Investigating the importance of different sources of organic matter as food for the benthos on three Arctic shelves

Abstract

Benthic communities on Arctic shelves are a key component of food webs, nutrient
cycling, and carbon burial. In ice-covered waters, they are fuelled by a complex assemblage
of organic matter from different sources, especially primary production from overlying
pelagic and sympagic (ice-associated) algae, and from terrestrial inputs in areas of high river
run-off. As the Arctic system is undergoing rapid changes, the relative contributions of these
sources of organic matter to benthic food webs are also changing. It is therefore crucial to
understand what fuels the Arctic benthos, to understand how resilient it can be to these
shifting food sources. Three Arctic shelves were studied: the Barents Sea, the Northeast
Greenland Shelf, and the Beaufort Sea Shelf, representing the often-described inflow,
outflow, and interior shelves, respectively. Highly branched isoprenoids – lipid biomarkers
produced with distinct structures in sea ice and the water column – were used to calculate
the ratio of sympagic-to-pelagic organic matter assimilated by the benthos in the Barents Sea
and Northeast Greenland Shelf. In the Barents Sea, the proportion of organic matter from
sympagic sources ranged from 0.4% to 95.8%. In the summer, a strong spatial pattern
(r2=0.754) associated with sea ice duration was found, with the benthic community feeding
on more sympagic organic matter in areas with longer ice cover. In the winter, the benthos
of the Barents Sea fed mostly on pelagic organic matter. On the Northeast Greenland Shelf,
where sea ice cover is much longer (~10 months) sympagic carbon constituted a larger
portion of diets (90%) throughout the study area, and sea ice duration had very little effect.
In the fjords, assimilation of sympagic carbon was higher (93.7%) than on the open shelf
(83.3%), due to the different type of sea ice (land-fast vs drift ice) and nutrient availability.
On the Beaufort Sea Shelf, stable isotopes were used to investigate seasonal and spatial
trophic structure change and terrestrial influence. Benthic trophic structure remained
seasonally quite stable but changed spatially with the strength of pelagic-benthic coupling.
Stronger pelagic-benthic coupling was found in shallower regions and an area of upwelling
and higher primary productivity. Terrestrial influence did not show many spatial or seasonal
variations in the benthos. The results here indicate that sympagic organic matter can be a
temporally and spatially important food source, and that the delivery of different sources of
organic matter to the Arctic benthos is not controlled by the same environmental factors on
different Arctic shelves. There are still gaps in our understanding of Arctic benthic food
webs, with different methods often showing inconsistent results. Future research should
focus on combining these methods within the same studies, in order to better understand the
roles of different sources of organic matter to the Arctic benthos.

Date of Award	9 May 2024
Original language	English
Awarding Institution	University of the Highlands and Islands
Supervisor	Bhavani Narayanaswamy (Supervisor) & Kim Last (Supervisor)