Leslie et al. 1997

Leslie AJ, Crisman TL, Prenger JP, Ewel KC. 1997. Benthic macroinvertebrates of small Florida pondcypress swamps and the influence of dry periods. Wetlands 17: 447-455.

These authors examined the faunas of three pondcypress swamps in northeastern Florida, over about 18 months from 1993 to 1995. Pondcypress swamps are wetlands with unpredictably-fluctuating water levels and a canopy formed by pondcypress trees (Taxodium distichum var. nutans). These swamps may remain wet for years or dry repeatedly within a single year, presenting a very challenging abiotic environment to aquatic organisms. Prior to this study, there had been few papers describing the biodiversity of pondcypress swamps, particularly during dry periods referred to as “drawdowns”.

The three ponds differed from each other in area, depth, and most dramatically emergent macrophyte vegetation. All three contained large abundances of Sphagnum mosses, and responded to local weather conditions similarly, tending to be driest at the same time, in June 1994. All ponds were highly acidic, with pH ranging between 3.4 and 4.4, and changing over time.

Benthic macroinvertebrates were collected by cores in a random fashion, to allow calculation of species richness and individual density per unit area, in this case reported as per square meter. Collected animals were identified to genus when possible using a range of identification keys from the literature (e.g. Pennak 1978). The beetle families Dytiscidae and Hydrophilidae and the fly family Chironomidae contributed large numbers of genera, and two genera of chironomids and one amphipod genus (Crangonyx) contributed the majority of individuals. Up to 52% of total individuals collected were specimens of Crangonyx.

Total species richness varied between the three pools, with the pool with least emergent vegetation having the lowest richness. Species richnesses were higher than those reported for other wetland habitats in the same region.

The authors report some surprise at the levels of biodiversity maintained even during dry periods. There is some speculation based on Barlocher et al. (1978) that drawdown periods are accompanied by an increase in bacterial and fungal activity, with a net increase in protein levels and therefore food quality in the detritus that forms the bulk of the food of most of the collected animals. Crangonyx and many of the other species found are generalists, and can cope with unpredictable dry periods by burrowing into wet soil or escaping to other habitats, while many of the species excluded during dry periods are obligate aquatics or have restricted life cycles and require a certain minimum period of wetness to complete one or more stages of the life cycle.

The high species richness of Dytiscidae and high population densities of Crangonyx are good indications for my project, as I expect to visit such habitats during the summer of 2008.