EcoLight - Ecosystem functions controlled by sea ice and light in a changing Arctic
Sea ice plays a fundamental role in the Arctic ecosystem through complex physical and bio-geochemical interactions and feedbacks. The sea ice matrix offers a protected habitat for microbial life, particularly for algae, which together with phytoplankton form the base of the Arctic marine food web, sustaining the sea ice associated macrofauna and part of the pelagic zooplankton. The growth of sea ice algae and phytoplankton depends in large parts on light availability, which is strongly dependent on the sea ice and under ice water properties. On the other hand, the ice underside provides a high variable and heterogeneous habitat for different ice-associated macrofauna, i.e. the zooplankton communities whose vertical migration is often triggered by food availability and periodic changes in light availability.
As the Arctic is changing, it is no longer dominated by thick multi-year ice (MYI), but it is a regime dominated by thinner, more dynamic, first year ice (FYI). At the same time, the length of the melt season has increased, leading to earlier retreat and later ice formation, changes in snow accumulation and freshwater input to the Arctic Ocean. These changes have important implications for the in-ice and under-ice biota, influencing light availability, ocean properties, and the timing of sea ice algae and phytoplankton blooms. In other words, changes in sea ice can alter phenology of carbon supply to the ecosystem. We are still in the process of elucidating these complexities, but our fundamental understanding of ecosystem function, sea ice, and upper ocean processes in the Arctic Ocean has been overwhelmingly derived from a MYI setting, rather than the FYI dominated Arctic of recent years. As a result, our current state of knowledge and the validity of many of the parameterisations presently embedded in models become more questionable. For example, most GCMs use a formulation of sea ice light transmission for MYI. However, such treatment can lead to underestimation of the under-ice light conditions. Furthermore, recent measures have shown that the transition from a MYI to FYI summer ice cover corresponds to a 50% increase in light absorption in sea ice and an increase of 200% in light transmittance into the upper ocean.
Today the availability of PAR (photosynthetically-available radiation) is very different to what it was when the Arctic was dominated by MYI, consequently we have to expect ecosystem function and associated timings will adapt as well. If we are to understand and predict ecosystem function in this ‘new Arctic’, we must understand and correctly parameterise the light climate under FYI. To do this we need a holistic approach that seamlessly brings biology, optics, sea ice and ocean physics, together with remote sensing and cutting-edge modelling. Our inter-disciplinary programme combines modelling, remote sensing and observational expertise to observationally constrain and improve the parameterisations of processes of light transmission in this ‘new’ Arctic.
To predict the consequences of changing sea ice for Arctic ecosystem function, it is critical that we understand the links between sea ice, upper ocean physics and ecosystem dynamics across diverse areas of the Arctic marine environment. For this reason we propose to perform a focused set of multidisciplinary autonomous measurements in the Beaufort/Chukchi Sea, a region that has witnessed the greatest loss of MYI. These types of autonomous measurements are only now achievable because the UK, Germany and other nations have invested heavily in robotic systems to perform the autonomous and long-term measurements. Our observations will provide coincident data on sea ice and snow properties, light, primary production and zooplankton dynamics over an entire annual cycle: from freeze up (autumn), through the polar night (winter), to emergence of the sun in late winter, to the snow melt and melt pond formation (spring), and finally ice break-up and melt (summer). Once robust relationships between sea ice conditions and ecosystem parameters (e.g. thickness, melt pond fraction, snow depth, primary production, ice algae and phytoplankton standing stocks) are established, we can then use the same satellite-derived quantities to map pan-Arctic light availability, and the ecosystem parameters related to it. These relationships will be used to improve and update parameterisations in numerical models to simulate the under-ice light regime and the sympagic ecosystem on pan-Arctic scale.
Our overall goal is to demonstrate how the Arctic ecosystem may change in the future, because of changes in timing and duration of primary production events and grazing habits of zooplankton. We will evaluate how these ecosystem functions mirror changing snow and sea ice regimes in the Arctic Ocean, as they continue their transition from thick MYI to thinner and more dynamic FYI.
The role of OASys in EcoLight:
In this framework the contribution of OASys will be to better understand and parametrize the physical components of the system as they determine the under ice light and enery regime. This will contribute to the improved understanding and simulating of the biological components and the biogeochemocal functions of the ocean. Based on new observations as well as existing data Oasys will derive new and improve existing parameterisations for the flux of energy and light through the Arctic sea ice, test and apply them on the pan-Arctic scale. This will be done in close cooperation with the project partner at the Alfred Wegener Institute for Polar and Marine Science. The parametrisations will finally be implemented in the Finite Element Sea ice and Ocean Model (FESOM), coupled to the SIMBA and RecoM submodels. The work consists of four main tasks: 1. Development of improved parametrisations of the radiative transfer between Arctic atmosphere, snow, seaice and oceans, based on new observations. 2. Tests of the new/improved parametrisations and validation with independent observations. 3. Implementation of the improved parameterisations into FESOM. 4. Quantification of the effects of the new parametrisations on the pan-Arctic light and energy regime under sea ice, and the state of the system.
EcoLight is a collaborative project with the Alfred Wegener Institute for Polar and Marine Research, the British Antarctic Survey and CPOM at the University College London. It is funded jointly by the British NERC and the German BMBF agencies.
EcoLight is part of the 'The Changing Arctic Ocean' programme of NERC.