Phytoplankton Responses to Changes in Nutrient Stoichiometry in the Eastern South Pacific

Franz, Jasmin, Hauss, Helena, Sommer, Ulrich and Riebesell, Ulf (2010) Phytoplankton Responses to Changes in Nutrient Stoichiometry in the Eastern South Pacific [Talk] In: Ocean Sciences Meeting 2010 "Oxygen Minimum Zones and Climate Change: Observations and Prediction IV", 22.02.-26.02.2010, Portland, Oregon, USA.

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Oxygen-deficient water masses in the ocean, so-called oxygen minimum zones (OMZ), are said to expand in the course of global climate change. The biogeochemistry in such areas of both the water column and the sediment are highly influenced by the vanishing concentrations of dissolved oxygen: Loss of inorganic nitrogen (N) occurs through denitrification and anaerobic ammonium oxidation (anammox). In contrast, phosphate (P) molecules bound to
metal oxides in the sediment are released under reducing conditions into the water column.
These water masses, rich in P and with relatively poor concentrations of N, deviate significantly from the canonical N:P Redfield ratio of 16 and are transported by upwelling processes into the productive surface layer. The question arouses how these changes in the nutrient stoichiometry are affecting the phytoplankton, in respect of its elemental as well as of its taxonomical composition. On this account, nutrient limitation experiments with on-board mesocosms were carried out during the M77/3 cruise on German RV Meteor from Guayaquil (Ecuador) to Callao (Peru) from the 26.12.2008 – 21.01.2009. 12 meso-scale vessels, distributed over 4 gimbals-mounted water baths on deck of Meteor, were filled each with 70 liters of natural seawater from 10 meters depth. 4 different treatments of inorganic N:P ratios were adjusted by adding ammonium nitrate and/or phosphate. N:P ratios ranged from 16
down to 2.8 and the addition of silicate (Si) to all treatments prevented Si limitation.
Differences in the supply of inorganic N:P could be retrieved in the stoichiometry of particulate organic nitrogen (PON) to phosphorus (POP). The ratio was up to 30 % higher at N:P of 16 than compared to the lowest N:P treatment. In contrast, no variations between the treatments could be observed in the ratio of particulate organic carbon (POC) to PON and in the PON concentrations in general. The striking result, that there is no difference in the size of the PON pool, independent of whether N:P stoichiometry meets Redfield ratio or is close to N:P of 1 can be explained with an increasing contribution by diazotrophic cyanobacteria.
Growth of filamentous cyanobacteria, demonstrated by measurement of the marker pigment aphanizophyll, was stimulated under nutrient conditions depleted in N and rich in P, thus compensating the lack of dissolved inorganic nitrogen. In addition to the effect of nutrient
limitation on nitrogen-fixing cyanobacteria, a significant response by diatoms and dinoflagellates could be discovered. Under N-replete conditions diatom biomass doubled and the concentration of autotrophic dinoflagellates increased up to 4-fold compared to the
treatments that had relatively low amounts of N available. Accordingly, decreasing inorganic N:P ratios in the oxygen-deficient waters of the Eastern South Pacific can induce a shift in the taxa dominating the phytoplankton community, favouring the growth of diazotrophic cyanobacteria and inhibiting expansion of diatoms and dinoflagellates. Furthermore, the observed changes in the elemental stoichiometry (PON:POP) of the phytoplankton community can have an impact on remineralisation processes as well as on the nutritional value of organic matter for higher trophic levels.

Document Type: Conference or Workshop Item (Talk)
Keywords: Marine Biology; phytoplankton;
Research affiliation: OceanRep > GEOMAR > FB3 Marine Ecology > FB3-EOE-N Experimental Ecology - Food Webs
OceanRep > SFB 754 > B2
OceanRep > SFB 754
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-BI Biological Oceanography
Projects: SFB754
Date Deposited: 05 Aug 2010 09:51
Last Modified: 23 Feb 2012 05:16

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