Effects of sandy vs muddy sediments on the vertical distribution of microphytobenthos in intertidal flats of the Fraser River Estuary, Canada.

Benthic algae or microphytobenthos (MPB) in intertidal flats play an important role in the sediment and overlying water ecosystems. We hypothesize that there are effects of sediment texture on the vertical distribution of MPB using chlorophyll a (chl a) as a proxy for MPB biomass and present results over a 2.5-year period. Four sites were sampled monthly: two sandy sites (A10 and A12) and two muddy sites (A0 and A14) on the intertidal flats of the Fraser River Estuary. At the two sandy sites, pigments were distributed down to 10 cm. High ratios of depth-integrated chl a to phaeopigments suggest that the chl a had been recently buried. In contrast, at the muddy sites, pigments were limited to the top 4 cm, with MBP in the top 1 cm contributing up to 60 % of the whole sediment core pigments. As a result, the depth-integrated chl a values were on average 2,044 mg m(-2) (160-4,200) at A10 and 882 mg m(-2) (183-2,569) at A12, the two sandy sites, and much higher than at the two muddy sites where averages of 84 mg m(-2) (41-174) and 235 mg m(-2) (77-854) were measured at A0 and A14, respectively. Despite these lower concentrations at the muddy sites than at the sandy sites, particulate organic carbon (POC) and nitrogen (PON) concentrations showed a homogenous vertical distribution at the two sandy sites. Such a homogeneous vertical distribution of chl a, POC, and PON suggests that vertical transport mechanisms were actively transporting organic material into and out of the sediment. These results suggest that MBP on sandy sediments play a very active role in providing food for herbivores and are interacting with the overlying water column in the sediment-water exchange processes during tidal cycles.