Connecting spatial structure in subtidal benthic communities with temporal variability in bottom temperature and dissolved oxygen along an upwelling coast

While spatial patterns in the structure of intertidal communities have been linked to changes in surface-ocean conditions indicative of coastal advection, nutrient supply, and productivity, less is known as to environmental features that, in addition to substrate type, are most influential in structuring subtidal benthic communities. Here we analyze data from subtidal benthic surveys conducted at six sites spanning ca. 180 km of shoreline along the upwelling coast of central Chile. Using continuous records of temperature and dissolved oxygen in bottom waters from each site, we tested whether among-site differences in community structure can be better explained by considering features of local environmental variability in addition to changes in substrate type. Starting from the NMDS ordination of benthic sites based on the presence and relative abundance of 29 species, we used PERMANOVA to test the explanatory power of substrate type and nine indices of environmental variability derived from in situ records. Three indices, namely the high-frequency variability and 95th percentile of bottom temperatures together with an index for the duration of hypoxia events, explained individually as much of the total variance as substrate type alone (ca. 30%). When combined into a single PERMANOVA analysis, these four factors explained 84% of the total variance in the NMDS-based ordination of benthic communities across the study region. A comparison of the benthic species ordination and environment-based site ordination suggests that soft-bottom species such as the razor clam Ensis macha, the snail Chorus giganteus, and the crab Cancer setosus, might be physiologically more tolerant to more rigorous conditions in terms of more intense warming and longer hypoxia events. Hard bottom species such as the gastropods Concholepas concholepas and Fissurella sp. and the barnacle Austromegabalanus psittacus thrive in environments where the temperature is lower on average but highly variable and where hypoxic conditions are intense but more intermittent. Our findings highlight the structuring role of the variability and the occurrence of extreme conditions, rather than the most frequent levels, in bottom-water properties such as temperature and dissolved oxygen. These two properties are closely intertwined in this and other coastal upwelling regions and are susceptible to change widely under future climate scenarios.