SEASONAL-VARIATION, EXPORT DYNAMICS AND CONSUMPTION OF FRESH-WATER INVERTEBRATES IN AN ESTUARINE ENVIRONMENT

In the Aber Estuary, North Wales, significant numbers of freshwater be nthic invertebrates occurred in the tidal freshwater area. Distinct se asonal patterns were observed in their longitudinal zonation which app eared to be unrelated to variations in tidal inundation. The December extension downstream of freshwater taxa is hypothesized to be in respo nse to decreasing water temperatures. In April, larvae/nymphs of the T richoptera (caddisflies), Ephemeroptera (mayflies) and Plecoptera (sto neflies) ranged as far as a site inundated by 80.9% of all high tides, and larval Elmidae and Chironomidae (midges) occurred at the most mar ine site (inundated twice daily by all high tides). In July, with the exception of the Chironomidae, the range of most aquatic insects had c ontracted to the upper estuary. Although, in general, densities of aqu atic insects decreased towards the lower estuary, significant densitie s persisted there. For example, maxima of 3514 chironomid larvae and 4 8 caddisfly larvae m(-2) were recorded at the 80.9% inundation site. A n estimated 31 x 10(6) freshwater invertebrates (weighing 62.6 kg), pe r annum, passed from fresh water into salt water across any given tran sect along the estuary. In comparison, the annual influx of invertebra tes carried upstream by incoming tides was estimated to be 1.9 x 10(6) (6.2%; weighing 2.5 kg). Predominant in the downstream drift were the larvae/nymphs and/or pupae of chironomids, mayflies, stoneflies and c addisflies. The 'reverse' drift comprised mainly copepods, ostracods, amphipods and oligochaetes. Mites and the brackishwater amphipod Gamma rus zaddachi commonly moved in both directions. Highest drift densitie s occurred in July, whereas the lowest densities occurred in late autu mn and winter. Multiple regression analysis showed no relationship bet ween total drift or 'reverse' drift densities and any of the measured environmental variables. Many of the freshwater invertebrates appeared not to die upon passing into tidal sections but resumed a benthic exi stence by virtue of varying degrees of salt tolerance. Of the three fi sh species common in the estuary, eel, common goby and flounder, the l ast two preyed measurably on freshwater taxa. Whereas gobies tended to be opportunistic feeders, depending on the section of estuary that th ey occupied, flounder were more restricted to the upper estuary where they fed selectively on chironomid larvae. On the latter diet, between March and September, the mean wet weight of flounders increased by mo re than 100 times (from 5 to 540 mg). Gobies were more numerous in the estuary from September to February, and although they ate insects the ir primary prey was G. zaddachi.