The relevance of epilithic foraminifera in ecological studies
crossref(2020)
摘要
<p>This study aims to emphasize the ecological importance of foraminifera contained within the > 1000 µm sediment fraction. Stones and gravel offer a habitat for diverse and rich epilithic foraminiferal communities. However, gravel-rich sediments > 1000 µm 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 lack of knowledge about the biology and ecology of epilithic foraminifera, specifically about agglutinated species.</p><p>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 a relatively large portion of gravel and shell fragments, which were densely populated by monothalamus, agglutinated foraminifera.</p><p>A faunal analysis of foraminifera in the > 1000 µm sediment fraction showed, that the most abundant species (> 85 %, 54 ind. 10 cm<sup>-3</sup>) in > 1000 µm consisted of <em>Tholosina vesicularis</em>, an unicellular agglutinated species that can reach up to 4 mm diameter. SEM-analysis revealed, that large quantities of partially decomposed diatom frustules were embedded within the protoplasm of <em>T. vesicularis</em> individuals, which were supposedly the remains of the foraminiferal diet. The sediment fraction of 125 - 1000 µm was dominated by <em>Ammonia</em> species (58 %, 190 ind. 10 cm<sup>-3</sup>), a genus known for its fast turnover of diatom-derived carbon and as key-players in benthic nutrient fluxes. Preliminary biovolume analysis of the two genera (<em>T. vesicularis</em>: n = 74, <em>Ammonia</em> spp.: n = 110) resulted in far higher values for <em>T. vesicularis</em> (853 ± 944 µm<sup>-3</sup> 10 cm<sup>-3</sup>, med. = 506 µm<sup>-3</sup> 10 cm<sup>-3</sup>), than for <em>Ammonia</em> spp. (117 ± 56 µm<sup>-3</sup> 10 cm<sup>-3</sup>, med. = 96 µm<sup>-3</sup> 10 cm<sup>-3</sup>), within the uppermost sediment layer of 0 - 1 cm. Therefore, <em>T. vesicularis</em> could be a main driver of benthic carbon turnover in gravel-dominated sediments. We hypothesize, that the epilithic fauna, when present is at least as relevant as sediment-dwelling species.</p><p>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.</p><p>This is most likely due to the motility of the free-living forms of 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 <em>T. vesicularis </em>and <em>Ammonia</em> spp..</p><p>In conclusion, this study shows, that if present, the > 1000 µm fraction can contain important information for the interpretation of ecological studies on the communities of live foraminifera and their role in marine ecosystems.</p>
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