Factors controlling plankton community production, export flux, and particulate matter stoichiometry in the coastal upwelling system off Peru
Authors: Bach Lennart Thomas , Paul Allanah Joy , Boxhammer Tim , von der Esch Elisabeth , Graco Michelle , Schulz Kai Georg , Achterberg Eric , Aguayo Paulina , Arístegui Javier , Ayón Patrizia , Baños Isabel , Bernales Avy , Boegeholz Anne Sophie , Chavez Francisco , Chavez Gabriela , Chen Shao-Min , Doering Kristin , Filella Alba , Fischer Martin , Grasse Patricia , Haunost Mathias , Hennke Jan , Hernández-Hernández Nauzet , Hopwood Mark , Igarza Maricarmen , Kalter Verena , Kittu Leila , Kohnert Peter , Ledesma Jesus , Lieberum Christian , Lischka Silke , Löscher Carolin , Ludwig Andrea , Mendoza Ursula , Meyer Jana , Meyer Judith , Minutolo Fabrizio , Ortiz Cortes Joaquin , Piiparinen Jonna , Sforna Claudia , Spilling Kristian , Sanchez Sonia , Spisla Carsten , Sswat Michael , Zavala Moreira Mabel , Riebesell Ulf 
Published: October 2020

Abstract. Eastern boundary upwelling systems (EBUS) are among the most productive
marine ecosystems on Earth. The production of organic material is fueled by
upwelling of nutrient-rich deep waters and high incident light at the sea
surface. However, biotic and abiotic factors can modify surface production
and related biogeochemical processes. Determining these factors is important
because EBUS are considered hotspots of climate change, and reliable
predictions of their future functioning requires understanding of the
mechanisms driving the biogeochemical cycles therein. In this field
experiment, we used in situ mesocosms as tools to improve our mechanistic
understanding of processes controlling organic matter cycling in the coastal
Peruvian upwelling system. Eight mesocosms, each with a volume of
∼55 m3, were deployed for 50 d ∼6 km
off Callao (12∘ S) during austral summer 2017, coinciding with a
coastal El Niño phase. After mesocosm deployment, we collected subsurface
waters at two different locations in the regional oxygen minimum zone (OMZ)
and injected these into four mesocosms (mixing ratio ≈1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal
developments of organic matter production, export, and stoichiometry in the
individual mesocosms. The mesocosm phytoplankton communities were initially
dominated by diatoms but shifted towards a pronounced dominance of the
mixotrophic dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in
surface layers. The community shift coincided with a short-term increase in
production during the A. sanguinea bloom, which left a pronounced imprint on organic
matter C : N : P stoichiometry. However, C, N, and P export fluxes did not
increase because A. sanguinea persisted in the water column and did not sink out during
the experiment. Accordingly, export fluxes during the study were decoupled
from surface production and sustained by the remaining plankton community.
Overall, biogeochemical pools and fluxes were surprisingly constant for most
of the experiment. We explain this constancy by light limitation through
self-shading by phytoplankton and by inorganic nitrogen limitation which
constrained phytoplankton growth. Thus, gain and loss processes remained
balanced and there were few opportunities for blooms, which represents an
event where the system becomes unbalanced. Overall, our mesocosm study
revealed some key links between ecological and biogeochemical processes for
one of the most economically important regions in the oceans.