COS 66-4 - Evidence of deterministic community assembly from fossil diatoms in ancient Lake Towuti, Indonesia

Background/Question/Methods

Although many classical studies on diatoms viewed them as opportunistic and stochastically-distributed species, others argued that diatom species are not randomly distributed, but that they closely track environmental changes, suggesting that these communities may follow deterministic patterns of assemblage. The fossil record analyzed from the sediment cores from ancient Lake Towuti, Indonesia ( >1My) identified two long-term planktonic diatom communities, known as planktonic maxima, each persisting for ~50kyr. These maxima were separated by a period ~100 ky of where diatoms were almost completely absent and both maxima were dominated by the same five provisional taxa of Aulacoseira. The aim of this study is to investigate whether Aulacoseira taxa were stochastically or deterministically distributed within these planktonic maxima. To date, we processed 43 sediment samples from planktonic maxima from two deep sediment cores, each ~135 m in depth. We counted a total of ~5,700 diatom valves via light microscopy to calculate relative abundances of each taxa. In order to determine changes in Aulacoseira species composition through time, we used nonmetric multidimensional scaling (NMDS). Stratigraphically constrained hierarchal clustering analysis (CONISS) was used to identify stratigraphic zones with similar diatom assemblages.

Results/Conclusions

In our preliminary results we have found that the upper layers of each planktonic maxima were dominated by A. towutiensis (provisional name) (67.9%±16.9). The lower layers of each maximum were dominated either by A. pseudomuzzanensis in the upper planktonic maxima (64.5%±26.7%), or A. granulata sensu lato in the lower planktonic maxima (56.0%±14.7). These findings were also reflected in the CONISS and NMDS results which showed similar patterns in species composition through time. The establishment and re-establishment of a repeated temporal pattern in diatom community composition, separated by a period of ~100ky when these taxa were almost absent, suggests that diatom communities do follow deterministic patterns of assemblage. This pattern was not only observed over an extensive period of geological time, but it also occurred under natural conditions, long before any potential disturbance by modern humans in the Anthropocene.