Abstract
AbstractEight Ice‐Tethered Profilers were deployed in the Arctic Ocean between 2011 and 2013 to measure vertical distributions of photosynthetically active radiation (PAR) and other bio‐optical properties in ice‐covered water columns, multiple times a day over periods of up to a year. With the radiometers used on these profilers, PAR could be measured to depths of only ∼20–40 m in the central Arctic in late summer under sea ice ∼1 m thick. At lower latitudes in the Beaufort Gyre, late summer PAR was measurable under ice to depths exceeding 125 m. The maximum depths of measurable PAR followed seasonal trends in insolation, with isolumes shoaling in the fall as solar elevation decreased and deepening in spring and early summer after insolation resumed and sea ice diminished. PAR intensities were often anomalously low above 20 m, likely due to a shading effect associated with local horizontal heterogeneity in light transmittance by the overlying sea ice. A model was developed to parameterize these complex vertical PAR distributions to improve estimates of the water column diffuse attenuation coefficient and other related parameters. Such a model is necessary to separate the effect of surface ice heterogeneity on under‐ice PAR profiles from that of the water column itself, so that euphotic zone depth in ice‐covered water columns can be computed using canonical metrics such as the 1% light level. Water column diffuse attenuation coefficients derived from such autonomously‐collected PAR profile data, using this model, agreed favorably with values determined manually in complementary studies.
Highlights
Eight Ice-Tethered Profilers were deployed in the Arctic Ocean between 2011 and 2013 to measure vertical distributions of photosynthetically active radiation (PAR) and other bio-optical properties in ice-covered water columns, multiple times a day over periods of up to a year
Under ice (Perovich 2003; Katlein et al 2014), and sea ice can alter the vertical distribution of light in the underlying water column, creating apparent subsurface maxima at locations where light attenuation by the ice layer above the sensor is stronger than it is in sea ice nearby (Frey et al 2011)
Even though we do not believe the Frey et al (2011) model is an appropriate parameterization to apply to these ITPenabled PAR profiles for the reasons detailed above, for completeness we examined how well Eq 2 fit these PAR profile data in a statistical sense using our original restrictions on profile upper depth and length
Summary
Instrumentation and deployments Long-term time series of under-ice irradiance profiles were collected in the Arctic Ocean using a variant of the ITP (Toole et al 2006; Krishfield et al 2008) In this variant, the standard conductivity-temperature-depth (CTD) sensor on the ITP’s profiling vehicle was augmented with a custom sensor package (a “biosuite”) consisting of a cosine radiometer for the photosynthetically active radiation (PAR) wavelengths between 400 nm and 700 nm (PAR-LOG-d, Satlantic) and a three-channel active sensor (FLBBCD, WETLabs) that measured the fluorescences of chlorophyll and dissolved organic matter and the optical backscattering at 700 nm. By sampling in the upper 200 m primarily and by reducing sampling effort over wintertime, an ITP deployed in late summer can reserve adequate power to resume high-resolution profiling at 6 h intervals the following spring, providing it physically survives ice ridging, floe degradation, damage of the
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