Effects of brackish water incursions and diel phasing of tides on vertical excursions of the keystone predator Pisaster ochraceus |
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Authors: | Corey Garza Carlos Robles |
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Institution: | (1) Division of Science and Environmental Policy, California State University, Monterey Bay, 100 Campus Center, Seaside, CA 93955, USA;(2) Department of Biological Sciences, California State University, Los Angeles, 5151 State University Drive, Los Angeles, CA 90032, USA |
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Abstract: | The physical factors that constrain the vertical foraging excursions of the keystone predator, the sea star Pisaster ochraceus, hold considerable interest because they indirectly shape the vivid patterns of zonation of rocky shore communities by impeding
or enhancing the ability of P. ochraceus to traverse the intertidal zone. In this paper, we describe a study conducted in the Pacific Northwest of North America in
which we examined, in the field and laboratory, the abiotic factors that can affect vertical excursions by P. ochraceus. Our field observations revealed that the extreme upward reach and average shore level height reached by P. ochraceus were significantly lower for daylight high tides than nocturnal high tides. Based on diver observations following a severe
storm, it would also appear that these diurnal movements can be impeded by freshwater incursions into the intertidal zone;
a regularly occurring event in the Pacific Northwest. As part of an experimental investigation into this phenomenon, we observed
that sea stars maintained in tall cylindrical aquaria, without tidal flux, remained near the bottom during daylight and moved
to the top of the column at night, suggesting that photoperiod alone can influence the cycle of vertical movement. Adding
a freshwater layer to the aquaria restricted these vertical excursions. Our results suggest that on rocky coastlines susceptible
to fresh water incursions, the suppression of foraging may be an important factor in the spatial and temporal variation in
the intensity of predation. Furthermore, given the relative increase in frequency and intensity of freshwater incursions in
the Pacific Northwest and the intolerance of P. ochraceus to lowered salinity, there is the long-term potential to significantly alter patterns of species zonation in this essential
marine habitat. |
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