Surf-zone hydrodynamics alter phytoplankton subsidies affecting reproductive output and growth of tidal filter...

Surf zones, classified from reflective to dissipative, separate the ocean from shore and subsidies from the coastal ocean must pass through surf zones to reach the shore. We have observed that variations in phytoplankton concentrations in the water over the intertidal zone varied with surf-zone hydrodynamics and we hypothesized that this variation would alter growth rates, population structure, and reproductive output of Mytilus californianus and Balanus glandula. From May 2016 to April 2017, along 7 km of Cape Arago, Oregon, USA surf-zone phytoplankton concentrations were determined weekly at nine sites with varying surf-zone hydrodynamics as indicated by surf-zone widths. Throughout the year, concentrations of phytoplankton in wider, more dissipative surf zones were, on average, 16× higher than in narrow, more reflective surf zones. Similar to previous observations, surf-zone width explained >90% of the variability in phytoplankton concentrations in the surf-zone. On average, ~83% of B. glandula had egg lamellae at more dissipative shores compared to only 8% at more reflective. An index of potential reproductive output by barnacle populations as measured by ash free dry mass (AFDM) of egg lamellae was ~243× larger at more dissipative than at reflective shores and surf-zone phytoplankton concentrations and surf-zone width explained ~96 and 92% of the variability in this index. On average, density of M. californianus was ~2× higher at more reflective shores, but 60% of these individuals were smaller and non-reproductive compared to only 24% at the more dissipative sites. The gonad tissue mass/m² of medium sized mussels was ~5× greater at more dissipative than reflective shores. Surf-zone phytoplankton concentrations and surf-zone width explained ~80% and 65% of the variability in individual mussel gonad mass and ~69% and 56% of the variability in mussel population gonad mass, respectively. M. californianus were out-planted to assess growth rates and, after 5 months, average body mass was ~3× greater at more dissipative than reflective shores. Surf-zone phytoplankton concentrations and width explained ~85% and 92% of the mass increase, respectively. Phytoplankton subsidies varied with surf-zone hydrodynamics altering the growth and reproductive output of two ecologically important intertidal filter feeders.