To probe the conditions of incorporation or rejection of biomass during ice growth requires observations on both sides of the ice-water interface repeatedly through time, without disruption of the system, and with fine vertical resolution. We are acquiring such observations by freezing a rectangular, optical-grade polycarbonate (transparent) tunnel vertically into sea ice near our core sites. Within the tunnel, we lower a commercial oceanographic fluorometer (the WetLabs ECO) and other instruments, oriented so as to observe chlorophyll, as a proxy for biomass, just outside the tunnel. We profile over a range of depths both within the water column and within the ice skeletal layer. 

We prevent and remedy the problem of algal growth on the surface of the tunnel by a combination of growth inhibition and automated mechanical cleaning. 

Fluorometric observation of chlorophyll inside sea ice presents a different problem. The light that mediates fluorometric observation, both excitation and emission, is much more strongly scattered and absorbed in sea ice than in seawater. The relation between fluorometric data and fluorophore concentration is thus more complicated in the ice than in the water, and must be interpreted with the aid of ancillary data. We calibrate the fluorometric data using a combination of reflection spectrometry data, optical scattering theory, and replicate analytical measurements of Chl a from melted ice core segments and CDOM from extracted brine.

Scientific results based on in situ fluorometry will surpass those based solely on established methods in two important ways. First, time series of autotrophic biomass, finely resolved in the vertical and acquired at one undisturbed location, will support the first quantitative diagnoses of couplings between physical and biological processes in the critical lower 5 cm of ice. Second, in situ fluorometry will provide a basis for future automated systems to observe, for the first time, events that export algae and (presumably) other organic carbon into the water, where they figure in large-scale ecological processes. This development strongly supports the larger, and longer-term, scientific objectives of projects such as SEARCH.