Ecology of detached seaweeds (KOL 06 d)

Manipulative field experiments to quantify effects of detached seaweeds on the structure, diversity, and function of faunal soft-bottom communities. Direct effects include mechanical disturbance on and food supply for fauna. Indirect effects include modification of predator presence and pressure as well as competitive interactions.

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  • field work


  • marine biology

Project Keywords

  • oceans / marine biology / marine invertebrates
  • oceans / marine biology

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Few studies addressed the connectivity between hard- and soft-bottom habitats in the Arctic. Detached seaweeds are one vector connecting rocky and sedimentary habitats. Their impact will very likely increase in the light of climate change, due to a postulated increase in storminess, an observed 20 - 40 % increase in seaweed abundance in Spitsbergen, and loss of protective sea-ice during the harsh winter season. Kelps are the large, sessile organisms that cannot escape, which makes them prone to detachment at exposed sites or during stormy periods. Detached kelp either accumulate on the shore or cross coastal habitats to reach deeper fjord parts. We observed accumulations of detached kelp that persisted for > 2 months in soft-bottom habitats adjacent to rocky shores, from where their originate. The effects of such seaweed accumulations on the structure, diversity, and function of the communities that dwell in soft-bottoms has been studied by us in summer, but we do not know if the effects will be stronger during winter, i.e. the season when kelp detachment rate should be highest. The soft-bottom fauna plays a pivotal role in food webs as it captures suspended biomass from pelagic sources and turns it into biomass that serves higher trophic levels of the benthos and fishes as food. Our project will assess experimentally the effects of accumulations of detached seaweeds at Brandal (ca. 1 km away from the nearset kelp bed at Stuphallet), where our summer experiment has been conducted. In addition, we will set up this experiment at Stuphallet to test if soft-bottom communities close to the source area of detached kelp will be stronger affected than those further away at Brandal. In September/October 2016, we will install 15 plots without manipulation (= control), 15 plots where football nets (50 x 50 cm) will be filled with 1.5 kg detached seaweeds (= treatment), and 15 plots where empty football nets will be fixed to the seafloor (= treatment control). In addition, we will mark 200 kelp attached to the rocky shore at Stuphallet. In March 2017 we will quantify remaining marked kelp and take sediment samples from all 90 experimental plots to quantify the abundance of each animal species in each sample. Furthermore, the number of seaweed mat-associated consumers (Hyas, fish, whelk) will be quantified. Afterwards, we will install 60 complete cages to enclose 3 Hyas (= treatment), 60 partial cages (= treatment control), and no cages in remaining plots (= control). In July 2017, the number of recovered seaweeds will be recorded. Moreover, a sediment core will be taken from each plot and the number of consumers inside seaweed mats quantified. This data will allow us to compare the effects of seaweed mats between seasons (summer, winter, spring) and whether seaweed mats interact with consumer effects, as consumers may be attracted by seaweed mats as refuge habitat. In 2018 and 2019, we plan to set up a competition experiment in the field and in the laboratory, in which we will manipulate the density of Euchone analis, which dominates patches of soft-bottoms, indicating that this polychaete can act as a suprior competitor. The interactive effects of detached seaweed will also be tested. All experiments will be also conducted in sedimentary habitats in the German Bight. This will allow a latitudinal comparison of the separate and interactive effects of detached seaweeds, consumers and competitors on soft-bottom communities. Besides structural responses, like species composition, species richness and evenness, we will test functional traits of the faunal soft-bottom community, i.e. productivity and filtration rates.

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