Description
Giant kelp (Macrocystis pyrifera) can greatly alter current velocity through and around kelp forests, and as a result, the delivery and settlement of kelp-associated organisms may differ depending on location. Our objective was to determine whether patterns of invertebrate settlement differed spatially within giant kelp forests. Eighteen sites, distributed across six transects perpendicular to shore, were established in kelp forests off Point Loma, San Diego, California. At each site, invertebrate settlement was quantified biweekly at ~ 7 m depth in the water column from July -- October in 2009 and 2010 and at 2 m above the seafloor over the same months in 2010. A total 21 taxa were identified, 8 of which were sufficient in abundance for further analysis. There was an overall difference in cross-shelf patterns of settlement, largely driven by settlement of the most abundant taxon, orthogastropods (snails), which settled in higher abundance along the outer edges and the interior than along the inner edges of kelp forests. However, different patterns occurred among taxa, and most invertebrate groups did not differ spatially in cross-shelf settlement. Temporally, inter-annual variability in settlement was detected in 4 of the 8 taxa analyzed, but cross-shelf settlement did not differ between years. Settlers of all taxa were less abundant near the sea floor than at 7 m depth in the water column, but patterns of cross-shelf settlement were similar at both depths. Finally, gradients in cross-shelf settlement were more apparent in relatively low vs. high pulses of settlement of taxa that varied in abundance by an order of magnitude. Further research on the underlying mechanisms for these patterns should provide for a better understanding of the interaction between biogenic structure and settlement of invertebrates.
