Blog from RV Lance: Old Clams Provide New Insight About Arctic Change
Most benthic organisms are relatively stationary as adults so they have no possibility to move if conditions change – they must adapt or die. Organisms employ different strategies to cope with the extreme environments of the Arctic. So studying how benthic organisms adapt and respond to environmental variability can tell us about how the physical drivers, regional oceanography and the marine ecosystem are linked, and may allow us some insight into how the system may respond to the climate changes that models predict may occur in this century.
One major problem, however, is the time element. Our three week-long ICE cruise may seem like a long time to those of us living and working on Lance, but most of the data collected during this field campaign will provide only an instantaneous glimpse of events occurring at this moment - a brief snapshot of the full length film of the changing Arctic. There is a great need, therefore, for us to extend and broaden our observational timescales.
As a solution to this problem, my colleagues and I have been working for several years at unlocking the secrets contained in bivalve shells. Clams provide great potential as climatic recorders due to the sequential deposition of their shell material through time as the animal grows. The patterns of shell material and its chemical composition provide uninterrupted records of growth histories, metabolism and environmental conditions experienced during the deposition of that shell material over the life of the organism. With lifespans of several decades, even more than a century for some species, these “trees of the sea” are providing a window into the past environment of the Arctic that will help predict potential ecosystem consequences of Arctic climate change.
During this ICE cruise we have collected samples of clams, both using the dive team (Haakon Hop, Peter Leopold, Ireen Vieweg, Sanna Markkula) and by trawling the bottom. We have collected samples of Clinocardium ciliatum (Hairy cockle (English), hjerteskjell (norsk)) and Mya truncata (common name (English), sandskjell (norsk)) from Raudfjorden, northwest Svalbard, and Serripes groenlandicus (Greenland Smooth cockle (English); (grondlandsskjell norsk)), from Moffen. These samples will be added to the archive of shells we have been building since 1996 from the Svalbard and Barents Sea area. Moffen could be particularly interesting because it is the home to Svalbard’s largest colony of walrus, and clams make up the vast majority of their diet. So better understanding how clams react to climatic variation over time can reveal possible consequences to the ungainly tusked beast.
By identifying and measuring each growth ring of a bivalve shell, we can not only determine an exact age for the sample, but also estimate growth during each year over the lifespan of the individual. If the animal was collected alive at a known time, we can then assign a calendar year to each growth year by counting backward from the collection date, and compare to environmental conditions occurring at that time. In this way, we can determine the suite of environmental conditions associated with enhanced or limited growth within a given year.
We have been working with clams up to 50 years in age, yielding chronologies dating from the early 1960’s to present. But we also are working to extend these chronologies further into the past, and have some preliminary indications that there are species in this region with lifespans over 100 years. Our analyses of shells to date have allowed us to identify strong linkages between clam growth patterns and oscillating atmospheric conditions. These large scale drivers influence local weather and oceanographic conditions, leading to variations in the timing, magnitude and fate of primary production. Proximity to Atlantic-derived water, which is warmer and more productive than Arctic derived water, has a strong influence in growth rates, with populations in or near Atlantic water growing faster than those living in Arctic conditions.
We can also examine the shell geochemistry to provide information about specific environmental conditions when shell material was laid down. Ratios of trance elements and stable isotopes in the shells has allowed us to examine seasonality in growth, identify the timing and magnitude of primary production, and traced the influence of river discharges to growth (in the southern Barents Sea).
In addition to shell material, we can examine bivalve tissues to provide additional information about ecological processes and anthropogenic impacts. Stable isotopic signatures of carbon and nitrogen of the tissues can reveal food web dynamics such as differences in food sources and trophic position. Additionally, analyses of anthropogenic contaminants can provide information on transport pathways of contaminants to and within the Arctic and biomagnification in the food web.
Thus, the analysis of bivalves and their shells can expand our toolbox in understanding the current structure of marine ecosystems and how they may react to change. But it is a large puzzle that we’re still putting together. But each piece we managed to fit allows us to better understand the marine ecosystem structure and how it will react to variability and change.