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

Abstract

From April 1994 to December 1997, we studied the microphytobenthic assemblages in sur- face (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 de- crease 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 µg 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 µg 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 Navi- cula sp. was saturated at a light intensity of 165 µE m -2 s -1 at 21°C. No photoinibition was found at higher light intensities up to 400 µE m -2 s -1 . The relationship between temperature and photosynthetic rate was positive and linear within a temperature range between 10 and 35°C at 55 µE 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.