Abstract
The Arctic archipelago of Svalbard is a hotspot of global warming and
many fjords experience a continuous increase in seawater temperature and
glacial melt while sea-ice cover decline. In 1996/98 and 2012-14
macroalgal biomass and species diversity were quantified at study
siteHansneset, Kongsfjorden (W-Spitsbergen) in order to identify
potential changes over time. In 2021, we repeated the earlier studies by
stratified random sampling (1x1 m2, n=3) along a
sublittoral depth transect (0, 2.5, 5, 10 and 15m) and investigated the
lower depth limits of dominant brown algae 220m. The fresh weight
maximum was 11.5 kg m-2 at 2.5m and kelp blades stored
277g carbon m-2 and 18g nitrogen m-2at this depth. Although biomass did not significantly change since
2012/13, the ‘Digitate Kelps’ community (Laminaria
digitata /Hedophyllum nigripes ) changed to an Alaria
esculenta dominated kelp forest in 2021 and a balanced age structure of
kelps (juveniles plus many older kelp individuals) was only apparent at
2.5m. In addition, the abundances and lower depth limits of all dominant
brown algae declined noticeably over the last 25 years while the deep
red algae flora remained stable. As we revealed that biomass allocation
to blades, stipes and holdfasts as well as the ability to store blade
carbon and nitrogen was kelp species-specific, a pronounced shift in the
functionality of the kelp forest had occurred over time. We propose that
the observed changes in the macroalgal community are mainly driven by
alterations in underwater light climate as in situ data confirmed
increasing turbidity and decreasing irradiance since 2012 and 2017,
respectively. As a consequence, the overall retreat of the kelp forest
to lower depth levels seems to be a result of strong glacial melt and
will possibly continue in future with unforeseen consequences for Arctic
coasts and their socio-ecological fjord systems.