Water chemistry variability in the lower intertidal zone of an estuary in the Seto Inland Sea, Japan: seasonal patterns of nutrients and particulate compounds

From April 1994 to April 1996, we carried out monthly surveys on the physic al and chemical variability of ebbing water at two intertidal stations near the emerged tidal flat and surface water at a nearby subtidal station, in a tidal estuary of the Seto Inland Sea, Japan. The fresh water runoff was a major source of new nitrogen (nitrate + nitrite = -2.1 x salinity + 74.3, r(2) = 0.76, p < 0.001, at the inner intertidal station). In contrast, the absence or weak correlation of ammonium, phosphate and silicate with salini ty in ebbing water highlighted the importance of in situ biologically-media ted processes, including the excretory activity of intertidal dominant biva lves (Ruditapes philippinarum and Musculista senhousia) and primary produce r nutrient uptake. Among suspended particulate matter (SPM), the negative c orrelation of Chl a with salinity (p < 0.001) suggested that an upper inter tidal zone, where high microphytobenthic assemblages occur, may represent a considerable source of microalgal biomass. Whereas, a stronger correlation of POC with pheo-pigments ( r(2) greater than or equal to 0.92) than with Chl a (r(2) less than or equal to 0.44), a low fraction of living phyto-Car bon (ca. 5%) and a high pheo-pigment/Chl a ratio (ca. 3) indicated a high p ortion of refractory algal material and a close water-sediment coupling on the flat, in which abundant macrozoobenthos is likely to play an important role. In spite of the strong variability of this ecosystems, this study als o demonstrated that highly significant seasonal patterns can be recognized in our study area. However, patterns varied depending on the different nutr ient species, SPM and sites. In ebbing water, ammonium concentrations were significantly higher in the cold period (i.e. November-April: water tempera ture 10.0 +/- 4.4 degrees C), when primary producer biomass decreases, whil e Chl a and pheo-pigment content were 4.3 and 4.8 higher in the warm period (May-October: water temperature 23.5 +/- 4.2 degrees C), respectively. At the subtidal site, nutrients were 1.9 (silicate)-2.9 (phosphate) and SPM wa s 1.8 (POC)-2.1 (pheo-pigments) times higher in the warm period. On an annu al basis, mean pheo-pigment and POC content was 12-25 times and 4-8 times h igher on the intertidal zone than on the subtidal zone, respectively, while nutrient concentrations were within the same order of magnitude. Such a qu alitative difference (i.e. nutrients vs. SPM) between sites suggests that p article deposition and/or removal is likely to occur along the estuary, res ulting in a limited SPM content at the surface layer of the subtidal site, while nutrients are more directly transported into the surface layer of the subtidal zone by lower salinity water mass intrusion from the intertidal z one, most importantly during the warm period.