Disentangling the impacts of heat wave magnitude, duration and timing on the structure and diversity of sessile marine assemblages.

Dan A Smale,Anna L E Yunnie,Stephen Widdicombe,Thomas Vance

doi:10.7717/peerj.863

Abstract

Extreme climatic events, including heat waves (HWs) and severe storms, influence the structure of marine and terrestrial ecosystems. Despite growing consensus that anthropogenic climate change will increase the frequency, duration and magnitude of extreme events, current understanding of their impact on communities and ecosystems is limited. Here, we used sessile invertebrates on settlement panels as model assemblages to examine the influence of HW magnitude, duration and timing on marine biodiversity patterns. Settlement panels were deployed in a marina in southwest UK for ≥5 weeks, to allow sufficient time for colonisation and development of sessile fauna, before being subjected to simulated HWs in a mesocosm facility. Replicate panel assemblages were held at ambient sea temperature (∼17 °C), or +3 °C or +5 °C for a period of 1 or 2 weeks, before being returned to the marina for a recovery phase of 2–3 weeks. The 10-week experiment was repeated 3 times, staggered throughout summer, to examine the influence of HW timing on community impacts. Contrary to our expectations, the warming events had no clear, consistent impacts on the abundance of species or the structure of sessile assemblages. With the exception of 1 high-magnitude long-duration HW event, warming did not alter not assemblage structure, favour non-native species, nor lead to changes in richness, abundance or biomass of sessile faunal assemblages. The observed lack of effect may have been caused by a combination of (1) the use of relatively low magnitude, realistic heat wave treatments compared to previous studies (2), the greater resilience of mature adult sessile fauna compared to recruits and juveniles, and (3) the high thermal tolerance of the model organisms (i.e., temperate fouling species, principally bryozoans and ascidians). Our study demonstrates the importance of using realistic treatments when manipulating climate change variables, and also suggests that biogeographical context may influence community-level responses to short-term warming events, which are predicted to increase in severity in the future.

Highlights

Ecosystems the world over have responded to climate change, with major implications for humanity’s use of ecological goods and services (Rosenzweig et al, 2008; IPCC, 2014)
In conjunction with gradual warming trends, discrete extreme climatic events are increasing in frequency and intensity as a consequence of anthropogenic climate change (IPCC, 2012; Coumou, Robinson & Rahmstorf, 2013)
Based on existing knowledge we proposed and tested the following hypotheses: (1) high magnitude, long duration heat waves (HWs) will be detrimental to some marine taxa and cause shifts in community structure (Garrabou et al, 2009; Wernberg et al, 2013); (2) high magnitude, long duration HWs will be less detrimental to non-native species compared with native species, as non-native species often exhibit wide thermal tolerances and high resilience to environmental stress (Stachowicz et al, 2002; Sorte, Fuller & Bracken, 2010; Sorte, Williams & Zerebecki, 2010; Diez et al, 2012), and; (3) these patterns will be broadly consistent across different HWs that occur during seasonal thermal maxima

Summary

Introduction

Ecosystems the world over have responded to climate change, with major implications for humanity’s use of ecological goods and services (Rosenzweig et al, 2008; IPCC, 2014). Links between a changing climate and shifts in species distributions and the structure of communities and ecosystems have been documented convincingly for many taxa across many regions (Parmesan & Yohe, 2003; Wernberg et al, 2011; Pinsky et al, 2013; Poloczanska et al, 2013). As such, understanding and predicting biological responses to ‘events’, rather than ‘trends’, is becoming increasingly important, event-based research still lags behind trend-based work (Jentsch, Kreyling & Beierkuhnlein, 2007; Thompson et al, 2013). It is clear that discrete climatic events can drive step-wise changes in species’ distributions and, ecosystem structure and functioning. Floods and heat waves, for example, can have catastrophic effects on both marine and terrestrial ecosystems (Jentsch, Kreyling & Beierkuhnlein, 2007; IPCC, 2012), with substantial socio-economic ramifications

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