UC San Diego

Conventional sampling systems (nets, pumps, acoustics, and most optical imaging systems) are inadequate to study planktonic trophic interactions. However, Zooglider, a novel endurance zooplankton sensing glider, is shown here to be uniquely capable of resolving planktonic trophic interactions at 5 cm vertical resolution.

In March 2017, Zooglider’s optical (Zoocam) and acoustic backscatter (Zonar) systems were compared against conventional ship-based nets (MOCNESS) and acoustics (EK80). Zoocam observed similar abundances of robust organisms (chaetognaths, euphausiids, and nauplii) and greater abundances of both smaller and more delicate zooplankton. Compared to the MOCNESS, Zoocam observed significantly more smaller appendicularia and copepods, while simultaneously observing significantly more larger gelatinous predators (ctenophora and hydromedusae) and mineralized protists (foraminifera, phaeodaria, and mostly acantharia). Furthermore, Zoocam revealed in situ local maxima in organismal abundances that were not resolvable by the coarser net resolution. Zonar matched the relative distributions and magnitude of ship-based acoustics, without the disadvantage of reduced signal-to-noise ratios in deeper depths.

From seven deployments spanning 15 months, Zooglider revealed the limitations of solely using chlorophyll-a fluorescence (Chl-a) as a proxy for herbivorous zooplankton prey. Zoocam observed that marine snow, small particles, and many zooplankton taxa, i.e., appendicularians, copepods, and large mineralized protists (primarily acantharia), have bilinear and nonlinear relationships with Chl-a concentrations. In most cases, zooplankton showed improved overlap with distributions of small suspended particles than with Chl-a. Furthermore, marine snow and small particles were determined to be the primary explanatory variables for zooplankton abundances, whereas Chl-a was either secondary or insignificant. No relationship was found between maximum water column stability and zooplankton or prey abundances.

Zooglider also detected size-dependent zooplankton predator-prey interactions. Size-dependent vertical distributions were found for three prey taxa and five predatory taxa and differential size-dependent diel vertical migration behavior was detected for copepods and chaetognaths. Zoocam images showed in situ predator-prey encounters (co-occurrence of predator and prey within a 250 mL sample volume). Analysis of these encounters revealed that abundances of smaller predatory zooplankton (chaetognaths, ctenophores, siphonophores, and trachymedusae) have stronger relationships with abundances of smaller prey and that smaller predators have greater observed probabilities of encountering smaller prey.