A regional-scale approach for modeling primary production and biogenic silica export in the Southern Ocean.

Persistent uncertainties in the representations of net primary production (NPP) and silicate in the Southern Ocean have been noted in recent assessments ofthe ocean biogeochemical components of Earth system models (ESMs). Consequently, more mechanistic studies at the regional scale are required. To reduce these uncertainties, we applied a one-dimensional (1D) marine ecosystem model to different bioregions in the Southern Ocean: the Polar Frontal Zone in the Pacific sector, the seasonal sea ice zone in the northwestern Ross Sea, and the inner shelf of Terra Nova Bay. To make the existing ecosystem model applicable to the Southern Ocean, we modified the phytoplankton physiology (stoichiometry depending on species) and the silicate cycle (dissolution rate of biogenic silica (BSi) depending on latitude) in the model. We quantified and compared seasonal variations in several limitation factors of NPP, namely, iron, irradiance, silicate and temperature, in the three regions. The simulation results showed that dissolved iron plays the most significant role in determining the magnitude of NPP and the phytoplankton community structure during summer. Additionally, the modified model successfully reproduced the vertical flux of BSi and particulate organic carbon (POC). The POC export efficiency was high in the inner shelf zone, which had high levels of iron concentration, NPP, and Phaeocystis biomass. In contrast, BSi export occurred most efficiently in the Polar Frontal Zone, where diatoms are dominant, the BSi dissolution rate is low, and NPP is extremely low. Our results from the integrated mechanistic framework at the regional scale demonstrate which specific processes should be urgently included in ESMs for better representation of the biogeochemical dynamics in the Southern Ocean.

Ecological and economic effects of COVID-19 in marine fisheries from the Northwestern Mediterranean Sea.

The SARS-CoV-2 coronavirus pandemic starting at the end of 2019 impacted many human activities. We analysed the abrupt reduction in fishing pressure of the mixed small-scale and industrial fisheries in the Catalan Sea, Spanish Mediterranean, and resulting ecological and economic impacts during the first half of 2020. We used detailed fisheries data on fishing effort, landings, revenues, landings per unit of effort (LPUE) and revenues per unit of effort from January to June 2020, and complemented it with the outcomes of a marine ecosystem model. We analysed data from 2017 to 2019 and compared these to 2020 to characterise changes in the fishing activity from before (January-February) to during (March-May) the lockdown. Fishing effort during the lockdown dropped by 34%, landings were down by 49% and revenues declined by 39% in comparison with the same period in 2017-2019. LPUEs did not show significant changes during the lockdown, with the exception of shrimp species, especially the deep-water rose shrimp, which significantly increased in LPUE during March-May. These increases may reflect positive effects of reduced fishing on fast-growing species. Positive effects mostly disappeared in June 2020 with the relaxation of the lockdown. In agreement, the ecological simulations projected slight short-term increases of biomass for fast-growing, small-sized organisms during 2020, which quickly vanished when fishing resumed, and which had no detectable ecosystem effects. Three additional alternative ecological simulations illustrated that to substantially recover commercial species and ensure ecosystem sustainability in the study area, a sustained and notable reduction of fishing activity would be needed.