VERNAL POOLS

Cantonati M. 2005. Algae. In: F. Stoch (Ed.), Pools, ponds and marshes. Quaderni habitat 11: 28-37. English Edition: Italian Habitats. Ministero dell’Ambiente e della Tutela del Territorio. Museo Friulano di Storia Naturale. ISBN 88 88192 22 0

Background

The California Coastal Prairie has the highest biodiversity of North America's grasslands, but also has the highest percentage of urbanization.  The most urbanized part of the California Coastal Prairie is its southernmost area, in Los Angeles County.  This southernmost region, known as the Los Angeles Coastal Prairie, was historically dotted with vernal pools, and has a unique biodiverse composition.  More than 99.5% of its estimated original 95 km (23,475 acres), as well as almost all its vernal pool complexes, have been lost to urbanization.

The Madrona Marsh Preserve, in Torrance, California, safeguards approximately 18 hectares (44 acres) of Los Angeles Coastal Prairie and includes a complex of vernal pools.  Its aquatic biodiversity had been studied, predominantly to genus level, but its terrestrial macro-invertebrates were virtually unknown, aside from butterfly, dragonfly, and damselfly observations.

Results

In order to better understand the biodiversity at the Madrona Marsh Preserve, a minimally-invasive macro-invertebrate inventory was conducted. The results of this inventory, with 689 invertebrate organisms recorded, covering eight phyla, 13 classes, 39 orders, and 222 families, are presented in this document.

We compare diet and feeding behaviour of Snapping Turtles (Chelydra serpentina) and Midland Painted Turtles (Chrysemys picta marginata) in Algonquin Provincial Park, Ontario, Canada. We observed young Chelydra and Chrysemys turtles feeding on insect and amphibian larvae in ephemeral ponds, adult Chrysemys terrestrially foraging on odonate larvae, and adult Chelydra consuming aquatic vegetation and seeds. These and other observations highlight the importance of seasonally available habitat and food for juvenile turtles. We also discuss the evidence for, and importance of, turtles as seed-dispersal agents for aquatic vegetation. Illustrative video recordings accompany our dietary observations.

1. Hydroregime (duration, frequency and predictability of the aquatic phase) is a key feature of temporary aquatic habitats that not only moulds community structure and diversity (species sorting) but also life history characteristics of the inhabitants (natural selection). However, since hydroregime is a complex multidimensional entity that cannot be estimated from short term observations, morphometric variables are commonly used as proxies for hydroregime, making it impossible to separate effects of habitat size and hydroregime on biota.2. We have used a simple hydrological model, validated with recent (average r2 = 96%) and historic water level observations (average r2 = 81%), to accurately reconstruct hydroregime based on historical rainfall and evaporation data in a cluster of 36 temporary rock pools in central South Africa.3. Using the model output, we demonstrated that both hydroregime and habitat size had unique and shared effects on temporary pool biota and that these effects depended on the dispersal modes of the taxa. Model-generated hydrological data explained more variation in community patterns than short-term observations of hydroperiod. Hydroregime was more important for passive dispersers than for active dispersers that can migrate when pools dry up. Basin morphometry was a relatively poor predictor of pool hydroregime. We concluded that simple modelling may greatly improve the resolution of studies linking hydroregime to biological variables.4. An accurate account of hydrological variation provides a firm foundation to understand community and population structure and dynamics in temporary aquatic habitats. Since many of these habitats have been destroyed or degraded in recent decades, our findings and tools may contribute to the development of reliable conservation guidelines.

Vernal pools are small, temporary ponds that fill are filled by winter rains and dry up by late spring. Often the ponds are well defined, sometimes functioning more or less independently, but usually they are interconnected by shallow drainages, especially in rainy years. In some cases the entire habitat consists of drainages and swales with the pool or pond component less noticeable. Occasionally the ponds are large enough to be considered vernal lakes, the center of which may actually hold water year around or at least long enough for a fresh water marsh to develop. Pools form in flat or gently sloping landscapes over an impervious soil layer, usually clay, which can be underlain by basalt, granitic rocks or a hardpan formed of clays or cobbles cemented together. In San Diego County, the majority of the pools, have in addition to clay surface soils, a hardpan layer beneath them made up of cobbles held together by a combination of iron and silica.