Factors controlling plankton community production, export flux, and particulate matter stoichiometry in the coastal upwelling system off Peru

Abstract

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.

Controls on the Silicon Isotope Composition of Diatoms in the Peruvian Upwelling

“…Phytoplankton production is fueled by upwelling of nutrient-rich subsurface waters and high incident light at the sea surface and is generally highest in austral summer (Jan.-March; our study was conducted in February 2013; e.g., Echevin et al, 2008). Upwelling of dSirich subsurface waters (up to 40 µmol L −1 ) drives diatom blooms in coastal surface waters, which leads to bSi concentration of up to 6 µmol L −1 (this study; Franz et al, 2012;Bach et al, 2020). The Si isotope signature of the dSi in upwelled waters, mainly the PCUC, is well characterized and relatively constant along with the Peruvian Shelf (+1.5 ± 0.2 , mean ± 1 s.d.).…”

Nitrogen loss processes in response to upwelling in a Peruvian coastal setting dominated by denitrification – a mesocosm approach

“…The current experiment started on 22 February 2017 with the deployment of eight Kiel Off-Shore Mesocosms for Future Ocean Simulations (KOSMOS) about 4 nautical miles (nm) off the Peruvian coast, close to San Lorenzo Island at 12.0555 • S and 77.2348 • W, and ended on 16 April 2017 after 50 d of sampling. Full details on the experimental set-up, the sampling, manipulation, and maintenance schedule can be found in Bach et al (2020a).…”

“…To minimise changes to deep water gas concentrations during injection, water was pumped from 2 m depth out of the deep water bags. For further details on collection and addition, see Bach et al (2020a).…”

“…The onset of this perturbation was estimated by the sudden increase in total particulate phosphorus deposition rates in the sediment traps, most likely from excrement, as phosphate concentrations during this time were relatively constant (Fig. S2; for details see Bach et al, 2020a). Total N loss was then estimated by summing up hourly in situ rate estimates (Eq.…”

Silicon Isotope Signatures of Radiolaria Reveal Taxon-Specific Differences in Isotope Fractionation

“…During this event, coastal waters off Peru showed a pronounced positive sea surface temperature anomaly of up to 4 • C (Echevin et al, 2018;Garreaud, 2018), as a result of strong local alongshore wind anomalies and equatorial Kelvin waves propagating towards the Peruvian coast (Echevin et al, 2018;Peng et al, 2019). Due to a deepening of the thermocline, nutrients, such as DSi, do not upwell to the surface, causing nutrient-depleted conditions and a shift in the phytoplankton community from diatom-dominance (Blasco, 1971;Estrada and Blasco, 1985;DiTullio et al, 2005) to species such as dinoflagellates (Sanchez et al, 2000;Bach et al, 2020). During the coastal EN in 2017 remained lower in the surface ocean ranging on average from 4 to 16 µmol L −1 from north to south, with lowest DSi of 4 to 6 µmol L −1 north of 3.5 • S and higher concentrations of 9.0 to 15.9 µmol L −1 between 10 and 15 • S (Figure 3D).…”