Linking environmental variability to diatom assemblages in seasonal wetlands

Seasonal wetlands undergo recurrent inundation and contraction, creating strong hydrochemical variability and non–independent observations within pans. We tested whether this variability translated into diatom differentiation in five pans within each of two South African Ramsar wetland systems, Makuleke and Nylsvley, during cool–dry and hot–wet seasons. Two field replicates per pan–season yielded 40 diatom samples and 40 environmental records. Environmental replicates were averaged within pan–season; diatom relative abundances were calculated within each replicate and averaged with equal weight. Fifty–three taxa were detected. Makuleke had higher temperature, pH, total dissolved solids, conductivity and several major ions than Nylsvley, whereas temperature, total dissolved solids, conductivity, nitrate and sodium differed between seasons. Hierarchical permutation tests detected no community difference between systems or system by season interaction, and the seasonal pattern approached but did not reach the 0.05 threshold. Rarefied diversity and the six variable canonical correspondence model were also non–significant. Thus, hydrochemical separation did not yield supported assemblage separation at the sampled scale. These results caution against inferring ecological status from nested samples and identify repeated, substrate consistent monitoring as necessary for diatom–based assessment of seasonal wetlands.