The importance of diel vertical migrations of mesozooplankton for supporting a mesopelagic ecosystem: an Inverse Modeling Approach in the California Current

We used linear inverse models (LIM) and data from two California Current Ecosystem LTER cruises to model carbon fluxes between the epipelagic and mesopelagic layers in this system. A Linear Inverse Model (LIM) reconstructs possible flux networks based on field observations and metabolic constraints. Measurements constraints on the LIM are: ¹⁴C primary productivity, dilution-based protozoan grazing rates, gut pigment-based mesozooplankton grazing rates, ²³⁴Th:²³⁸U disequilibrium, sediment traps, and metabolic requirements of fish, zooplankton, and bacteria. A likelihood approach (Markov Chain Monte Carlo) was used to determine the most likely structure while also estimating the resulting rate uncertainties from a sample (n=1000) of randomly generated flux networks.

Diel vertical migrations by mesozooplankton transport a significant quantity of carbon to depth, which is both grazed by non-vertically migrating mesopelagic plankton and fish species and respired or excreted at depth as CO₂ or DOC. Although no direct flow between small mesozooplankton (SMZ) and large POM exists in the model, SMZ process ~70% of the carbon that eventually flows into the large POM pool, which indicates an important role for SMZ in the biological carbon pump of this ecosystem. Within the mesopelagic layer, bacterial remineralization is ~20% relative to net respiration, whereas remineralization rates in the epipelagic were upwards of 80%, indicative of reduced carbon cycling and a less dynamic food web at depth.