Effects of physical processes on structure and transport of thin zooplankton layers in the coastal ocean

Abstract

Thin layers of plankton are recurrent features in a variety of coastal systems. These layers range in thickness from a few centimeters to a few meters. They can extend horizontally for kilometers and have been observed to persist for days. Densities of organisms found within thin layers are far greater than those above or below the layer, and as a result, thin layers may play an important role in the marine ecosystem. The paramount objective of this study was to understand the physical processes that govern the dynamics of thin layers of zooplankton in the coastal ocean. We deployed instruments to measure physical processes and zooplankton distribution in northern Monterey Bay; during an 11 d period of persistent upwelling-favorable winds, 7 thin zooplankton layers were observed. These zooplankton layers persisted throughout daylight hours, but were observed to dissipate during evening hours. These layers had an average vertical thickness of 1.01 m. No layers were found in regions where the Richardson number was < 0.25. In general, when the Richardson number is < 0.25 the water column is unstable, and incapable of supporting thin layers. Thin zooplankton layers were also located in regions of reduced flow. In addition, our observations show that the vertical depth distribution of thin zooplankton layers is modulated by high-frequency internal waves, with periods of 18 to 20 min. Results from this study clearly show an association between physical structure, physical processes and the presence of thin zooplankton layers in Monterey Bay. With this new understanding we may identify other coastal regions that have a high probability of supporting thin layers. KEY WORDS: Thin layer · Physical processes · Transport · Zooplankton · Coastal circulationResale or republication not permitted without written consent of the publisher

Evolution of the spatial structure of a thin phytoplankton layer into a turbulent field

“…However, there was an across-isobath, offshore-directed, near-surface current which advected near-surface warm water from onshore to offshore. The tidal current was relatively weak, estimated to be less than 5 cm s -1 throughout the experiment (McManus et al 2005). Maximum tidal displacement during the experiment was 1.5 m, changing from high to low during the experiment.…”

“…Thin layers have now been observed in a variety of coastal marine environments, including a shallow fjord (East Sound, WA, USA; Alldredge et al 2002), the continental shelf (Monterey Bay, CA, USA; McManus et al 2005), and an enclosed sea (Baltic Sea; Nielson et al 1990). Although recent advances in high resolution optical and acoustical sensors have provided evidence of thin layers at multiple coastal ocean sites (McManus et al 2005), little direct research has occurred on the role of the very smallest scale physical processes on thin layers.…”

“…Although recent advances in high resolution optical and acoustical sensors have provided evidence of thin layers at multiple coastal ocean sites (McManus et al 2005), little direct research has occurred on the role of the very smallest scale physical processes on thin layers. One of the principal objectives of the Layered Organization in the Coastal Ocean (LOCO) experiment was to examine this role.…”

Zooplankton vertical distribution and migration off Central Peru in relation to the oxygen minimum layer

“…Although there is a plethora of studies describing vertical distribution and migration in marine zooplankton (see recent reviews in Hays 2003;Pearre 2003), only a few studies were conducted explicitly considering and testing the Xuctuations in cline depths (Haury et al 1983;Trevorrow 1998;McManus et al 2005), or testing the eVect of speciWc abiotic variables on migrating organisms (Cohen and Forward 2002;Renz and Hirche 2006). Previous studies on vertical distribution and migration of zooplankton conducted oV Peru have not yet explicitly tested the hypotheses related to speciWc abiotic variables and Xuctuations in cline depths.…”

Stratification and mixing regimes in biological thin layers over the Mid‐Atlantic Bight

“…Thin layers are marked by an increase in population over small vertical scale (order 1 m); they are temporally coherent, last hours to days, and may span distances as great as order 10 km in the horizontal (Cowles et al 1998;McManus et al 2008;Moline et al 2010). Bacteria, phytoplankton, and zooplankton have all been observed to form thin layers (McManus et al 2003;Bochdansky and Bollens 2004;Benoit-Bird 2009). This range of scale in organism size is suggestive of the large range in processes that may be a factor in layer formation (from primarily physical or biological controls to complex bio-physical interactions).…”