Effects of nutrient enrichments on oligotrophic phytoplankton communities: a mesocosm experiment near Hawai‘i, USA

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.

A tale of two nutrients: how nitrogen and phosphorus differentially control marine biomass production and stoichiometry

“…S5). All treatments saw a decline in the relative abundance of Prochlorococcus as is commonly observed in long-term incubation studies 22,23,30 , but Synechococcus persisted throughout the incubation period (Fig. S6).…”

“…1a,d, S3a,d), attributable to new phytoplankton production stimulated by N supply. Incubation studies from both the North Paci c 22,23 and North Atlantic 24,25 gyres agree that increasing nutrient N supply alone generally stimulates growth equaling N and P co-addition, including deep seawater addition 22 , although growth is sometimes enhanced by the addition of P with N in the Atlantic when phosphate concentrations are low 24 . Particulate C:N ratios remained similar across the + N-P and + N + P treatments (Fig.…”

Changes in size-dependent Chlorophyll a concentration and group-specific picophytoplankton abundance in short-term nutrient-addition experiments in the Equatorial Eastern Indian Ocean

“…Due to the small incubation volumes in our study, the potential "bottle effect" would affect the small phytoplankton, especially on picophytoplankton (Calvo-Dıáz et al, 2011). When cultured water samples in small bottles, larger cells would be underestimated, the trophic interaction would be disturbed and exchange of nutrients and metabolites in bottles with surrounding waters would be prevented (Calvo-Dıáz et al, 2011). Böttjer-Wilson et al (2021 compared the results of mesocosm (60 m 3 ) and microcosm (20 L) experiments, and found that the Prochlorococcus abundance in microcosm was only 60% of those in mesocosm after a two-day incubation.…”

“…In contrast, Mouriño-Carballido et al (2016) found that abundances of Synechococcus and eukaryotic picophytoplankton (Peuk) were enhanced with the elevation of nitrate and other nutrient concentrations in the northwestern Mediterranean Sea. Böttjer-Wilson et al (2021) indicated that the insufficient phosphate levels led to a greater decrease in Prochlorococcus abundance in oligotrophic oceans near Hawai'i. These findings demonstrate that the short-term community dynamics of picophytoplankton are divergent in different regions and under different nutrient additions, which likely affects the longterm responses.…”

Pelagic ecosystem research incubators (PERIcosms): optimized incubation tanks to investigate natural communities under long term, low nutrient, and low metal conditions

“…This desire to control external variables comes with limitations since incubations are inherently simplified and therefore can be difficult to scale up to the ecosystem under study (Schindler 1998). Even so, incubations are routinely used to approximate primary productivity in natural systems (Steemann Nielsen 1952), to understand community responses to events such as altered nutrient supply (e.g., Lebaron et al 2001;Mahaffey et al 2012;Turk-Kubo et al 2015;Böttjer-Wilson et al 2021), climate change (e.g., Bach et al 2016;Boyd et al 2022), and anthropogenic pollutants (e.g., Menzel and Case 1977;Kuiper and Hanstveit 1984).…”