Fig 1.
The inset globe shows the geographic region of interest outlined in blue. The detailed map shows the numbered CTD stations sampled in August 2019, overlayed on bathymetric contours. We use the 200m bathymetric contour line around 92°W to separate Lancaster Sound and Barrow Strait. The ship track is shown in red. The complete list of geographic coordinates sampled is available in the Polar Data Catalogue (doi: 10.5884/12715). The map was produced in Matlab (R2020a) using the publicly available m_map package [41].
Table 1.
Commonly referred to photophysiological terms and abbreviations.
Fig 2.
Summary of oceanographic conditions.
Hydrographic conditions, phytoplankton biomass and biological oxygen saturation measured in surface waters (∼ 7 m depth) along the cruise track. Shaded area on Aug. 13–14 marks measurements made in Barrow Strait.
Fig 3.
Summary of photophysiological conditions.
Semi-continuous FRRf measurements of photophysiology (black dots) are superimposed over the in-situ surface PAR (blue line). A loess smoothing function was applied to photo-physiological measurements (red line). Shading denotes the Barrow Strait portion of the transect.
Fig 4.
Recent light history effects on photophysiology measured under low light.
FRRf-derived photophysiological parameters measured under low light are plotted against in-situ surface PAR at the time of sample acquisition. Panels (a) and (b) show Fv/Fm and σPSII measurements made after 5 min of low light exposure. Panels (c) and (d) show NPQ and residual quenching measured after low light treatment. Lines of best fit are shown for Barrow Strait (solid line) and Lancaster Sound (dashed line). Full regression analyses results are reported in S2 Table. Error bars show standard error, but are often concealed by size of data symbols.
Table 2.
Correlation of underway photophysiological variables and surface PAR.
Fig 5.
Recent light history effects on photophysiology measured under low light.
The PSII photochemical efficiency measured under 150 μmol quanta m-2 s-1 in relation to natural surface irradiance at the time of sample acquisition. Lines of best fit are shown for Barrow Strait (solid line) and Lancaster Sound (dashed line). Full regression analyses results are reported in S2 Table. Error bars show standard error, but are often concealed by size of data symbols.
Fig 6.
Photosynthesis-irradiance curves derived by ETRa and ETRk.
Consolidated mean ETRa (red) and ETRk (black) estimates at each light step of the 25 reprocessed P-I curves. Error bars represent the standard error of all individual measurements from all curves at each light step. Curves were produced using the photosynthesis-irradiance function described by Webb et al. (1974).
Fig 7.
ETRk plotted against ETRa derived from photosynthesis-irradiance curves and colored by Fv/Fm.
Each point represents the mean ETR value at a given light intensity within a Photosynthesis-Irradiance curve, and error bars are the standard error. The dashed line indicates a 1:1 relationship.
Fig 8.
Parameters contributing to the ETRk:ETRa ratio measured in Lancaster Sound (LS) versus Barrow Strait (BS).
Horizontal lines within each boxplot represent the median. The upper and lower edge of each box demarks upper and lower quartiles, respectively, while whiskers extend over the entire data range, excluding outliers. Outliers, determined as data points falling over 1.5x the interquartile distance away from box edges, appear as unfilled circles. p values in each subplot are results from 2-group Kruskal-Wallis tests. All data shown here was collected during the 25 reprocessed P-I curves.
Fig 9.
Spatial distribution of riverine nutrient inputs and photochemical efficiency (Fv/Fm) along the ship track.
River contributions of nitrate and nitrite are indicated by the size of grey bubbles. The largest inputs of nitrate and nitrite in the region are concentrated in the strait between Cornwallis and Devon Islands, coincident with observations of raised Fv/Fm values. Larger circles are used to denote night-time measurements of Fv/Fm, whereas smaller triangles denote day-time measurements.