PEANUTS

Description

In the Arctic, there is a very strong seasonality in the availability of light, due to both the formation of sea ice and also changing day length – from the perpetual darkness of winter to the mid-night sun – enabling accumulation of nutrients close to the surface in winter and so implying a strong seasonal cycle in nutrient fluxes to the surface layer. Furthermore our own previous turbulence measurements have shown mixing in the Arctic to be highly intermittant (and in consequence fluxes varying by up to 3 orders of magnitude) on timescales as short as an hour. These facts imply that in order to quantify the flux of nutrients from intermediate depths towards the sea surface, and subsequent impacts on primary productivity, measurements are required which resolve a range of timescales, from hourly to seasonally.

The aim of this project then, is to test the hypothesis that observed increased primary production is promoted by increased availability of nutrients resulting from increased nutrient fluxes. Data will be collected to test this hypothesis, including employing novel acoustic Doppler techniques developed at Bangor University, to make turbulent mixing rate and nutrient flux estimates using profilers and from moorings at contrasting locations around the Arctic shelf break and interior, on timescales from hourly to the full seasonal cycle. These will then be compared to baseline measurements made at these locations by ourselves and others during the recent 2007/8 International Polar Year. The new measurements will be integrated with coincident fluorescence timeseries measurement, within the framework of a biogeochemical ecosystem model, to quantify the impact of the observed changes in the nutrient environment, on net primary productivity, and to deduce intra-seasonal ecosystem responses to specific flux events. Thus specific project objectives are:

1. Quantify current turbulence mixing rates at the base of the Arctic surface polar mixed layer over a seasonal cycle around the Arctic shelf break, and so elucidate the physical processes that drive seasonal changes in stratification.
2. Quantify nitrate fluxes into the shelfbreak polar mixed layer over a seasonal cycle to resolve the variability of nitrate supply to the euphotic zone, and compare this to bottle samples of phosphates,and dissolve organic nitrates and carbon in order to infer nutrient uptake and cycling.
3. Quantify the response of the various shelfbreak marine ecosystems through subsurface chlorophyll-A concentrations over the full seasonal cycle to events driving nitrate fluxes into the polar mixed layer, and synthesise these with NPZ models of primary productivity.
4. Compare and contrast the active-mixing regimes of the Arctic shelf break with the central Arctic by resolving turbulent mixing, resulting nutrient fluxes and ecosystem response from the MOASIC drift during the spring bloom leg (June-July 2020).