Abstract
Ongoing changes along the northeastern Atlantic coastline provide an opportunity to explore the influence of climate change and multitrophic interactions on the recovery of kelp. Here, vast areas of sea urchin‐dominated barren grounds have shifted back to kelp forests, in parallel with changes in sea temperature and predator abundances. We have compiled data from studies covering more than 1,500‐km coastline in northern Norway. The dataset has been used to identify regional patterns in kelp recovery and sea urchin recruitment, and to relate these to abiotic and biotic factors, including structurally complex substrates functioning as refuge for sea urchins. The study area covers a latitudinal gradient of temperature and different levels of predator pressure from the edible crab (Cancer pagurus) and the red king crab (Paralithodes camtschaticus). The population development of these two sea urchin predators and a possible predator on crabs, the coastal cod (Gadus morhua), were analyzed. In the southernmost and warmest region, kelp forests recovery and sea urchin recruitment are mainly low, although sea urchins might also be locally abundant. Further north, sea urchin barrens still dominate, and juvenile sea urchin densities are high. In the northernmost and cold region, kelp forests are recovering, despite high recruitment and densities of sea urchins. Here, sea urchins were found only in refuge habitats, whereas kelp recovery occurred mainly on open bedrock. The ocean warming, the increase in the abundance of edible crab in the south, and the increase in invasive red king crab in the north may explain the observed changes in kelp recovery and sea urchin distribution. The expansion of both crab species coincided with a population decline in the top‐predator coastal cod. The role of key species (sea urchins, kelp, cod, and crabs) and processes involved in structuring the community are hypothesized in a conceptual model, and the knowledge behind the suggested links and interactions is explored.
Highlights
Transitions between kelp forests and sea urchin‐dominated barren grounds have been studied for decades
Wide‐scale kelp recovery is occurring along distinct regions of the Norwegian coast (Norderhaug & Christie, 2009; Rinde et al, 2014), offering a rare opportunity to explore important scientific and man‐ agement questions related to the processes and mechanisms in‐ volved in the return and persistence of recovered kelp forests
We explored the sea urchins’ use of bedrock and cobblestone, FIGURE 3 Map of northern Norway showing the distribution of the 11 sampling areas from Vega (~65.5oN, 12.5oE) to the Russian border in the northeast with relative abundance of sea urchins (Strongylocentrotus droebachiensis, green columns) and kelps (Laminaria hyperborea and Saccharina latissima, brown columns) based on a total of 1,249 stations
Summary
Transitions between kelp forests and sea urchin‐dominated barren grounds have been studied for decades (reviewed by Filbee‐Dexter & Scheibling, 2014; Lawrence, 1975). Both kelp forests and bar‐ rens have been described as stable states (Elner & Vadas, 1990; Marzloff et al, 2013), with several reinforcing feedback mecha‐ nisms, making a shift to the alternative state difficult (Filbee‐Dexter & Scheibling, 2014; Ling et al, 2015). Wide‐scale kelp recovery is occurring along distinct regions of the Norwegian coast (Norderhaug & Christie, 2009; Rinde et al, 2014), offering a rare opportunity to explore important scientific and man‐ agement questions related to the processes and mechanisms in‐ volved in the return and persistence of recovered kelp forests
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