Pigment measurements were obtained from the website of BATS (http://bats.bios.edu), resampled at regular monthly intervals using a linear interpolation between measurements, and converted to relative Chl a components from different phytoplankton groups as in . Briefly, each phytoplankton group is associated with signature pigments that have relatively constant ratios with total Chl a. Signature pigment concentrations from each phytoplankton group, obtained via HPLC analysis, were converted to Chl a concentration using these ratios. This method has been verified in the North Pacific and the North Atlantic subtropical gyres . We focused on measurements from the upper water column (top 30 m), consistently within the mixed layer , but also examined trends and variability integrated over the depth of the euphotic zone (∼ 140 m) to verify congruence with the top 30 m of the water column.

Haptophyte pigments specifically were calculated using 19′-hexanoyloxyfucoxanthin (19′-hex) pigments and 19′-butanoyloxyfucoxanthin (19′-but). Coccolithophores and other prymnesiophytes contain 19′-hex and negligible amounts of 19′-but, while some other phytoplankton (e.g., chrysophytes and Phaeocystis) contain significant amounts of both 19′-hex and 19′-but . Based on the relative concentrations of 19′-but to 19′-hex measured at BATS, we subtracted out the phytoplankton pigment contribution that contains both 19′-hex and 19′-but, as in , and multiply the remaining 19′-hex concentration by a 19′-hex to Chl a ratio found in calcifying haptophytes . The result is the haptophyte chlorophyll a fraction, which in this study, we assume to be mainly from coccolithophores. Particularly at BATS, the dominant haptophyte group has been reported to be coccolithophores by, e.g., and , but these references offer no direct evidence for this assumption. We did, however, find a significant correlation (p < 0.0000001, r2 = 0.69) between coccolithophore cell counts published in and calculated Chl a from haptophytes at BATS (Fig. S1 in the Supplement). More generally, reported that coccolithophores often dominate the haptophyte community in open-ocean environments, such as BATS.

In order to explore whether phytoplankton population dynamics are driven by carbonate chemistry parameters, we used the Mocsy Fortran 90 package to solve the full carbonate chemistry system using available measurements at BATS . Using dissociation constants from , carbonate chemistry output from Mocsy agrees with other current carbonate system packages available . Input includes average concentrations of total alkalinity and DIC along with mean temperature and salinity in the top 30 m. Output includes pHT, CO32- concentration, bicarbonate (HCO3-) concentration, and aqueous CO2+carbonic acid(CO2(aq)+H2CO3=H2CO3*).

We used satellite observations of level 3, monthly binned PIC and Chl a from the SeaWiFS (1997–2007; limited data availability after 2007) and MODIS Aqua (2003–2014) on a 9 km (5 min) grid obtained from the NASA Ocean Color distributed archive (http://oceancolor.gsfc.nasa.gov/). We calculated the mean satellite-derived PIC concentration in the BATS region that contains > 95 % of pigment measurements (Fig. a).

Chl a from haptophytes (Chl ahapto; assumed to be primarily from cocccolithophores; see Sect. 2) is present throughout the euphotic zone at BATS and displays a pronounced seasonal cycle. Concentrations of Chl ahapto surpassed 100 µgm-3 during periods of high abundance, with ∼35 µg Chl ahapto m-3 during periods of relatively low abundance, such as between 2000 and 2004 (Fig. b). The bulk of haptophyte pigments occurred around ∼80 m of depth, but pigments were also abundant in the upper 30 m, especially during spring. Haptophyte pigment concentration is low below depths of ∼140 m. During the mid-90s and last 6 years of the data set, Chl ahapto was more concentrated, especially in the upper 30 m of the water column (Fig. b).

Haptophytes comprise roughly 30 % of the Chl a in the upper 30 m of the water column at BATS (Fig. ), a percentage that persists from the start of measurements (∼1990) to the end of our data set (∼2012). However, a period of low haptophyte abundance occurred between 2000 and 2004, reducing their relative contribution to Chl a to 15 %. Synechococcus Chl a is variable, ranging from 20 to 70 % of the total Chl a in the upper 30 m. Unfortunately, signature pigment concentrations necessary to calculate Synechococcus Chl a were missing from the BATS data set at the beginning of our time series as well as during a 5-year segment from ∼1997 to ∼2002 (shown as hatched area in Fig. ). In contrast to generally high Synechococcus and haptophyte pigment abundance, Prochlorococcus and diatom pigments contribute relatively small fractions to the total Chl a in the upper 30 m. Correlations with potentially influential oceanographic drivers can aid in explaining variable abundance of different phytoplankton Chl a fractions in these surface waters.

Correlation coefficients between detrended, deseasonalized anomalies of Chl ahapto, Chl a from Synechococcus (Chl asyn), Chl a from Prochlorococcus (Chl apro), and Chl a from diatoms (Chl adiatoms) and anomalies in other oceanographic measurements/indices are presented in Fig. . Chl ahapto shows strong positive correlations with DIC and HCO3- (explaining nearly 20 % of the variability). Chl apro showed similar, but somewhat weaker, correlations with DIC and HCO3-. Conversely, Chl asyn displayed a strong negative correlation with DIC and HCO3- (Fig. ). All phytoplankton pigment groups, except Synechococcus, were positively correlated with nitrate (NO3-) variability (measurements also include nitrite, NO2-; NO3-+NO2- is referred to hereinafter as NO3-). CO32- concentration, the saturation state of aragonite (Ωarag), and temperature were negatively correlated with Chl ahapto, opposite to Chl asyn. Indeed, Synechococcus and haptophyte pigments display inverse correlations for nearly every variable tested, including temperature (Fig. ).

Temperature is negatively correlated with Chl ahapto (as well as Chl apro and Chl adiatoms) and positively correlated with Chl asyn (Fig. ). Furthermore, when the NAO index is in a positive phase, temperatures over this region of the North Atlantic are generally warmer . Therefore, in line with the temperature correlations, the NAO index is negatively correlated with Chl ahapto, opposite again to Chl asyn. MLD, an indicator of both temperature and nutrient availability, shows corresponding correlations with Chl a components. When the MLD is deeper, there is more Chl ahapto and less Chl asyn. Both methods of calculating MLD (MLDtemp and MLDsigma) showed similar correlations (Fig. ).

Haptophyte and Synechococcus pigments generally show opposing correlations with the variables tested (Fig. ). This is supported by the opposing dominance of either Synechococus or haptophyte Chl a throughout the time series (Fig. ). Indeed Chl ahapto and Chl asyn show a significant negative correlation (p<0.00001). During periods of low haptophyte pigment abundance (e.g., 2000–2004), Synechococcus pigments dominate the water column. Later in the time series, however, Synechococcus pigments decline and haptophyte pigments increase. Prochlorococcus pigments (mostly low-light Prochlorococcus; see Discussion) at BATS reside mostly in the deep chlorophyll maximum at ∼100 m of depth and are a relatively minor component of the Chl a in the upper 30 m of the euphotic zone. Diatom presence, usually associated with cold, high nutrient environments, is sporadic and generally low in this oligotrophic oceanic region of the subtropical North Atlantic, according to their signature pigments (Fig. ).

In general, pigment analyses indicate that Synechococcus and haptophytes are the most abundant phytoplankton groups at BATS (Figs.  and ). However, the category classified as other (includes chrysophytes, dinoflagellates, and prasinophytes, among others) in Fig.  comprises a large portion (> 30 %) of the phytoplankton biomass during certain periods, indicating certain conditions may favor neither Synechoccocus nor haptophytes. The opposing correlations Synechoccocus and haptophyte pigments exhibit with DIC, HCO3-, CO32-, NO3- and temperature could lead to interesting trends in phytoplankton abundance in an ocean increasingly influenced by anthropogenic climate change.

A time series of Chl ahapto at BATS shows that haptophytes (mainly coccolithophores; see Sect. 2) have been increasing significantly since 1990 (p<0.01 for 30 m integral; p<0.001 for 140 m integral; Fig. a, b). Mean concentration of Chl ahapto in the upper 30 m of the water column has increased by 0.848 µgm-3yr-1 (standard error = 0.332), corresponding to a 68 % increase over the course of the BATS time series (1991–2012; an overall increase of 17.8 µgm-3), while the 140 m integral of Chl ahapto has increased by 0.103 mgm-2yr-1 (standard error = 0.0307) corresponding to a 37 % increase (an overall increase of 2.2 mgm-2). We assess the sensitivity of these trends to interannual variability by performing trend calculations for a range of start and end years over the time series. The resulting trend pattern shown in Fig. a shows mostly positive trends in Chl ahapto, except for end years in the 2000–2004 period. Total chlorophyll a (Chl atotal) also shows a significant positive trend over the time series (p<0.05 for 30 m integral; p<0.001 for 140 m integral; Fig. ; upper right corner of Fig. c). Figure c shows trends in Chl atotal for a range of start and end years, displaying a different pattern than that of the trends in the Chl ahapto component of Chl a. For instance, for end years in the 2000–2004 period, Chl atotal shows positive trends (but nonsignificant), whereas Chl ahapto shows significant negative trends (Fig. ). Mean Chl a in the upper 30 m primarily exhibits negative trends in the later part of the time series and most trends in Chl a are nonsignificant (Fig. c). Unfortunately, missing Synechococcus pigment data did not allow for long-term trend analysis of this group of phytoplankton (see hatched area in Fig.  and white area in Fig. ).

In line with the results of our correlation analysis, the trends in HCO3- for various start and end years show a similar pattern to the trends in Chl ahapto (Fig. b): mostly positive trends with slightly negative (but nonsignificant) trends in HCO3- concentration for end year in the 2000–2004 period, a low point in haptophyte pigments. Conversely, the trend pattern for the substrate-inhibitor ratio, [HCO3-] / [H+], is distinctly different from that of Chl ahapto, exhibiting all negative trends (Fig. d). Trends in Chl ahapto, assumed here to be mainly representative of coccolithophores (see Sect. 2), can be further corroborated with PIC measurements from the satellite record.

Significant correlations were detected between Chl ahapto (30 m integral) measured at BATS and PIC derived from each satellite. The SeaWiFS-derived PIC correlated somewhat better with Chl ahapto than MODIS-derived PIC (SeaWiFS PIC-Chl ahapto p=0.0075, r2=0.19 vs. MODIS PIC-Chl ahapto p=0.050, r2=0.12). This difference is likely inherent in the different algorithms used to estimate PIC from each satellite (see following paragraph). Nevertheless, these correlations demonstrate correspondence between Chl ahapto measurements and satellite PIC, both of which relate to relative coccolithophore abundance.

Two radiance-based PIC algorithms can be used to relate water-leaving radiance to calcite absorption and scattering properties: a two-band algorithm and a three-band algorithm . The North Atlantic subtropical gyre exhibits relatively low PIC concentrations year-round. During the SeaWiFS–MODIS overlap period (2003–2007), PIC estimated from the two satellites revealed stark differences (Fig. c), possibly explained by the differences in algorithm performance in this region (i.e., sensitivity to low/background PIC concentrations). The low correspondence between the two estimates of PIC prevented the generation of a single, merged PIC time series. We therefore report trends in PIC separately over the respective satellite eras (Fig. ).

Chl a measured at BATS (30 m integral) and Chl a derived by satellite were significantly correlated (p<0.01, r2=0.16). In this case, Chl a measured by satellite displayed good correspondence between the two satellite eras and could be merged into one time series. Following the regression technique of , we generated one continuous record of Chl a from 1998 to 2014 by applying linear regression over the 2003–2007 SeaWiFS–MODIS overlap period to predict these variables from 2008 to 2014 (Fig. c).

Linear trends in PIC derived from satellite observations are positive for most of the North Atlantic subtropical gyre (Fig. ). Nearly all significant trends (p<0.05) in PIC concentration are positive, especially during the MODIS era (Fig. c, d; 1998–2007 for SeaWiFS and 2003–2014 for MODIS). However, unlike at the BATS site, Chl a does not appear to be increasing in most of the gyre (Fig. ). There are slight positive trends in Chl a around the BATS region (Fig. a), but these are not statistically significant (Fig. b). Indeed, most of the North Atlantic subtropical gyre shows a slight negative trend in Chl a or no trend at all. A trend of a subset of the satellite Chl a from 1998 to 2012 shows a slight, but nonsignificant, upward trend in Chl a in the BATS region (Fig. ; 0.0009 mgm-3yr-1, p>0.05), just as the corresponding grid box for Chl a at BATS in Fig. .

In this study, we observed that coccolithophore populations, based on pigment data for haptophytes, are increasing at BATS and are positively correlated with DIC and HCO3- (Figs. b, , a). We observed opposite correlations for DIC and HCO3- with Synechococcus, the other major member of the phytoplankton community at BATS. Some studies have suggested that photosynthesis and growth of the coccolithophore, E. huxleyi, is carbon limited and could possibly benefit from increasing CO2 . Since CO2(aq) and HCO3- concentrations increase with increasing DIC/ocean acidification, both photosynthesis and calcification could be stimulated in coccolithophores, which primarily use CO2 for photosynthesis and HCO3- for calcification . The results presented in this study support the hypothesis that coccolithophores are responding positively to increasing carbon availability, perhaps increasing their competitive ability in oligotrophic settings such as BATS. However, a threshold H+ ion concentration could be reached with further ocean acidification, eventually constraining coccolithophore growth.

Synechococcus, on the other hand, was negatively correlated with increasing carbon in the upper mixed layer and positively correlated with temperature. In laboratory experiments, Synechococcus showed only a slight, non-significant increase in growth rate under elevated CO2 conditions, but increased growth 2.3-fold with increasing CO2 and temperature . Sea surface temperature in the upper 30 m at BATS has not increased significantly over the time period of this study (0.04 ∘Cyr-1 trend, p=0.22). However, positive temperature anomalies were recorded during the 2000–2004 period, a period of increased Synechococcus pigment abundance (and low haptophyte pigment abundance). Conditions that could favor Synechococcus may eventually arise with further warming, increasing the competitive ability of Synechococcus.

In order to examine redundancy in our correlations, we performed multiple linear regressions between Chl ahapto and several of the driver variables with which it showed the strongest correlations (not shown). When DIC and HCO3- were regressed together with Chl ahapto, all the statistical power of DIC was removed, indicating that HCO3- is the primary driver of the two for Chl ahapto variability. Furthermore, when temperature and NO3- were regressed together with Chl ahapto, the statistical effect of temperature was removed. This indicates that temperature is not a controlling factor for coccolithophore variability, but rather is a proxy for nutrient concentration in relation to coccolithophore growth. A multiple linear regression of Chl ahapto with both HCO3- and NO3- explained > 50 % of the variance in Chl ahapto (r2=0.52). Other factors, such as competition or grazing, could perhaps account for some of the remaining variability. Both HCO3- and NO3- have increased significantly over this time period (p<0.001). However, NO3- measurements are highly variable and near zero, making their accuracy questionable, and the trend is only significant if the last 2 years are included in the time series (Fig. S2). This is in contrast to HCO3-, which shows largely positive trends over this time period matching quite well with those of Chl ahapto (Fig. ).

The positive trend in HCO3- concentration in the upper mixed layer of the water column at BATS is most likely due to increasing absorption of anthropogenic CO2 from the atmosphere. The upper 30 m of the water column is particularly inundated with anthropogenic CO2 in the North Atlantic . From 1991 to 2012, DIC concentration in the upper 30 m at BATS increased by a rate of 1.4 µmolkg-1yr-1, which is roughly the expected rate of increase given the rise in atmospheric CO2 see Chapter 10 in. Increasing inorganic carbon supply could also be accompanied by warmer sea surface temperatures, increased stratification, and decreased nutrient supply over the next century . Enriched coccolithophore growth by this additional carbon, as well as other predicted oceanic changes with global warming, could lead to shifts in phytoplankton community structure at BATS.

Coccolithophores are not the only phytoplankton that may be responding positively to additional inorganic carbon. Trichodesmium, the filamentous N2-fixing cyanobacteria, has been shown to increase growth and N2 fixation under increasing CO2 , yet other drivers, such as sea surface temperature, nutrients, and species diversity, tend to exert more control on their growth in situ . Trichodesmium has been reported to be a common component of the phytoplankton assemblages in the subtropical North Atlantic , but was not specifically resolved in this study. Trichodesmium contain a similar suite of pigments as Synechococcus , and therefore could be included in the Chl asyn fraction of our calculations. This would aid to explain the negative correlation between Chl asyn and NO3- (Fig. ), and further explain why Chl asyn is more abundant than Chl ahapto in the upper water column during warmer, more stratified periods.

Whatever the exact components of Chl asyn, Chl ahapto generally shows opposing abundance with this group. We hypothesize that when the Synechococcus component (∼ 40 % of Chl a on average at BATS) is abundant (perhaps due to a positive temperature anomaly), photosynthesis accompanying an increase in Synechoccocus draws down DIC (thus, HCO3-). Low DIC could provoke carbon limitation of the coccolithophore population , hindering their competitive ability. Overall, however, the increase in DIC experienced at BATS during the past 2 decades could lead to slightly increased growth rates of coccolithophores , perhaps bolstering their competitive ability within the phytoplankton community. If coccolithophores are becoming more competitive at BATS due to a lessening of carbon limitation, then they could continue to exert greater competitive stress on Synechococcus, which appear to be competing with coccolithophores for a similar niche. However, if the surface waters continue to warm in this region of the Atlantic, as predicted , then Synechococcus could regain its competitive edge. Furthermore, increasing H+ ion concentrations could eventually constrain coccolithophore growth . Small changes in DIC, nutrients, pH, or temperature could, in combination, influence which phytoplankton group (Synechococcus or haptophytes) dominate at any given time at BATS.

The Prochlorococcus group, designated by Chl apro, shows similar correlations with oceanic driver variables as for Chl ahapto (Fig. ). Prochlorococcus have been shown to lack a CO2(aq) uptake mechanism and therefore rely on HCO3- uptake for photosynthesis , possibly explaining similar behavior to coccolithophores (also positively correlated with HCO3-), which use HCO3- for calcification. Prochlorococcus reside mainly in the deep chlorophyll maximum, comprising a rather small portion of the Chl a in the upper 30 m at BATS (Figs. , ). This could be, however, due to the relatively high Chl b to Chl a ratio used in our pigment calculations , which is more representative of low-light Prochlorococcus . Yet, since ∼2005, Chl apro has been more common in the upper 30 m, resulting in an overall positive trend in Chl apro over the entire time series (p<0.001).

Consistent with colder, high nutrient environments in which diatoms are normally found, Chl adiatoms showed a strong positive correlation with NO3- and a negative correlation with temperature (Fig. ). If predicted trends in sea surface temperature and nutrient supply with further stratification are realized, then diatoms could become a reduced component of the phytoplankton assemblage at BATS . Combining fine-scale phytoplankton dynamics from BATS with the satellite record can help to elucidate what changes are occurring over large spatial scales.

Unlike at BATS, Chl a in the North Atlantic subtropical gyre is not increasing (Fig. ). PIC, on the other hand, shows mainly positive trends over the whole gyre, in agreement with data from BATS – Chl ahapto shows overall increasing trends, despite some periods of low abundance. Together with an absence of Chl a trend, this implies that coccolithophores are increasing in abundance relative to other types of phytoplankton in the subtropical gyre. Accompanying this conclusion, however, are uncertainties associated with the satellite-derived PIC estimates (see Sect. 4.3 on Limitations of this Study). Even so, ratios relating Chl ahapto to PIC can further elucidate confidence in satellite-derived PIC estimates.

Data on the amount of Chl a per coccolithophore cell allows for the calculation of cell concentration of coccolithophores in the surface waters at BATS. Using a value of 0.26 pg Chl a per cell , mean coccolithophore cell concentration in the upper 30 m is 143×103 cellsL-1 (corresponding to the mean value of 37.3 µg Chl ahapto m-3 in the upper 30 m). Employing a ratio of PIC to coccolith of 0.26 and considering 15 coccoliths per cell (a minimal monolayer of coccoliths covering the cell) under nutrient replete conditions and 100 coccoliths per cell under severe nutrient limitation , we arrive at a PIC concentration range of 0.56 to 3.73 mg PIC m-3. This range corresponds well to the average satellite-derived PIC concentration in the BATS region (Fig. a) over the study period: 2.71 mg PIC m-3 for SeaWiFS (standard deviation = 0.50) and 2.66 mg PIC m-3 (standard deviation = 0.39) for MODIS. The relatively high satellite-derived PIC concentration further suggests that coccolithophores may be experiencing nutrient limitation at BATS and producing additional coccoliths in response;. Therefore, even given the multiple sources of error involved with satellite-derived PIC estimates and pigment analyses (see below), we feel the strong predominance of positive trends in PIC, along with the BATS Chl ahapto data, suggests that coccolithophores are proliferating in this region (Fig. ).

There are several caveats of this study that must be discussed before these results can be put into context. First, a primary assumption in this paper is that the haptophyte group is mainly composed of coccolithophores. Though high haptophyte diversity has been reported in open-ocean regimes , this does not necessarily contradict the assumption that coccolithophores are the dominant type of haptophyte. While several studies describe coccolithophores to be the dominant haptophyte at BATS see Sect. 2;, this claim remains unsupported by specific data. We use the significant positive correlation between Chl ahapto and cell counts from to support our assumption that Chl ahapto is mainly representative of coccolithophores (Fig. S1). However, as with any study that derives phytoplankton community composition from signature pigments, inherent uncertainties are associated with changes in pigment content within a phytoplankton group over time.

Overall pigment and PIC concentration per coccolithophore cell may be influenced by environmental conditions. For example, photo-acclimation and nutrient limitation can invoke changes in pigment composition or calcification that are not necessarily associated with changes in overall abundance . Dominant species shifts within a phytoplankton group could also influence pigment and/or PIC measurements. Nevertheless, given upward trends observed for both Chl ahapto and PIC (Figs. , ), we feel the most probable explanation of these observations is increases in overall coccolithophore abundance. However, satellite-derived PIC measurements also contain inherent uncertainties.

Radiance-based algorithms for deriving PIC from satellite reflectance data are formulated to capture the light-scattering properties of the numerically dominant coccolithophore, E. huxleyi, but also capture detached or detrital coccoliths . PIC concentrations in the North Atlantic subtropical gyre are comparatively low, generally ∼ 2.7 mg m-3, compared to other coccolithophore bloom regions, which have PIC concentrations between 10 and 100 mg m-3 . The low concentrations of PIC observed in the North Atlantic subtropical gyre could be within background error or nearing the sensitivity threshold of the instrument. Errors in satellite-derived PIC can arise from atmospheric correction, inclusion of other suspended minerals (such as silica; opal contamination), and/or the influence of chlorophyll or colored, dissolved organic matter see. However, these errors can be minimized by binning in space and time, as we have done in this study (using monthly, 9 km data rather than daily, 4 km data). It is also curious that SeaWiFS-derived PIC data better match the Chl ahapto estimates from BATS than the MODIS PIC (Fig. ). On the one hand, this may be indicative that other 19′-hex-containing haptophytes were responsible for the increase in Chl ahapto at BATS during the last several years of the time series. On the other hand, the predominance of upward trends in MODIS-derived PIC for areas around BATS (Fig. b, d) suggests increases in calcifying haptophytes (coccolithophores). It should be noted that MODIS PIC from the BATS region (see Fig. ) still shows a significant correlation with Chl ahapto (see Sect. 3, Sect. 3.5).

Finally, we are limited in our trend analysis by the length of the time series data. Figure  demonstrates that different start and end years can influence the sign and magnitude of our trends in, e.g., Chl ahapto. In this study, we report trends in pigments from 1990 to 2012 and trends in PIC from 1998 to 2014, both of which, when employing the full time series of data, imply increases in coccolithophore populations in the subtropical North Atlantic.

This is not the only study to suggest that coccolithophores are increasing in abundance in the North Atlantic. Using Continuous Plankton Recorder ship measurements, Rivero-Calle et al. (2015) document an increase in coccolithophore occurrence from ∼2 to > 20 % in the North Atlantic from 1965 to 2010, which they attribute to increasing CO2 concentrations. The data region in their study extends from ∼40 to ∼ 65∘ N (subpolar gyre), just north of the subtropical gyre region focused on in this study. Thus, these studies combined add robustness to the conclusion that coccolithophores in the North Atlantic are increasing in abundance and are likely stimulated by additional carbon from anthropogenic sources.

Ocean acidification may eventually hinder the growth and calcification of coccolithophores, however. Recently, and introduced the substrate–inhibitor ratio, describing the dependence of calcification on HCO3- (the substrate) and H+ (the inhibitor) concentrations. When this ratio falls below a critical level (i.e., intercellular to extracellular H+ concentration ratio too low) coccolithophore calcification will be hindered, unless they evolve a mechanism for coping with low pH . demonstrated that pH starts to have a negative impact below pHT 7.7, whereas the BATS average pHT is ∼8.1. Thus, critical pH levels will not likely happen for several thousand years . Other factors besides carbonate chemistry, such as light availability, temperature, and nutrients, likely influence coccolithophore growth in present-day oceans.

Increases in coccolithophore abundance in the North Atlantic could have far-reaching ecological, biogeochemical, and climate effects. A shift in phytoplankton community structure could change trophic dynamics, ultimately resulting in ecosystem shifts . For example, though the evolutionary purpose of coccolithophore shells is unclear, some studies speculate they could protect against grazing see chapter on functions of coccoliths in. Shifts to relatively more coccolithophores in a phytoplankton assemblage could reduce trophic energy available for grazers. Coccolithophore shells also function as a ballast material, sinking faster due to increased weight of the CaCO3 shell, and sequestering organic matter in the deep ocean Sarmiento and Gruber, 2006; for a recent study of export at BATS, see. Increases in coccolithophore abundance may have a positive impact on export production, thus a negative feedback on increasing atmospheric CO2. In addition to bringing carbon to the deep ocean, coccolithophores produce the marine trace gas dimethyl sulfide DMS;, which affects cloud formation and climate. Therefore, overall increases in coccolithophore abundance could increase marine DMS production. Furthermore, DMS production by the coccolithophore, E. huxeyi, has been shown to increase with increasing temperature and ambient CO2 . Thus, changes in coccolithophore abundance could have a multitude of effects on marine ecosystems in the North Atlantic, as well as global carbon cycling and climate. These effects could be further amplified if other ocean basins show similar shifts in phytoplankton composition.

In the subtropical North Atlantic, the upper mixed layer contains particularly high levels of anthropogenic CO2 . We speculate that this rise in DIC is contributing to the increases in coccolithophore pigments and PIC documented in this study. However, it is not clear if phytoplankton communities in other similar oceanic ecosystems, e.g., the North Pacific subtropical gyre, will show similar changes as atmospheric CO2 concentrations continue to increase and inundate the upper mixed layer. The aforementioned ecosystem and carbon cycle effects of coccolithophore increases could become even more prevalent in the world's ocean, or, alternatively, coccolithophore growth could be further modulated by temperature, nutrients, and light. In any case, monitoring the response of natural coccolithophore populations to increasing DIC/ocean acidification is essential for understanding effects of anthropogenic carbon emissions on the world's oceans.