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Biological correlates of sea urchin recruitment in kelp forest and urchin barren habitats

B. Weitzman*, B. Konar

*Corresponding author:

ABSTRACT: Shifts between the alternate stable states of sea urchin barren grounds and kelp forests correspond to sea urchin density. In the Aleutian Archipelago, green sea urchins (Strongylocentrotus polyacanthus) are the dominant herbivores that graze kelp forests. Sea urchin recruitment is an important driver that influences sea urchin density, particularly in the absence of top-down control from a keystone predator such as the sea otter (Enhydra lutris). To understand how the biological community may influence patterns of sea urchin recruitment, we compared sea urchin recruit (size ≤ 20 mm) densities with biomass of other benthic organisms in both barren ground and kelp forest habitats at nine islands across the Aleutian Archipelago. Patterns of biological community structure between the two habitats did not explain patterns of sea urchin recruits; however, the same ten specific taxa were found to correlate with sea urchin recruits in each habitat. Taxa that showed strong positive correlations included Codium, Constantinea, Schizymenia, and hydrozoans, while strong negative correlations were observed with Pachyarthron and Pugettia. Weak-positive correlations were observed with Alcyonidium and Ascidaceans in both habitats, while weak-variable relationships were detected with Polysiphonia and Corallina between habitats. The observed species-specific relationships may be due to small sea urchin displacement by larger conspecifics, larval responses to settlement cues, post-settlement survival via biogenic refugia, or potentially by predation. These potential species-specific interactions were apparent, regardless of habitat, and it can be inferred that they would be preserved in the presence or absence of keystone predation.