Abstract
A large-volume mesocosm-based nutrient perturbation experiment was conducted off the island of Hawai‘i, USA, to investigate the response of surface ocean phytoplankton communities to the addition of macronutrients, trace metals, and vitamins and to assess the feasibility of using mesocosms in the open ocean. Three free-drifting mesocosms (~60 m3) were deployed: one mesocosm served as a control (no nutrient amendments); a second (termed +P) was amended with nitrate (N), silicate (Si), phosphate (P), and a trace metal + vitamin mixture; and a third (termed -P) was amended with N, Si, and a trace metal + vitamin mixture but no P. These mesocosms were unreplicated due to logistical constraints and hence differences between treatments are qualitative. After 6 d, the largest response of the phytoplankton community was observed in the +P mesocosm, where chlorophyll a and 14C-based primary production were 2-3× greater than in the -P mesocosm and 4-6× greater than in the control. Comparison between mesocosm and ‘microcosm’ incubations (20 l) revealed differences in the magnitude and timing of production and marked differences in community structure with a reduced response of diatoms in microcosm treatments. Notably, we also observed pronounced declines in Prochlorococcus populations in all treatments, although these were greater in microcosms (up to 99%). Overall, this study confirmed the feasibility of deploying free-drifting mesocosms in the open ocean as a potentially powerful tool to investigate ecological impacts of nutrient perturbations and constitutes a valuable first step towards scaling plankton manipulation experiments.
Highlights
The availability of dissolved inorganic nutrients is a key determinant of the abundance and activity of photosynthetic microorganisms in the global ocean, Publisher: Inter-Research · www.int-res.comAquat Microb Ecol 87: 167–183, 2021 mineralization processes that return nutrients from particulate to dissolved forms and can lead to the net sequestration of carbon (C) from the atmosphere (Moore et al 2013)
Vertical profiles of temperature, salinity, pH, oxygen, and fluorescence were obtained from each mesocosm (~20 m depth) using a handheld CTD with a 5 Hz sampling rate; data were only used from the downcast
Understanding and predicting ecosystem responses to environmental perturbations are fundamental goals in ecology (Moore et al 2013, Browning et al 2017), yet the designs of ecosystem perturbation experiments in the open ocean are challenging because physical and biological interactions occur over broad scales of time or space and with ecological complexity that is difficult to examine in small-scale studies (Carpenter 1996, Schindler 1998)
Summary
Aquat Microb Ecol 87: 167–183, 2021 mineralization processes that return nutrients from particulate to dissolved forms and can lead to the net sequestration of carbon (C) from the atmosphere (Moore et al 2013) For this reason, nutrient perturbations aimed at manipulating ocean production and potentially enhancing C export to the interior waters are of enormous public and scientific interest (Boyd 2008, Taucher et al 2018). Artificial enrichments of the surface ocean with deep, nutrientrich waters have been proposed as a mechanism to stimulate phytoplankton blooms and support the drawdown of atmospheric carbon dioxide (CO2) (Lovelock & Rapley 2007, Vaughan & Lenton 2011) These deep waters are enriched in CO2 that would be released into the atmosphere when brought to the surface and counter the intended C sequestration (Takahashi et al 1997, Karl & Letelier 2008). Karl & Letelier (2008) argued that the chemical composition of the source water, and the nitrogen:phosphorus (N:P) ratio, utilized in nutrient perturbation experiments can be critical to achieving net export and in avoiding unintended ecological consequences such as the formation of toxin-producing microorganisms or shifts in the planktonic community structure that might alter trophic interactions in unforeseen ways
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