(Home) Nova Scotia seaweed decline, July 2005 - Debbie MacKenzie
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.)
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