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Nova Scotia seaweed decline, July 2005
- Debbie MacKenzie
Disappearing belt of Irish moss (Chondrus
crispus) below a lush belt of rockweed, in an area exposed to moderate
wave action, June 2005.
Early summer 2005: Surveys in June and early July showed a continuation of
changing trends in seaweeds that have been previously described here (see
Shifting color in Irish Moss,
Seaweed photo galleries and article on
"pseudo-eutrophication." ) A substantial
decline is noted in the amount of Irish moss (Chondrus crispus). Granite
areas, that long supported Irish moss thick enough to be harvested commercially,
are increasingly appearing as bare rock. This is noted near the low tide level,
where Irish moss is normally exposed to air for a time. My best guess is that
the Irish moss has finally left these areas as a result of winter-kill
associated with exposure to cold air. These areas supported close cropped, very
bleached moss in recent summers, but now it
seems that the hold-fasts have been virtually eliminated. The resistance of
seaweeds to all environmental stressors, including cold air, will predictably
decline if their general fertility declines, so this pattern of loss of Irish
moss might not be unexpected. This pattern of seaweed loss is seen in relatively sheltered parts of the historical
habitat of the species. Where wave action is more intense, Irish moss is still
seen in the lower intertidal zone. When I first noticed this pattern of loss in
Chondrus, a couple of years ago, it was much less extensive and I assumed that
the seaweed had disappeared as a result of ice scour in a few places. However,
ice scour does not seem to provide an adequate explanation for these
observations, because the belt of rockweed above the moss remains unscathed (and
because these shorelines, affected by moderate wave action, have not been
observed to have winter ice driven against them). Also, areas of granite
shoreline are observed where ice scour has obviously removed the rockweed, but
the Chondrus belt remains intact. (see photos below) Chondrus is not uniquely vulnerable to ice,
but increasingly fragile under-fertilized plants may be more vulnerable to
damage from exposure to cold air.
Besides noting a further decline in Irish moss in 2005, I have seen a
continuing decline in kelp (at the sheltered extremes of its range) and,
similarly, in subtidal rockweeds. The novel view of
"white rocks" just below the surface in sheltered inlets is becoming ever
wider. Again, the loss of subtidal kelp and rockweed in these areas cannot be
explained by ice scour (the explanation that I also initially assumed for this
pattern of seaweed disappearance). Decaying older rockweeds, and kelp stumps,
are seen on now-visible white granite boulders in shallow water, that were historically
obscured by a canopy of dark seaweed. A contraction of the depth range of rockweeds has
been reported in other parts of the world in recent years (e.g. the Baltic Sea).
In these other areas the loss of subtidal rockweeds has been attributed to
decreased light availability resulting from a pollution-induced increase in
phytoplankton biomass. However, this does not seem to be a plausible explanation for the
loss of rockweeds observed here. Rockweeds have a fairly high tolerance to air
exposure, and photosynthesis in these plants actually occurs most efficiently in
this situation (partial drying). Therefore, rockweeds growing in the intertidal
zone might be expected to survive for the longest time under a scenario of
declining nutrient availability, and the recently observed range contraction of
these seaweeds might be predictable if ocean fertility should fall.
(Click on thumbnails below to view larger images.)
Disappearance of Irish moss at the low tide mark
June, 2005, East Dover, Nova Scotia
Opportunistic green annual seaweeds
...occupy space where Irish moss once grew thick enough for commercial harvesting. Mussel spat and barnacle spat also settle heavily in some of these areas.
Was winter wind the cause?
Irish moss inside a sheltered nook at the low tide mark survived whatever stressor removed it from more exposed rocks. If ice had removed the moss, the same effect should have been felt by the rockweed.
Ice scour removes rockweed
...small, new rockweed "recruits" are seen in the zone where the canopy of mature rockweeds was rubbed off by winter ice. Note that the Irish moss belt below the rockweed does not seem to have been damaged significantly by the ice.
Subtidal Irish moss beds
...are increasingly overgrown and replaced by a turf of coralline algae, and by many types of soft annual seaweeds as shown here. A membranous red seaweed here appears white in June, a signal of low fertility.
Tiny blue mussels
...successfully invade the Irish moss belt in areas with heavier wave action. This trend has been increasing in recent years. These mussels, however, do not seem to grow to large sizes - some patches are decimated by starfish and dogwhelks, and others just disappear by the next summer. An increasing coverage of shoreline habitat by blue mussels tends to be interpreted by scientists as an indicator of "eutrophication"...but multiple other signs are pointing in the opposite direction (for instance, the low pigmentation of the Irish moss...).
Classic arrangement of rockweed belt, Irish moss belt, and kelp belt
...with bare granite showing in the moss zone only, just above the low tide mark. This is now a common observation in this area.
Closer view of rock shown in previous slide:
A few Irish moss holdfasts are left. Below the bare granite zone, Irish moss is being replaced by coralline algae.
The same rock shown in the previous two photos, underwater view.
Aerial view of shallow granite seabed, June 2005.
Yellowish area seen through the water is bare granite, that was historically obscured by a heavy canopy of kelp and rockweeds. This photo was taken after a period of heavy rain, which explains the brownish discoloration of the water.
Decaying subtidal rockweeds
...seen in the area where the pale granite is increasingly becoming exposed.
Unidentified stringy brown seaweed,
with small, pale kelp specimens, in the area where the granite bottom is becoming increasingly visible.
Kelp:
remnants where a heavy canopy of kelp once grew. No sea urchins have been seen in this area for many years. Periwinkle grazing of the kelp occurs here, but this cannot explain the massive disappearance of seaweed from this area.
Similar to previous photo,
the disappearance of kelp in this area obviously cannot be attributed to ice scour. June 2005.
