“INTERNAL TIDE POOLS” AND THEIR INFLUENCE ON THE DISTRIBUTION OF HYPOXIA IN THE KELP FOREST

In coastal upwelling systems, severe hypoxic pulses occur often on the inner shelf. The primary driver of coastal hypoxic events in Monterey Bay is internal wave transport of upwelled cold oxygen depleted water from within Monterey canyon. Most research on internal waves has been conducted over smooth bottoms, and interaction with complex topographies such as kelp forests/rocky reef systems has been largely unstudied. Previously, we have shown that spatial dissolved oxygen variability at small (10m) scales is strongly influenced by phasing with local internal wave activity, and hypothesized that the observed variability is a consequence of flow-reef interactions. Here we present evidence for an “internal tide pool” phenomenon, where the relaxation of internal waves leaves pools of dense hypoxic water retained in depressions in the subtidal reef. We show that following an internal wave event, the recovery dissolved oxygen and temperature within these depressions to pre-event levels, can be delayed by 6+ hours behind the water column as a whole, during which, oxygen and temperature inside the pool may differ from surrounding areas by 5mg/L and 3°C respectively. This delay is strongest in areas of greatest concavity (“bowl-shaped”). Further, we show that this delay, is a direct result of “pooling” of dense hypoxic, water within these depressions, i.e. salinity and flow patterns indicate that upwelled water settles within topographical depressions and slowly drains along topographic contours. Finally we show that the diversity of pooling topographies within a small area creates a highly “patchy” dissolved oxygen landscape following a water column hypoxic event, where, across a small spatial scale, individual depressions in the reef may vary widely in their instantaneous oxygen and temperature content, which is likely to impact the habitat quality and distributions of local species.