Seasonal and vertical dynamics in the trophic structure of a temperate zooplankton assemblage

Abstract

We determined the stable nitrogen isotope composition (δ15N values) and body size of taxonomic groups in a zooplankton community in the Cantabrian Sea (southern Bay of Biscay) to explore seasonal and depth (0–2000 m) variations in the size‐based trophic structure and their coupling to the production cycle. The positive linear relationship between δ15N values and log‐transformed body size reflects the dominance of new vs. regenerated production. The slope of the relationship (b) is high during productive periods and low when herbivory declines and the food web is more dependent on recycled production. This variation can be attributed to high δ15N values of the smallest plankton after repetitive cycles of microbial degradation. Downward transport of organic matter after the spring phytoplankton bloom was captured by a steady variation from low values of b at the surface to high values at the bathypelagic zone, where the imprint of the spring production pulse could be detected. Variation in b reveals that the mesopelagic and bathypelagic zooplankton communities are as dynamic as their epipelagic counterparts. This shows the efficiency of δ15N vs. body size relationships to capture fast, transient ecosystem processes without need for lengthy incubations or complex rate measurements.

Large deep-sea zooplankton biomass mirrors primary production in the global ocean

“…This enrichment was significant in the Trades biome provinces but not in provinces of the Coastal biome, thus suggesting the existence of regional differences in the links between zooplankton and surface productivity. Maximum enrichment at depth could be expected when the food web largely depends on repeatedly recycled nitrogen, as found in low productivity regimes 20 . In contrast, seasonally productive ecosystems display large variations in plankton isotopic signatures that are attributed to changes in the amount of organic matter channeled through microbial pathways, particularly after the peaks in primary production 20,21 .…”

“…Maximum enrichment at depth could be expected when the food web largely depends on repeatedly recycled nitrogen, as found in low productivity regimes 20 . In contrast, seasonally productive ecosystems display large variations in plankton isotopic signatures that are attributed to changes in the amount of organic matter channeled through microbial pathways, particularly after the peaks in primary production 20,21 . In addition to in situ feeders, bathypelagic zooplankton communities include seasonal ontogenetic migrants overwintering at depth and not feeding there 22 , and organisms using the deep sea as a refuge from predators 16 .…”

Using ecological partitions to assess zooplankton biogeography and seasonality

“…Areas with a high biomass of large phytoplankton cells (> 20 µm) are more likely to support a more direct trophic pathway to meso-zooplankton which are dominated by herbivores-omnivores (Dećima 2022). With larger phytoplankton dominating the spring bloom production there is a much more direct coupling and efficient transfer of carbon between phytoplankton and zooplankton (Cushing, 1989;Romero-Romero et al, 2019). In the tropics and equatorial regions the trophic pathways are often longer and dominated by carnivory which is more difficult to predict (Dećima 2022) and was not included in this version of the Darwin model.…”