The Influence of Physical Factors on Kelp and Sea Urchin Distribution in Previously and Still Grazed Areas in the NE Atlantic

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.

Indicator-Based Assessment of Marine Biological Diversity–Lessons from 10 Case Studies across the European Seas

“…Kelp forests along the Norwegian and Russian coast were indeed dramatically grazed during the early 1970s and replaced by barren grounds dominated by sea urchins (Norderhaug and Christie, 2009). Though a progressive northward recovery of kelp forests extent is observed, its recovery status is still partial in northern Norway (Sivertsen, 2006;Rinde et al, 2014). (2) In northernmost part of the Barents sea, sea-ice extent is undergoing a particularly dramatic decrease (Parkinson et al, 1999) with a significant decrease rate of −3.5% per decade of winter ice extent (Sorteberg and Kvingedal, 2006) as a response to climate warming (Boitsov et al, 2014).…”

Status, trends and drivers of kelp forests in Europe: an expert assessment

“…In some smaller areas on the West coast of Norway a spatial predictive model was built. The southern border of Laminaria hyperborea kelp loss caused by sea urchin grazing was assessed based on field observations in 2011, as described in Rinde et al (2014). A grid cell was defined as ''reduction'' (i.e.…”

“…On the other hand, the increase in seawater temperature has also resulted in unfavourable conditions in some areas for overgrazing by sea urchins, resulting in kelp forest recovery in large areas of the NE Atlantic (e.g. Norderhaug and Christie 2009;Rinde et al 2014). Storms affect kelp mortality (Christie et al 1998, Smale and, and future increases in storm frequencies due to changes in climate are also likely to affect kelp forest distribution.…”

Biodiversity and ecosystem services in Nordic coastal ecosystems

“…The rise in seawater temperature may be part of the explanation, making unfavorable conditions for sea urchin larvae, with resultant decreased grazing pressure allowing for kelp forest recovery (e.g. Rinde et al, 2014).…”

“…The reason for this development is not fully understood, but might relate to both stochastic and cyclic events. However, in the last decade, a gradually northward recovery of kelp has been observed Rinde et al, 2014). This recovery is partly explained by the negative effects of warming on sea urchin recruitment (Fagerli et al, 2013) and to some degree from increased predation by northward expanding Cancer pagurus and Carcinus maenas crabs .…”