LATERAL MOVEMENTS VERSUS STATIONARITY - ADAPTIVE ALTERNATIVES IN BENTHIC INVERTEBRATES TO THE SEASONAL ENVIRONMENT IN A BOREAL RIVER by

Benthic invertebrates inhabiting boreal rivers are exposed to very large sea­ sonal variations in their physical environment. The extremes are in winter when the littoral area freezes solid and in spring when water flow increases rapidly. In the North Swedish river Vindelälven, the invertebrates fell into three main categories according to their seasonal lateral distribution. One category of animals was stationary in the littoral zone and let itselt freeze into the ice, adopting a "take it" alternative. The category consisted of many species be­ longing to several higher taxa. By overwintering in ice, the animals avoided predation for nearly half the year and they were in the productive littoral at the same time as they thawed out from the ice in spring. On the other hand the animals had to withstand sub-zero temperatures. A typical representative for this category of animals was the semivoltine snail Gyraulus acronicus. It is a less mobile species connected to dense stands of macrophytes, which are found only in the littoral zone of the river. Nearly the whole population was found overwinte­ ring successfully enclosed in ice. Its shell and epiphragm could serve as mecha­ nical protection when frozen into the ice. G. acronicus was cold-hardy only dur­ ing late autumn and early winter, but it could stand prolonged sub-zero exposure during the proper time. A second category of animlas avoided being frozen by per­ forming lateral movements to deeper parts of the river, adopting the Vleave it" alternative. No species tested in this category were found cold-hardy. It consis­ ted of mobile species known to utilize sedimentated detritus which was only found in greater amounts in the littoral zone of the river. In springtime, prior to spring flood peak, these species colonized promptly the former frozen zone. This behaviour was most pronounced in several lentie mayfly species. An extreme case of migratory behaviour was found in the mayfly Pararneletus chelifer which not only moved towards the river bank but continued up into small tributaries. The shore­ ward movements of mayflies both allowed the nymphs to avoid the high current velo­ cities in the central part of the river during spring flood time and to utilize the food resources in the flooded areas. A third category of animals avoided the ice by living stationary in the sublittoral zone, adopting the "never face it" al­ ternative. This category was dominated by filter feeders.