Effects of formalin preservation on carbon and nitrogen stable isotopes of seaweeds: A foundation for looking back in time

Abstract Stable isotope analysis of preserved natural‐history specimens from long‐term storage can provide useful data about energy flow and trophic structure in ecosystems of the past. Nitrogen isotope values from primary producers are used as critical food web baselines, while both nitrogen and carbon isotopes expose the presence of sewage contamination and nutrient runoff. Seaweed specimens being prepared for long‐term storage are often fixed in formalin prior to being dried or transferred into a storage liquid, but the effects of this fixative on nitrogen and carbon stable isotope values of marine primary producers have not been investigated. We evaluated the effects of formalin on macroalgal and cyanobacterial stable isotope values. For this purpose, we analyzed δ 15 N and δ 13 C values of 41 pairs of formalin‐preserved vs. dried seaweed specimens stored for up to 5 yr. While neither isotope was consistently altered by formalin when all seaweeds were pooled, δ 15 N values were overall less variable than δ 13 C values. However, formalin preservation lowered δ 13 C values in Ochrophyta by 0.2% on average. Neither isotope was affected by duration of formalin preservation. Because formalin‐exposed samples show similar δ 15 N and δ 13 C values to dried samples, formalin‐exposed specimens can be used to accurately assess nutrient dynamics and food webs of historic marine ecosystems provided that one has sufficient sample sizes of seaweeds.