Temporal variability and bio-physical coupling in the pelagic fauna of Monterey Bay

Temporal variability is an important feature of aquatic ecosystems that is often difficult to measure. To this end, a stationary, upward-facing scientific echosounder was used to record the vertical distribution of pelagic fauna in Monterey Bay, California, for 18 months. To characterize these distributions, we developed and tested a suite of metrics quantifying the distribution of biomass in the water column. These metrics recorded a strong seasonal cycle, with total backscatter reaching a minimum during the spring upwelling season and a maximum in the fall and winter. Variability was greatest at long time scales and displayed a power spectrum similar to that of a turbulent fluid, with discrete peaks at 12- and 24-hour periods, corresponding to the semidiurnal tide and diel vertical migration. Pelagic animals also responded to short-term oceanographic variability, moving up in the water column and decreasing in abundance in the days following upwelling events. At longer lags, consistent with reproduction of small zooplankton (38 days), their abundance increased. Primary production appeared to propagate down the water column at rates consistent with sinking marine snow. Animal density throughout the water column was positively correlated at 0-2 day lags with sea level. Active acoustics are a useful addition to ocean observatories. The metrics presented here provide a useful and objective set of descriptors for stationary acoustic data. Such data provides insight into pelagic ecosystem dynamics unavailable with other sampling methods, and has great potential for future research.