Variations in surfzone hydrodynamics alter phytoplankton subsidies and further affect growth rate, reproductive output, and size class distribution of Mytilus californianus mussels

Background/Question/Methods: Surfzone hydrodynamics can affect the composition and biodiversity of coastal marine populations. Shores are classified on a spectrum from reflective to dissipative depending on surfzone width, beach slope, sand grain size, and rip current presence. Reflective and dissipative surf zones connect the ocean to the shore, acting as semipermeable barriers through which subsidies from the coastal ocean must pass. Surfzones are an important marine environment to study as this barrier is present wherever waves come into contact with land. One important food subsidy which must pass through the surfzone, phytoplankton, has been previously linked to variations in surfzone hydrodynamics. I hypothesized this variation may alter growth rates, reproductive output, and size class distributions of intertidal filter feeders, since phytoplankton is an essential food source for sessile filter feeders. During Spring/Summer 2016, I enumerated phytoplankton weekly at 10 shores with varying surfzone widths within a 7-km region near Charleston, Oregon. I also conducted transect surveys to gauge reproductive output and assess population sizes of a common intertidal filter feeder, the mussel Mytilus californianus. In tandem, I carried out a 5-month growth rate experiment of M. californianus.

Results/Conclusions: Wider, more dissipative surf zones had, on average, ~8x the concentration of coastal phytoplankton than where surf zones were narrow, and reflective. After executing transect surveys at each shore, I concluded that individuals at more dissipative shores had significantly higher reproductive output than those at more reflective shores; average M. californianus gonad tissue mass/m² was ~6x greater at more dissipative shores and size-class distributions of M. californianus were significantly different depending on surfzone width (Smirnoff-Kolmogorov: D (900) = 0.364, p < 0.0001). From transect data, ~2x more M. californianus were counted at reflective shores, but the frequency of larger M. californianus was ~2x higher at more dissipative shores. After 5 months, I found mean M. californianus mass was ~3x greater for mussels that had grown along dissipative shores than at reflective shores and this mass increase was not due to an increase in shell dimensions. Surfzone hydrodynamics are an important coastal ocean phenomena, capable of altering the basic components of intertidal populations. This study showed that surfzone hydrodynamics are affecting food subsidies over a short spatial scale and modifying an intertidal organism at various life stages. Ultimately, surfzone hydrodynamics research is furthering our understanding of ocean subsidies and highlights that the ecological implications of surf zones need to be considered when examining coastal marine communities.