Temporal and spatial occurrence of thin phytoplankton layers in relation to physical processes

Abstract

In 1996 three cruises were conducted to simultaneously quantify the fine-scale optical and physical structure of the water column. Data from 120 profiles were used to investigate the temporal occurrence and spatial distribution of thin layers of phytoplankton as they relate to variations in physical processes. Thin layers ranged in thickness from a few centimeters to a few meters. They may extend horizontally for kilometers and persist for days. Thin layers are a recurring feature in the marine environment; they were observed and measured in 54% of our profiles. Physical processes are important in the temporal and spatial distribution of thin layers. Thin layer depth was closely associated with depth and strength of the pycnocline. Over 71% of all thin layers were located at the base of, or within, the pycnocline. The strong statistical relationships between thin layers and physical structure indicate that we cannot understand thin layer dynamics without understanding both local circulation patterns and regional physical forcing.

Layered organization in the coastal ocean: An introduction to planktonic thin layers and the LOCO project

“…For example, non motile or passive organisms/particles such as diatoms or marine snow may form thin layers by settling on or through strong density gradients (Alldredge et al, 2002), and motile organisms may actively aggregate into thin layers by responding to chemical and/or physical cues, seeking food, for sexual reproduction or for defense from predators (Dekshenieks et al, 2001;Leising, 2001;Holliday et al, 2003;McManus et al, 2003;Sullivan et al, this issue;Ryan et al, this issue). Other biological mechanisms such as in-situ growth, production, grazing and predation can also contribute to thin layer formation and persistence.…”

“…Such sets have been proposed for phytoplankton in East Sound, WA (Dekshenieks et al, 2001) and Monterey Bay, CA (Sullivan et al, this issue), as well as for acoustically measured zooplankton layers in Monterey Bay, CA (Cheriton et al, 2007;Benoit-Bird et al, 2009) as well as several other sites along the west coast of the US (Cheriton et al, 2007). Each of these definitions is distinct and customized to the type of organism in the layer, the particular instrument being used to detect the layer, and the region being studied.…”

“…For example, it has been shown that fish feed preferentially on intense zooplankton thin layers, affecting the depth distribution and behavior of fish in Monterey Bay (Benoit-Bird, in review). While interactions within thin layers are just beginning to be investi gated, thin layers are likely to be important for a variety of biological processes, including growth rates, reproductive success, grazing, predator-prey encounters, nutrient uptake and cycling rates, as well as toxin production (Lasker, 1975;Mullin and Brooks, 1976;Sieburth and Donaghay, 1993;Donaghay and Osborn, 1997;Cowles et al, 1998;Hanson and Donaghay, 1998;Dekshenieks et al, 2001;Rines et al, 2002;McManus et al, 2008). In addition, the layering of the ocean and its plankton species into persistent thin structures acts to diversify and expand available ecological niche space.…”

“…First, far from being unusual or rare, it is now clear that thin layers of plankton are common features and can be found in a wide variety of environments (Derenbach et al, � 1979;Bjørnsen and Nielsen, 1991;Donaghay et al, 1992;Cowles and Desiderio, 1993;Carpenter et al, 1995;Holliday et al, 1998;Dekshenieks et al, 2001;McManus et al, 2003;Ryan et al, 2008). As thin plankton layers can often contain 50-75% (or more) of the total biomass in the water column (Holliday et al, 1998;Cowles et al, 1998;Sullivan et al, this issue), thin layers are likely to be concentrated areas of intense biological activity, likely playing a critical role in the life histories and evolutionary trajectories of many species that are found in or interact with thin layers.…”

“…The term 'thin layer' is used to describe highly-concentrated patches of organ isms, or particles, that have vertical extents on the order of centimeters to a few meters, yet can extend horizontally for many kilometers and persist for hours to weeks (e.g. Donaghay et al, 1992;Cowles et al, 1998;Dekshenieks et al, 2001;Holliday et al, 2003).…”