Water flow is the main hydrological factor determining the phytoplankton structure (named potamoplankton
, in rivers), mainly because it can act on numerous environmental factors controlling the community dynamics (Reynolds, 2000).
In large rivers like the Lower Mississippi River (LMR), abiotic features, such as discharge and light, are usually considered the major factors influencing the biomass and composition of potamoplankton
Most rotifer species that occurred exclusively in the pre-impoundment phase are littoral and frequently associated with river potamoplankton
(Paggi and Jose de Paggi, 1990).
Although primary production is minimal in such streams, the algal contribution represents the inocula for the potamoplankton
in high-order streams.
Keywords: lotic ecosystem, potamoplankton, Iguacu River.
Frequently, the potamoplankton density is inversely proportional to the river's discharge owing to the dilution and the physical changes caused by the intense flux, which is one of the factors that alters the rivers' turbulence (Zalocar de Domitrovic et ah, 2007).
Potamoplankton is composed of algae capable of surviving different selective forces acting on these ecosystems (Margalef, 1983; Reynolds, 1988; Reynolds and Descy, 1996; Zalocar de Domitrovic et ah, 2007), and is characterised by a high proportion of rare species (Rodrigues et ah, 2009).
Keywords: spatial heterogeneity, functional groups, potamoplankton, reservoirs, seasonality.
High densities of potamoplankton have been recorded in eutrophic rivers during periods of lower discharge and slower water flow (Grabowska and Mazur-Marzec, 2011).
The potamoplankton forms can include species of limnoplankton, which originate from lake catchments or from periphyton (Reynolds and Descy, 1996).
The higher turbulence and lower light intensity in rivers cause differentiations in the composition of the potamoplankton, promoting higher abundance of diatoms and green algae, which grow quickly and have a greater ability to thrive in turbulent environments (Reynolds, 1994).
Understanding potamoplankton biodiversity in connection with environmental factors is extremely important for monitoring water quality (Rodrigues et al.