Depth-area-volume and hydroperiod relationships of ephemeral (vernal) forest pools in southern New England

Abstract

Ephemeral or ''vernal'' pools occur commonly throughout the forests of the northeastern United States and adjacent eastern Canada. These pools are critical breeding habitat for a number of amphibian species and support a diverse invertebrate community. The hydroperiod or duration of surface water of vernal pools affects faunal composition and reproduction. We conducted bathymetric surveys of 34 vernal pools located in central Massachusetts in early spring when the pools were at maximum extent after receiving snowmelt runoff. With these data, we estimated maximum pool depths, surface areas, perimeters, volumes, and basin profile coefficients. We calculated relative hydroperiod indices for the pools based on the presence or absence of surface water during periodic pool visits over the three-year study. The ranges of estimated pool morphological parameters were 0.11-0.94 m for maximum depth, 68-2941 m 2 for maximum surface area, 6-506 m 3 for maximum volume, and 30-388 m for maximum perimeter. Basin profile coefficients ranged between 0.60 (convex) and 2.24 (concave), with a median value of 1.02 (straight slope). Maximum pool depth was positively correlated with area and perimeter, but the correlations were only moderately strong, and there were many shallow pools with large surface areas. Correlations between basin profile coefficients and other morphological parameters were weak or non-significant. Maximum pool volume was proportional to the product of area and depth, but the proportionality constant was dependent on the basin profile coefficient. Relative hydroperiod was weakly correlated with pool morphometry; the strongest rela- tionship was found between hydroperiod and maximum pool volume. In general terms, pools with a maxi- mum depth greater than 0.5 m, a maximum surface area larger than 1000 m 2 , or a maximum volume greater than 100 m 3 had surface water more than 80% of the times they were visited. In contrast, shallower pools, smaller pools, or pools with lesser volumes had varying hydroperiods. The weak relationships between pool morphometry and hydroperiod indicate that other factors, including temporal patterns of precipitation and evapotranspiration and ground-water exchange may have significant influence on vernal pool hydrology and hydroperiod.