Spatial patterns of invertebrate settlement in giant kelp forests

Settlement of kelp-associated organisms may vary as they are delivered to (and through) giant kelp (Macrocystis pyrifera) forests, with implications for local population dynamics and community structure. Previous work suggests that settlement of invertebrates with long pelagic durations would be reduced as they move from an offshore environment toward the interior of kelp forests due to dampened current flow and reduced larval delivery. We evaluated spatial variation in settlement across giant kelp forests in an extensive field study conducted over 2 yr. We collected and sorted >36000 settling organisms and had sufficient data to explore patterns in detail for 8 taxa. Orthogastropods (snails) were the most common invertebrates and exhibited a pattern of declining settlement from the outer (seaward) to inshore edge of kelp forests. Inverse patterns were observed for Crepidula spp. and carideans, and other abundant taxa (non-sessile polychaetes and pectinids) showed spatial structure that differed regionally and between years. Other taxa failed to exhibit significant spatial variation in settlement. In general, settlement was lower near the sea floor than in the upper water column, and similar across locations for most groups. For some taxa, spatial variation was more apparent when the magnitude of settlement was relatively low, which may suggest that kelp forests become ‘saturated’ with larvae during pulses of high settlement. Our results are in contrast to previous predictions, as we observed high settlement in the interior for several species with long pelagic durations. For taxa that settled evenly across kelp-forested reefs, differential distributions of adults may be attributed to post-settlement processes. The patterns we observed here warrant additional study to address potential mechanisms for differential settlement.