Supply and survival: glacial melt imposes limitations at the kelp microscopic life stage

High-latitude kelp beds may be at risk from increasing sedimentation rates due to glacial melt. Nereocystis luetkeana (hereafter Nereocystis) sporophytes (canopy-forming) occur infrequently downstream of glacial melt where thick layers of sediment accumulate, while Saccharina latissima (hereafter Saccharina) sporophytes (understorey) are common in these areas. We examined whether glacial melt conditions affect adult summer fecundity, how sedimentation affects early gametophyte survival and growth and whether competitive interactions among gametophytes are altered by the presence of sediment. Summer fecundity did not differ in relation to glacial discharge for either species. In single species treatments, there was an effect of sediment on gametophyte survival for Saccharina but not Nereocystis. In mixed species treatments when no sediment was added, Nereocystis had higher survival than Saccharina when Nereocystis was settled first. When sediment was added, Saccharina had higher survival than Nereocystis when Saccharina was settled first. There was no difference in gametophyte growth between the two species for any treatment. Settlement timing may explain the ability of Nereocystis to coexist with Saccharina and to occur in some locations downstream of glacial discharge. Climate change could result in the loss of Nereocystis by favoring Saccharina gametophytes in competitive interactions.