Seasonal Patterns of Sporophyte Growth, Fertility, Fouling, and Mortality of Saccharina latissima in Skagerrak, Norway: Implications for Forest Recovery
On the Skagerrak coast the kelp Saccharina latissima has suffered severe stand reductions over the last decade, resulting in loss of important habitats. In the present study, healthy kelp plants were transplanted into four deforested areas and their patterns of growth, reproduction, and survival were monitored through subsequent seasons. Our main objective was to establish whether the kelp plants were able to grow and mature in deforested areas. We observed normal patterns of growth and maturation at all study sites. However, heavy fouling by epiphytes occurred each summer, followed by high kelp mortality. The study shows that the seasonal variations and the life stage timing of S. latissima make formation of self-sustainable populations impossible in the present environment. Most noteworthy, we suggest that fouling by epiphytes is involved in the lack of kelp forest recovery in Skagerrak, Norway. 1. Introduction Saccharina latissima (Linnaeus) C. E. Lane, C. Mayes, Druehl, and G. W. Saunders is a large (1–3？m) brown alga in the order Laminariales (kelps). The species is common, and often dominates the subtidal vegetation on sheltered rocky shores in Norway. In 2002, a dramatic decline of S. latissima was observed along the south coast of Norway (Moy and Christie, submitted). Areas previously (in the mid 1990s and before) dominated by dense stands of kelps are now almost exclusively dominated by annual filamentous algae. A report published by The Norwegian Climate and Pollution Agency (Klif) in 2009 revealed the vegetational shift as a geographically widespread phenomenon along the entire south coast of Norway, from the Swedish border in the east to (and including) the M？re og Romsdal region in the west. When islands and fjords are included this stretch has approximately 34？500？km of coastline. According to Moy and Christie (submitted) the wave sheltered fjords have been more affected than wave exposed coastline. Scattered S. latissima individuals were observed at several of the stations in poor condition, but mainly at very shallow waters (0–2？m depth) (Moy and Christie, submitted). The mechanisms that drove the shift and the mechanisms currently preventing recolonisation of kelp have not been identified. Increasing water temperatures in Skagerrak over the past decades and a couple of particularly warm summers (in 1997 and 2002) preceding observations of deforestation has pointed toward elevated temperature as a probable culprit (Moy and Christie, submitted). Large-scale shift from perennial macrophytes to short lived ephemeral algae is a global
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