Seasonal boundary dynamics of a groundwater/surface-water ecotone

Ecology, Sept, 1998 by Brian G. Fraser, D. Dudley Williams

TABLE 2. Correspondence analysis summary for the ordination of the
combined, all-seasons data set. Only the values for the first four
ordination axes are given.

                            Cumulative %
                              variance
Axis       Eigenvalue         explained        Length (SD)

1            0.287              35.2              2.28
2            0.103              47.8              1.16
3            0.048              48.4              0.94
4            0.037              48.9              1.01

Note: Variance explained by each axis is the ratio of its eigenvalue
to that of the total inertia (i.e., all ordination axes combined).

Hydrogeology

Interstitial water flow within the component of the flow system sampled appeared to be typical of an effluent river (sensu Williams 1989) [ILLUSTRATION FOR FIGURE 4 OMITTED]. Measured hydraulic potential, or head, beneath the river was greater than that of the water level of the river, indicating that net water movement was generally upwards into the channel. In only one instance was this not the case: during the spring sampling period, the river head was greater than the head 20 cm below the bed surface at midriver, indicating the potential for net water flow into the interstices [ILLUSTRATION FOR FIGURE 4 OMITTED].

From spring through fall, between 60 and 100 cm below the surface of the river bed, at sampling stations 7, 8, and 9, the head data indicated that flow was virtually horizontal across the river rather than vertical into the river. The head data also indicated that the relative upward force of baseflow varied among seasons and within seasons across the transect. There was substantial seasonal variation in the hydraulic gradient (i) between the river and the subsurface (Table 1), i was greatest for the summer and winter sampling periods. Periods of high i were coincident with times of the year when discharge in the river was dominated by baseflow. For individual seasons, the head distribution was asymmetrical across the river (Table 1). For all seasons, i was greater from midriver to the south bank than from midriver to the north bank.

TABLE 3. Results of the multiple linear regression (correlation
coefficients) between CA axes 1 and 2 and measured environmental
parameters for which there was one significant correlation (P [less
than] 0.05).

Variable                  Axis 1          Axis 2

Alkalinity              0.364(***)          ...
Conductivity            0.323(***)          ...
Dissolved oxygen       -0.195(*)            ...
Nitrate                -0.163(*)         -0.181(*)
Sodium                    ...             0.161(*)

* P [less than] 0.05, ** P [less than] 0.01, *** P [less than]
0.001. Ellipses indicate that the correlation was not significant.

Ordination and relationship with measured environmental parameters

The primary and secondary ordination axes, which together explained nearly 50% of the variance in the data set, were 2.28 and 1.16 SD in length and represented changes in taxonomic composition of [approximately]85 and 55%, respectively (Table 2). Grouping of sampling sites was readily evident [ILLUSTRATION FOR FIGURE 5 OMITTED] and corresponded well with the distinction between groundwater and hyporheic sites generated by the level-1 TWINSPAN dichotomy. The ordination displayed a much stronger distinction between hyporheic and groundwater community subunits vs. seasonal changes in faunal composition. This is likely the result of the level of taxonomic identification used, as seasonal changes in the major taxa are more conspicuous at the genus level (Williams and Hynes 1974).