Progressive changes in the Western English Channel foster a reorganization in the plankton food web

doi:10.1016/j.pocean.2015.04.025

Progress in Oceanography

Volume 137, Part B, September 2015, Pages 524-532

https://doi.org/10.1016/j.pocean.2015.04.025 Get rights and content

Highlights

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We unveil regional progressive climate modifications fostering the entire plankton food web.
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Hydroclimate shifts have progressively impacted the seasonality of plankton.
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The profound plankton reorganization warns on the replacement of local species by cosmopolitan ones.

Abstract

Growing evidence has shown a profound modification of plankton communities of the North East Atlantic and adjacent seas over the past decades. This drastic change has been attributed to a modification of the environmental conditions that regulate the dynamics and the spatial distribution of ectothermic species in the ocean. Recently, several studies have highlighted modifications of the regional climate station L4 (50° 15.00′N, 4° 13.02′W) in the Western English Channel. We here focus on the modification of the plankton community by studying the long-term, annual and seasonal changes of five zooplankton groups and eight copepod genera. We detail the main composition and the phenology of the plankton communities during four climatic periods identified at the L4 station: 1988–1994, 1995–2000, 2001–2007 and 2008–2012. Our results show that long-term environmental changes underlined by Molinero et al. (2013) drive a profound restructuration of the plankton community modifying the phenology and the dominance of key planktonic groups including fish larvae. Consequently, the slow but deep modifications detected in the plankton community highlight a climate driven ecosystem shift in the Western English Channel.

Introduction

Time-series data have been shown to be fundamental in understanding marine ecosystem function in an era of global environmental change (Harris, 2010). In the last half-century, modifications of the natural environmental equilibrium in the global ocean have complicated our understanding of natural ecosystem dynamics. Resolving how marine communities deal with these changes challenges the sustainable management of resources, and therefore represents a major endeavour in marine science. In this regard, the thorough assessment of plankton dynamics appears essential to forecast pelagic food web changes. Indeed, by virtue of the role of plankton at the base of the food web and their non-linear responses to external forces, these organisms are valuable indicators of ecosystem health (Hays et al., 2005). Hence, understanding the temporal variability of plankton communities can help anticipate changes in the marine ecosystem (Behrenfeld, 2014).

The Western English Channel has a long history of marine ecosystem investigations. First reports appeared in the late 1880s and were followed by a number of investigations that focused on the effect of environmental and climatic variations on marine resources. Southward et al. (2004) provided a compelling overview of the long-term research in the Western English Channel and showed that field data are fundamental to identify climate-induced environmental changes and their effects on marine populations. Recent studies using the Plymouth Marine Laboratory dataset showed conspicuous changes in plankton over the period 1988–2007 (Eloire et al., 2010). These results portrayed an asymptotic trend in zooplankton abundance together with an increased species richness in pelagic copepods. These changes echoed modifications in the magnitude and length of the seasonal temperature influence and, consequently, in the timing and depth of the thermocline (Molinero et al., 2013). The latter authors further warned of the increasing climate variance that might change plankton species’ structure and function, thereby promoting bottom-up controls in food webs. Plankton resilience is of central importance for the entire marine ecosystem, for which a thorough understanding of structural changes, i.e. taxa/species replacement.

On the basis of the result found in Molinero et al. (2013), we here examine plankton community response to climate forcing over the period 1988–2012. As the strength of the climate influence on populations varies over time, we may expect different type of response between the different planktonic groups and species. To quantify the effects of climate cascades on the plankton ecosystem we first assessed the time-varying strength of the climate influence and identified regimes of environmental variability that governed the period investigated. Subsequently, we examined temporal changes of plankton taxa and identified structural changes related to dominant taxa, composition and phenology for the whole planktonic system, as well as for pelagic copepods.

Section snippets

Biological data

All biological data used in the study are from the Plymouth Marine Laboratory L4 dataset (www.westernchannelobservatory.org.uk). This is based on quasi-weekly sampling at station L4 (50° 15.00′N, 4° 13.02′W). The temporal resolution has been averaged for monthly resolution for all biological parameters to compensate for non-homogenous sampling effort (i.e. occasional missed sampling and variations in day of the week sampled).

Phytoplankton samples were collected from a depth of 10 m using a 10 L

Results

Climate conditions for the Western English Channel, as represented by PC1 of regional atmospheric variables, showed the expected inter-annual variability, with a trend indicating multi-annual cycles of 6–7 years (Fig. 1a). The OLS-based CUSUM test identified four major periods of forcing during the years examined (Fig. 1b): January 1988 to circa 1994, 1994 to circa 2001, 2001–2007 and 2008 onwards. These changes, primarily driven by the expanding climate variance envelope, i.e. interval of

Progressive changes in environmental conditions

Environmental conditions in the Western English Channel biotope are mainly controlled by global atmospheric (Beaugrand, 2009) and hydrographic (Hátún et al., 2005) processes that regulate the entire North Atlantic. The effect of these macro-processes varies between different North Atlantic marine ecosystems due to variations in basin shape and local meso-scale features such as river run-off, coastal orientation or interface with adjacent open seas areas. For example, the North Atlantic

Acknowledgements

The authors acknowledge the efforts over many years of staff at PML and MBA and the ships’ crew in taking and processing the samples. This work is dedicated to the memory of our colleague and friend Frederic Ibañez.

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