On the sensitivity of plankton ecosystem models to the formulation of zooplankton grazing.

Model representations of plankton structure and dynamics have consequences for a broad spectrum of ocean processes. Here we focus on the representation of zooplankton and their grazing dynamics in such models. It remains unclear whether phytoplankton community composition, growth rates, and spatial patterns in plankton ecosystem models are especially sensitive to the specific means of representing zooplankton grazing. We conduct a series of numerical experiments that explicitly address this question. We focus our study on the form of the functional response to changes in prey density, including the formulation of a grazing refuge. We use a contemporary biogeochemical model based on continuum size-structured organization, including phytoplankton diversity, coupled to a physical model of the California Current System. This region is of particular interest because it exhibits strong spatial gradients. We find that small changes in grazing refuge formulation across a range of plausible functional forms drive fundamental differences in spatial patterns of plankton concentrations, species richness, pathways of grazing fluxes, and underlying seasonal cycles. An explicit grazing refuge, with refuge prey concentration dependent on grazers' body size, using allometric scaling, is likely to provide more coherent plankton ecosystem dynamics compared to classic formulations or size-independent threshold refugia. We recommend that future plankton ecosystem models pay particular attention to the grazing formulation and implement a threshold refuge incorporating size-dependence, and we call for a new suite of experimental grazing studies.

Fate of floating plastic debris released along the coasts in a global ocean model.

Marine plastic pollution is a global issue, from the shores to the open ocean. Understanding the pathway and fate of plastic debris is fundamental to manage and reduce plastic pollution. Here, the fate of floating plastic pollution discharged along the coasts is studied by comparing two sources, one based on river discharges and the other on mismanaged waste from coastal populations, using a Lagrangian numerical analysis in a global ocean circulation model. About 1/3 of the particles end up in the open ocean and 2/3 on beaches. The input scenario largely influences the accumulation of particles toward the main subtropical convergence zones, with the South Pacific and North Atlantic being mostly fed by the coastal population inputs. The input scenario influences the number of beached particles that end up in several coastal areas. Beaching occurs mainly locally, although a significant number of particles travel long distances, allowing for global connectivity.

California coastal upwelling onset variability: cross-shore and bottom-up propagation in the planktonic ecosystem.

The variability of the California Current System (CCS) is primarily driven by variability in regional wind forcing. In particular, the timing of the spring transition, i.e., the onset of upwelling-favorable winds, varies considerably in the CCS with changes in the North Pacific Gyre Oscillation. Using a coupled physical-biogeochemical model, this study examines the sensitivity of the ecosystem functioning in the CCS to a lead or lag in the spring transition. An early spring transition results in an increased vertical nutrient flux at the coast, with the largest ecosystem consequences, both in relative amplitude and persistence, hundreds of kilometers offshore and at the highest trophic level of the modeled food web. A budget analysis reveals that the propagation of the perturbation offshore and up the food web is driven by remineralization and grazing/predation involving both large and small plankton species.