Changes in ecosystem engineers: the effects of kelp forest type on currents and benthic assemblages in Kachemak Bay, Alaska

In many temperate, rocky-reef coastal ecosystems, kelp forests modify alongshore currents, impact invertebrate settlement, and regulate understory algal abundances. Along the southern coasts of Alaska, in particular, two species of canopy-forming kelps with dissimilar morphologies, Nereocystis luetkeana and Eualaria fistulosa, co-occur but vary in their distributions and patterns of abundance, with one species often replacing the other at given sites in subsequent years. As a result, their differences in morphology may affect patterns of current flow through the forests, which could have strong impacts on invertebrate settlement and benthic diversity, particularly in Kachemak Bay, Alaska (59° 36′33ʺN, 151° 20′51ʺW), which experiences large tidal fluctuations resulting in strong alongshore currents. In this study, significant differences in current reductions were observed between the two kelp forest types within Kachemak Bay, but these reductions were small compared to the natural spatiotemporal variability of currents within the bay, and they were primarily limited to the upper few meters of the water column where the two species exhibit their greatest differences in morphology. Consequently, invertebrate settlement and abundance differed between the two kelp forest types as well as among study sites within the bay. In addition, there were differences observed in the composition and abundance of understory algae between the two forest types, which were presumably due to differences in shading abilities of the dominant kelps. Together, our results suggest that future changes in the distribution and abundances of these two kelp forest types may result in corresponding changes in hydrodynamic conditions within the forests, and thereby impact invertebrate and benthic algal assemblages.