Contribution of harbor seal derived nutrients to Saccharina beds assessed using δ15N

Although seaweed beds along the northern coast of Japan have generally deteriorated, algal beds have remained where nutrients are continuously supplied from inflows and/or animal aggregations. Here, we show that NH₄-N assimilated in kelp (Saccharina angustata) was derived from harbor seals (Phoca vitulina stejnegeri) on the reef of Cape Erimo, Hokkaido which is the largest breeding ground of this species in Japan, using isotopical and molecular techniques. Coastal waters of Erimo area generally have high nutrient concentrations in winter and low nutrient concentrations from spring to autumn. On the other hand, in the reef area of Cape Erimo where the harbor seal haul-out occurs, high concentrations of DIN and DIP, high contribution of NH₄-N to DIN continued after winter, and environmental DNA analysis revealed that seal-derived DNA was simultaneously high. The mean δ¹⁵N value of S. angustata in the reef area of Cape Erimo was 8.8 ± 0.9 ‰ (n = 14) which was enriched compared to the mean δ¹⁵N value of 6.9 ± 0.6 ‰ (n = 75) sampled from other coastal areas of Erimo. The mean TN content of S. angustata in the reef area of Cape Erimo was also 2.0 ± 0.1 % (n = 14), which was higher than the mean TN content of 1.6 ± 0.2 % (n = 75) sampled from other coastal areas of Erimo. The thallus size of S. angustata collected from the reef area of Cape Erimo was larger than samples collected from the other sampling points where the thallus size decreased from June to August. These results indicated that the harbor seal was a significant source of NH₄-N absorbed by kelp, acting as a vector for supplying nutrients to nutrient poor coastal ecosystem probably through excrement and detrital material from prey items. Thus we conclude that harbor seals, as the top predator of the food web, play an important role as a bottom-up control in the ecosystem.