Stable isotope evidence of long-term changes in the North Sea food web structure

A temporal change in stable isotope (δ15N) distributions in North Sea harbour porpoises Phocoena phocoena (L.) was documented by analysing stable isotope abundances in bone collagen from 88 skeletons of harbour porpoises stranded in the period between 1848 and 2002 along the southern North Sea coast. Porpoises collected after ~1960 had significantly lower δ15N than porpoises collected earlier. This change in δ15N implies that fundamental changes in food web structure in, or nutrient availability to, the North Sea have taken place over the last ~150 yr and that most of the change occurred over a relatively short period in the middle of the 20th century. Harbour porpoises are generalist feeders and their diet largely reflects the food available to them. Thus, one plausible explanation for the observed change in δ15N could be that harbour porpoises in the North Sea have, since the mid-20th century, been feeding at a lower trophic level than during the preceding century, i.e. animals from lower trophic levels may now be more dominant than they were prior to the middle of the 20th century. There is no a priori reason to suspect that a change in isotope distributions at the base of the food web has occurred during this period and we have not been able to find material that would allow us to test the assumption that there has been no temporal development of δ15N at the lowest levels of the food web. Thus, we cannot eliminate the possibility that the change in δ15N in harbour porpoise skeletons reported here may be a reflection of a change in the isotope signature of nitrogen entering the food web. Regardless of its underlying cause, the recorded change in isotopic signature in harbour porpoises is noteworthy as it represents the first fisheries-independent documentation of a long-term temporal change in the structure or function of the pelagic ecosystem in the North Sea.