Caveats on the use of paleolimnology to infer Pacific salmon returns
Limnol. Oceanogr., 52(5), 2007, 2053-2061 | DOI: 10.4319/lo.2007.52.5.2053
ABSTRACT: The ability of paleolimnology to reconstruct historical sockeye salmon (Oncorhynchus nerka) abundance was assessed at Fraser Lake, an important nursery lake in the interior of British Columbia (BC), Canada. Multiple sediment proxies of lake production, as well as nitrogen and carbon stable isotopes, all portray relatively complacent stratigraphies, despite well-monitored changes in salmon returns over the most recent 60 years. The separation of autochthonous from total sediment organic matter did not clarify the identification of the nitrogen isotopic fingerprint of marine-derived nutrients (MDN). Slight shifts in diatom assemblages are better interpreted as responses to either early human activities in the catchment or post-Little Ice Age climate warming. The low proportion of MDN in the lakes annual budget (3% N and 7% P) is the probable reason their influence is not expressed in lake sediments. Thus, paleolimnology is incapable of providing unequivocal inferences concerning historical salmon abundances in Fraser Lake, in contrast to lakes from southern Alaska, where the same techniques have yielded unambiguous results. Sediment MDN proxies, together with catchment characteristics and escapement data, were collated for an additional nine sockeye nursery lakes spanning southern Alaska to southern BC. Primary production and biogeochemical cycling in Alaskan nursery lakes appears largely driven by MDN from sockeye returns, whereas BC lakes are more strongly influenced by allochthonous organic matter and lake-water residence times. Alaskan lakes possess the limnological features that are prerequisite for meaningful salmon reconstructions using paleolimnology, while lakes from southern BC do not. This does not suggest that MDN are unimportant in southern BC lakes but rather that they are not readily disentangled from other factors that shape the paleolimnological record.