ALBERT

Description: The early life-history stages of polar marine invertebrates are understudied, particularly in deep water. We present the results from a long-term (1999 – 2017) colonization experiment at the LTER (Long-Term Ecological Research) observatory HAUSGARTEN in the Fram Strait (Arctic Ocean, 79⁰ N, 04⁰ E, 2500 m water depth). Recruitment panels were constructed from plastic and brick and deployed attached to a metal frame in 1999. The experiment was monitored using an ROV in 2003 and 2011 and recovered in 2017. Recruitment was very low, with only foraminiferans being visible after 4 years (2003) and one metazoan species, the hydroid Halisiphonia arctica, being visible on the panels after 12 years (2011). After 18 years underwater, panels were colonized by 13 metazoan species as well as calcareous and agglutinating foraminiferans. Recruitment was higher on brick panels than on plastic, but while some species were more common on panels at higher altitude (H. arctica and the crinoid Bathycrinus carpenterii), others were more common on panels closer to the seafloor (serpulid polychaetes, agglutinating foraminifera) or on panels in line with the predominant bottom current (small round white sponge). The most common recruiting species can be described as opportunistic and are rare or absent in older communities on natural substrata nearby. Meanwhile, large hexactinellid sponges that are common in natural communities did not recruit to our panels. These results suggest that succession in the Arctic deep sea takes decades and involves early dominance by opportunistic species, with slower-growing, structureforming species appearing later on.