Comparing Vertical Distributions of Chl-a Fluorescence, Marine Snow, and Taxon-Specific Zooplankton in Relation to Density Using High-Resolution Optical Measurements

Abstract

Interactions between predators and their prey are important in shaping planktonic ecosystems. However, these interactions are difficult to assess in situ at the spatial scales relevant to the organisms. This work presents high spatial resolution observations of the nighttime vertical distributions of individual zooplankton, chlorophylla fluorescence, and marine snow in stratified coastal waters of the Southern California Bight. Data were obtained using a planar laser imaging fluorometer (PLIF) augmented with a shadowgraph zooplankton imaging system (O-Cam) mounted along with ancillary sensors on a free-descent platform. Fluorometer and PLIF sensors detected two well-defined and distinct peaks: the subsurface chlorophyll maximum (SCM) and a fluorescent particle maximum (FPM) dominated by large marine snow. The O-Cam imaging system allows reliable estimates of concentrations of crustacean and gelatinous zooplankton groups; we found that grazers and their predators had well-structured nighttime distributions in and around the SCM and FPM in ways that suggested potential predator avoidance at the peak of the SCM and immediately above the FPM (where predatory hydromedusae, and to some degree euphausiids, were primarily located). Calanoid copepods were found above the SCM while cyclopoids were associated with the FPM. The locations of predator and grazer concentration peaks suggest that their dynamics may control the vertical gradients defining the SCM and FPM.

Zooglider‐measured association of zooplankton with the fine‐scale vertical prey field

“…Zooglider revealed that most suspension‐feeding omnivorous zooplankton were more closely associated with marine snow and with small particles rather than with Chl a fluorescence in the San Diego Trough. This result provides further evidence supporting previous studies (Herman 1983; Roman et al 1986; Napp et al 1988; Ohman and Runge 1994; Calbet and Saiz 2005; Briseño‐Avena et al 2020) that a broader spectrum of prey need to be considered, rather than the conventional focus on fluorescing Chl a bearing phytoplankton alone. We now consider this result in relation to our three primary taxa: planktonic copepods, appendicularians, and larger protists.…”

“…However, it is important to note that copepods do not always overlap with particle concentrations. Copepods were observed to be primarily located shallower than the FPM and SCML in the Southern California Bight (Briseño‐Avena et al 2020). Conversely, Greer et al (2020) found copepod concentrations, in the summer of 2016, to be the highest on the edges of a thin layer of diatoms, in the northern Gulf of Mexico.…”

“…Furthermore, the vertical resolution of conventional zooplankton samplers is insufficient to resolve small differences in vertical offsets of mesozooplankton and their potential prey (Möller et al 2012). Improved understanding of whether mesozooplankton are preferentially associated with a particular prey source will accelerate our ability to quantify trophic transfer rates (Greer 2013; Greer and Woodson 2016; Briseño‐Avena 2015; Briseño‐Avena et al 2020; Greer et al 2020), the fate of marine snow (Möller et al 2012; Biard and Ohman 2020), and mesozooplankton survival. Therefore, we pose the question: of potential food sources, are the vertical distributions of suspension‐feeding and flux‐feeding mesozooplankton better associated with Chl a , small particles (ECD: 0.25–0.45 mm), or marine snow (ECD ≥ 0.45 mm)?…”

Pelagic tunicate grazing on marine microbes revealed by integrative approaches

“…In situ imagery deployed with high temporal resolution can provide detailed vertical distribution data over diel cycles (Maas et al 2021). High-resolution imagery may also provide new insights into the settling behavior of discarded mucus, fecal pellets, and carcasses after release (e.g., Briseño-Avena et al 2020a). Direct in situ sampling of prey-laden mucus and fecal pellets as they sink could reveal the dynamics of settlement, fragmentation, and restructuring by the microbial community as this material descends to depth.…”

Active Carbon Flux of Mesozooplankton in South China Sea and Western Philippine Sea

“…The DVM of the total abundance, as well as dominant groups except for protozoan and tunicate, were higher in the WPS than that in the SCS. In general, the vertical distribution of zooplankton was closely related to Chl a (Madhupratap and Haridas, 1990;Onsrud and Kaartvedt, 1998;Steinberg et al, 2008a;Briseño-Avena et al, 2020). Steinberg et al (2008a) indicated that the deeper the maximum depth of Chl was, the stronger the DVM was performed.…”