Glynn Study and Other Algal Changes

Marine researchers have predicted that, in the absence of sea-level rise (indeed our surveys show no change in the height of intertidal benchmarks relative to sea level since 1930, see Sagarin et al. 1999), the upper limits of some intertidal species may shift toward lower tidal heights with warming climate (Lubchenco et al. 1993). Other historical studies done at HMS reveal large community-level changes that are consistent with this prediction. Beginning in spring 1994 we relocated seven small (10 cm2) plots established by Peter Glynn (1965) within the zone characterized by high densities of the barnacle Balanus glandula and the turf alga Endocladia muricata. In surveys, we found that the vertical range of Endocladia muricata has shifted lower by 0.18 meters at the upper boundary and 0.40 meters at the lower boundary. These tidal height changes were reflected in widespread changes in species' abundances and species diversity at the locations of Glynn's plots. At present, we cannot identify specific mechanisms that may have caused this tidal height shift. Work by Denny and Paine (1998) showed that an 18.6-year oscillation in the moon's orbital inclination may be responsible for large changes in time of emersion in the intertidal at HMS. Although such shifts in emersion time might be expected to drive shifts in intertidal zona-tion (with shifts to lower tidal heights seen in times of greater emersion due to greater desiccation), analysis of Denny and Paine's data shows that both lunar inclination and emersion times were similar between Glynn's study and the more recent studies. Whether the algal height shift is related to the long-term changes in climate at HMS remains to be seen, although it is certainly consistent with predictions of species responses to climate warming.

Table 3.1. Alternative Hypotheses, as Discussed in Barry et al. (1995) and Sagarin et al. (1999)

Alternate hypotheses Potential Influences

Supporting Evidence Opposing Evidence

Conclusion

Anthropogenic effects

Seasonal changes

Substratum changes

Trampling, collecting Pollution

• Hewatt's original sampling dates unknown, re-surveys done in spring and summer

Little evidence

• Shifts in sand cover may affect species' distributions (R. Paine, pers. comm. 1994)

• Hewatt (1934) reports many small moveable boulders in

• Seismic shifts affect tidal height distributions

• Erosion of substratum changes primary habitat transect—not apparent

• Loss of movable in recent study boulder habitats • Decline in abundance of 6 of 10 species which use under-rock habitat

• Establishment of Hopkins Marine life refuge, 1931

• Localized effects of canneries

• Both Glynn (1965) and Hewatt (1934) reported little seasonal variation in the HMS intertidal

• Surveying shows no shift in tidal height of Hewatt's transect relative to surrounding shore or other parts of the intertidal

• Granitic rock at HMS resists erosion. Substratum drawn in Hewatt's maps clearly matches substratum today

• Likely to be minor in affecting changes along the transect

• Seasonal changes unlikely to be responsible for large changes seen in many species

• Seismic effects unlikely

• Loss of boulders possibly responsible for some species declines continues

Table 3. I. Continued

Alternate hypotheses Potential Influences

Supporting Evidence Opposing Evidence

Conclusion

Return of otters

Return of oystercatchers

Return of harbor seals

• Predation on several intertidal invertebrate species

• Decline in 4 of 10 potential otter prey species including mussels, urchins, sea-stars, and crabs

• Intertidally foraging bird returned to HMS between Hewatt's study and the present

• Seal population has increased since 1970s at HMS—may affect intertidal invertebrates through crushing, shading, and feces

• Otters rarely seen foraging in area of transect

• One prey species declined in abundance

• No direct evidence— studies of effects are ambiguous (Boal 1980, Horng and Hayhurst, unpublished data)

• One prey species increased in abundance, 4 species showed no change in abundance

• Seals rarely haul out on rocks along the transect

• Re-surveyed transect plots do not include mussel beds where otter predation likely to be greatest

• Effect not likely to be great in area of transect re-surveyed. All prey species are cosmopolitan, so pattern in northern and southern species is not affected by otters

• Effect is minimal on observed population changes

• Effect is minimal on observed population changes

• El Niño manifests in California as increased northward transport and increased sea temperatures

• Southern species (especially fish) often found north of their range during El Niño years

Celestial mechanics

• An 18.6 y oscillation in the moon's orbital inclination changes the amount of time inter-tidal animals are exposed to air, on average (Denny and Paine 1998)

• Strong correlation between this oscillation and HMS water temperature suggests that changes we observed could have occurred over 18.6 y rather than 60 y

• Long-lived larvae (more likely to be affected by changes in transport processes) did not show greater changes (increases or decreases) than species with short or no plank-tonic phase

• El Niño patterns similar in years preceding and during Hewatt's study as years preceding and during current study (Fig. 3.8)

• Temperature has increased gradually throughout the 60 y period since Hewatt's study

• Intertidal animals were exposed to air longer on average during Hewatt's study.

• Hypothesis cannot be rejected—important, but as yet unknown feedbacks between long-term warming and El Niño events may benefit southern species

• Effects of El Niños on intertidal invertebrate populations are poorly under stood

• Points out the inability of our study to establish direct mechanistic links between species' changes and sea temperature changes

• Does not invalidate the finding of range-related species changes

Algal changes were also observed in our re-surveys of Hewatt's transect. Although Hewatt did not quantify algal populations directly, several references and photos in his thesis indicate that the area surrounding his transect was dominated by the rockweed Pel-vetia compressa. Hewatt reports 100% cover of this alga in a tidal zone where we discovered it to be largely absent or rare, with maximal cover of 30% in any plot within this zone (Sagarin et al. 1999). Photographic comparison of rocks and areas of the intertidal that Hewatt photographed also reveals a dramatic decline in Pelvetia (Figs. 3.12 and 3.13). Reports from 1948 indicate that this decline

Figure 3.12. View of Hewatt's transect area. (A) ca. 1932. (B) 1998 (at lower tide than A). Dark areas in A are almost exclusively the alga Pelve-tia compressa. Same areas in B are nearly devoid of this alga and characterized by turf algae such as Endocladia muricata.

Figure 3.12. View of Hewatt's transect area. (A) ca. 1932. (B) 1998 (at lower tide than A). Dark areas in A are almost exclusively the alga Pelve-tia compressa. Same areas in B are nearly devoid of this alga and characterized by turf algae such as Endocladia muricata.

Figure 3.1 3. Close-up of intertidal rock near Hewatt's transect. (A) ca. 1932. (B) 1998. Note that heavy layer of Pelvetia in lower half of A is completely absent in B, which shows only scattered turf algae, mostly

Endocladia muricata.

Figure 3.1 3. Close-up of intertidal rock near Hewatt's transect. (A) ca. 1932. (B) 1998. Note that heavy layer of Pelvetia in lower half of A is completely absent in B, which shows only scattered turf algae, mostly

Endocladia muricata.

was well under way at HMS at that time (Stephenson and Stephenson 1972), although we cannot be sure what populations have done between 1948 and 1993 when our studies began.This change would not be predicted based on our division of species into northern and southern categories, as Pelvetia is basically southern in range, and all of our southern invertebrates increased in abundance. We currently have no strong hypothesis to explain this change. It is interesting that Lepidochitona hartwegii, a southern chiton that uses Pelvetia almost exclusively for food and shelter (Andrus and Legard 1975, DeBevoise 1975), increased in abundance during the same period that this alga declined, indicating that habitat loss did not outweigh benefits to this species, which may include the warmer sea temperatures.

Our findings suggest that changes in algal composition and zonation may be associated with dramatic changes in faunal populations. Although some of the observed algal changes match simple predictions of species' responses to climate change, others do not. In light of this, the asymmetry between the detailed faunal observations by Hewatt (1934) and the paucity of algal observations is frustrating and highlights the need in historical studies to have data on as many components of the system as possible. Even the small amount of algal information provided by Hewatt proved extremely useful.

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