Phytoplankton pigments in the deep chlorophyll maximum of the Caribbean Sea and the western tropical Atlantic Ocean

We report on the distribution of chlorophylls 1986, Jochem & Zeitzschel 1993). The group includes and carotenoids in the deep chlorophyll maximum @CM) of the coccoid cyanobacteria, some prasinophytes and the western tropical Atlantic and the Caribbean Sea, focusother small eukaryotic algae (Johnson & Sieburth ing on pigments of picophytoplankton. Prochlorococcus-Like cells consistently were the dominant component of the DCM Gieskes & Kraay et 1986a)1 as phytoplankton community, with divinyl chlorophyll a being well as the recently-described prochlorophyte-like a nearly-constant 38% of total chlorophyll a. Coccoid cyanobacteria (Chisholm et al. 1988, 19921. The latter contain bacteria were 1 to 2 orders of magnitude less abundant at all unique, ~divinyl~ forms of both chlorophyll a and b stations and depths. Prochlorococcus-like cells in the deeper parts of the DCM contained large amounts of chlorophyll b, (Goencke & Repeta 1992). Because they contain chl b, often more than twice as much as divinyl chlorophyll a, sugthese ~ i c o ~ l a n k t e r s have been referred to as 'progesting the possibility of chromatic adaptation to increasing chlorophytes', though they are probably closely related proportions of blue-green light at the bottom of the euphotic to the coccoid cyanobacteria (Palenik & Haselkorn zone. Based on flow cytometry estimates of cell abundance, 1992, Urbach et 1992), They are widely distributed divinyl chlorophyll a per cell also mcreased dramat~cally through the DCM, with values ranging from 0.09 to 1.45 fg and 'pen Ocean waters, cell-l The carotenoids 1g1-hexanoy1-oxy-fucoxanthin and they also have been reported from temperate latitudes 19'-butanoyl-oxy-fucoxanthin increased with depth in the and in nearshore waters of the Great Barner Reef and DCM in relation to both total chlorophyll a and non-divinyl the ~ ~ d i ~ ~ ~ ~ ~ ~ ~ ~ ~ ( ~ i & wood 1988, olson et al. 1990, chlorophyll a, supporting the idea that eukaryotic nanoand picoplankters compnse a higher portion of the phytoplankVaulot et al. 1990, Campbell & Vaulot 1993, Goericke ton community In the deeper portions of the DCM in the & Repeta lg938 Goericke & Welschme~er lgg31 Shitropics. nlada et al. 1993). Some data suggest that in the tropics prochlorophytes are mainly associated with the DCM KEY P r o c h l o r O ~ h ~ t e ' C~anobacteria ' Picoplank(Shimada et al. 1993), while others have shown more ton . Chlorophyll . Divinyl . Carotenoid . Flow cytometry uniform vertical distributions of prochlorophytes in Deep chlorophyll maxima (DCM) are found throughout the world's oceans, but are deepest and most persistent in the tropics, where they occur at or near the base of the euphotic zone (reviewed by Cullen 1982). Picophytoplankton comprise a significant portion of the photosynthetic biomass in the DCM of the tropics (Takahashi & Hori 1984). These small phototrophs are abundant in most oceanic waters (Li et al. 1983, Takahashi & Hori 1984, Stockner & Antia 1986), and can account for more than half of the primary production in oligotrophic regions (e.g. Iturriaga & Mitchell waters possessing a DCM (Campbell & Vaulot 1993). A large number of studies have demonstrated the differences in species composition among phytoplankton in surface and DCM communities (cf. Venrick 1993 and references therein), but less attention has been paid to zonation within the DCM itself. Some evidence suggests that pico-eukaryotes are relatively more abundant below the DCM peak, and that they are better adapted to the spectral quality of light there (Furuya & Marumo 1983, Murphy & Haugen 1985, Glover et al. 1986a, b, 1987). We report here on the distribution of carotenoids, and of chlorophyll a, total chlorophyll b (normal and O Inter-Research 1994 Resale of full article not permitted 200 Mar. Ecol. Prog. Ser. 113: 199-206, 1994 divinyl forms), and divinyl chlorophyll a (hereafter referred to as chl a , , chl b, and chl a,, respectively) in neritic waters of the Caribbean Sea and the western tropical Atlantic Ocean. Our focus was on the DCM zone, ca 80 to 120 m below the surface, and on changes in the relative abundance of different chlorophylls and carotenoids with depth within it. Our aim was to use changes in pigment concentrations to evaluate the relative importance of prochlorophytes, zonation of different picophytoplankton populations within the DCM, and possible adaptation of cell pigment content to changing quantity and quality of Light at the base of the euphotic zone. Materials and methods. We participated in 2 cruises, in January/February 1991 and January 1992. The study area ranged from the Atlantic Ocean side of the southern Bahamas to Guadeloupe. Most samples were taken in nearshore waters on the Caribbean side off Guadeloupe, Montserrat, and the U.S. Virgin Islands (Fig. 1, Table 1). At each station, a rosette fitted with 10 or 30 1 Niskin sampling bottles, a CTD, and an in situ fluorometer (Sea Tech, Inc.) was deployed to a depth of approximately 200 m. The down trace of fluorescence was used to locate the DCM and samples were collected on the way up. Most often, we sampled at the bottom of the DCM, twice within it, and at its top. On some occasions, samples were also taken near the surface (5 m), or well below the DCM (e.g. 200 m). Some stations were sampled for nutrient concentrations; measured using standard methods (Parsons et al. 1984). Light attenuation was estimated at several stations with a 30 cm diameter white Secchi disk. In addition, 2 profiles of spectrally resolved downwelling irradiance were made, using a Biospherical Instruments MER 1000 spectral radiometer with a 27 Light sensor, 12 spectral bands plus PAR, and a pressure sensor for depth. The maximum depth for this instrument was 85 m. -