NUMBtR 40 . 3 7 



presumably related to the intense storminess and heavy cloud 

 cover of late winter and spring. 



One of the difficulties of potentially mapping yearly layers 

 solely from changes in carbonate density (due to intertilament 

 crystallization that is directly photosynthesis driven) is that inter- 

 filament crystallization is directly responsive to the light regime. 

 Although light tends to parallel temperature, ecological varia- 

 tion in the availability of light is extensive; perhaps more criti- 

 cal, shading by sea ice and snow cover on the Labrador Coast 

 and farther north greatly delays the return of light in the spring. 

 Although the winter temperature in shore waters at 10-20 m is 

 little different from northern Labrador (0°C to -1.8°C) to the 

 Gulf of Maine (0°C-1°C), the winter solar radiation under the 

 sea ice in northern regions is nil and is greatly reduced until at 

 least May, whereas in the Gulf of Maine and the Aleutian Islands, 

 solar radiation is considerable throughout the winter. In southern 

 Labrador, in shallow water (8-12 m), the period of significant 

 interfilament crystallization is mid-May through mid-September. 



Conceptacle-Based Growth Analyses 



As shown in Figures 2B and I2B, reproduction, especially 

 in C. compactum, can be patchy in time and space, limiting the 

 use of simple conceptacle-based analyses in archiving. Also, 

 damage to and even loss of yearly perithallial tissue from urchin 

 grazing and other physical factors commonly disrupts simple 

 seasonal layering. However, SEM imaging and mapping, accom- 

 panied by SEM-matched microprobe analysis, not only solves 

 this problem but concurrently increases ecological and climatic 

 discrimination. Laser ablation-inductively coupled plasma-mass 

 spectrometry (LA-ICP-MS) has been successfully employed on 

 C. compactum and C. nereostratum to detect changes in Mg/ 

 Ca (for temperature) and Ba/Ca (for salinity; Gamboa et al., 

 2010; Chan et al., 2011; Hetzinger et al., 2011), although with 

 considerable unidentified variability (Figure 3; Gamboa et al., 

 2010; Williams et al., unpublished). Kamenos et al. (2008) have 

 employed microprobe and LA-ICP-MS methods for identifying 

 Mg/Ca variation with time, and herein extensive SEM imaging 

 has identified structural and reproductive complexities. It seems 

 likely that SEM mapping of laser tracks, with parallel micro- 

 probe sensing wherever anatomical complexities appear, could 

 resolve most of the variation inherent in plant structure. This 

 would require specimen manipulation (e.g., back-to-back anal- 

 ysis) since geochemical methods require polished surfaces that 

 prevent detailed SEM analyses. 



Finally, although Clathromorpbiun coiupactiiin is primarily 

 an outer coast rather than a bay species, ecological and oceano- 

 graphic variation in temperature and salinity along the complex 

 rocky shores frequented by C. coiupactiim is a widespread and 

 inevitable source of variation. It is essential for accurate environ- 

 mental archiving that multiple samples from known depths and 

 localities be secured for analysis. The value of C. compactum as 

 a millennial climate archive will also likely be enhanced by lesser 

 growth rates in the lower SST of northern Labrador and the high 

 Arctic. Clathromorphum compactum occurs throughout the 



Arctic (Figure 29), and it is likely that this species can provide a 

 pan-Arctic climate archive of considerable significance. Fiigh Arc- 

 tic reconnaissance, collection, and study will need to be under- 

 taken to fully understand the extent of clathrostrome formation. 



Changing Ocean pH 



In the last decade, ocean acidification, resulting from in- 

 creasing CO, diffusion from the atmosphere, has been recognized 

 as a potential problem limiting organism calcification. Aragonite 

 formation and high-magnesium calcite, where the magnesium 

 concentration exceeds about 12 mol %, are susceptible to reduc- 

 tions in rates of formation and increase in rates of dissolution 

 (Morse et al., 2006; Anderson et al., 2008) at lower pH. The sea- 

 sonal MgC03 range in C. compactum in the southern Labrador 

 Sea is about 6-12 mol %, and therefore, the effects of reduced 

 carbonate saturation on cor-stromes may be minimal. In tropical 

 waters, where magnesium content is much higher, this may be an 

 issue of concern (however, see Nash et al., 20 13). 



Primary C. compactum calcification is metabolic, and as we 

 have discussed, the organism maintains significant physiologi- 

 cal control over both placement and dissolution of carbonate. 

 Decreased pH might reduce secondary interfilament precipita- 

 tion (and therefore skeletal density of summer growth). Fiow- 

 ever, secondary calcification takes place internally, within highly 

 photosynthetic tissues; it is light that limits skeletal density, and 

 how much of a role ambient water pH plays is uncertain (Roleda 

 et al., 2012). Chan et al. (unpublished) did not find a significant 

 decrease in Clathromorphum calcification rates in recent decades 

 in the North Pacific. 



The primary function of the calcium carbonate in corallines 

 is grazing protection. There are competing, noncalcified red spe- 

 cies that form crusts and are more or less abundant, but usually 

 not dominant, in Labrador Sea clathrostromes. These species 

 and the dominant macroalgae of the community (Agarum clath- 

 ratum, Desmarestia viridis, and Ptilota serrata) probably possess 

 protective chemicals (Adey and Fiayek, 2011). Perhaps lower 

 pH levels could shift the balance between competing crusts, so 

 that the noncalcified crusts would become dominant, greatly 

 limiting development of clathrostrome structure and secondary 

 production. 



Grazers are abundant on clathrostrome bottoms. Lnnpets 

 and chitons have a mutualistic relationship with C. compactum, 

 feeding on epithallial cells and epiphytes; not only do they rarely 

 impact the crusts on which they feed, but they likely enhance 

 productivity by preventing abundant algal and invertebrate epi- 

 phytism. Epithallium calcification is quite "chalky," probably as 

 a result of minimal inner-wall carbonate and the more "leafy" 

 interfilament calcification. Reduced calcification might increase 

 grazing depth, eventually reaching the meristem and causing 

 reduced growth. Flowever, in the short term, increased grazing 

 depth could remove additional decaying epithallial cells and in- 

 crease photosynthesis and secondary calcification. Sea urchins 

 can significantly damage coralline surfaces and primary calcifica- 

 tion. Flowever, adult sea urchins do not target corallines, and 



