Annual cycle dampening and decrease in predictability of water level fluctuations in a dam-regulated Neotropical floodplain

Abstract

The flood pulse concept refers to seasonal variation in river water levels promoting landscape spatial heterogeneity, connectivity, and nutrient cycling among nearby environments, in addition to contributing to the ecological functioning of river-floodplain systems. This seasonal variation is inherent to river-floodplain systems and responsible for establishing their natural pattern, as frequently observed in nature. In Brazil, the Upper Parana River and its floodplain constitute a river-floodplain system of interest, where an upstream reservoir cascade has altered the flood regime. This regime has been explored in the past; however, research has focused only on its time-domain. In this study, I sought to analyze the flood pulse in the Upper Paraná River Floodplain through a set of spectral (frequency-domain) and nonlinear analyses using a time-series of water level fluctuations (1968–2017) in the system. In doing so, I believe to bring novel information on this system flood regime in terms of its dynamic invariants. I divided the data into four periods: (i) natural regime period, (ii) transitional period, (iii) dam cascade period, and (iv) Primavera dam period. Spectral analysis demonstrated a decrease in the annual cycle amplitude, reflected in its power spectrum, which indicates a weakening in the difference between flood and drought events. Additionally, nonlinear dynamical analysis revealed a less deterministic and predicable behavior leading to more erratic fluctuations, thus jeopardizing the temporal heterogeneity of the system. Such altered dynamics is harmful for threatening the ability of organisms to harmonize their biological cycles with seasonal phenomena, consequently raising concerns about the maintenance of ecological integrity in the system.