A statistical model relating horizontal seston fluxes and bottom sediment characteristics to growth of Mercenaria mercenaria

Several field experiments have shown that bottom sediment type affects growth of the hard clam, Mercenaria mercenaria. A previous manipulative field experiment carried out within a moderate range of tidal current speed/seston concentration regimes in Great Sound, a coastal lagoon in southern New Jersey, USA, indicated that growth of M. mercenaria was more influenced by horizontal seston fluxes than sediment type. The present descriptive/correlative study was also conducted at Great Sound, during 1985 and 1986, and was designed to quantify the relationships between shell growth of M. mercenaria and a wide range of levels of tidal currents, seston, and bottom sediments. It showed: (1) near-bottom tidal currents and seston were best correlated with growth when combined as a horizontal seston flux rate, [i.e., current speed (cm s-1) x seston concentration dry wt cm-3=dry wt seston cm-2 s-1]; (2) the correlation between seston fluxes and growth was positive to some point, then became negative with further increases in seston fluxes; (3) sediment characteristics were correlated with growth; and (4) there may be an interaction between sediment characteristics and seston fluxes as they affect growth. These findings suggested a statistical model of the general form: SG=B0+B1(X1)+B2(X1)2+B3(X2)+B4(X1)(X2)+E, where SG=shell growth; B o=y-intercept; B i =coefficient of the ith term; X 1=mean horizontal seston flux; X 2=sediment characteristic; E=error term. Multiple regression techniques were used to determine values for the coefficients in the model, and associated R 2 values. R 2 values for models using various shell growth measurements, seston fluxes, and bottom sediment characteristics ranged from 0.69 (P=0.35) to 0.90 (P=0.07). Maximum growth occurred at moderate seston flux rates (e.g. 90 to 130 mg particulate organic matter cm-2 s-1) combined with sand sediments. Increased or decreased (relative to moderate rates) seston fluxes, and/or decreased grain size of the bottom sediments, resulted in reduced growth