2 * SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



Wherever rocky bottom occurs in the subhttoral Subarctic 

 (typically to 40 m, depending upon exposure and salinity), it is ir- 

 regularly coated by an extensive but patchily discontinuous crust 

 of biogenic calcium. This carbonate crust (typically 0.5-10 cm 

 thick, but to over 0.5 m in thickness; Lebednik, 1976) is present 

 on all but the most mobile or friable rock surfaces (Frantz et al., 

 2005; Adey and Hayek, 20 11). Because it is a high-magnesium 

 carbonate, laid down mostly by coralline red algae, this crust 

 can be called a cor-strome. It also occurs in the Arctic, where 

 it is built largely by Subarctic coralline species (specimens and 

 images in the U.S. National Herbarium); however, its continuity 

 is unknown. The depth zone immediately below the cor-strome, 

 where broken fragments accumulate and some species of Litho- 

 thamnion continue to grow in the mobile state as rhodoliths, has 

 received some attention (Freiwald and Heinrich, 1994), but the 

 cor-strome itself remains largely unstudied. 



In both the Atlantic and the Pacific, these carbonate- 

 encrusted bottoms have been widely referred to as "coralline, 

 sea urchin barrens" (Mathieson et al., 1991; Estes and Duggins, 

 1995; Steneck et al., 2002); however, this description is certainly 

 a misnomer, as they may well be among the richer shallow ma- 

 rine bottoms of the marine Holarctic (Himmelman, 1991; Che- 

 nelot et al., 2011). The Chenelot et al. (2011) study focused on 

 abundant invertebrates resident in Clathromorphum nereostra- 

 tum in the Aleutian Islands, but it is likely that a similar relation- 

 ship exists for cor-stromes in the northwestern North Atlantic. 

 Drying fragments of cor-strome on research vessel laboratory 

 benches generally release large quantities of invertebrates, espe- 

 cially grazing chitons and filter-feeding brittle stars. A Labrador 

 fjord Lithothamnion glaciale cor-strome produced 740 macro- 

 invertebrates from 637 cm' of crust (Adey, unpublished field 

 notes). Thus, there is little question of the coral reef-like richness 

 of these bottoms; what is lacking is geographically widespread 

 quantitative research on invertebrate diversity and biomass. 



Considerable debate exists concerning the primary eco- 

 logical drivers of ecological structure on Subarctic-Boreal rocky 

 bottoms (e.g., Steneck et al., 2002; Springer and Estes, 2003; 

 Estes et al., 2005; Trites et al., 2007); is it climate, top preda- 

 tors, overfishing, or some complex of the three that drives the 

 major fluctuations of the associated kelp beds observed over the 

 last half century? These issues carry special weight because of 

 the collapse of numerous major cold-water fisheries over the last 

 several decades, the currently expanding Arctic fisheries, and the 

 ongoing debate over climate change and its causes. Much of this 

 debate has focused on the Bering Sea and Aleutian Islands and 

 the Gulf of Maine and Nova Scotia. However, the carbonate 

 bottom, with the rich diversity it harbors, has been considered 

 at best a backdrop in the debate over large seaweeds, grazing 

 invertebrates, and sea mammals and at worst a "barren" created 

 by imbalances among the major, large players. As we will dis- 

 cuss at depth, the age of the cor-strome frequently reaches one to 

 three centuries, and the maximum age of specimens now under 

 study exceeds 800 years in the Aleutian Islands and 600 years 

 in the Labrador Sea. The biological constant in these regions of 



scientific dispute, a basal calcified coralline crust with centuries 

 of growth, is the least studied element. 



Several species each of the genera Clathromorphum, Litho- 

 thamnion, and Leptophytiim are the dominant producers of the 

 Subarctic-Arctic cor-strome. In the fully Subarctic northwestern 

 North Atlantic, Clathromorphum compactum is an abundant 

 species encrusting rocky bottoms in the mid-photic zone (10-20 

 m; Adey, 1966b), being largely replaced by Clathromorphum 

 circumscriptujn in shallower waters and Lithothamnion glaciale 

 in deeper waters. Lithothamnion glaciale also tends to replace 

 C. compactum as the dominant cor-strome producer in inner bays 

 and fjords, where siltier and fresher waters usually dominate. 

 In many localities, depressions within the rocky cor-strome bot- 

 toms are filled with mobile nodules (rhodoliths) largely derived 

 from fragments fractured from the Lithothamnion cor-strome by 

 waves and bioturbation. Extending below the rocky cor-strome 

 and continuing to significant depths (20-60 m), beds of rhodo- 

 liths, usually as part of a shelly gravel substrate, are widespread. 

 However, the full extent of these beds is poorly known. Two 

 species of Lithothamnion (L. glaciale and L. tophiforme) are the 

 dominant producers of rhodoliths, the former species being more 

 Subarctic and the latter more Arctic (Adey et al., 2005). 



As previous studies have demonstrated (Halfar et al., 2007, 

 2008, 201 la; Frantz et al., 2005; Hetzinger et al., 201 1; Williams 

 et al., 2011), these centuries-old carbonate formations, through 

 analysis of their temperature- and light-dependent growth layers 

 and the trace chemical structure of their carbonate cell walls, 

 can provide significant climate archives. The coralline provides 

 a surface for algal epiphytes, and both the corallines and the 

 epiphytes are grazed by many invertebrates. They are also very 

 important producers of secondary biomass, mostly by providing 

 a porous, reef-like habitat to innumerable invertebrates that uti- 

 lize mostly planktonic sources of primary production (Chenelot 

 et al., 2011). Additionally, they provide significant ecosystem 

 support for Subarctic-Arctic fisheries that are now expanding in 

 importance. These key Arctic and Subarctic ecosystems have a 

 detailed history written into their basal biogenic carbonate; to 

 read this history, we need only to extract the Rosetta stone of 

 anatomical and geochemical information. 



The Distribution of Clathromorphum Species 



Although a few species of the genus Clathromorphum 

 have been described for the colder Southern Hemisphere 

 (http://www.algaebase.org), their relationship to the abun- 

 dant Northern Hemisphere species is currently uncertain. In 

 the Northern Hemisphere, the four epilithic species are largely 

 restricted to the Subarctic and Arctic or its fringes, including 

 the Aleutian Islands (Lebednik, 1976). Two additional species, 

 C. parcum and C. reclinatum, are obligate epiphytes with limited 

 archival value, and we will not discuss them further; phyloge- 

 netic studies currently underway (Adey et al., unpublished) show 

 that these two species belong to a different clade. They are Bo- 

 real and fringe Subarctic in distribution only in the North Pacific. 



