Effects of impoundment and water-level fluctuations on macrophyte and macroinvertebrate communities of a dammed tidal river

Abstract

A freshwater headpond was created in 1968 when a causeway was builtacross a tidal portion of the Petitcodiac River, New Brunswick, Canada. In the32-year history of this tidal barrier, there have been numerous water-leveldrawdowns to manage risk of flooding (relatively minor and of short duration)and to permit passage of anadromous fishes past the dam structure (often ofseveral weeks duration). Following five years of routine management to preventflooding, there were three major water-level drawdowns that fully exposed thelittoral zone to air for varying lengths of time during the spring of 1997 (7days), 1998 (19 days), and 1999 (55 days). The 1998 and 1999 drawdowns resultedin significant decreases in biomass of macrophytes and benthicmacroinvertebrates in the littoral and sublittoral zones, compared to 1997. Thelargest decrease occurred between 1997 and 1998 when populations of mostspecieswere largely eliminated. The remaining species were those resilient toprolongedexposure to the atmosphere. The only macrophyte to increase in biomass over thethree years was Polygonum lapathifolium, which formedimpenetrable mats in 1999. Hexagenia limbata in thesublittoral zone was the only benthic invertebrate species to increase inbiomass. The age distribution of the eastern floater (Pyganodoncataracta) population suggested that a major drawdown accompanied bysalt-water intrusion had eliminated the entire freshwater community in 1988.Thegoals of allowing passage of anadromous fishes past the dam structure and thedevelopment of a stable, artificial, freshwater-lake community appear to beincompatible.