AQUATIC INSECTS IN AN ESTUARINE ENVIRONMENT - DENSITIES, DISTRIBUTIONAND SALINITY TOLERANCE

1. Aquatic insects were quantitatively surveyed at five sites along th e tidally influenced section of a river-dominated estuary in North Wal es. Site 1 was the furthest upstream and was established as a referenc e site as it was never inundated by salt water. Site 5 was the furthes t downstream and was inundated by all incoming tides. Numerically, ins ects made up 32% of the estuarine invertebrate fauna. 2. Although the densities of most insect taxa decreased towards the estuary mouth, the re were significant numbers present downstream for much of the year; f or example, in April at site 4 (which was inundated by 81% of all high tides), a mean of 3514 chironomid larvae were recorded per m(2) of es tuary bed. Even at site 5, which was inundated twice daily, there were 747 larvae per m(2). Among the larger aquatic insects, caddisfly and elmid beetle larvae, together with stonefly nymphs, were consistently taken at site 4 (e.g, maxima of forty-eight caddisfly larvae m(-2) in December and seventy elmids m(-2) in April), although their densities were lower than upstream. 3. There were seasonal shifts in the longitu dinal distribution of several taxa, most notably the extension of chir onomids down the estuary in April and July and the concentration of si muliid larvae and mayfly nymphs at site 2 in July. The total freshwate r benthos showed a downstream shift between September and December, wh ich was maintained through April and into the summer. The latter was d espite peak saltwater inundation (highest tides) in October, November and April. In June and July when saltwater intrusion was lowest, the r anges of many aquatic insects had contracted to sites 1 and 2. 4. Labo ratory experiments showed that virtually all individuals of nineteen s pecies of insects collected from site 1 (freshwater) survived a 4-h im mersion in 8.75 parts per thousand saltwater (25% strength seawater). Immersion in progressively more saline solutions reduced the survivors hip of first the mayflies, followed by the caddisflies Glossosoma conf ormis and Hydropsyche instabilis. After 4h in full strength seawater, all specimens of the stonefly Dinocras cephalotes, over half of the Pe rla bipunctata, and some individuals of nine species of caddisfly were alive. Four species of caddisfly (Sericostoma personatum, Odontocerum albicorne, Potamophylax cingulatus and Adicella reducta) survived a 2 4-h simulated tidal cycle of immersion. With the exception of P, cingu latus, a few individuals of these caddisfly species survived immersion in full-strength seawater for 24 h. For some individual species there was good agreement between their observed longitudinal distribution i n the estuary and laboratory-measured salinity tolerance; however, the re was no significant correlation, overall, for the fauna.