = 0.477, F(4,67) = 15.259, p < 0.000001. n = 72. Dependent variable

Explaining variable

Coefficient

t value

Adjusted r2

p

Bacterial Abundance

Intercept

8.870

30.704

0.445

0.000

−0.347 0.339 −0.383 0.373

−3.470 1.586 −2.617 2.218

+

NH4 Kd NO3SiO4−4

0.001 0.117 0.010 0.030

1.3 × 109 cells L−1 and 35.0 μg C L−1, respectively (Fig. 6 A-B). At Florianópolis transect, maximum values were again observed in the subsurface waters at the 100-m isobath (1.9 × 109 cells L−1 and 60.0 μg C L−1), in the frontal zone between PPW and STSW, and as expected, the values decreased in both directions: towards the coast and towards the shelf-break (Fig. 6 C-D). At Araranguá, the bimodal distribution observed in summer repeated in winter, with maximum values in the subsurface waters near the coast (up to the 50 m isobath − 40 km away from the coast) in the PPW, and in the subsurface waters of the 200 isobath in the frontal zone formed between PPW and TW (2.5 and 2.0 × 109 cells L−1, 55 and 40 μg C L−1, respectively) (Fig. 6 E-F). Average abundance and biomass values along the shelf were between 8.0 × 108 and 1.2 × 109 cells L−1, and 17.5–30 μg C L−1. Bacterial abundance and biomass were significantly higher in August (Fig. 7), when the PPW was well distributed over the continental shelf and higher amount of particulate organic carbon were present (Figs. 4 and 7). The highest bacterioplankton averages in the PPW were 1.15 × 109 cells L−1 and 28 μg C L−1 (Fig. 7B and D). PPW distribution over the continental shelf was detected only in winter in our study, and it was shown in the top 40 m deep waters. The most dominant water mass over the SBS is the STSW, which is present in the summer and winter; however, bacterial abundance and biomass doubled in winter compared to summer. Averages were 2.0 × 108 cells L−1 and 4 μg C L−1 in March, respectively, against 3.8 × 108 cells L−1 and 8 μg C L−1 in August (Fig. 7). This increase in the bacterioplankton in 34



dominate in downwelling events (Guenther et al., 2008). These studies support our results, confirming the importance of the microbial food web in August (winter), under the influence of cold, organic matter-rich waters. We conclude that the bacterioplankton distribution is strongly regulated by water mass intrusions over the Southern Brazilian Shelf, as reported for eukaryotic planktonic organisms such as phytoplankton, zooplankton and icthyoplankton. The high loads of particulate organic matter flushed over the SBS with the colder, turbid and silicate-rich PPW in winter influenced bacterioplankton by increasing its abundance and biomass by one order of magnitude. Under the lack of PPW in the area, coastal discharges and upwelling were of secondary importance and created the bimodal distribution of bacterioplankton within each water mass, where the peaks were near the coast and in the frontal zones around 150 km away from the coast. Thus, seasonality in the local hydrographic regime patterns and in the water mass properties regulate the dynamics of microbial carbon produced in the SBS, and, consequently, will affect the amount of energy to be transferred within the pelagic realm.

amount of carbon produced/removed from the water column; e.g. the northward propagation of the PPW throughout the S-SE Brazilian continental shelf is shown to increase suspended matter over the SBS (Muelbert et al., 2008), primary productivity (Brandini et al., 2000; Ciotti et al., 1995, 2010), sedimentation rates (Mahiques et al., 2004), copepods, fish larvae and eggs abundances (Muelbert et al., 2008). The structure of planktonic communities is also influenced by the PPW distribution, where higher densities of Engraulis anchoita larvae, (De Macedo-Soares et al., 2014, Piola et al., 2005; Zhu et al., 2013), number of benthic shrimps and crab larvae species (Brandão et al., 2015) are observed within the PPW. Foraminifera community structure also shifts under the influence of silicate+phosphate-rich PPW in winter (Eichler et al., 2008), as well as phytoplankton, where it shifts from diatomdominated in summer to phytoflagellate-dominated in winter, which reach up to 70% of total phytoplankton biomass (Brandini et al., 2014). As for these organisms, bacterioplankton was stimulated by the advection of PPW in the SBS during winter, where high POC and low in situ chlorophyll a levels were observed, suggesting that most of POC may be non-phytoplanktonic. Increasing light attenuation coefficient (Kd) values in winter were already discussed to be one of the variables that were related to higher bacterioplankton abundance and biomass in winter. Bacterial abundance and biomass showed a bimodal distribution for subsurface waters at Florianópolis and Araranguá transects in summer, but not at Babitonga. The southern transects are closer to the Cape Santa Marta, area of the SACW upwelling. The bacterioplankton parameters along the Babitonga transect appeared to be stimulated by continental runoffs over the shelf originated from the Babitonga Bay, and other nearby estuaries, as seen by the satellite images. A similar bimodal distribution for suspended matter (higher values near the coast and offshore subsurface waters) was shown in summer for the extreme south of SBS (Muelbert et al., 2008), whereas in winter it was homogeneously distributed, as a result from the advection of PPW (Muelbert et al., 2008). In winter, the lack of bimodal distribution in most transects reinforces the homogeneous distribution of PPW throughout the study area. Only Araranguá transect displayed a bimodal distribution of bacterioplankton abundance and biomass in winter, where the second peak was located within a thermic frontal zone at the 200 m isobath. The meeting of two water masses with different characteristics may have promoted the accumulation of cells and/or stimulated certain bacterial populations leading to increasing standing stocks, as observed by Taylor et al. (2012) in the California current system. Yokokawa et al. (2010) also showed substantial heterogeneity in the bacterial community within the same water mass, with more active and abundant populations present in the subsurface rather than in the bottom waters. Additionally, deep convective mixing brings up DOC to different water strata and then regulates both spatial and temporal patterns in bacterioplankton distribution (Doval and Hansell, 2000; Morris et al., 2005). The positive relationship between chlorophyll a, temperature, and bacterial biovolume of samples collected within the STSW in summer (Fig. 8) suggests predominant photoautotrophic organisms (such as prokaryotic and eukaryotic cells) in summer compared to winter. The higher nitrogen: phosphorus (NP) ratios found in summer of 16:1 in the SACW and 24:1 in the STSW, compared to 5:1 in the PPW and 8:1 in the STSW in winter, reinforce our hypothesis since ideal NP ratios for phytoplankton fall around 16:1 (Redfield, 1958), or more specifically 25:1 for photosynthetic picoeukaryotes Prymnesiophyceae and 12:1 for Chrysophyceae (Kirkham et al., 2013). “Warm-water” ecotypes of prokaryotes such as Cyanobacteria, SAR11 and Prochlorococcus are dominant in warm tropical waters (Seymour et al., 2012), and when these organisms dominate, nutrients are likely to play an important role (Brown et al., 2014). In the coastal waters of Frio Cape, Rio de Janeiro state, located above the northern end of SBS (latitude of approximately 21°S), in tropical waters, larger phytoplanktonic cells predominate under upwelling of nutrient-rich SACW, while microbial food webs

Acknowledgements The authors acknowledge the staff of Centro de Pesquisa e Gestão de Recursos Pesqueiros do Litoral Sudeste e Sul (CEPSUL) of Instituto Chico Mendes de Conservaç ão da Biodiversidade (ICMBIO), especially Roberta A. dos Santos for the logistic support during cruises. We also thank all undergraduate students that were involved on sampling and laboratory analysis, in special Fernando Ritter, Afonso, and Fernando Pacheco, and the crew of the R.V. Soloncy Moura. Melissa Carvalho and Maria L. S. Fontes thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the respective grants (Proc. Num. 2719/09-6 and 0195/08-1). Grants were also provided by INCT TMCOcean CNPq/INCT/Proc.no 573.601/2008-9. A.S. Freire benefited from a CNPq grant (312644/2013-2). References Acha, E.M., Mianzan, H.W., Guerrero, R.A., Favero, M., Bava, J., 2004. Marine fronts at the continental shelves of austral South America. J. Mar. Syst. 44, 83–105. Agogué, H., Lamy, D., Neal, P.R., Sogin, M.L., Herndl, G.J., 2011. Water mass‐specificity of bacterial communities in the North Atlantic revealed by massively parallel sequencing. Mol. Ecol. 20, 258–274. http://dx.doi.org/10.1111/j.1365-294X.2010. 04932.x. Barrera-Alba, J.J., Gianesella, S.M.F., Moser, G.A.O., Saldanha-Corrêa, F.M.P., 2009. Influence of allochthonous organic matter on bacterioplankton biomass and activity in a eutrophic, sub-tropical estuary. Estuar. Coast. Shelf Sci. 82, 84–94. Biddanda, B., Ogdahl, M., Cotner, J., 2001. Dominance of bacterial metabolism in oligotrophic relative to eutrophic waters. Limnol. Oceanogr. 46, 730–739. Braga, E.S., Chiozzini, V.C., Berbel, G.B.B., Maluf, J.C.C., Aguiar, V.M.C., Charo, M., Molina, D., Romero, S.I., Eichler, B.B., 2008. Nutrient distributions over the Southwestern South Atlantic continental shelf from Mar del Plata (Argentina) to Itajaí(Brazil): winter– summer aspects. Cont. Shelf Res. 28, 1649–1661. Brandão, M., Garcia, C., Freire, A., 2015. Large-scale spatial variability of decapod and stomatopod larvae along the South Brazil Shelf. Cont. Shelf Res. 107. http://dx.doi. org/10.1016/j.csr.2015.07.012. Brandini, F.P., Fernandes, L.F., 1996. Microalgae of the continental shelf of Paraná State, southern Brazil: review of studies. Rev. Bras. De. Oceanogr. 44 (1), 69–80. http://dx. doi.org/10.1590/S1413-77391996000100008. Brandini, F.P., 1990. Hydrography and characteristics of the phytoplankton in shelf and oceanic waters off southeastern Brazil during winter (July/August 1982) and summer (February/March 1984). Hydrobiologia 196, 111–148. Brandini, F.P., Boltovskoy, D., Piola, A., Kocmur, S., Rottgers, R., Abreu, P.C., Lopes, R.M., 2000. Multianual trends in fronts and distribution of nutrients and chlorophyll in the southwestern. Atl. (30-62 S) Deep-Sea Res. 47 (Part 1), 1015–1033. Brandini, F.P., Nogueira, M., Simião, M., Codina, J.C.U., Noernberg, M.A., 2014. Deep chlorophyll maximum and plankton community response to oceanic bottom intrusions on the continental shelf in the South Brazilian Bight. Cont. Shelf Res. 89, 61–75. Brown, M.V., Ostrowski, M., Grzymski, J.J., Lauro, F.M., 2014. A trait based perspective on the biogeography of common and abundant marine bacterioplankton clades. Mar. Genom. 15, 17–28. http://dx.doi.org/10.1016/j.margen.2014.03.002. Campos, E.J.D., Lorenzzetti, J.A., Stevenson, M.R., Stech, J.L., de Souza, R.B., 1996. Penetration of waters from the Brazil–Malvinas Confluence region along the South American continental shelf up to 231S. An. da Acad. Bras. De. Ciências 68 (1), 49–58. Campos, P.C., Moller Jr., O.O., Piola, A.R., Palma, E.D., 2013. Seasonal variability and

35



chemical properties at the Subtropical Shelf Front Zone in the SW Atlantic Continental Shelf. Cont. Shelf Res. 28, 1662–1673. Nagai, R.H., Ferreira, P.A.L., Mulkherjee, S., Martins, V.M., Figueira, R.C.L., Sousa, S.H.M., Mahiques, M.M., 2014. Hidrodinamic controls on the distribution of surface sediments from the southeast South American ontinental shelf between 23°S and 38°S. Cont. Shelf Res. 89, 51–60. http://dx.doi.org/10.1016/j.csr.2013.09.016. (ISSN 0278-4343). Norland, S., 1993. The relationship between biomass and volume of bacteria. In: KEMP, P.F., Sherr, B.F., Sherr, E.B., Cole, J.J. (Eds.), Handbook of Methods in Aquatic Microbial Ecology. Lewis Publishers, pp. 303–307. Palma, E.D., Matano, R.P., 2009. Disentangling the upwelling mechanisms of the South Brazil Bight. Cont. Shelf Res. 29, 1525–1534. Pereira, M.D., Schettini, C.A.F., Omachi, C.Y., 2009. Caracterização de feições oceanográficas na plataforma de Santa Catarina através de imagens orbitais. Rev. Bras. De. Geofísica 27 (1), 81–93. Piola, A.R., Campos Jr., E.J.D., O.O.M, Charo, M., Martinez, C., 2000. Subtropical Shelf Front off eastern South America. J. Geophys. Res. 105, 6565–6578. Piola, A.R., Matano, R.P., Palma, E.D., Möller Jr., O.O., Campos, E.J.D., 2005. The influence of the Plata River discharge on the western South Atlantic shelf. Geophys. Res. Lett. 32, L01603. http://dx.doi.org/10.1029/2004GL021638. Piola, A., Moller Jr, O., Guerrero, R., Campos, E., 2008a. Variability of the subtropical shelf front off eastern South America: winter 2003 and summer 2004. Cont. Shelf Res. 28, 1639–1648. Piola, A., Romero, S., Zajaczkovski, U., 2008b. Space-time variability of the Plata plume inferred from ocean color. Cont. Shelf Res. 28, 1556–1567. Porter, K.G., Feig, Y.S., 1980. The use of DAPI for identifying and counting aquatic microflora. Limnol. Oceanogr. 25, 943–948. SCOR-UNESCO, 1966. Determinations of Photosynthetic Pigments in Seawater, Monographs on Oceanographic Methodology. UNESCO Publications, Paris, pp. 11–18. Redfield, A.C., 1958. The biological control of chemical factors in the environment. Am. Sci. Sept. 205–221. Samo, T.J., Pedler, B.E., Ball, G.I., Pasulka, A.L., Taylor, A.G., Aluwihare, L.I., Azam, F., Goericke, R., Landry, M.R., 2012. Microbial distribution and activity across a water mass frontal zone in the California Current Ecosystem. J. Plankton Res. 34 (9), 802–814. http://dx.doi.org/10.1093/plankt/fbs048. Schettini, C., 2002. Near bed sediment transport in the Itajaí-Açu River estuary. South. Braz. Proc. Mar. Sci. 5, 499–512. http://dx.doi.org/10.1016/S1568-2692(02) 80036-5. Seymour, J., Doblin, M., Jeffries, T., Brown, M.V., Newton, K., Ralph, P., Baird, M., Mitchell, J., 2012. Contrasting microbial assemblages in adjacent water masses associated with the East Australian Current. Environ. Microbiol. Rep. 4, 548–555. http://dx.doi.org/10.1111/j.1758-2229.2012.00362.x. Stramski, D., Reynolds, R.A., Babin, M., Kaczmarek, S., Lewis, M.R., Rottgers, R., Sciandra, A., Stramska, M., Twardowski, M.S., Franz, B.A., Claustre, H., 2008. Relationships between the surface concentration of particulate organic carbon and optical properties in the eastern South Pacific and eastern Atlantic Oceans. Biogeosciences 5, 171–201. Taylor, A.G., Goericke, R., Landry, M.R., Selph, K.E., Wick, D.A., Roadman, M.J., 2012. Sharp gradients in phytoplankton community structure across a frontal zone in the California Current Ecosystem. J. Plankton Res. 34 (9), 778–789. http://dx.doi.org/ 10.1093/plankt/fbs036. Treusch, A.H., Vergin, K.L., Finlay, L.A., Donatz, M.G., Burton, R.M., Carlson, C.A., et al., 2009. Seasonality and vertical structure of microbial communities in an ocean gyre. ISME J. 3, 1148–1163. Vasconcellos, M., Gasalla, M.A., 2001. Fisheries catches and the carrying capacity of marine ecosystems in southern Brazil. Fish. Res. 50, 279–295. Vieira, R.P., Gonzalez, A.M., Cardoso, A.M., Oliveira, D.N., Albano, R.M., Clementino, M.M., Martins, O.B., Paranhos, R., 2008. Relationships between bacterial diversity and environmental variables in a tropical marine environment, Rio de Janeiro. Environ. Microbiol. 10, 189–199. Weber, T.S., Deutsch, C., 2010. Ocean nutrient ratios governed by plankton biogeography. Nature 467 (7315), 550–554. Yokokawa, T., De Corte, D., Sintes, E., Herndl, G., 2010. Spatial patterns of bacterial abundance, activity and community composition in relation to water masses in the eastern Mediterranean Sea. Aquat. Microb. Ecol. 59, 185–195. Zar, J.H., 1999. Biostatistical Analysis, 4th ed. Prentice-Hall, New Jersey, USA. Zhu, C., Wagner, T., Talbot, H.M., Weijers, J.W.H., Pan, J.M., Pancost, R.D., 2013. Mechanistic controls on diverse fates of terrestrial organic components in the East China Sea. Geochimica et Cosmochimica Acta 117, 129–143. http://dx.doi.org/10. 1016/j.gca.2013.04.015. ISSN 0016-7037.

coastal upwelling near Cape Santa Marta (Brazil). J. Geophys. Res.: Oceans 118, 1420–1433. Carlson, C.A., Morris, R., Parsons, R., Treusch, A.H., Giovannoni, S.J., Vergin, K., 2009. Seasonal dynamics of SAR11 populations in the euphotic and mesopelagic zones of the northwestern Sargasso Sea. ISME J. 3, 283–295. Ciotti, Á.M., Garcia, C.A.E., Jorge, D.S.F., 2010. Temporal and meridional variability of satellite-estimates of surface chlorophyll concentration over the Brazilian continental shelf Pan-American. J. Aquat. Sci. 5, 236–253. Ciotti, Á.M., Odebrecht, C., Fillmann, G., Moller Jr, O.O., 1995. Freshwater outflow and Subtropical Convergence influence on phytoplankton biomass on the southern Brazilian continental shelf. Cont. Shelf Res. 15, 1737–1756. Cotner, J.B., Biddanda, B.A., 2002. Small players, large role: microbial influence on biogeochemical processes in pelagic aquatic ecosystems. Ecosystems 5, 105–121. Doval, M.D., Hansell, D.A., 2000. Organic carbon and apparent oxygen utilization in the western South Pacific and the central Indian Oceans. Mar. Chem. 68, 249–264. Finkel, Z.V., Beardall, J., Flynn, K.J., Quigg, A., Rees, T.A.V., Raven, J.A., 2009. Phytoplankton in a changing world: cell size and elemental stoichiometry. J. Plankton Res. fbp098. Eichler, P., Gupta, B.S., Eichler, B., Braga, E., Campos, E., 2008. Benthic foraminiferal assemblages of the South B.razil: relationship to water masses and nutrient distributions. Cont. Shelf Res. 28, 1674–1686. http://dx.doi.org/10.1016/j.csr.2007.10. 012. Fortunato, C.S., Crump, B.C., 2011. Bacterioplankton community variation across river to ocean environmental gradients. Microb. Ecol. 62. Fuhrman, J.A., Steele, J.A., 2008. Community structure of marine bacterioplankton: patterns, networks, and relationships to function. Aquat. Microb. Ecol. 53, 69–81. Giovannoni, S.J., Vergin, K.L., 2012. Seasonality in ocean microbial communities. Science 335, 671–676. Grasshoff, K., Ehrhardt, M., Kremling, K., 1999. Methods of seawater analysis, 3 ed. Wiley-VCH, Weinhein (New York, USA). Gregoracci, G.B., Nascimento, J.R., Cabral, A.S., Paranhos, R., Valentin, J.L., Thompson, C.C., Thompson, F.L., 2012. Structuring of bacterioplankton diversity in a large tropical bay. PLoS One 7. Guenther, M., Gonzalez-Rodriguez, E., Carvalho, W.F., Rezende, C.E., Mugrabe, G., Valentin, J.L., 2008. Plankton trophic structure and particulate organic carbon production during a coastal downwelling-upwelling cycle. Mar. Ecol. Progress. Ser. 363, 109–119. Heileman, S., Gasalla, M.A., 2008. South Brazil large marine ecosystem. In: Sherman, K., Hempel., G. (Eds.), The UNEP World's Large Marine Ecosystem Report: A Perspective on Changing Conditions in LMEs of the World ́ s Regional Seas. United Nations Environmental Program, Nairobi, pp. 723–734. Hewson, I., Steele, J.A., Capone, D.G., Fuhrman, J.A., 2006. Remarkable heterogeneity in meso‐ and bathypelagic bacterioplankton assemblage composition. Limnol. Oceanogr. 51. http://dx.doi.org/10.4319/lo.2006.51.3.1274. Kirchman, D., 1994. The uptake of inorganic nutrients by heterotrophic bacteria. Microb. Ecol. 28, 255–271. Kirkham, A.R., Lepère, C., Jardillier, L., Not, F., Bouman, H., Mead, A., Scanlan, D.J., 2013. A global perspective on marine photosynthetic picoeukaryote community structure. ISME J. 7 (5), 922–936. http://dx.doi.org/10.1038/ismej.2012.166. De Macedo-Soares, L.C.P., Garcia, C.A.E., Freire, A.S., Muelbert, J.H., 2014. Large-Scale ichthyoplankton and water mass distribution along the South Brazil shelf. PLoS One 9 (3), e91241. http://dx.doi.org/10.1371/journal.pone.0091241. Mahiques, M.Md, Tessler, M.G., Ciotti, Á.M., Silveira, I.C.Ad, Sousa, S.Hd.Me, Figueira, R.C.L., Tassinari, C.C.G., Furtado, V.V., Passos, R.F., 2004. Hydrodynamically driven patterns of recent sedimentation in the shelf and upper slope off Southeast Brazil. Cont. Shelf Res. 24, 1685–1697. Massana, R., Gasol, J.M., Bjornsen, P.K., Blackburn, N., Hagstrom, A., Hietanen, S., Hygum, B.H., Kuparinen, J., PedrosAlio, C., 1997. Measurement of bacterial size via image analysis of epifluorescence preparations: description of an inexpensive system and solutions to some of the most common problems. Sci. Mar. 61, 397–407. Matsuura, Y., 1986. Contribuição ao estudo da estrutura oceanográfica da região sudeste entre. Cabo Frio (RJ) e Cabo De. St. Marta Gd. (SC) Ciência Cult. 38, 1439–1450. Moller, O.O., Piola, A.R., Freitas, A.C., Campos, E.J.D., 2008. The effects of river discharge and seasonal winds on the shelf off southeastern South America. Cont. Shelf Res. 28, 1607–1624. Morris, R.M., Vergin, K.L., Cho, J.C., Rappe, M.S., Carlson, C.A., Giovannoni, S.J., 2005. Temporal and spatial response of bacterioplankton lineages to annual convective overturn at the Bermuda Atlantic Time-series Study site. Limnol. Oceanogr. 50, 1687–1696. Muelbert, J.H., Acha, M., Mianzan, H., Guerrero, R., Reta, R., Braga, E.S., Garcia, V.M.T., Berasategui, A., Gomez-Erache, M., Ramírez, F., 2008. Biological, physical and

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