Abstract
Kelp forests produce large amounts of macroalgal detritus, ranging from whole plants to small particles (1 mm). The role of this kelp detritus in fueling deep-sea communities adjacent to healthy kelp forests was investigated in a region in the north of Norway by comparing the community structure and biodiversity of meio-, macro-, and megafauna in two deep (450 m) areas with different expected input of kelp detritus: a deep fjord basin surrounded by kelp forests and the adjacent continental shelf 15 km offshore from the kelp forests. The results showed that, although the fjord received a significantly higher amount of large kelp detritus (i.e. blades) than the shelf area, the amount of small kelp detritus available on the sediment was similar in both areas. There were significant differences in the multidimensional scaling analyses on the community structure for meio-, macro-, and megafauna between the fjord and the shelf. Significant differences were also found in biomass, abundance and biodiversity indices for some groups. However, no clear pattern emerged in the community structure and biodiversity between the fjord and the shelf, and the observed differences could not be linked directly to kelp detritus availability. The similar amounts of small particles of kelp detritus in the fjord and shelf area suggest that kelp detritus can provide organic matter to ecosystems further away than initially hypothesized, thus potentially shaping the structure and functioning of deep benthic communities distant from the kelp forests. Yet, the direct (trophic) links of kelp detritus and the studied benthic fauna need to be further analysed. The results are discussed in relation to current global changes in kelp forest, including regime shifts from healthy kelp reefs to turfs or barren areas, which reduce drastically the amount of macroalgal detritus produced and exported.
Highlights
Deep-sea ecosystems are characterized by the lack of photosynthetically-available light
We showed that kelp detritus is produced by distal erosion of blades, dislodgement of blades and whole plants, grazing from sea urchins and other fauna, and phenologically-driven whole blade loss in spring (Pedersen et al, 2019)
The results show that the rank correlation coefficient ƿ is maximised for median grain size (MGS) (0.71), followed by a combination of MGS and total organic matter (TOM) (ƿ = 0.66), and MGS, total nitrogen (TN) and TOM (ƿ = 0.61)
Summary
Deep-sea ecosystems (commonly considered below 250 m depth and often coinciding with the shelf break, Thisle, 2003) are characterized by the lack of photosynthetically-available light. Deep-sea communities are predominantly heterotrophic, with their metabolism depending on the breakdown of complex organic molecules derived from photosynthetic production in the euphotic zone (Gage, 2003). This particulate organic matter enters the system through the water column as “marine snow”, as well as advected down the margins or as sporadic falls of large parcels of organic matter (e.g. fish, whales, wood and macroalgae) (Ramirez-Llodra et al, 2010). Because of the hetero trophic nature of most deep-sea communities, the quantity and quality of food supply to the deep-sea floor plays a major role in shaping species composition, and the spatio-temporal patterns of abundance and species richness (Billett et al, 2001; Stuart et al, 2003; Smith et al, 2008; Costello and Chaudhary, 2017)
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