1. Distribution of phytoplankton in the tropical Eastern South Pacific at 10°S in relation to hydrographic and nutrient conditions Jasmin Franz*, Gerd Krahmann*, Gaute Lavik** and Ulf Riebesell* *Leibniz Institute of Marine Sciences (IFM-GEOMAR), Kiel, Germany **Max Planck Institute for Marine Microbiology, Bremen, Germany jfranz@ifm-geomar.de 2. Study site Fig. 1. Cruise track of M77/3 cruise on RV Meteor running from Guayaquil (Ecuador) to Callao (Peru) from Dec. 2008 – Jan. 2009. Red dots denote CTD cast stations. Blue frame denotes sampled transect at 10°S. Fig. 3. Hydrography along the 10°S transect derived from vertical CTD casts (see fig. 1 and 2). (A) Temperature and (B) O 2 distribution show upwelling of cold O 2 - deficient intermediate water, lifting the upper boundary of the OMZ Fig. 4. (A) Upwelling of NO 3 - -rich water into the euphotic zone above the narrow shelf enabled "new" production (denoted by green dashed line). (B) Subsurface NH 4 + max. within the pycnocline off the shelf potentially produced by zooplankton grazing and bacterial decomposition indicate "regenerated" production in this area (denoted by red dashed line). Black dots indicate sampled depths. 4. Primary production Fig. 5. PFTs restricted in their distribution primarily to the eutrophic surface layer of the narrow inner shelf. (A) Diatoms, in terms of biomass the most dominant PFT along the transect, (B) cryptophytes, (C) prasinophytes (subdivision of the chlorophytes) and (D) autotrophic dinoflagellates. 5.1 Inner shelf 5. Distribution of phytoplankton functional types (PFTs) B A Fig. 6. PFTs occurring in the surface inner shelf waters (N-source: NO 3 - ; see fig. 4A), and in addition in the subsurface water column off the shelf (N-source: NH 4 + ; see fig. 4B). (A) Chlorophytes, (B) haptophytes (probably Phaeocystis globosa), (C) chrysophytes and (D) the picocyanobacterium Synechococcus. Deep Chl a max. formed by these PFTs (denoted by red dashed area) coincided with the subsurface NH 4 + max. within the offshelf pycnocline (denoted by red lined area ) (see (E)). 5.2 Inner shelf and shelf slope 5.3 Open ocean Fig. 7. (A) A low-light adapted Prochlorococcus strain was discovered in the oligotrophic open ocean region of the transect, colonizing the water body right below the lower limit of the 6. Summary 1. Hydrographic structure of the water column along 10°S is controlling the system of primary production - NO 3 - -based "new" production above the narrow shelf - NH 4 + -based "regenerated" production west of the shelf 2. Subsurface NH 4 + maximum within the thermocline potentially formed by bacterial decomposition and zooplankton grazing of phytoplankton transported offshore 3. Partioning of PFTs along the transect in types that exclusively grow in the NO 3 - -rich surface layer above the shelf (Fig. 5), other groups that additionally colonize the subsurface in the realm of the NH 4 + max. (Fig. 6), and the picocyanobacterium Prochlorococcus in the westerly open ocean region of the transect (Fig. 7). 4. Low-light adapted Prochlorococcus strain occurred along the lower boundary of the oxycline and upper boundary of the OMZ, respectively (Fig. 7A and 3B). Thanks to Peter Fritsche, Kai G. Schulz, the CTD-Team and the Crew of German RV Meteor for their great support during the M77/3 cruise. This work is a contribution of the Sonderforschungsbereich 754 "Climate-Biogeochemistry interactions in the Tropical Ocean" (www.sfb754.de) which is supported by the Deutsche Forschungsgemeinschaft. Acknowledgements subsurface NH 4 + max. within the thermocline "regenerated" production AB C D oxycline or the upper boundary of the OMZ, respectively (see (3B)). Yellow dashed line mark upper limit of OMZ. Exclusive photoautotrophic species detected in the light-limited water region below the pycnocline. (B) A second Prochlorococcus population adapted to higher irradiances than the one below the pycnocline (see fig. 7A). A B © Kerstin Nachtigall D "new" production coastal upwelling of NO 3 - (O 2 < 20 µmol/kg) to 10-20 m depth in the area next to the coast. In contrast, the water column west of the shelf was strongly stratified. Green arrows denote coastal upwelling. 1. Introduction The tropical Eastern South Pacific is oceanographically a highly diverse region, with upwelling of nutrient-rich midwater into the surface layer above the narrow shelf, an O 2 -deficient intermediate water body (so-called OMZ) and a pronounced stratification of the water column west of the shelf area. Nutrient cycling is significantly affected by the hydrographic conditions of the water column. Nutrient availability is in turn the major factor controlling primary production and the taxonomical composition of the phytoplankton community. In this study we investigated linkages between hydrography, nutrient chemistry and photoautotrophic production along an east-west transect at 10°S off the Peruvian coast. A B D E A B C D … calculated from measured phytoplankton marker pigments (HPLC= High Pressure Liquid Chromatography) with the program CHEMTAX (Mackey et al., 1997; Veldhuis & Kraay, 2004) References Mackey, M. D., Higgins, H. W., Mackey, D. J. and Wright, S. M. (1997) CHEMTAX - a program for estimating class abundances from chemical markers: application to HPLC measurements of phytoplankton pigments. Marine Ecology Progress Series 144, 265-283. Veldhuis, M. J. W. and Kraay, G. W. (2004) Phytoplankton in the subtropical Atlantic Ocean: towards better assessment of biomass and composition. Deep-Sea Research 51, 507-530. 3. Hydrography Fig. 2. For collecting hydrographic data as well as water samples for biogeochemical analyses, vertical casts were conducted using a rosette device equipped with CTD (conductivity-temperature-depth), O 2 and fluorescence sensors, and 24 x 10-L Niskin bottles. subsurface NH 4 + maximum E RV Meteor M77/3 cruise Ecuador - Peru Jan. 2009