Zooplankton dynamics in a lowland river along temporal and spatial gradients

Abstract

Studies on zooplankton ecology have traditionally dealt with lentic systems, while research on lotic assemblages has lagged far behind. It is especially in the last decades that riverine zooplankton has become the focus of an increasing number of investigations, showing that large lowland rivers often host extremely rich and abundant plankton communities. Surveys conducted in rivers all over the world highlighted the major role played by abiotic constraints in shaping the development of riverine zooplankton, while much fewer studies have dealt with biotic factors, which are generally thought to play a minor role in the main current of rivers. Despite this growing amount of research, questions still remain to be answered regarding the origin of river plankton, how these communities are able to persist in the current and the major mechanisms involved in regulating their spatio-temporal dynamics. The present work deals with different aspects of the ecology of zooplankton in a large lowland river (Po River, Northern Italy). A multi-level approach was adopted, combining different surveys carried out at distinct spatial and temporal resolutions, based on the following main research questions: 1. What are the main abiotic and biotic factors controlling the spatio-temporal dynamics of zooplankton abundance and composition in the potamal reach of a lowland river? What is the effect of disturbance events like floods on community structure? Is the community able to exhibit regular successional patterns? These research topics have been addressed by means of a two-year sampling campaign carried out in the potamal reach of the Po River. Results confirmed the role of abiotic constraints related to seasonality and hydrology, together with that of trophic conditions, as the major drivers of zooplankton dynamics. The comparison of two hydrologically different years suggested that the uncoupling between seasonality and hydrology can significantly influence community density and diversity temporal patterns. The relationship between discharge and zooplankton abundance is not univocally negative, as increases in river flow may at times bring about a net increment of the assemblage, when resuspension of organisms from the river bottom and/or littoral zones prevails over advective losses. Discharge fluctuations strongly affect zooplankton diversity too, both taxonomical and functional, with higher diversity associated with increases in river flow, up to a threshold over which destructive effects dominate. However, the impact of hydrodynamic forcings in shaping lotic zooplankton appears to lose importance in favour of that of seasonality and trophic state when moving down the longitudinal dimension of the river system, and a clear downstream shift towards a truly planktonic community occurs, especially during summer low-flow conditions. 2. Can biotic interactions (predation/competition) within the zooplankton become crucial drivers of community structure under advective conditions? A short-term, high-frequency sampling campaign was carried out in summer 2005 in the potamal stretch of the Po River. To test for the presence of association patterns among zooplankters, which might be suggestive of potential trophic interactions (predation/competition), taxa were aggregated into functional groups according to their feeding ecology, and time series of their abundances were analysed by means of a variance ratio test and multivariate autoregressive models, which revealed the occurrence of compensatory dynamics among functional groups under low and stable, although truly advective, discharge conditions. Evidence on the importance of predator-prey interactions and intra-population regulation mechanisms also came from further analyses on the dominant predator's gut contents and on its main prey's population dynamics. Results showed that, under certain conditions, zooplankton is able to exhibit internal, self-regulatory mechanisms also in the main current of a large river and the view of riverine zooplankton as a mere assemblage of taxa exclusively abiotically controlled is therefore oversimplified. 3. What are the main changes within the plankton community during its downstream transport and how can the observed longitudinal dynamics be explained? What is the contribution of the major tributaries to the main channel zooplankton assemblage? These questions have been addressed by means of a Lagrangian sampling experiment carried out in May 2010 on a 330-km river stretch. Results highlighted how water residence time under spring conditions is too short to allow longitudinal development of zooplankton, which is merely transported downstream without significant changes in abundance and composition. One of the tributaries, the Mincio River, hosted an exceptionally abundant zooplankton community, which resulted in a significant increase in total zooplankton densities in the Po River downstream of its inflow. The Mincio lower course is highly regulated and artificially modified. The result is a system where current is extremely impaired and slowed down, creating favourable conditions for phyto- and zooplankton massive development. As a consequence, the influence of this tributary on the Po River zooplankton during the survey was still detectable as far as 100 km downstream of its mouth and the community composition recorded downstream of the Mincio inflow was more similar to the assemblage found in the tributary than to that found at the upstream stations of the Po River itself. 4. Does the presence of an hydrological discontinuity along the river course, such as a man-made reservoir, induce relevant changes in zooplankton composition and abundance? Are there significant differences in community structure along cross-channel transects (middle channel versus river banks) or is the river a completely well-mixed environment with homogeneously distributed plankton assemblages? A sampling campaign was carried out in summer 2009 at four stations respectively upstream, inside (mid-channel vs. shore) and downstream of the artificial impoundment of Isola Serafini, located in the middle reach of the Po River. The study revealed that longitudinal and lateral hydrogeomorphic heterogeneity may affect community abundance and diversity, so that the assemblage found at one site along the river may also be related to local hydroecological features and not only to processes taking place further upstream. However, in a rectified and channelized river like the Po, this appears to be the case only at extremely low discharge rates and high water residence time. Results of the surveys carried out along extremely artificialized traits of both the Po River (Isola Serafini dam) and its tributaries (lower section of the Mincio River) showed how anthropogenic modifications of the river's natural hydrogeomorphic features may have strong impacts also on biotic components that are usually overlooked by standard monitoring programs, even though their role in the functioning of lotic systems is still not completely understood.