Vertical Distribution of Pisaster ochraceus larvae in a halocline in the presence and absence of a food patch
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Thin phytoplankton layers are characterized by dense concentration of cells a few centimeters to several meters thick at haloclines. They extend horizontally for kilometers and can persist for days. These dense concentrations of phytoplankton have the potential to influence the behavior, feeding success, and predation by higher trophic levels. This study looked at changes in the vertical distribution and persistence of two algae Isochrysis galbana and Rhodomonas sp. at concentrations of 12,000-66,000 cells/ml, at and below a halocline in the presence and absence of a predator Pisaster ochraceus larvae. Three treatments with two replicates per treatment were set up, columns with, a) algae at the halocline + larvae, b) algae at the halocline – larvae, c) no algae + larvae in halocline. In each case, approximately 100 larvae were gravity fed to the bottom of each column. Patch width was measured 1, 3, and 6 hour after introduction into the columns. Patch width for Rhodomonas sp. remained near one cm for six hours while patch width for I. galbana increased to two cm in six hrs. For both algal species, cell concentration at the halocline declined significantly in the presence of Pisaster larvae. A greater decrease in cell concentration was observed for I. galbana when the food was at the bottom of the column rather than at the halocline. This was observed in the presence and absence of larvae. This indicates that for I. galbana, the decline in algal cell concentration at the bottom of the column might be due to cell movement towards the halocline. Interestingly, larvae remained around the halocline in the presence or absence of food for 6 hours. Living in stratified layers where thin phytoplankton layers might exist can be advantageous for P. ochraceus larvae by providing them with resources to feed, grow, and metamorphose but might also expose them to predation by higher trophic levels.