Rocky islands in a sea of mud: biotic and abiotic factors structuring deep-sea dropstone communities

Abstract

Stones released by melting icebergs are called dropstones, and these stones constitute island-like hard-bottom habitats at high latitudes. In 2012, dropstone megafauna in the HAUSGARTEN observatory in the Fram Strait was sampled photographically. We tested the hypothesis that dropstones would have the same species distribution patterns as terrestrial islands, using 5 patterns commonly found in the classical island literature. Higher richness, diversity, and abundance of fauna occurred on larger stones and on stones near a deep-water rocky reef. These patterns can be explained by the greater surface area of larger stones, the exposure of larger stones to faster current higher in the benthic boundary layer, and increased larval supply from the rocky reef. Some pairs of morphotypes (12 pairs out of 56 morphotypes and 1540 possible pairs) co-occurred less often than expected by chance. While similar patterns have been attributed to interspecific competition in the classical island literature, we offer alternative mechanisms for dropstones. Non-random co-occurrence on dropstones may be explained by larval dispersal. Dropstone fauna had an overdispersed (clumped) distribution, so pairs of morphotypes may have negative non-random co-occurrence simply because short larval life and limited dispersal ability prevent them from having randomly overlapping distributions. In addition, we found 8 morphotype pairs that co-occurred more often than expected by chance because of epibiontism. The patterns found in dropstone communities resemble terrestrial islands, but different mechanisms may be responsible.

Glacial dropstones: islands enhancing seafloor species richness of benthic megafauna in West Antarctic Peninsula fjords

“…This is consistent (albeit weakly) with the findings of Meyer et al (2016) for Arctic dropstone communities, in which dispersal-limitation caused by increasing distance from a large, hardsubstrate larval source (i.e. a 'mainland') explained most of the variability in the communities (Meyer et al 2016). Dropstones within WAP fjords likely have a residence time of years because the sedi mentation rate in the fjords is 0.5−1 cm yr −1 , while dropstones on the outer shelf likely remain exposed for decades be cause the sedimentation rate is 2 orders of magnitude lower (~0.02 cm yr −1 ; Grange & Smith 2013).…”

“…fjord walls), which is expected for island habitats. This is consistent (albeit weakly) with the findings of Meyer et al (2016) for Arctic dropstone communities, in which dispersal-limitation caused by increasing distance from a large, hardsubstrate larval source (i.e. a 'mainland') explained most of the variability in the communities (Meyer et al 2016).…”

“…The correlations between dropstone area and species richness and abundance, and the increase in the proportion of dropstones colonized with increasing dropstone size, match the predicted island-versus-size relationships of island biogeog- raphy. Recent investigations of Arctic dropstone communities in Svalbard yielded similar relationships between dropstone size and species richness (Meyer et al 2016). It is clear from our study that dropstone communities in fjords along the WAP and on the open Antarctic shelf were not spacelimited; all dropstones contained open space, and uncolonized stones often were covered with sediment, indicating they had been at the seafloor for a relatively long time.…”

“…Characteristics of the environment, such as the magnitude of overlying productivity which controls the flux of organic matter to the seafloor, may influence the community structure and especially the functional diversity of the communities between sites. The extent to which dropstone communities function as habitat islands has recently been investigated in Svalbard fjords (Meyer et al 2016) but remains unassessed in most glaciomarine fjords, including along the WAP, which harbors novel patterns of biodiversity and community structure. While a positive correlation was found between dropstone size and species richness in Svalbard fjords, the mechanisms behind this island-like phenomenon are likely to be related to dispersal and not the same mechanisms that produce these patterns in terrestrial island communities, such as disturbance and competition (Meyer et al 2016).…”

Major impacts of climate change on deep-sea benthic ecosystems

“…This disturbance leads to scale-dependent recolonization of scoured areas and an increased input of dropstones (Smale and Barnes, 2008). These processes will enhance seafloor heterogeneity and create hard substrates for sessile megafauna (Schulz et al, 2010;Meyer et al 2015Meyer et al , 2016. Dropstones also create diverse microhabitats for meiofauna, allowing for greater trophic and functional diversity around stones (Hasemann et al, 2013;Gooday et al, 2015).…”