The intertidal zone of the North-East Atlantic region: pattern and process.

Abstract

The north-east Atlantic region is an area where clades originating in the north Pacific (fucoids, balanoids, littorinids, thaids, laminarians) collide with clades from further south in the Atlantic (e.g., patellids, trochids, chthamalids). At high latitudes in the north, seaweeds dominate the midshore zone of all but the most exposed shores. Further south, midshore space-occupying invertebrates (mussels and barnacles) win, facilitated by grazing by patellid limpets that controls algal recruitment; propagule pressure is much less as fucoids become rarer, and juvenile growth is slower due to environmental stress, thereby reducing the probability of escapes from grazing (Figure 2.4) (Ferreira et al., 2014, 2015a, 2015b). Low on the shore seaweeds dominate space by forming algal turfs or kelp or fucoid canopies. These algae outpace the ability of grazing limpets to control them in the low-intertidal zone. L. digitata canopies can lead to rock covered by encrusting algae and sponges, facilitating limpets. If canopy is removed, then colonising ephemeral algae and turf-forming algae swamp the limpets. There is usually too much water movement immediately either side of low water for effective foraging by sea urchins. Psammechinus miliaris and Echinus esculentus only appear in the subtidal, and Paracentrotus lividus is confined to refuges in burrows relying mainly on the drift of food (Benedetti-Cecchi and Cinelli, 1995;Boudouresque and Verlaque, 2007; Jacinto and Cruz, 2012). High on the shore, physical factors dominate. At high latitudes in the north of the Atlantic, ephemeral algae are present all year round. Further south they are only present in the winter, dying-off in the summer. Grazing has limited effects, only occurring around refuges that littorinids maintain (Stafford and Davies, 2005; Skov et al., 2010, 2011). Patterns are also strongly modified by mesoscale processes driven by upwelling that influences nutrient and larval supply (North Africa, Iberia) and coastal configuration, where embayed versus headlands also strongly influence larval supply (France northwards). In high-recruitment areas, interactions can be intense between spaceoccupying species, also driving predator abundance (e.g., dog whelks). Connell’s (1961a) classic paper on competition was possible on the Isle of Cumbrae because space was almost saturated; elsewhere lower larval supply would have created less intense interactions, as shown by Gordon and Knights (2017) in Plymouth. The north-east Atlantic has faster rates of warming than any other ocean, although the region south of Greenland and Iceland is undergoing cooling due to a climate-driven slowdown in the Atlantic meridional overturning circulation, causing a weakening in the Gulf Stream (Rahmstorf et al., 2015). Species are responding to rapid alterations in the marine climate by adapting or exhibiting range shifts, or by becoming locally extinct. There is a high degree of spatial and temporal heterogeneity in the resultant impacts on marine communities due to the idiosyncratic responses of individual species. Warming seas have resulted in biogeographic range shifts of intertidal and subtidal species in coastal waters of the northeast Atlantic. The leading range edges of Lusitanian species are expanding, while the trailing edges of boreal species are retracting to higher latitudes, but with some cold-water species showing surprising resilience (Southward et al., 1995; Mieszkowska et al., 2006, 2014b; Lima et al., 2007; Hawkins et al., 2008, 2009; Wethey and Woodin, 2008; Mieszkowska and Sugden, 2016). In addition to changes in the distribution ofspecies, community structure is also altering as species dominance and interactions change (Poloczanska et al., 2008; Hawkins et al., 2008, 2009; Mieszkowska et al., 2014b). In a warming world the midshore of France and the British Isles are likely to show much less cover by large canopy-forming fucoids as harsher warmer, drier and stormier conditions coupled with increased grazing pressure from more grazing species reduces the probability of fucoids recruiting to form adult populations. Lowshore kelp forests will likely change with less L. digitata and A. esculenta and more S. polyschides. The late autumn to early spring window of dense ephemeral algal growth high on the shore (Hawkins and Hartnoll, 1983a) will also constrict, except in the north and in extreme exposure. These changes will have consequences for biodiversity (Thompson et al., 1996; Smale et al., 2013; Teagle et al., 2017) and productivity (Hawkins et al., 1992) – particularly the decrease in export of algal detritus (Notman et al., 2016). More shores will become dominated by suspension-feeding barnacles and mussels. Thus, there will be switches on many mid-latitude shores as many become net importers rather than exporters of energy (Hawkins et al., 2008, 2009).