Nature and ecological implications of algal pigment diversity on the Molenplaat tidal flat (Westerschelde estuary, SW Netherlands)

Depth profiles of chlorophyll a (chl a), its degradation products, and accessory pigment markers from 5 sites with differing sediment properties highlighted the role of physical and biological processes in re-distributing microalgal biomass both horizontally and vertically around the Molenplaat tidal flat. Intermediate-scale distribution of chi a was highly correlated with sedimentological parameters, with biomass greatest at siltier sites. Fucoxanthin:chl a ratios of 0.35 to 1.60 indicated that the microphytobenthos community was dominated by diatoms in both silty and sandy sediments. Cyanobacteria (zeaxanthin) were also present, mainly in September. Other sources of algae came in the form of deposited water column material. Peridinin, 19'hexanoyloxyfucoxanthin, and to a lesser extent chl b were the main pigments, primarily found at silty sites. Between June and September, the relative amounts of accessory pigments increased suggesting that inputs of material from the water column occurred during this period. Although benthic and water column pigments were found in the surface layers of silty and sandy sites, there were pronounced differences in biomass and vertical distribution. At sandy sites, homogenous vertical distributions of chi a in the upper 2 cm of sediment, together with low phaeopigment levels and the absence of water colum pigments below the surface layer, suggest that accumulation of in situ and allochthonous sources of microalgae is insignificant. Material is subjected to a continuous cycle of burial and resuspension during each tidal cycle and through the action of bioturbators such as Arenicola marina. There was a pronounced peak in chi a biomass at the silty sites in June. Rapidly decreasing chi a over the top 1 cm suggests that degradation was more rapid than sediment mixing. Large amounts of phaeopigments were produced, with the relative contribution switching from phaeophytins (March/April) to phaeophorbides (September) as the role of the herbivorous community in chi a cycling became more significant. An increase in relative contributions of accessory pigments both at the surface and in deeper layers between spring and autumn suggest that silty sites are a sink for sedimenting algae from the water column during this time. During the late autumn-winter, chi a biomass and pigment diversity decreased considerably, coincident with reduced biological activity and increased physical mixing.