The Resistance of Waters to Trophic Growth as a Result of Processes in Their Ecotone Areas (According to Phytoplankton Indicators)

Abstract—According to the saprobity index, estimated by the phytoplankton biomass, the degree of organic pollution of ecotone areas (the mouth of the Il’d River and the littoral of the Volga reach of the Rybinsk Reservoir) has been determined. The saprobity index in protected shallow waters was 1.83 in 2009 and 2.07 in 2010; in the pelagial, it was 2.14 and 2.13 in 2009 and 2010, respectively. In the river–reservoir system, the lowest average values of the saprobity index were noted in the mouth area of the Il’d River: in 2009 it was 1.94, in 2010 1.87, and in 2011 1.88. In the river and the reservoir, its values were higher: 2.05 and 2.07 in 2009, 1.96 and 2.08 in 2010, and 1.97 and 1.99 in 2011, respectively. The decrease in saprobity in shallow waters and at the mouth of the river, compared to the bordering water, indicates a higher intensity of self-purification processes, which can ensure the sustainability of the ecosystem of the reservoir as a whole. Abnormal weather phenomena, in particular, high summer water temperatures, can lead to the destabilization of the features of ecotone algocenoses and, as a result, to a violation in the process of self-purification of the reservoir.