The current prevailing theory of diel vertical migration (DVM) of zooplankton is focused largely on two biotic drivers: food and predation. Yet recent evidence suggests that abiotic drivers such as damaging ultraviolet (UV) radiation and temperature are also important. Here we integrate current knowledge on the effects of abiotic factors on DVM with the current biologically based paradigm to develop a more comprehensive framework for understanding DVM in zooplankton. We focus on ''normal'' (down during the day, up at night) DVM of holoplanktonic, primarily herbivorous zooplankton. This new transparency-regulator hypothesis differentiates between structural drivers, such as temperature and food, that vary little over a 24-h period and dynamic drivers, such as damaging UV radiation and visual predation, that show strong variation over a 24-h period. This hypothesis emphasizes the central role of water transparency in regulating these major drivers of DVM. In less transparent systems, temperature and food are often optimal in the surface waters, visual predators are abundant, and UV radiation levels are low. In contrast, in more transparent systems, vertical thermal gradients tend to be more gradual, food quality and quantity are higher in deeper waters, and visual predator abundance is often lower and damaging UV radiation higher in the surface waters. This transparency-regulator hypothesis provides a more versatile theoretical framework to explain variation in DVM across waters of differing transparency. This hypothesis also enables clearer predictions of how the wide range of ongoing transparency-altering local, regional, and global environmental changes can be expected to influence DVM patterns in both inland and oceanic waters of the world.Diel vertical migrations (DVM) of zooplankton in the world's lakes and oceans comprise some of the most widespread and massive migrations of animals on Earth. These striking migrations across strong vertical habitat gradients have inspired numerous ecological and evolutionary studies addressing mechanisms of habitat selection and consequences for species interactions. These migrations also have important implications for water quality and fisheries production as well as biogeochemical cycling. Natural and anthropogenic environmental changes, such as shifting local land use patterns and regional to global climate change, are altering water transparency and have the potential to affect DVM in lakes and oceans worldwide. The purpose of this article is to provide a common theoretical framework for understanding variation in the drivers of DVM across transparency gradients with a particular emphasis on recent advances in our understanding of the role of ultraviolet radiation (UV).Many environmental factors are recognized as providing both important proximate cues as well as ultimate consequences of adaptive significance for DVM, including light, temperature, food availability, and predation pressures (Table 1; Lampert 1989;Ringelberg 1993;Hays 2003). In spite of the wides...

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As a field test of the hypothesis that variable diel vertical migration in zooplankton is a response to variations in the abundance of visually orienting predators, we sampled juvenile and adult planktivorous fish inhabiting a temperate fjord (Dabob Bay, Washington) concurrently with the vertical distributions of a population of adult female Calanus pacificus (Copepoda: Calanoida) known to exhibit seasonally variable diel vertical migration. Results of nighttime trawling on seven dates between April 1985 and October 1986 and subsequent stomach content analyses showed that eight species of fish, comprising 45 different size classes, fed on adult female C. pacificus; all species of fish showed marked seasonal and interannual variations in their abundances. The extent to which the population of C. pacificus exhibited diel vertical migration was determined for each cruise date by calculating the proportion of the population crossing 75‐m depth on a diel cycle. A statistically significant regression was obtained between strength of diel vertical migration in C. pacificus and abundance of fish actively preying on the copepod. We propose that the timing and magnitude of changes in the migration behavior of C. pacificus in Dabob Bay are directly linked to the year‐class strengths of the dominant species of planktivorous fish.

The "Wind Events and Shelf Transport" (WEST) program was an interdisciplinary study of coastal upwelling off northern California in 2000 was comprised of modeling and field observations. The primary goal of WEST was to better describe and understand the competing influences of wind forcing on planktonic productivity in coastal waters. While increased upwelling favorable winds lead to increased nutrient supply, they also result in reduced light exposure due to deeper surface mixed layers and increased advective loss of plankton from coastal waters. The key to understanding high levels of productivity, amidst these competing responses to wind forcing, is the temporal and spatial structure of upwelling. Temporal fluctuations and spatial patterns allow strong upwelling that favors nutrient delivery to be juxtaposed with less energetic conditions that favor stratification and plankton blooms.