title for learn-about section of A SNAIL'S ODYSSEY
  Life in the intertidal zone
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Water-chemistry effects

  Topics dealing with life in the intertidal zone include water-chemistry effects considered here, and TEMPERATURE EFFECTS, HEAT-SHOCK PROTEINS, GAS EXCHANGE, WAVE EFFECTS, SALINITY EFFECTS, and TRANSLOCATION STUDIES, considered in other sections.
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Research study 1

There is growing concern over rising global atmospheric CO2 levels and increasing ocean acidity, and the deleterious effect that these will have on marine species such as mussels and other molluscs, calcareous seaweeds, corals, and so on, that have dissolvable calcareous skeletons.  Laboratory experiments have  provided useful data on the acute effects of pH on certain physiological processes in selected marine organisms, but their major drawback is that exposure to atmospherically related pH change in seawater will not be acute; rather, it is an effect that will be drawn out over time-spans measured in decades.  This may allow adaptation to occur.  A different and more dynamic field approach is used by researchers at the University of Chicago, Illinois who record pHs in ocean water around Tatoosh Island, Washington over an 8yr period, then attempt to relate pH changes with aspects of growth and population dynamics of several common intertidal species including mussels Mytilus californianus and M. trossulus.  Results show that pH is not constant, but fluctuates not only daily in response to photosynthesis and respiration, but also seasonally and yearly, with a significant overall decline being recorded over the period of study (see graph).  Through application of sophisticated association-type statistics the authors determine that abundances and sizes of several shore species, such as mussels and goose barnacles Pollicipes polymerus, decrease significantly with declining pH levels.  In contrast, other species including acorn barnacles and several types of seaweeds (including, unexpectedly, the calcified red alga Corallina vancouveriensis), increase in size and graph showing pH values over an 8yr period at Tatoosh Island, Washingtonabundance with declining pH.  A possible explanation for the positive responses of these other species is that they may be released from competitive disadvantage as the dominant space-inhabiting mussels and goose barnacles. The authors note that a greater understanding of pH effects at both physiological and ecological levels will be required to enable reliable predictions of long-term effects of rising atmospheric CO2 levels.  Wooton et al. 2008 Proc Nat Acad Sci 105 (48): 18848.

NOTE  although any open-ocean location would have been suitable for the study, this one has enjoyed a particularly rich history of intertidal study, primarily by researchers from the University of Washington, Seattle

NOTE  these include multispecies Markov-chain models combined with principle-components analyses.  Note that these associations are correlative, not cause-and-effect.  There may be many other causes for the changes observed

pH data (N = about 25,000) obtained at Tatoosh Island
suggest that the rate of decline in world oceans may
be much faster than predicted by current models

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Research study 2

graphs showing relationship of delta-13-Carbon levels with time in mussels Mytilus californianusLike other molluscs, mussels build their shells from chemical elements taken from the surrounding seawater.  The shells, therefore, may provide a record of environmental change in water chemistry from past times through to the present.  This interesting idea is examined by researchers from the University of Chicago, Illinois using modern shells of mussels Mytilus californianus from Tatoosh Island, Washington, shells from "archival" collections dated about 1975, and 11 ancient shells from middens courtesy of the Makah People of northern Washington.  From analyses of isotopic carbon (delta13C)  and oxygen (delta18O) in the shells, the researchers are able to assess historical changes in shell chemistry likely related to decreasing pH levels associated with increasing oceanic carbon-dioxide content.  Results show, indeed, a significant decline in delta13C concentrations in the shells (see graph) that is unexplained by changes in coastal food webs, or in upwelling patterns or other aspects of ocean circulation.  The drop in delta13C in mussels parallels a similarly steep decline in delta13C in algae in the Gulf of Alaska.  In the same mussels, values for delta18O are elevated over time, but not significantly so (data not shown).  The authors note that the decline in delta13C during 1999-2009 is actually faster by 2.5-fold  than can be explained by equilibration with rising carbon-dioxide concentration in the atmosphere.  They discuss possible reasons for this, not considered here.  The method provides reliable proxy-chemical records spanning longer time intervals than available from standard instrument-derived records, and is sure to be adopted by other researchers.   Pfister et al. 2011 PLoS ONE 6 (10): e25766. 

NOTE  these 2009 mussels are approximately 10yr old, as estimated from counts of growth lines in prepared sections of the shells.  Other shells of 1000-1340ybp are examined from Makah Nation middens located in northern Washington.  By analysing portions of recent shells from when an individual was young to when it was 10y old, data are provided for an 11y time span from 1999-2009 for each individual

NOTE  use of multiple values from each shell to encompass an 11yr time span is not as statistically “robust” as increasing the number of shells analysed by 11-fold so that each shell contributes just a single value.  The reason is lack of independence of the data. This would have  been possible for  Tatoosh, where mussels are abundant, but probably not for the “archival” or midden collections where availability of shells is limited

The top graph shows delta13C values from about 650-2009ybp, with the data for
1999-2009 compressed. The bottom graph shows these last data on an expanded
scale . A vertical shaded line with a single dot for each of the 3 data sets indicates
mean and variance. The small inset graph on the top Left shows regression lines
calculated for each data set. Only the Tatoosh regression has a significant slope (-ve)

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Research study 3

graph comparing shell thicknesses of sea mussels Mytilus californianus from ancient middens, archival collections, and modernA follow-up study done by several of the same investigators in RS2 above uses the same time-based approach to assess effects of ocean acidification in sea mussels Mytilus californianus, in this case on shell calcification. Actual shells used are the same as in the previous study, including ones from an archival collection done in 1975, 11 archeological-midden shells from Tatoosh Island provided by the Cultural and Research Center of the Makah tribe (dated by radiocarbon method as 1000-1340ybp), and fresh shells (2009-2011) from 2 sites in northwestern Washington, Tatoosh Island and Sand Point. The shells are sliced and thicknesses of the inner prismatic (calcite) layer measured and compared. The authors hypothesise that through reduction of bicarbonate ions in oceans from increasing levels of atmospheric CO2 and concomitant decreasing pH levels, shells have become thinner. Results show, indeed, that thicknesses have decreased significantly from ancient-midden to modern times (see sample graph). On average, midden shells of millennial age and aarchival shells from the 1970s are about 30% thicker than modern shells. The authors discuss several possible explanations for the thinning, including temperature, food availability, wave exposure, and others, but anthropogenic increases in atmospheric CO2 concentrations with associated reductions in pH appear the most likely. Apart from causes, a trend of shell-thinning over time could have serious consequences for this ecologically important species. Pfister et al. 2016 Proc R Soc B 283: 20160392. Photographs courtesy Cathy Pfister and the Pfister Lab.

NOTE this approach bypasses the shortcomings of most other contemporary investigations of effects of ocean acidification that use short-term (days to weeks) acute exposures of a single generation of organisms to seawaters of varying pHs. In contrast, the present approach employs a thousand-year timeline encompassing perhaps 100 generations of M. californianus. Recent multigenerational studies have been done on marine organisms, but thus far and for obvious reasons seem to be restricted to fast-maturing phytoplankton species, and a European polychaete cultured under variable pH levels for 39 generations

NOTE a bivalve shell has 3 layers: an outer proteinaceous periostracum, an middle prismatic (calcite) layer, and an inner nacreous laye

photograph of shell slice of sea mussel Mytilus californianus

photograph of Tatoosh Island, Washington
Section of a sea-mussel Mytilus californianus cut through the umbo region illustrating the thick prismatic (chalky, calcite) layer. Drill holes indicate where samples have been removed for chemical analyses in Research Study 2 above. The noticeable dark bandings are annual growth lines and can be counted for age estimates (ranging from 6-10yr for all shells used in the study) Tatoosh Island, Washington has been the site of many studies by researchers from the University of Washington over the past 6 decades, including seminal studies by Robert Paine and his students on ochre-star predation on sea mussels Mytilus californainus. In fact, Paine himself and at least 3 of his former doctoral students (Pfister, Wooton, and Suchanek) are among the 7 co-authors of this Research Study 3. Tatoosh Island is owned by the Makah tribe and has long been used by them as a fishing and shellfish-gathering spot
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