For kelp, finallyfaith said it beautifully with her opening thread:
the "laminaria" species of kelp is the species highest in Iodine content. this includes any kelp product labeled laminaria digitata, l. japonica, l. saccharina, and l. longissima. the laminaria species of kelp can be 5% or more of Iodine per dry weight.
"frontier" brand kelp is laminaria digitata, and therefore has a high Iodine content.