Factors related to autotrophic potamoplankton, heterotrophic protists and micrometazoan abundance, at two sites in a lowland temperate river during low water flow

Abstract

The potamoplankton of the Middle Loire was studied every two weeks from June to October 1997, a period of low water flow. The downstream increase in dissolved oxygen, suspended matter, BOD₅ and chlorophyll a, and the decrease in nutrients were associated with algae growth, principally of chlorophytes and bacillariophytes. Algae reached maxima of 54 × 10⁶ cells l⁻¹ and 78 × 10⁶ cells l⁻¹ up- and downstream respectively. A large ciliate community of oligotrichs and peritrichs also developed, with densities up to 24 × 10³ cells l⁻¹ upstream and 49 × 10³ cells l⁻¹ downstream. These maxima were succeeded by large numbers of heterotrophic flagellates that reached 690 × 10³ cells l⁻¹ upstream following a peak of rotifers, and 2430 × 10³ cells l⁻¹ downstream following the ciliate maximum. Nevertheless, these flagellates constituted only a small proportion of the total protozoan biomass, which was dominated by ciliates. The autotrophic plankton remains the dominant component for most of the time, confirming that lowland rivers support significant phytoplankton production. The influence of anthropic disturbances on rotifer development was debated with regard to the results obtained in previous years. The maximum rotifers density, due to brachionids and trichocercids reached 3500 ind l⁻¹ upstream and 2750 ind l⁻¹ downstream, a decrease similar to that observed the previous year. In 1997, addition of suspended matter was probably responsible for the increase in food levels, which enhanced the development of the predators, thus illustrating that potamoplankton is largely dependent on trophic relationships and local influences, which highlights the need for a careful evaluation of all assessment criteria.