The relevance of epilithic foraminifera in ecological studies
Authors: Wukovits Julia , Wollenburg Jutta , Nehrke Gernot , Glock Nicolaas , Heinz Petra , Roy Alexandra-Sophie 
Published: March 2020

&lt;p&gt;This study aims to emphasize the ecological importance of foraminifera contained within the &gt; 1000 &amp;#181;m sediment fraction.&amp;#160;Stones&amp;#160;and gravel offer a habitat for diverse and rich epilithic foraminiferal communities.&amp;#160;However, gravel-rich sediments &gt; 1000 &amp;#181;m are usually not the subject of quantitative benthic foraminifera studies, because most foraminifera species used as proxies or bioindicators are sediment-dwelling and found within smaller sediment size fractions. Therefore, there is a current&amp;#160;lack of knowledge about the biology and ecology of&amp;#160;epilithic foraminifera, specifically about agglutinated species.&lt;/p&gt;&lt;p&gt;During a cruise at the Gullmarfjord/Sweden in September 2018, we retrieved a core at a station at 7 m water depth, which contained organic-rich material and&amp;#160;a relatively large portion of gravel and shell fragments, which were densely populated by monothalamus, agglutinated foraminifera.&lt;/p&gt;&lt;p&gt;A faunal analysis&amp;#160;of foraminifera in the &gt; 1000 &amp;#181;m sediment fraction showed, that the most abundant species (&gt;&amp;#160;85 %, 54 ind. 10 cm&lt;sup&gt;-3&lt;/sup&gt;) in &gt; 1000 &amp;#181;m consisted of &lt;em&gt;Tholosina vesicularis&lt;/em&gt;, an unicellular agglutinated species that can reach up to 4&amp;#160;mm diameter. SEM-analysis revealed, that large quantities of partially decomposed diatom frustules were embedded within the protoplasm of &lt;em&gt;T. vesicularis&lt;/em&gt;&amp;#160;individuals, which were supposedly the remains of the foraminiferal diet. The&amp;#160;sediment&amp;#160;fraction of 125 - 1000 &amp;#181;m was dominated by &lt;em&gt;Ammonia&lt;/em&gt;&amp;#160;species&amp;#160;(58&amp;#160;%, 190 ind. 10 cm&lt;sup&gt;-3&lt;/sup&gt;), a genus&amp;#160;known for its&amp;#160;fast turnover of diatom-derived carbon and as key-players in benthic nutrient fluxes.&amp;#160;Preliminary biovolume analysis of the two genera (&lt;em&gt;T. vesicularis&lt;/em&gt;: n = 74, &lt;em&gt;Ammonia&lt;/em&gt;&amp;#160;spp.: n = 110) resulted in far higher values for &lt;em&gt;T. vesicularis&lt;/em&gt;&amp;#160;(853 &amp;#177;&amp;#160;944 &amp;#181;m&lt;sup&gt;-3&lt;/sup&gt;&amp;#160;10 cm&lt;sup&gt;-3&lt;/sup&gt;, med. = 506 &amp;#181;m&lt;sup&gt;-3&lt;/sup&gt;&amp;#160;10 cm&lt;sup&gt;-3&lt;/sup&gt;), than for &lt;em&gt;Ammonia&lt;/em&gt;&amp;#160;spp. (117 &amp;#177;&amp;#160;56 &amp;#181;m&lt;sup&gt;-3&lt;/sup&gt;&amp;#160;10 cm&lt;sup&gt;-3&lt;/sup&gt;, med. = 96 &amp;#181;m&lt;sup&gt;-3&lt;/sup&gt;&amp;#160;10 cm&lt;sup&gt;-3&lt;/sup&gt;), within the uppermost sediment layer of 0 - 1 cm. Therefore, &lt;em&gt;T. vesicularis&lt;/em&gt;&amp;#160;could be a main driver of benthic carbon turnover&amp;#160;in&amp;#160;gravel-dominated sediments. We hypothesize, that&amp;#160;the&amp;#160;epilithic fauna, when present&amp;#160;is at least as relevant as sediment-dwelling species.&lt;/p&gt;&lt;p&gt;Additionally, biodiversity indices (species diversity, Shannon-Index, Evenness), show a slightly higher diversity and a more even distribution within the softer substrate, compared to the stones.&lt;/p&gt;&lt;p&gt;This is most likely due to the motility of the free-living forms of&amp;#160;the smaller size fraction, which allows them a flexible response to spacial competition. However, niche separation between hard and soft-substrate allows the co-existence of the opportunistic species &lt;em&gt;T. vesicularis&amp;#160;&lt;/em&gt;and &lt;em&gt;Ammonia&lt;/em&gt; spp..&lt;/p&gt;&lt;p&gt;In conclusion, this study shows, that if present, the &gt; 1000 &amp;#181;m fraction can contain important information for the interpretation of ecological studies on the communities of live foraminifera and their role in marine ecosystems.&lt;/p&gt;