Seasonal and interannual patterns of intertidal microphytobenthos in combination with laboratory and areal production estimates

From April 1994 to December 1997, we studied the microphytobenthic assemblages in surface (0 to 0.5 cm) and subsurface (0.5 to 2 cm) sediments at spring low tide along a transect of 5 stations in an estuarine sandflat of the Seto Inland Sea, Japan. At the innermost sampling station, microphyto-benthos biomass (chl a) was also investigated in a vertical profile to 10 cm depth from December 1994 to April 1996. The chl a contents at the 2 uppermost layers were well correlated with each other, with a mean decrease of 34% from the surface to subsurface layer. Chl a tended to decrease rapidly through the vertical profile and was reduced to 3.2 +/- 1.4% SD in the 9 to 10 cm layer. There was a progressive decrease in the chl a content every year in fall and the occurrence of major peaks in early spring and/or summer. This was accompanied by a significant increase in microphytobenthos biomass from 1994 to 1995 and from 1995 to both 1996 and 1997. The microphytobenthos biomass in surface sediments (mean of 5 stations) ranged between 27.7 (October 1994) and 120 mg chl a m(-2) (July 1997), or between 3.9 (November 1994) and 20.3 mug chl a g(-1) dry wt (July 1996). Annual mean (1995 to 1997) biomass was 72.3 +/- 27.1 mg chl a m-2 and 11.0 +/- 4.3 mug chl a g(-1) dry wt. These values rank in the mid-upper range of microphytobenthic biomass for intertidal sediments. In addition to the field investigations, we conducted laboratory experiments on a dominant diatom species, Navicula sp. The photosynthetic rate of Navicula sp. was saturated at a light intensity of 165 muE m(-2) s(-1) at 21degreesC. No photoinibition was found at higher light intensities up to 400 muE m(-2) s-1. The relationship between temperature and photosynthetic rate was positive and linear within a temperature range between 10 and 35degreesC at 55 muE m(-2) s(-1). Areal 'Potential' primary production of microphytobenthos was between 0.32 (December 1994) and 3.0 g C m(-2) d(-1) (July 1997), with an annual mean of 1.2 g C m(-2) d(-1). Uni- (summer) or bi-modal (spring and summer) peaks of microphytobenthos biomass and primary production highlighted a marked interannual variability. Marked seasonal patterns were also recognizable, with primary production of microphytobenthos significantly higher both in spring and summer than in winter and fall.