Abstract:
Sea surface planktonic assemblages were sampled using environmental DNA at 1.5° latitudinal increments (from 39.5 to 11.5°S) following the 110°E meridian in the Eastern Indian Ocean to reveal factors structuring eukaryotic diversity. Metabarcoding the v4 region of the 18S rRNA gene revealed a eukaryotic assemblage over-dominated by amplicon sequence variants representing just four taxa that comprised 50% of reads, which were from copepods. Eukaryotes were predominantly heterotrophs and mixotrophs, with the presence of individual taxa responding to nutrient and temperature/oxygen gradients. Assemblage richness and diversity was greater at the northern end of the transect, but this diversity change occurred mostly around the sub-tropical front (∼35°S) with an unusually high degree of uniformity and non-monotonic progression in the alpha and beta diversity over the majority of the transect despite extending through distinct water masses, i.e., Indonesian Throughflow with tropical, low salinity waters, Subtropical Surface Water of high salinity waters and varying temperature, and Sub-Antarctic Mode Water with cooler and low salinity water. However, piecewise linear models of environmental and biotic data and fuzzy c-means clustering (with k = 2, 3 or 4) identified some discontinuities in the transect that were clumped at the southern end of the transect, although these cluster boundaries lacked consensus, being often poorly defined between the different data types. This eDNA data was largely concordant with morphological analyses, that also found the eukaryotic assemblage became largely stable northwards of the southernmost sampling points. These results contrast to other observations of diversity increasing monotonically toward the equator; i.e., eukaryotic communities were contrary to the “latitudinal diversity gradient hypothesis”. This accords with observations that extremely oligotrophic conditions can stabilise planktonic assemblages, despite significant latitudinal differences in temperature of as much as 16 °C.
