L. Forró szerk.: Miscellanea Zoologica Hungarica 9. 1994 (Budapest, 1994)

Andrikovics, S.; Hadnagy, T.: Ecological evaluation of aquatic invertebrates distribution of Apátkút Creek (Visegrádi Mountain, Hungary) in winter

The reason is not the competition of the two species but their completely different microhabitat use, the different feeding type and way of life. Hydropsyche pellucidula can be found among stones and catches its prey with net and prefers fast flowing water. Halesus can be found in calm reaches among detritus on branches and wood, from which it builds its case. The distribution of insect larvae is influenced not only by the pollution, water current, food, competition but also by several other factors, e. g. method of egg-laying. The occurrence of Rhithrogena semicolorata group along the creek is also the result of that females lay their eggs not in masses but they drop them one by one to the water flying above the water surface. The water current and dissolved oxygen content have obvious influence on the number of insect larvae. Dissolved oxygen content depends on velocity of water. The number of individuals follows the values of water velocity to a certain extent, because the strong current washes away the animals. It was found that optimum water velocity for larvae is about 2 m/sec, and on reaches with smaller and greater values of water velocity the number of individuals decreases or different species can be found. Higher number of individuals together with increasing water velocity has two main reasons: higher water velocity means larger amount of dissolved oxygen, and the stronger velocity washes more food to the larvae. The close relationship between water velocity and number of individuals can be altered only by pollution. On the third of sampling place at Pilisszentlászló, in spite of high water velocity (2.4 m/sec) the number of individuals is small (20 larvae/m ), while on the 11th sampling site with similar water velocity values (2.3 m/sec^ the number of individuals is almost 13 times larger than at the previous one (250 larvae/m ). It démontrâtes the effect of pollution on the abundance very well. To summarize the above mentioned facts, the pollution is obviously shown by low level of dissolved oxygen and low number of individuals of larvae. Altogether 5.3% of all aquatic insect larvae and amphipods were among and under stones. Stones are the most frequent substrates in the creek. The relation between distribution of larvae and size of stones is discussed below. In the case of sample sites with 1 m/sec flow-velocity, Fig. 5 demonstrates the number of individuals and size of stones belonging to them together. The distribution of larvae is influenced by stone size beside the primary effect of velocity. Most of the larvae can be found among or under 15-20 cm stones. The number of individuals on larger and smaller stones is considerably lower. There is an additional ecological factor, water-depth, which has a great influence on the larva distribution. Although every maximum number of individuals is associated with 15-20 cm stone-size, there are some samples with lower abundance of larvae. The reason is the deeper creek-bed in these places. The usual velocity, on 18 sampling places was 2 m/sec. It can be observed that the maximum abundance of larvae always belong to the stones with 15-20 cm diameter everywhere. Those numbers of individuals, which is associated with this stone-size, are going to be taken from the 18 samples. Eight samples remained out of the 50 ones, where velocity and stone-size are constant (2 m/sec and 15-20 cm), so the effect of water depth on distribution of larvae can be determined apart from the previous factors. The curves of abundance and water-depth in Fig. 7 are almost symmetrical to each other. Greater water-depth results in lower number of larvae and more shallow water in bigger number of larvae.