Video survey of deep benthic macroalgae and macroalgal detritus along a glacial Arctic fjord: Kongsfjorden (Spitsbergen)

In Kongsfjorden (Spitsbergen), we quantified the zonation of visually dominant macroalgal taxa and of detached macroalgae
from underwater videos taken in summer 2009 at six transects between 2 and 138 m water depth. For the first time, we
provide information on the occurrence of deep water red algae below the kelp forest and of detached macroalgae at water
depth > 30 m. The presence and depth distribution of visually dominant red algae were especially pronounced at the outer
fjord, decreased with proximity to the glacial front and they were absent at the innermost locations. Deepest crustose coralline
red algae and foliose red algae were observed at 72 and 68 m, respectively. Brown algae were distributed along the entire fjord
axis at 2–32 m. Green algae were only present at the middle to inner fjord and at areas influenced by physical disturbance
at water depths of 2–26 m. With proximity to the inner fjord the depth distribution of all macroalgae became shallower and
only extended to 18 m depth at the innermost location. Major recipients of detached macroalgae were sites at the shallower
inner fjord and at the middle fjord below the photic zone at depths to 138 m. They may either fuel deep water secondary
production, decompose or support carbon sequestration. Univariate and community analyses of macroalgal classes including
detached macroalgae across transects and over depths reveal a considerable difference in community structure between the
outermost sites, the central part and the inner fjord areas, reflecting the strong environmental gradients along glacial fjords.