Quantitative analysis of food web dynamics in a low export ecosystem

Abstract

Food webs trace the flow of organic matter and energy among producers and consumers; for pelagic marine food webs, network complexity directly influences the amount and form of carbon exported to the deep ocean via the biological pump. Here we present a synoptic view of mixed layer food web dynamics observed during the late summer 2018 EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign in the subarctic Northeast Pacific at the long-running time-series site, Ocean Station Papa. Carbon biomass reservoirs of phytoplankton, microzooplankton, and bacterioplankton, were approximately equal while mesozooplankton biomass was 70% lower. Live organisms composed ~40% of the total particulate organic carbon within the mixed layer: the remainder was attributed to detritus. Rates of carbon transfer among reservoirs indicated production and assimilation rates were well balanced by losses, leaving little organic carbon available for export. The slight positive net community production rate generated organic carbon that was exported from the system in the form of food web byproducts, such as large fecal pellets generated by mesozooplankton. This characteristically regenerative food web had relatively slow turnover times with small-magnitude transfers of carbon relative to standing stocks that occurred amidst a high background concentration of detrital particles and dissolved organic matter. The concurrent estimation of food web components and rates revealed that separated processes dominated the transfer of carbon within the food web compared to those that contributed to export.

Small particles and heterotrophic protists support the mesopelagic zooplankton food web in the subarctic northeast Pacific Ocean

“…This uncertainty can be similar in magnitude to the number of trophic steps we are interested in resolving (about 1), hindering the ability to assign statistical significance during hypothesis testing. Yet, estimates of the number of protistan trophic steps in the euphotic zone (mean: 1.3 AE 0.3), where the role of protists is well constrained, agree well with the accepted paradigm for microzooplankton food web structure (Miller 1993;McNair et al 2023). We also see that at times both TP GlxÀPhe and TP trÀsrc give unrealistically low estimates of TP (i.e., TP GlxÀPhe of small zooplankton is < 1 higher than that of particles through much of the mesopelagic zone).…”

“…P) is located in the subarctic Northeast Pacific, a high nutrient, low chlorophyll region characterized by small phytoplankton, low primary productivity, and a tight coupling between phytoplankton growth and microzooplankton grazing (Landry et al 1993;Miller 1993;McNair et al 2021). The result is a highly regenerative food web with low export potential, organized around efficient utilization of organic matter (McNair et al 2023). Below the euphotic zone, the food web is detritus based (Goldblatt et al 1999), and active microzooplankton grazing has been observed down to the euphotic-mesopelagic transition (McNair and Menden-Deuer 2020).…”

Assessment of Oceanographic Conditions during the North Atlantic EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) Field Campaign

“…The first was conducted in the Subarctic North Pacific Ocean near Ocean Station P (August-September 2018; Siegel et al, 2021). This high nutrient, low chlorophyll site can be characterized as a highly recycled, low export ecosystem where organic matter was tightly retained in the surface-ocean mixed layer and zooplankton populations mediated much of the transport of organic matter to depth (Stamieszkin et al 2021;Steinberg et al 2023;McNair et al 2023). The recent EXPORTS North Atlantic Ocean deployment (EXPORTS NA; May 2021) was conducted to provide a contrasting end member by focusing on the export associated with the North Atlantic spring bloom where all three export flux pathways are likely to be important (Dam et al 1993;Omand et al, 2015;Martin et al 2011).…”

“…A major operational goal of the EXPORTS science plan was to conduct core food web rate and export flux determinations in a Lagrangian frame of reference to minimize the influence of advective processes on their interpretation (Siegel et al 2016). During the North Pacific EXPORTS field campaign, the weak horizontal currents and spatial gradients in biogeochemical fields and low level of temporal variability that characterize the fall season at the Ocean Station P site made this goal achievable to a large degree (Siegel et al 2021;McNair et al 2023).…”

Microbial Respiration in Contrasting Ocean Provinces via High-Frequency Optical Assays

“…Linear regressions show that respiration positively correlated with small cell phytoplankton biomass (r 2 =0.50), weakly correlated to coinciding program measurements of particulate organic carbon (r 2 =0.21) and exhibited an inverse relationship with NPP. These findings reflect the low biomass, low productivity, and highly recycled nature of the Station Papa ecosystem in late summer (Meyer et al 2022;McNair et al 2023).…”