Comment. What drives plankton seasonality in a stratifying shelf sea? Some competing and complementary theories

Abstract

The Plymouth L4 time plankton series in the Western English Channel is a textbook example of a shallow, stratifying shelf sea system. Over its 30 yr of weekly sampling, this site has provided a diverse and contrasting suite of numerical and conceptual models of plankton bloom formation, phenology, and seasonal succession. The most recent of these papers, Kenitz et al. (2017) has initiated this comment, partly because we feel that it has presented a slightly misleading picture of the plankton composition at this site, and of a robust, recurring seasonal succession. We address this by illustrating the extent of inter‐annual variability in phenology that occurs at the site, and which needs to be captured better within models. However our main aim is to foster a much better integration of the variety of top‐down and bottom‐up processes that have all been suggested to be key in driving seasonal succession. Some of these, particularly the multiple grazing and growth controls contributing to the so‐called “loophole hypothesis” may be complementary, but others, such as the role of copepod feeding traits in driving species succession (Kenitz et al. 2017) offer testable competing hypotheses. The basic assumptions and outputs of all these models need to be validated more critically, both against time series data and process studies that include the finding of unselective feeding. We suggest that the variability in plankton phenology (and not just mean timing and amplitude) could be used to diagnose the performance of alternative models of plankton succession.

Response to comment: “What drives plankton seasonality in a stratifying shelf sea? Some competing and complementary theories”

“…emerge from the complex interactions of the ecosystem itself where any given player is exposed to top‐down and bottom‐up pressures. In a comment to our recent manuscript (Kenitz et al ), Atkinson et al () rightly point out that no single mechanism is likely to explain observed variations, and we support their call to carefully reconsider how seasonal plankton dynamics are described and modeled.…”

“…In reality, variability in species distribution is controlled by multitude of interacting processes that swing in and out of prominence on temporal and spatial scales. In our response, we would like to echo the main message of Atkinson et al () comment: there is a tremendous need to combine the suggested mechanisms to investigate their cumulative effects on shaping plankton community structure. In this, the L4 station time series provides an invaluable resource that traces 30 years of highly resolved physical and biological observations.…”

Increasing nutrient stress reduces the efficiency of energy transfer through planktonic size spectra

“…by size-based analysis, are not captured in conceptual or simulation models of L4 seasonality (Atkinson et al 2018). The models tend to overemphasize the spring diatom bloom, a relatively indistinct feature in the size spectra.…”

Integration of temporal environmental variation by the marine plankton community

“…Overall, plankton and environmental data from Station L4 did not neatly conform either to the singleor the double-integration models proposed by Di Lorenzo & Ohman (2013) for long-lived krill (1−2 yr) in the California Current Ecosystem. The California Current region is an upwelling system with strong decadal changes in geostrophic transport (Bograd et al 2001), whereas Station L4 is a dynamic inshore, albeit seasonally stratified continental shelf ecosystem (Atkinson et al 2018). It is therefore not surprising that these systems, with strongly contrasting physical dynamics and plankton communities, would have different pathways linking atmospheric and ecological variability.…”