Differences in the benthic-pelagic particle flux (biodeposition and sediment erosion) at intertidal sites with and without clam (Ruditapes philippinarum) cultivation in eastern China

An annular flume or flux system (AFS) was deployed to measure the biodeposi tion and sediment resuspension processes at four stations in the mid-shore and low-shore zones along both natural and Manila clam (Ruditapes philippin arum) farming transects in Xuejiadao intertidal area located in Jiaozhou Ba y, eastern China. The results showed that there was a significant correlati on between biodeposition rates and the density (r = 0.984, P < 0.05) and bi omass (r = 0.977, P < 0.05) of the suspension feeding Manila clam. The effe ct of biodeposition at the farming transect was more intensive than at the natural transect where the biodeposition effect was lower than that resulti ng from natural sedimentation. In contrast, the biodeposition rate at the c ulture site with the high density of Manila clams was four times the sedime ntation rate. Following the addition of Manila clams to increase their dens ity in natural sediments, there was a marked increase in both clearance and biodeposition rates at all four sites. At the site with the highest natura l bivalve biomass, the biodeposition rate increased 1.5-fold, while at the site with the least bivalve biomass, it increased by 40 times compared with the biodeposition rate by natural biota. The mean clearance rate of Manila clam was 0.90 +/- 0.34 l h(-1) ind.(-1) and the biodeposition rate was cal culated to be 0.06 +/- 0.01 g h(-1) ind.(-1). The critical erosion velociti es of intertidal sediment at Xuejiadao were from 17.4 to 20.4 cm s(-1). Rel ationships describing suspended particulate matter (SPM) vs. current veloci ty were analysed by linear regression following log transformation of the S PM. Statistical analysis of the slopes of the regression lines revealed tha t there were distinct differences between the low-shore and mid-shore(P < 0 .05) at both transects, respectively. Then was also distinct difference bet ween the two mid-shore stations (P < 0.05) possibly due to differences in t he densities of bioturbators (e.g. Macoma incongrua etc.), however, there w as no significant difference between the two low-shore stations (P > 0.05). Furthermore, it was found that the site differences in sediment erodibilit y were nor significantly correlated with measured physical properties of se diments and biota factors such as total macrofauna biomass, total abundance and macrofauna densities, Chl-a and Ph-cc (P > 0.05). However, there was a significant correlation between sediment erodability and both median grain size (P < 0.01) and the density of the bioturbator M. incongrua (P < 0.05) . There was also evidence of a slight increase in sediment stability after 3 h of air exposure, but the effect was not overcome following the addition of Manila clams. It is hypothesised that the lower stability of sediments at the mid-shore level was probably associated with higher densities of bio turbators and with disturbance by shrimp fanning near the high-shore. (C) 2 001 Published by Elsevier Science B.V.