Quantifying phytoplankton productivity and photoinhibition in the Ross Sea Polynya with large eddy simulation of Langmuir circulation

Abstract

Southern Ocean phytoplankton assemblages acclimated to low‐light environments that result from deep mixing are often sensitive to ultraviolet and high photosynthetically available radiation. In such assemblages, exposures to inhibitory irradiance near the surface result in loss of photosynthetic capacity that is not rapidly recovered and can depress photosynthesis after transport below depths penetrated by inhibitory irradiance. We used a coupled biophysical modeling approach to quantify the reduction in primary productivity due to photoinhibition based upon experiments and observations made during the spring bloom in Ross Sea Polynya (RSP). Large eddy simulation (LES) was used to generate depth trajectories representative of observed Langmuir circulation that were passed through an underwater light field to yield time series of spectral irradiance representative of what phytoplankton would have experienced in situ. These were used to drive an assemblage‐specific photosynthesis‐irradiance model with inhibition determined from a biological weighting function and repair rate estimated from shipboard experiments on the local assemblage. We estimate the daily depth‐integrated productivity was 230 mmol C m−2. This estimate includes a 6–7% reduction in daily depth‐integrated productivity over potential productivity (i.e., effects of photoinhibition excluded). When trajectory depths were fixed (no vertical transport), the reduction in productivity was nearly double. Relative to LES estimates, there was slightly less depth‐integrated photoinhibition with random walk trajectories and nearly twice as much with circular rotations. This suggests it is important to account for turbulence when simulating the effects of vertical mixing on photoinhibition due to the kinetics of photodamage and repair.

The interactive effects of stratospheric ozone depletion, UV radiation, and climate change on aquatic ecosystems

“…150 bloom, was about 30% lower with mixing, than without (decreasing from 11% inhibition with no mixing to 7% in its presence). 197 Mixing lessened inhibition because phytoplankton were circulated between the inhibitory near-surface zone and the recovery-promoting irradiance environment of the mid-depth zone. Accurate simulation of Langmuir circulation, however, required a computationally intensive hydrodynamic model, which limits a more general assessment of mixing effects in oceans.…”

Environmental effects of ozone depletion, UV radiation and interactions with climate change: UNEP Environmental Effects Assessment Panel, update 2017

“…This is illustrated in the time-dependent inhibition by UV radiation of photosynthesis in Antarctic phytoplankton. 342 In this context, Smyth et al 343 modeled mixing and inhibition of photosynthesis by UV radiation in the Ross Sea Polynya, Antarctica. The focus of the study was vertical transport by Langmuir Circulation, a form of deep mixing created by the interaction of wind and waves in most open ocean (and lake) surface waters.…”

Calving Event Led to Changes in Phytoplankton Bloom Phenology in the Mertz Polynya, Antarctica

“…Photoinhibition and photodamage might also have a negative impact on phytoplankton (Alderkamp et al, 2011(Alderkamp et al, , 2012van de Poll et al, 2006;Villafañe et al, 2008) although it usually only lowers primary production by up to 10% in certain areas in the Southern Ocean (Western Antarctic Peninsula, Moreau et al, 2015) Both grazing and photoinhibition were not accounted for in the present study and could explain, in part, the temporal and spatial distribution of the bloom mean and maximum and why they are not explained by environmental parameters. Previous work highlighted the impact of such factors on phytoplankton growth in polynyas (Deibel & Daly, 2007;Smyth et al, 2017;Yang et al, 2019) where grazing leads to a biomass decrease, shortening the bloom period, and photoinhibition reduces the photosynthetic capacity. Both photoinhibition and grazing should be considered in future studies to fully resolve the drivers of bloom phenology.…”