Phytoplankton functional community structure in Argentinian continental shelf determined by HPLC pigment signatures

Estuarine, Coastal and Shelf Science 100 (2012) 72e81

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Phytoplankton functional community structure in Argentinian continental shelf determined by HPLC pigment signatures D. Vega Moreno a, *, J. Pérez Marrero a, J. Morales b, C. Llerandi García a, M.G. Villagarcía Úbeda c, M.J. Rueda a, O. Llinás c a b c

Department of Oceanography, Canary Institute of Marine Sciences, Canary Islands, Spain IFAPA Centro “Agua del Pino”, Apartado 104, 21071 Huelva, Spain Oceanic Platform of the Canary Islands, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 September 2010 Accepted 16 January 2012 Available online 1 February 2012

The Patagonian Sea in Argentina is known as an area with high primary production and biodiversity. The complex hydrodynamic environment resulting from the interaction between the MalvinaseBrazil convergence and the waters over the continental shelf and slope enhances the development of high chlorophyll concentrations, especially in frontal and coastal areas. The composition, distribution and variability of several phytoplankton functional types (PFTs) derived from diagnostic pigments were studied in relation to the local hydrographical conditions, using data from a research cruise carried out on board the RV Bio Hesperides at the end of the Summer season (March 2008). Phytoplankton cell size and PFT distributions were found to be highly influenced by the physical and chemical characteristics of the studied environments. Thus large cells, mainly diatoms, were the dominant size fraction in the southern frontal areas, where the SubAntarctic Surface Waters (SASW) from the Malvinas Current meet shelf waters. However, other groups of microphytoplankton (mPF), mainly dinoflagellates, were also detected in the shallow waters zone influenced by the tidal regime near the Valdes Peninsula. Picophytoplankton (pPF) was an important contributor to the floristic composition in the southern frontal zones, while nanophytoplankton (nPF) was dominant in the stations located over the continental slope, and in the oligotrophic area near Mar del Plata. The ratio between photoprotective and photosynthetic pigments (PPC:PSC) and the photoprotection index (PI) indirectly provide information about the environment and its effect on the PFTs composition. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: phytoplankton biomarker pigments Argentinian continental shelf HPLC size group distribution

1. Introduction The Patagonian shelf in Argentina shows complex hydrodynamic processes influenced by tides and the confluences of two western boundary currents (Brazil and Malvinas Currents) (Acha et al., 2004). At the shelf-break, the cooler and more saline waters of the Malvinas Current deriving from the Antarctic Circumpolar Current meet the shelf waters (Ferreira et al., 2009). The front formed between the Argentinian shelf waters and the Malvinas Current flow shows a conspicuous band of high biomass concentration (García and García, 2008; Piola et al., 2010), that extends more than 3000 km northwards and southwards along the

* Corresponding author. E-mail address: [email protected] (D.V. Moreno). 0272-7714/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2012.01.007

continental shelf-break until it converges in the discharge of Rio de La Plata (Fig. 1). Extensive studies performed in the last 15 years over the Patagonian Sea have described this complex region (Glorioso, 1987; Carreto et al., 1995; Piola et al., 2010). The Malvinas Current has higher nutrient concentration than the warmer Brazil Current (Piola and Falabella, 2009); another relevant pattern is the existence of intensive thermohaline neritic fronts (Franco et al., 2008; Rivas and Pisoni, 2010), which act like concentration barriers for phytoplankton biomass (Bakun, 1996; Campagna et al., 2000; Rivas, 2006; Paparazzo et al., 2010). Some of these oceanic fronts show seasonal patterns (e.g. the Valdes Peninsula tidal front in Spring and Summer), while others present some permanent features like the Argentina shelf front (Acha et al., 2004; Piola et al., 2008). Marine phytoplankton plays an important role in the carbon withdrawal from the atmosphere, and in the primary production and biogeochemical fluxes (Legendre and Rassoulzadegan, 1996);

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Fig. 1. Sampling locations plotted over a composite of one week of satellite MODIS data, showing the chlorophyll a concentration (mg m3) distribution over the Argentinian continental shelf during BLOOM 2008 cruise.

however, the variability of the phytoplankton community composition modulates the influence in the functioning of the pelagic ecosystem and in the marine biogeochemical cycles (Anderson, 2005; Nair et al., 2008). The use of satellite imagery (SST and ocean colour) has provided a mesoscale synoptic view of the hydrodynamics in the area and its relationship with the regional productivity, closely related to the high phytoplankton biomass developed next to frontal zones (Saraceno et al., 2005; Rivas et al., 2006; Romero et al., 2006; Carranza, 2009; Dogliotti et al., 2009; Lutz et al., 2010). The distribution and succession of phytoplankton in the ocean are the result of the microalgal groups adaptation to the environmental conditions (temperature, nutrients, light field, turbulence, etc.) (Margalef, 1978). Turbulent waters and/or high nutrient environments are generally favourable for the development of larger phytoplankton cells (Margalef, 1978; Reul et al., 2006; HueteOrtega et al., 2010), while stratified waters and/or low nutrient areas favour the development of smaller algal types (Cushing, 1989; Chisholm, 1992; Kiorboe, 1993, Marañon, 2009). Sieburth et al. (1978) grouped the phytoplankton cell sizes in three main categories: microphytoplankton (20e200 mm ESD, Equivalent Spherical Diameter), nanophytoplankton (2e20 mm ESD) and picophytoplankton (0.2e2 mm ESD). Using this classification as reference, Vidussi et al. (2001) proposed a method to distribute phytoplankton populations in categories according to their HPLCpigment signatures (Sieburth et al., 1978). Changes in the phytoplankton community structure, composition (Gibb et al., 2000; Barlow et al., 2007) or in their physiological state (Trees et al., 2000; Veldhuis and Kraay, 2004) can be characterized by specific biomarker pigments or by particular pigment ratios (Fishwick et al., 2006; Aiken et al., 2008, 2009). Chlorophyll a has been commonly used as a proxy to phytoplankton biomass, responding more rapidly than other pigments to environmental changes (Margalef, 1967). From a functional perspective, the abundance of photosynthetic pigments, and

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especially photosynthetic carotenoids (PSCs), is considered a good indicator of high productivity waters; however, the dominance of photoprotective carotenoids (PPCs) has been related to ecosystems with low productivity (Barlow et al., 2002). The proportion of certain accessory pigments is used to determinate the phytoplankton groups which are present, following the equations developed by Vidussi et al. (2001) who proposed an equation set for a cell size classification of natural samples from phytoplankton communities using diagnostic pigments. Those equations were updated by Uitz et al. (2006) changing the initial condition formulation by Vidussi et al. (2001), who assigns an equal weight to each accessory pigment marker aiming to obtain a better statistical approach based in Gieskes et al. (1988). For this study, a Photoprotection Index (PI) is proposed as an environmental indicator of the phytoplankton communities reaction to variations in some habitat conditions such as water transparency, light intensity and water column stratification (Griffith et al., 2010). PI represents the ratio between the main photoprotection pigments and chlorophyll a (Table 1). The environmental conditions defined by high PI values at the surface are mostly found under oligotrophic conditions (e.g. open ocean, intensively stratified coastal waters, etc.), whilst low PI values are generally expected in high productive waters (well-mixed coastal waters, fronts, etc.) (Ras et al., 2008). In March 2008, a research cruise was carried out over the Argentinian continental shelf (Patagonia region, South America) on board of the R/V Hesperides, aiming to typify the phytoplankton functional composition through its pigment signatures. The study extended from 38
S to 53
S of latitude, including the confluence zone between the Malvinas and Brazil currents (Pereira-Brandini et al., 2000). It included coastal environments as well as open ocean ecosystems, with the main objective of contributing to improve our current limited knowledge about the spatial and vertical distribution of phytoplankton communities in the Patagonian Sea during the austral Autumn. Phytoplankton Functional Groups (PFG), derived from diagnostic pigments and optical indices, are analyzed and interpreted in relation to regional hydrodynamics, in order to increase our knowledge about the complex biological patterns existing in the Patagonian shelf at the end of the Summer period.

Table 1 The symbols, names, formulae and selected taxonomic designations (Jeffrey et al., 2005) for chlorophylls, carotenoids, pigment sums and pigment indices. Symbol

Pigment

Chla Chlb Allo But Caro Diad Diato Fuco Hex Peri Zea TChla PPC PSC DP PI mPF

Chlorophyll a (plus divinyl Chla) Chlorophyll b (plus divinyl Chla) Alloxanthin 190 -butanoyloxyfucoxanthin Carotenes (a- plus b-carotenes) Diadinoxanthin Diatoxanthin Fucoxanthin 190 -hexanoyloxyfucoxanthin Peridinin Zeaxanthin Total chlorophyll a Photoprotective carotenoids Photosynthetic carotenoids Total diagnostic pigments Photoprotection index Microphytoplankton proportion factor Nanophytoplankton proportion factor Picophytoplankton proportion factor

nPF pPF

Designation Chlorophytes Cryptophytes Haptophytes (major)

Diatoms (major) Haptophytes Dinoflagellates Cyanophytes and prochlorophytes Chla þ divinyl Chla Allo þ Diad þ Diato þ Zea þ Caro But þ Fuco þ Hex þ Peri PSC þ Allo þ Zea þ Chlb (Diad þ Diato þ Zea)/Chla (Fuco þ Peri)/DP (Hex þ But þ Allo)/DP (Zea þ Chlb)/DP

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2. Materials and methodology 2.1. Study area and samples collection Sampling was performed in the Argentinian Sea (from 38
S to between the 13th and the 28th of March 2008. Seven vertically resolved transects were sampled, showing in Fig. 1 the location of the 27 sampling stations and their distribution in the seven transects. Temperature, salinity and in vivo fluorescence, using a Fluorescence Sensor Seapoint (low pressure, 470 nm excitation685 nm emission), were profiled during CTD (Seabird-25) deployments at every station. Water samples were collected at a minimum of 4e5 selected depths in the range of 0e100 m using 12 Niskin bottles; these samples were used for analyses of pigments and nutrients and to calibrate a salinity sensor. Pigment samples (500 ml) were filtered on board onto 47 mm GF/F filters to collect phytoplankton, and were stored frozen in liquid nitrogen until the analysis was conducted ashore. Nutrient samples were frozen (20
C) and analyzed ashore by standard auto-analyser techniques (Llinas et al., 1999). 53
S)

2.2. Pigment analysis Pigments were extracted in 100% methanol (5 ml) with the use of ultrasonication and clarified by centrifugation. Then analysed by reverse phase HPLC procedure (Wright et al., 1991), using a Waters Spherisorb 5 mm ODS2 4.6  250 mm C18 column. Pigments were

detected at 436 nm (excitation) and 680 nm (emission) with a fluorometer detector and at 440 nm with a PDA detector. Chlorophyll a (Chla) and chlorophyll b (Chlb) standards were obtained from SigmaeAldrich Ltd., and other pigment standards were purchased from the DHI Institute for Water and Environment (Denmark). The method used for the analysis does not separate divinyl and monovinyl chlorophyll a nor divinyl and monovinyl chlorophyll b. Limits of detection were 0.001 mg L1. 2.3. Pigment indices Photopigment indices were derived to assess the changing contribution of chlorophylls and carotenoids to the total pigment pool (Barlow et al., 2007). The carotenoid pigments were discriminated as photosynthetic carotenoids (PSCs) and photoprotective carotenoids (PPCs). The photopigment indices symbolised as TChla, PSCs, PPCs, DPs (Diagnostic Pigments) and PI (Photoprotection Index) are described in Table 1. Phytoplankton size classes, symbolised as mPF (microphytoplankton), nPF (nanophytoplankton) and pPF (picophytoplankton) respectively (Table 1) were derived from diagnostic pigment indices (DP) (Vidussi et al., 2001), in order to be used as a cell size functional grouping of the phytoplankton natural populations. Nevertheless, the diagnostic indices used in this study are not definitive for phytoplankton classification as has been previously shown by Barlow et al. (2007), though they are useful for indicating the major groups which contribute to the phytoplankton

Fig. 2. Vertical profile of temperature (
C) and salinity (&) for transect 1 (stations 1e4) and transect 4 (stations 13e16).

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community. Aiken et al. (2008) defined ecological niches according to phytoplankton size classes. Microphytoplankton is predominant in high nutrient environments (upwelling zones and Spring blooms), presents large Chla concentrations and a high value of the TChla/TP pigment index (where TP stands for total pigments); nanophytoplankton grows in regions showing some inorganic nutrients and recycled nutrients (organic and inorganic), and has moderate TChla concentrations and TChla/TP values. Lastly, picophytoplankton is dominant in low nutrient zones (e.g. oligotrophic gyre) and is associated to low TChla concentrations and TChla/TP indexes. 3. Results 3.1. Oceanographic Cruise BLOOM’s 2008 The studied area covers the Patagonian shelf from 52
S to 38
S, a vast extension with two conspicuous environmental areas: a coastal zone and an offshore frontal zone. During the oceanographic cruise BLOOM 2008 (March 2008), 27 oceanographic stations grouped in seven transects were sampled (Fig. 1). Hydrographical measurements revealed that most of the water column over the Argentinian shelf presented a two-layer stratification structure (Fig. 2, Stations 13e16), although some areas showed a good vertical mixing (Fig. 2, Stations 1e4) (Bloom Technical Report, 2009). The upper layer, warmer and lighter, encompasses most of the proportion of phytoplankton biomass,

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around 90% of phytoplankton concentration on average in all the stations sampled. Nutrients (nitrates, phosphates and silicates) were generally almost depleted near the surface, due to the consumption by the high phytoplankton concentrations. In contrast, the bottom layer (up to 40 m) exhibits relatively high concentration of nutrients and low phytoplankton biomass levels (Fig. 3). Fig. 4 shows relationships between temperature and salinity (A), nitrate and chlorophyll a (B), phosphateenitrate ratio (C) and silicateenitrate ratio (D). Values were transformed to their standard normal distributions (calculated by subtracting the sample mean and dividing by the corresponding sample standard deviation, Wilks, 1995) to better compare magnitudes regardless of the units of individual variables. This statistical approach facilitates the study of tendencies and similarities between parameters, but it has also proved to be a good tool for zonal discrimination, since geographical areas emerge as clusters in all the scatter plots. The whole studied hydrographical relationships show good agreement (above 90%) with the standard normal distribution except in the case of phosphate, where the Kolmogorov Smirnov test gave a value below critical for 0.1 confidence level. The temperatureesalinity (TeS) distribution reflects the relative influence of SubAntarctic Surface Water (SASW) (Fig. 4A), which represents the most important input of nutrients in the region. High productivity areas involve intensive consumption of nutrients (Fig. 4B: upper red line which represents higher change rate) whilst in zones where chlorophyll concentrations are low, nutrient

Fig. 3. Vertical profile of nitrate (mmol/L), phosphate (mmol/L), silicates (mmol/L) and chlorophyll a (mg/L) for transect 4 (stations 13e16).

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Fig. 4. General hydrography of the area under study give as standardized normal distributions (typified values): [A] TemperatureeSalinity diagram, [B] Chlorophyll a versus nitrate, [C] Phosphate Nitrate relationship, [D] Silicate nitrate relationship. Symbols are assigned to transects, hence depicting geographical variability of the parameters.

consumption is weaker (Fig. 4B: lower blue line). Phosphate and silicate concentrations are always above 0.25 mmol L1, and thus these nutrients are not a limiting factor (Fig. 4C and D). The minimum nitrate concentration was 0.08 mmol L1, which is probably a limiting factor regulating phytoplankton growth in some areas (Fig. 4B). Stations with low nitrate concentrations are always located within the first 30 m of the water column. The Nitrate/Phosphate concentrations ratio presents a linear relationship except in transect 5 (Fig. 4C); in some cases the Redfield ratio is close to 1, which converts the nitrate in the limiting factor for algal blooms development. The Nitrate/Silicate ratio is less uniform in transects 4, 5 and 6 (Fig. 4D), depicting noticeable deviations, that points out to the nitrate concentration as the critical macronutrient regulating phytoplankton composition, and explaining an important presence of low nitrogen hapthophyte species, identified by high concentrations of 190 -Butanoxyfucoxanthin and 190 -Hexanoxyfucoxanthin (Jeffrey et al., 2005).. Those results were confirmed by the spatial phytoplankton composition.

Fig. 5 shows vertical profiles of nine biomarker pigments in the seven transects. A general pattern in all transects except transect 7 is a clear two-layer stratification structure located around 30e40 m deep; nonetheless, transect 7 shows a remarkable vertical homogeneity that may have been caused by tidal currents. Chlorophylls have a similar vertical distribution for all transects, showing high concentration values in surface and subsurface layers and a marked concentration decrease below 40 m. In contrast, accessory pigments present a variable vertical distribution in the different transects, showing in some cases an increase of accessory pigment concentrations in deeper waters (e.g. fucoxanthin, 190 -butanoxyfucoxanthin and alloxanthin). 3.2. Phytoplankton pigment data Based on the Sieburth et al. (1978) phytoplankton size classification, Jeffrey et al. (2005) distributed phytoplankton groups as follows:

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Fig. 5. Vertical profiles of the nine phytoplankton pigments (concentration unit: mg L1) studied versus depth in the seven different transects (average by transect) where samples were collected.

-

Microphytoplankton (20e200 mm ESD): diatoms, some dinoflagellates and silicoflagellates. Nanophytoplankton (2e20 mm ESD): haptophyta and chrysophyta. Picophytoplankton (0.2e2 mm ESD): cyanophyta, prochlorophyta and chlorophyta.

Fig. 6 shows the cumulative concentration of phytoplankton pigments integrated in the first 200 m of the water column or its maximum depth if it was lower (Fig. 6A: peridinin, fucoxanthin, 190 -

hexanoyloxyfucoxanthin, 190 -butanoyloxyfucoxanthin, chlorophyll b and chlorophyll a; Fig. 6B: total carotene, diatoxanthin, alloxanthin, diadinoxanthin and zeaxanthin) in the 27 sampled stations. Diagnostic pigments are used to determine the composition of the cell size phytoplankton community (Table 2) and the spatial distribution of the major phytoplankton groups in the seven transects (integrated values for the water column). - Transect 1 (stations 1e4): The phytoplankton community size structure is dominated by picophytoplankton (pPF, 39.5%),

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- Transect 4 (stations 13e16): As shown in transect 3, the highest ratio of phytoplankton populations corresponds to mPFs (51.8%). However, in this case dinoflagellates are the dominant group (high peridinin concentrations, Table 1). Picophytoplankton is mainly composed by cyanophyta instead of chlorophyta (very low concentration of chlorophyll b), as shown in Table 1. Nanophytoplankton mainly consists of haptophyta in this transect (high concentration of 190 -hexanoyloxyfucoxanthin). This is by far the area with the highest pigment concentrations (Fig. 5); however, nutrient limitations can be observed in some stations (Fig. 4B). - Transect 5 (stations 17e20): This transect shows a marked decrease of nutrient concentrations in stations 17 and 18 (Fig. 4). Nitrate is an important limiting factor in these stations (Fig. 4B), which could explain the observed reduction of mPF (30.3%) and the pPF increase (33.3%), since many cyanobacteria species have the capacity of atmospheric nitrogen fixation (Ohlendieck et al., 2000). - Transect 6 (stations 21e24): There is a high abundance of nPF (65.2%), principally due to the presence of haptophyta species (high concentration of 190 -hexanoyloxyfucoxanthin and 190 butanoyloxyfucoxanthin). At station 23, low nitrate and silicate concentrations determine the development of nano- and picophytoplankton and explain the low mPF concentration. - Transect 7 (stations 25e27): The phytoplankton group distribution shows a similar pattern to transect 6, i.e. a high concentration of haptophyta with the presence of cryptomonads (high concentration of alloxanthin).

Fig. 6. Accumulate integrated concentration in the first 200 m (or maximum depth) (mg m2) of pigments: peridinin, fucoxanthin, 190 -hexanoyloxyfucoxanthin, 190 butanoyloxyfucoxanthin, chlorophyll b and chlorophyll a [A] and total carotene, diatoxanthin, alloxanthin, diadinoxanthin and zeaxanthin [B] for all stations sampled.

mainly composed by chlorophyta according to the chlorophyll b concentrations (Fig. 5), and microphytoplankton (mPF, 48.0 %), dominated by diatom species (Fig. 6). Station 1 is different, since a relevant dinoflagellate concentration is detected according to the peridinin concentration found. - Transect 2 (stations 5e8): The average community cell size diminishes (Table 2) due to an increase in the nanophytoplankton (nPF) proportion and a decrease of mPF. Picophytoplankton has a similar ratio to transect 1 (Table 2), and it is dominated by chlorophyta (high concentration of chlorophyll b). - Transect 3 (stations 9e12): In this case, the composition by size is controlled by a high proportion of mPF (mainly diatoms and to a lesser extent dinoflagellates), though a nitrate limitation can be observed (Fig. 4B).

Table 2 Phytoplankton size distribution (%). Integrated values in the first 200 m or maximum depth per station (average by transect) derived from diagnostic pigments (uncertainty error 0.2%). Transect

Picophytoplankton 0.2e2 mm (%)

Nanophytoplankton 2e20 mm (%)

Microphytoplankton 20e200 mm (%)

1 2 3 4 5 6 7

39.5 38.1 26.9 17.7 33.3 11.0 23.2

12.5 26.6 18.0 30.5 36.4 65.2 52.4

48.0 35.3 55.1 51.8 30.3 23.8 24.4

3.3. Phytoplankton pigment indexes Diagnostic pigments used to estimate the relative abundances of different phytoplankton groups are considered independent of each other, since they are biomarkers for different phytoplankton groups (Wright and Van den Enden, 2000). The PPC (photoprotective carotenoids), PSC (photosynthetic carotenoids) and DP (total diagnostic pigments) indexes provide a physiological condition estimation of the phytoplankton community, resulting from the environmental and trophic conditions, e.g. under light stress conditions PPC is greater than PSC (Barlow et al., 2007). This situation is particularly remarkable in transects 1 and 2 (Table 3), which are zones with clean and shallow waters. The DP index shows high pigment concentration in all areas, measuring the highest record in transect 5. The PPC ratio (Barlow et al., 2008) shows some peaks in transects 4, 5 and 7 (Table 3), in correspondence with coastal environments exhibiting shallow depths. The index shows an increasing trend from deep layers to surface where the irradiance is higher, as shown by an increase of the proportion between photoprotective carotenoids and chlorophyll a. This fact confirms the existence of intensive light fields which would force phytoplankton species to raise the concentration of photoprotective carotenoids (Barlow et al., 2007; Vijayan et al., 2009). Table 3 Concentrations (mg/L) and standard deviation of photosynthetic carotenoids (PSC), photoprotective carotenoids (PPC), total diagnostic pigments (DP) and photoprotection index (PI) in the 7 transects. Transect

[PSC]

1 2 3 4 5 6 7

0.69 0.66 1.30 1.99 1.84 1.46 1.52

      

[PPC] 0.015 0.013 0.025 0.032 0.029 0.027 0.030

0.70 0.77 0.62 1.07 1.26 0.60 1.12

      

[DP] 0.009 0.011 0.009 0.016 0.018 0.008 0.015

1.37 1.26 1.97 2.62 2.96 2.08 2.44

PI       

0.023 0.021 0.031 0.038 0.043 0.032 0.040

1.33 1.14 1.57 3.12 3.03 1.00 2.97

      

0.015 0.014 0.015 0.034 0.033 0.011 0.028

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PPC:PSC distribution shows a remarkable horizontal -rather than vertical- grouping of the sampled stations (Fig. 7A and B, black diamonds), indicating that in the Patagonian shelf and during the transition period between Summer and Autumn the horizontal variability of pigment distribution is the relevant grouping factor, rather than the vertical sample position in the water column. In most cases the PPC:PSC ratio approaches to the unity linear relationship (Fig. 7, red line), with some exceptions which are generally associated with high productivity waters (Fig. 7A, stations 14 and 23). This is the case in the shallow waters of the Valdes Peninsula, where the PPC:PSC ratio is very low. A good correlation is observed between high values of PPC:PSC, an indicator of stress conditions and low nutrient concentrations; such situations are frequently observed in shallow waters (Fig. 7B, grey numbers which represent depth values). When irradiance increases and nutrient concentrations decline, there is a significant increment in the proportion of photoprotective carotenoids (Barlow et al., 2007), and consequently in the PPC:PSC index. 4. Discussion The hydrographic structure of the Patagonian shelf has been largely described in the last years (Guerrero and Piola, 1997; Bianchi et al., 2005; Palma et al., 2008). Many authors have associated the presence of intensive phytoplankton biomass developments to fronts and other regional hydrodynamic processes (Carreto et al., 1995; Acha et al., 2004; Romero et al., 2006; García et al., 2008). Hydrodynamic processes (vertical and horizontal circulation) seem to be the main driving force for algal bloom developments in the Patagonian region, while macronutrients availability is a crucial factor in phytoplankton communities composition (Gomez et al., 2011). The Argentinian continental shelf shows a complex hydrodynamic pattern at the end of the Summer (Romero et al., 2006; Palma et al., 2008); thus, the intrusion of subantarctic water into the southern area (transect 1) causes temperature and salinity minima, depicting a good mixing of the water column and low stratification. In the North-Eastern sector (transect 6) a strong thermal front presenting a long horizontal temperature gradient is observed, while coastal areas (see transect 4) are characterized by

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higher water temperatures, low nutrient concentrations and elevated phytoplankton biomass. Our in situ results confirm those obtained by Romero et al. (2006) through the analysis of five years of remotely sensed ocean colour data of the same area. It is especially remarkable the persistence of blooms during the Summer between 37
S and 51
S of latitude, and notably the bloom near Valdes Peninsula measured in station 14. The environmental linkage between algal biomass development and oceanic fronts has been frequently cited in the literature (Franks, 1992; Yamamoto et al., 1998; Smayda, 2002; Figueiras et al., 2006; Ryan et al., 2008). In the Patagonian shelf, some studies link the presence of high phytoplankton biomass to fronts produced by salinity and thermal fronts (Gayoso, 1998; Lutz et al., 2010). Our study reveals that thermal fronts (e.g. transect 6) create two distinct environments for phytoplankton development, i.e. a cold side of the front with rich nutrient waters and a warm side of lower nutrient concentrations. Some phytoplankton groups like haptophyta have a general presence in all transects while the presence of other groups is strongly related to environmental conditions, i.e. nitrate availability or fronts. In our case, picophytoplankton communities were dominant in transects 1 and 2, due to the low nitrate concentrations in the first stations (stations 1 and 2); however, the macronutrient concentrations increase northward, and consequently the phytoplankton composition shows a higher proportion of microphytoplankton species. Vertical distribution of accessory pigments, showing in some cases concentration increase with depth would indicate a concentration of large phytoplankton species near the seasonal thermocline, and there are two possibilities to explain this fact: (1) settlement of larger species during the stratification period; and (2) an active photoprotective migration of the most sensitive phytoplankton species (Richter et al., 2007). Diatoms are commonly found in frontal and high nutrient areas while dinoflagellates, although present in transects 3, 5 and 7 in low concentrations, were especially abundant in the first 40 m of depth in transect 4. This is due to a strong stratification in this period (Summer) which favours the presence of mobile species in the upper water layers, as reported by Bianchi et al. (2009). Recent studies have

Fig. 7. Photoprotective (PPC) vs Photosynthetic (PSC) Carotenoids by station number (a), shallow samples are presented as open circles while deep samples are presented as open squares; by nitrate concentration (b, black diamonds) and depth (b, grey numbers which represents depth value), only low nitrate samples are presented.

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related the persistence of dinoflagellates algal blooms to the level of stratification in southern Patagonian waters during Spring and Winter (Gomez et al., 2011). The proportion of nPF and pPF increases near the thermocline, as compared with the ratios found in shallower waters. Some areas without nutrient limitation (as in transect 2) and a weak stratification show a low presence of microphytoplankton in the upper water layers; this behaviour has been also observed by Marañon (2009) in the northern Atlantic waters. The distribution of phytoplankton groups in relation to seasonal stratification shows a close relationship with phytoplankton size. Generally, the presence of nPF and mPF is highly promoted in stratified waters (Margalef, 1978), as observed in our results. The highest proportion of mPF appeared in transects 3 and 4, which corresponded to warmer coastal waters with strong stratification and high nutrient inputs. The nPF maximum ratio is observed in transect 6, that is located in the Malvinas current front and characterized by cold and nutrient-rich waters and a high stratification during this period. Some authors as Reul et al. (2006) and HueteOrtega et al. (2010) reported that the large phytoplankton cells remain in the upper layers of the photic zone only under moderate turbulence, which keeps them from sinking. Our observations of mPF near the surface under stratification conditions could be explained by the motility of some groups (dinoflagellates and silicoflagellates), that would delay or even completely avoid their sedimentation (Herrera and Escribano, 2006). The PPC:PSC ratio between accessory pigments can be used as a classification tool; in this sense, the higher proportions of PPC relative to PSC were found in the southern frontal zones (transects 1 and 2). On the other hand, the maximum relative proportion of PSC was found in station 14 (Valdes area, transect 4), associated with an absolute chlorophyll a maximum in an area dominated by dinoflagelates. The shallow station 23 (transect 6) is located in the slope frontal zone and showed high relative amounts of PSC too, though in this case they are associated to moderate chlorophyll a concentrations. When there is an increase of PPC, the photoprotection index (PI) describes environmental stress conditions (intensive irradiance, low nutrient concentrations, etc.), which are not suitable for phytoplankton growth (Barlow et al., 2007). Moreover, PI can also be used to identify high productivity areas through an inverse relationship with chlorophyll a concentration (PI ¼ PPC/TChla). , This implies that the lowest PI values are mostly found in high productivity waters with the highest concentration of chlorophyll a (Ras et al., 2008), as generally observed in our study area (Table 3) except for station 14 (transect 4). 5. Conclusions Large cells tend to dominate in frontal zones generated by the confluence of subantarctic surface waters with shelf waters, as in transects 1 and 3. Nevertheless, near the Valdes Peninsula (transect 4) a relatively high proportion of dinoflagellates are found, in coincidence with the highest chlorophyll a concentrations measured. The largest amounts of phytoplankton (Chla concentration) were found in surface waters in transects 3 and 4, which exhibit a differentiate hydrography and showed a varied composition of communities. Although both areas are dominated by large cells, diatoms are more frequent in the frontal zone of transect 3, while dinoflagellates are the main contributors in shallow waters of the Valdes area. Picophytoplankton population was relevant in the southern frontal zones, i.e. transects 1 and 2. Intermediate cell sizes (nPF) were majority in the frontal zone associated to the continental slope and also near Mar del Plata (transect 7), where the most oligotrophic conditions were found.

The PPC:PSC ratio between accessory pigments has proven to be an interesting classification tool, given that it can be related both to phytoplankton populations and hydrography. Surface nitrate depletion tends to be associated with growing amounts of PPC in southern frontal zones (transects 1 and 2). If the PPC proportion can be explained as a physiological response to potentially destructive radiation, it may be assumed that this adaptation does not arise with the same intensity in all studied environments. Special attention might be given to the relationship between PPC and nitrate availability.

Acknowledgements All samples of this work were collected during the Oceanographic Cruise BLOOM carried out in 2008 (Project CTM 200420132-E), and analysed at the Canary Institute of Marine Sciences (Instituto Canario de Ciencias Marinas, ICCM), a research institute pertaining to the Agencia Canaria de Investigación e Innovación, Presidencia del Gobierno de Canarias, Spain. We are grateful to the RV Hesperides’ captain and crew for their invaluable help and collaboration during the BLOOM cruise. The work of Dr. Daura Vega Moreno has been supported by a Postdoc fellowship from Las Palmas de Gran Canaria University.

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 physical and ecological processes. Journal of Marine Systems 44, 83e105. Aiken, J., Hardman-Mountford, N., Barlow, R., Fishwick, J., Hirata, T., Smyth, T., 2008. Functional links between bioenergetics and bio-optical traits of phytoplankton taxonomic groups: an overarching hypothesis with applications for ocean colour remote sensing. Journal of Plankton Research 30 (2), 165e181. Aiken, J., Pradhan, Y., Barlow, R., Lavender, S., Poulton, A., Holligan, P., HardmanMountford, N., 2009. Phytoplankton pigments and functional types in the Atlantic Ocean: a decadal assessment, 1995e2005. Deep-Sea Research II 56, 899e917. Anderson, T.R., 2005. Plankton functional type modelling: running before we can walk? Journal of Plankton Research 27, 1073e1081. Bakun, A., 1996. Patterns in the Ocean: Ocean processes and Marine Population Dynamics. University of California Sea Grant, San Diego, in cooperation with Centro de Investigaciones Biológicas de Noroeste, La Paz, Baja California Sur, Mexico, California, USA, 323 pp. Barlow, R.G., Aiken, J., Holligan, P.M., Cummings, D.G., Maritorena, S., Hooker, S., 2002. Phytoplankton pigment and absorption characteristics along meridional transects in the Atlantic Ocean. Deep-Sea Research I 49, 637e660. Barlow, R., Stuart, V., Lutz, V., Sessions, H., Sathyendranath, S., Platt, T., Kyewalyanga, M., Clementson, L., Fukasawa, M., Watanabe, S., Devred, E., 2007. Seasonal pigment patterns of surface phytoplankton in the subtropical southern hemisphere. Deep-Sea Research I 54, 1687e1703. Barlow, R., Kyewalyanga, M., Sessions, H., Van den Berg, M., Morris, T., 2008. Phytoplankton pigments, functional types, and absorption properties in the Delagoa and Natal Bights of the Agulhas ecosystem. Estuarine, Coastal and Shelf Science 80, 201e211. Bianchi, A.A., Bianucci, L., Piola, A.R., Ruiz Pino, D., Schloss, I., Poisson, A.R., Balestrini, C.F., 2005. Vertical stratification and air-sea CO2 fluxes in the Patagonian shelf. Journal of Geophysical Research 110, C07003. doi:10.1029/ 2004JC002488. Bianchi, A.A., Ruiz Pino, D., Isbert Perlender, H.G., Osiroff, A.P., Segura, V., Lutz, V., Clara, M.L., Balestrini, C.F., Piola, A.R., 2009. Annual balance and seasonal variability of sea-air CO2 fluxes in the Patagonia Sea: their relationship with fronts and chlorophyll distribution. Journal of Geophysical Research 114, C03018. doi:10.1029/2008JC004854. Bloom Technical Report, 2009. Informe de Campaña e Campaña Oceanográfica Bloom 2008. http://www.oceanografiaiccm.es/images/stories/Informes/Informe_Bloom_ 08.pdf. Campagna, C., Rivas, A., Marin, M., 2000. Temperature and depth profiles recorded during dives of elephant seals reflect distinct ocean environments. Journal of Marine Systems 24, 299e312. Carranza, M., 2009. Indicadores del estado del ambiente marino patagónico en áreas frontales. Tesis de Licenciatura en Oceanografía. Buenos Aires University. 115 pp.. Carreto, J., Lutz, V., Carignan, M., Colleoni, A., Demarco, S., 1995. Hydrography and chlorophyll a in a transect from the coast to the shelf break in the Argentinian sea. Continental Shelf Research 15, 315e336.

D.V. Moreno et al. / Estuarine, Coastal and Shelf Science 100 (2012) 72e81 Chisholm, S.W., 1992. Phytoplankton size. In: Falkowski, P.G., Woodheard, A.D. (Eds.), Primary Productivity and Biogeochemical Cycles in the Sea. Plenum Press, New York, pp. 213e237. Cushing, D.H., 1989. A difference in structure between ecosystems in strongly stratified waters and those that are only weakly stratified. Journal of Plankton Research 11, 1e13. Dogliotti, A.I., Schloss, I.R., Almandoz, G.O., Gagliardini, D.A., 2009. Evaluation of SeaWIFS and MODIS chlorophyll-a products in the Argentinean Patagonian continental shelf (38
Se55
S). International Journal of Remote Sensing 30 (1), 251e273. Ferreira, A., Garcia, V.M., Garcia, C.A., 2009. Light absorption by phytoplankton, nonalgal particles and dissolved organic matter at the Patagonia shelf-break in spring and summer. Deep-Sea Research I 59, 2162e2174. Figueiras, F.G., Pitcher, G.C., Estrada, M., 2006. Harmful algal blooms dynamics in relation to physical processes. In: Graneli, E., Turner, J.T. (Eds.), Ecology of Harmful Algae. Springer Verlag, Berlin, pp. 127e138. Fishwick, J.R., Aiken, J., Barlow, R., Sessions, H., Bernard, S., Ras, J., 2006. Functional relationships and bio-optical properties derived from phytoplankton pigments, optical and photosynthetic parameters, a case study of the Benguela ecosystem. Journal of the Marine Biological Association of the U.K. 86, 1267e1280. Franco, B.C., Piola, A.R., Rivas, A.L., Baldoni, A., Pisoni, J.P., 2008. Multiple thermal fronts near the Patagonian shelf break. Geophysical Research Letters 35, L02607. doi:10.1029/2007GL032066. Franks, P.J.S., 1992. Phytoplankton blooms at fronts: patterns, scales and physical forcing mechanisms. Reviews in Aquatic Sciences 6 (2), 121e137. García, C., García, V., 2008. Variability of chlorophyll-a from ocean color images in the La Plata continental shelf region. Continental Shelf Research 28, 1568e1578. García, V., García, C., Mata, M., Polley, R., Piola, A., Signorini, S., McClain, C., IglesiasRodríguez, M., 2008. Environmental factors controlling the phytoplankton blooms at the Patagonia shelf-break in spring. Deep-Sea Research: Part I 55, 1150e1166. Gayoso, A.M., 1998. Long-term phytoplankton studies in the Bahia Blanca estuary, Argentina. ICES Journal of Marine Sciences 55, 655e660. Gibb, S.W., Barlow, R.G., Cummings, D.G., Rees, N.W., Trees, C.C., Holligan, P., Suggett, D., 2000. Surface phytoplankton pigment distributions in the Atlantic Ocean: an assessment of basin scale variability between 50
N and 50
S. Progress in Oceanography 45, 339e368. Gieskes, W., Kraay, G., Nontji, A., Setiapermana, D., Sutomo, 1988. Monsoonal alternation of a mixed and layered structure in the phytoplankton of the euphotic zone of the Banda Sea Indonesia): a mathematical analysis of algal pigment fingerprints. Netherlands Journal of Sea Research 22, 123e137. Glorioso, P.D., 1987. Temperature distribution related to shelf-sea fronts on the Patagonian shelf. Continental Shelf Research 7 (1), 27e34. Gómez, M.I., Piola, A.R., Kattner, G., Alder, V.A., 2011. Biomass of autotrophic dinoflagellates under weak vertical stratification and contrasting chlorophyll levels in subantarctic shelf waters. Journal of Plankton Research 33 (8), 1304e1310. Griffith, G.P., Vennell, R., Williams, M.J., 2010. An algal photoprotection index and vertical mixing in the Southern Ocean. Journal of Plankton Research 32, 515e527. Guerrero, R.A., Piola, A.R., 1997. Masas de agua en la plataforma continental. In: El Mar Argentino y Sus Recursos Pesqueros, vol. 1. Inst. Nac. de Invest. y Desarrolla Pesquero, Mar del Plata, Argentina, pp. 107e118. Herrera, L., Escribano, R., 2006. Factors structuring the phytoplankton community in the upwelling site off El Loa River in northern Chile. Journal of Marine Systems 61, 13e38. Huete-Ortega, M., Marañon, E., Varela, M., Bode, A., 2010. General patterns in the size scaling of phytoplankton abundance in coastal waters during a 10-year time series. Journal of Plankton Research 32 (1), 1e14. Jeffrey, S., Mantoura, R., Wright, S. (Eds.), 2005. Phytoplankton Pigments in Oceanography. Guidelines to Modern Methods. UNESCO. Kiørboe, T., 1993. Turbulence, phytoplankton cell size, and the structure of pelagic food webs. Advances in Marine Biology 29, 1e72. Legendre, L., Rassoulzadegan, F., 1996. Food-web mediated export of biogenic carbon in oceans: hydrodynamic control. Marine Ecology Progress Series 145, 179e193. Llinás, O., Rodriguez de León, A., Siedler, G., Wefer, G., 1999. ESTOC Data Report 1995/1996. In: Informes Técnicos ICCM. ISSN: 1136-193X, vol. 7, p. 152. Lutz, V.A., Segura, V., Dogliotti, A.I., Gagliardini, D.A., Bianchi, A.A., Balestrini, C.E., 2010. Primary production in the Argentine Sea during spring estimated by field and satellite models. Journal of Plankton Research 32 (2), 181e195. Marañon, E., 2009. Phytoplankton size structure. In: Steele, J.H., Turekian, K.K., Thorpe, S.A. (Eds.), Encyclopedia of Ocean Sciences, second ed. Elsevier, Amsterdam, pp. 4249e4256. Margalef, R., 1967. Some concepts relative to the organization of plankton. Oceanography and Marine Biology: Annual Review 5, 257e289. Margalef, R., 1978. Life forms of phytoplankton as survival alternatives in an unstable environment. Oceanologica Acta 1 (4), 493e509. Nair, A., Sathyendranath, S., Platt, T., Morales, J., Stuart, V., Forget, M.H., Devred, E., Bouman, H., 2008. Remote sensing of phytoplankton functional types. Remote Sensing of Environment 112, 3366e3375. Ohlendieck, Ute, Stuhra, Annegret, Siegmund, Heike, 2000. Nitrogen fixation by diazotrophic cyanobacteria in the Baltic Sea and transfer of the newly fixed nitrogen to picoplankton organisms. Journal of Marine Systems 25 (3-4), 213e219.

81

Palma, E.D., Matano, R.P., Piola, A.R., 2008. A numerical study of the Southwestern Atlantic shelf circulation: stratified ocean response to local and offshore forcing. Journal of Geophysical Research 113, C11010. doi:10.1029/2007JC004720. Paparazzo, F.E., Bianucci, L., Schloss, I.R., Almandoz, G.O., Solís, M., Esteves, J.L., 2010. Cross-frontal distribution of inorganic nutrients and chlorophyll-a on the Patagonian continental shelf of Argentina during summer and fall. Revista de Biología Marina y Oceanografía 45 (1), 107e119. Pereira-Brandini, F., Boltovskoy, D., Piola, A., Kocmur, S., Röttgers, R., Cesar Abreu, P., Mendes Lopes, R., 2000. Multiannual trends in fronts and distribution of nutrients and chlorophyll in the southwestern Atlantic (30e62
S). Deep-Sea Research I 47, 1015e1033. Piola, A.R., Moller, O.O., Guerrero, R.A., Campos, E.J., 2008. Variability of the subtropical shelf front off eastern South America: winter 2003 and summer 2004. Continental Shelf Research 28, 1639e1648. Piola, A.R., Falabella, V., 2009. El mar Patagónico. In: Falabella, V., Campagna, C., Croxall, J. (Eds.), El Mar Patagónico: Especies y Espacios. Wildlife Conservation Society e Birdlife International, Cambridge, pp. 56e75. Piola, A.R., Avellaneda, N.M., Guerrero, R.A., Jardon, F.P., Palma, E.D., Romero, S., 2010. Malvinas-slope water intrusions on the northern Patagonia continental shelf. Ocean Sciences 6, 345e359. Ras, J., Claustre, H., Uitz, J., 2008. Spatial variability of phytoplankton pigment distributions in the Subtropical South Pacific Ocean: comparison between in situ and predicted data. Biogeosciences 5, 353e369. Reul, A., Rodríguez, J., Blanco, J.M., Rees, A.P., Burkill, P.H., 2006. Control of microplankton size structure in contrasting wáter columns of Celtic Sea. Journal of Plankton Research 28 (5), 449e457. Richter, P.R., Hader, D., Goncalves, R., Marcoval, M., Villafa, V., Helbling, E., 2007. Vertical migration and motility responses in three marine phytoplankton species exposed to solar radiation. Photochemistry and Photobiology 83, 810e817. Rivas, A.L., 2006. Quantitative estimation of the influence of surface thermal fronts over chlorophyll concentration at the Patagonian shelf. Journal of Marine Systems 63, 183e190. Rivas, A.L., Dogliotti, A.I., Gagliardini, A.D., 2006. Seasonal variability in satellitemeasured surface chlorophyll in the Patagonian shelf. Continental Shelf Research 26, 703e720. Rivas, A.L., Pisoni, J.P., 2010. Identification, characteristics and seasonal evolution of surface thermal fronts in the Argentinean continental shelf. Journal of Marine Systems 79, 134e143. Romero, S.I., Piola, A.R., Charo, M., Garcia, C.A.E., 2006. Chlorophyll-a variability off Patagonia based on SeaWiFS data. Journal of Geophysical Research 111, C05021. doi:10.1029/2005JC003244. Ryan, J.P., McManus, M.A., Paduan, J., Chavez, F.P., 2008. Phytoplankton thin layers caused by shear in frontal zones of a coastal upwelling system. Marine Ecology Progress Series 354, 21e34. Saraceno, M., Provost, C., Piola, A.R., 2005. On the relationship between satelliteretrieved surface temperature fronts and chlorophyll a in the western South Atlantic. Journal of Geophysical Research 110, C11016. doi:10.1029/ 2004JC002736. Sieburth, J.M., Smetacek, V., Lenz, J., 1978. Pelagic ecosystem structure: heterotrophic compartments of the plankton and their relationship to plankton size fractions. Limnology and Oceanography 23, 1256e1263. Smayda, T.J., 2002. Turbulence, watermass stratification and harmful algal blooms: an alternative view and frontal zones as “pelagic seed banks”. Harmful Algae 1, 95e112. Trees, C.C., Clark, D.K., Bidigare, R.R., Ondrusek, M.E., Mueller, J.L., 2000. Accessory pigments versus chlorophyll a concentrations within the euphotic zone: a ubiquitous relationship. Limnology and Oceanography 45, 1130e1143. Uitz, J., Claustre, H., Morel, A., Hooker, S.B., 2006. Vertical distribution of phytoplankton communities in open ocean: an assessment based on surface chlorophyll. Journal of Geophysical Research 111, 1e23. Veldhuis, M.J.W., Kraay, G.W., 2004. Phytoplankton in the subtropical Atlantic Ocean: towards a better assessment of biomass and composition. Deep-Sea Research I 51, 507e530. Vidussi, F., Claustre, H., Manca, B., Luchetta, A., Marty, J., 2001. Phytoplankton pigment distribution in relation to upper thermocline circulation in the eastern Mediterranean Sea during winter. Journal of Geophysical Research 106, 19939e19956. Vijayan, A.K., Yoshikawa, T., Watanabe, S., Sasaki, H., Matsumoto, K., Saito, S.-i., Takeda, S., Furuya, K., 2009. Influence on non-photosynthetic pigments on light absorption and quantum yield of photosynthesis in the western equatorial pacific and the subarctic north pacific. Journal of Oceanography 65, 245e258. Wilks, D.S., 1995. Statistical Methods in the Atmospheric Sciences. In: International Geophysics Series, vol. 59. Academic Press, San Diego. Wright, S., Jeffrey, S., Mantoura, R., Llewellyn, C., Bjornland, T., Repeta, D., Welschmeyer, N., 1991. Improved HPLC method for the analysis of chlorophylls and carotenoids from marine phytoplankton. Marine Ecology Progress Series 77, 183e196. Wright, S., Van den Enden, R., 2000. Phytoplankton community structure and stocks in the East Antarctic marginal ice zone (BROKE survey, JanuaryeMarch 1996) determined by CHEMTAX analysis of HPLC pigment signatures. Deep-Sea Research II 47, 2363e2400. Yamamoto, T., Nishizawa, S., Taniguchi, A., 1998. Formation and retention mechanisms of phytoplankton peak abundance in the Kuroshio front. Journal of Plankton Research 10 (6), 1113e1130.