The influence of physical factors on kelp and sea urchin distribution in previously and still grazed areas in the NE Atlantic.

Eli Rinde,Kjell Magnus Norderhaug,Camilla W. Fagerli,Hege Gundersen,Trine Bekkby,Hartvig Christie,Dag Ø. Hjermann,João Miguel Dias

doi:10.1371/journal.pone.0100222

Eli Rinde, Kjell Magnus Norderhaug + Show 6 more

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https://doi.org/10.1371/journal.pone.0100222

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Abstract

The spatial distribution of kelp (Laminaria hyperborea) and sea urchins (Strongylocentrotus droebachiensis) in the NE Atlantic are highly related to physical factors and to temporal changes in temperature. On a large scale, we identified borders for kelp recovery and sea urchin persistence along the north-south gradient. Sea urchin persistence was also related to the coast-ocean gradient. The southern border corresponds to summer temperatures exceeding about 10°C, a threshold value known to be critical for sea urchin recruitment and development. The outer border along the coast-ocean gradient is related to temperature, wave exposure and salinity. On a finer scale, kelp recovery occurs mainly at ridges in outer, wave exposed, saline and warm areas whereas sea urchins still dominate in inner, shallow and cold areas, particularly in areas with optimal current speed for sea urchin foraging. In contrast to other studies in Europe, we here show a positive influence of climate change to presence of a long-lived climax canopy-forming kelp. The extent of the coast-ocean gradient varies within the study area, and is especially wide in the southern part where the presence of islands and skerries increases the area of the shallow coastal zone. This creates a large area with intermediate physical conditions for the two species and a mosaic of kelp and sea urchin dominated patches. The statistical models (GAM and BRT) show high performance and indicate recovery of kelp in 45–60% of the study area. The study shows the value of combining a traditional (GAM) and a more complex (BRT) modeling approach to gain insight into complex spatial patterns of species or habitats. The results, methods and approaches are of general ecological relevance regardless of ecosystems and species, although they are particularly relevant for understanding and exploring the corresponding changes between algae and grazers in different coastal areas.

Highlights

Kelp forest and sea urchin dominated barrens are considered to be two alternative stable states of an ecosystem [1,2] and the presence of the two states is mutually exclusive
The focus of this study is how the distribution of the kelp Laminaria hyperborea and the sea urchin Strongylocentrotus droebachiensis is affected by physical factors
After decades of destructive grazing and barren ground formation, sea urchin populations decline and kelp forests recover along parts of the Norwegian coast in the NE Atlantic

Summary

Introduction

Kelp forest and sea urchin dominated barrens are considered to be two alternative stable states of an ecosystem [1,2] and the presence of the two states is mutually exclusive. Due to reinforcing feedback mechanisms both states have a high degree of resilience; the kelp forest inhabits sea urchin predators that may prevent overgrazing, whereas the barren areas lack suitable habitats for sea urchin predators [2]. The transition from sea urchin dominated barrens to kelp seems to occur when sea urchin density decrease below critical thresholds [2,3,4]. Since the 1970s, the sea urchin Strongylocentrotus droebachiensis (O.F. Muller) has occurred in high densities in sheltered or moderately wave exposed areas between 63uN and 71uN [5,6,7,8].

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