Abstract
The distribution of plankton in the ocean is patchy across a wide range of spatial and temporal scales. One type of oceanographic feature that exemplifies this patchiness is a ‘thin layer’. Thin layers are subsurface aggregations of plankton that range in vertical thickness from centimeters to a few meters, which may extend horizontally for kilometers and persist for days. We undertook a field campaign to characterize the biological, physical, and chemical properties of thin layers in Monterey Bay, California (USA), an area where these features can be persistent. The particle aggregates (marine snow) sampled in the study had several quantifiable properties indicating how the layer was formed and how its structure was maintained. Particles were more elongated above the layer, and then changed orientation angle and increased in size within the layer, suggesting passive accumulation of particles along a physical gradient. The shift in particle aggregate orientation angle near the pycnocline suggests that shear may also have played a role in generating the thin layer. Pseudo-nitzschia spp. were the most abundant phytoplankton within the thin layer. Further, both dissolved and particulate domoic acid were highest within the thin layer. We suggest that phosphate stress is responsible for the formation of Pseudo-nitzschia spp. aggregates. This stress together with increased nitrogen in the layer may lead to increased bloom toxicity in the subsurface blooms of Pseudo-nitzschia spp. Several zooplankton groups were observed to aggregate above and below the layer. With the knowledge that harmful algal bloom events can occur in subsurface thin layers, modified sampling methods to monitor for these hidden incubators could greatly improve the efficacy of early-warning systems designed to detect harmful algal blooms in coastal waters.
Highlights
The spatial distribution of plankton in the ocean is patchy in both space and time
Persistent thin layer of phytoplankton was observed in northeastern Monterey Bay on 5 July 2010
The average vertical dimension of the thin layer was 1.16 m, which is similar to other studies in highly stratified coastal systems
Summary
Mar Ecol Prog Ser 678: 17–35, 2021 observing methods such as remote sensing, bottle samples, or discrete-depth instruments. Understanding the biological, physical, and chemical mechanisms driving this patchiness will allow us to predict patterns of heterogeneity in coastal areas around the globe This is critical knowledge as we move into an unprecedented period in history, as our oceans’ biological populations, chemistry, and physical structures change with rising average global temperatures and other sources of anthropogenic influence. Since their discovery in the 1960s (Strickland 1968), thin layers have been detected in both coastal and open ocean waters worldwide. The bulk of these observations were made from the late 1990s to 2014, driven by a rapid advancement in sampling methods and technologies involving optical sensing (Cowles et al 1998, Dekshenieks et al 2001, McManus et al 2003), acoustic sensing (Holliday et al 2003, McManus et al 2005, Cheriton et al 2007, Sevadjian et al 2010, Benoit-Bird & McManus 2012), underwater imaging (Alldredge et al 2002, Greer et al 2013), towed vehicles (Cheriton et al 2009, 2010), autonomous underwater vehicles (Ryan et al 2008, 2010, Hodges & Fratantoni 2009), and airplane-based light detection and ranging (Churnside & Donaghay 2009)
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