Tracking solar radiation doses in moving aquatic organisms and particles: a novel irradiance Lagrangian module of the Connectivity Modeling System

While water and solar radiations are the basis of life, short radiation wavelengths such as ultraviolet radiations (UVR, λ < 400 nm) are some of the most ubiquitous stressors of living organisms damaging their DNA. Planktonic organisms are also exposed to this stressor, despite that UVR penetration in the water decreases exponentially with depth, following the coefficient of absorption Kd. For example, high doses of UVB (280 < λ < 320 nm) can be lethal for the early life stages of marine larvae and UVA (320 < λ < 400 nm) exposure reduces the photosynthesis by up to 70% in phytoplankton. However, given the exponential decrease of UVR, these wavelengths are assumed to be only lethal to marine organisms living at or near the surface. In addition, all marine organisms from plankton to fish larvae and whales move in the water column based on their density, shape, and/or swimming migration behavior, thus creating variable exposure rates of UVR over time and space. While studies have attempted to estimate UVR exposure on marine organisms, they were focused on specific locations and depths. Due to climate change, UVR is predicted to increase along with temperatures; measuring the dose of UVR received by organisms or particles such as microplastics in the ocean becomes critical to inform on several processes often constrained to be studied in laboratory experiments. Notwithstanding, there is so far no method to compute the incident dose of radiation (e.g. visible light or UVR) on moving particles in the ocean. Here, we developed a module within the Connectivity Modeling System (CMS) Lagrangian framework to estimate the incident dose of light individual virtual particles would receive as they move and are transported in the ocean. Tracking solar radiation dose exposure is based on the intensity of the surface irradiance from Global Earth Models, the coefficient of absorption of light in the water (Kd), and the depth of the particle at each time step. The coefficient of absorption can be defined explicitly in time and space, to take into account the seasonal and geographical variations in irradiance incidence on Earth. Here, we use the net shortwave radiation from the earth system model NAVGEM to demonstrate the efficiency of this novel irradiance module by simulating particles in different regions of the Atlantic, Indian and Pacific Oceans, where the coefficients of absorption of PAR and UVA (λ=380 nm) were available from bio-ARGO floats. Particles are released from these regions throughout the year to test for the effect of contrasting irradiance and UVA intensities on the particles. In one set of simulations, particles remain at constant depths from 0 to 30 m, while on another, they follow a diel vertical migration behavior between 30 m during the day and the surface during night time. To account for vertical micro-turbulence, we add a stochastic, vertical diffusion to the larvae vertical distribution. Our novel CMS application offers an operational framework to estimate the incidence of UVR on organisms or particles. This method can be used at a range of time and spatial scales to study any process related to UVR incidence on moving organisms, such as to determine the risk of changing UVR on photodamage and photodegradation, or to estimate the available PAR for phytoplankton or symbiotic organisms.