Abstract
Designing environmental flows in lowland river sections and estuaries is a challenge for researchers and managers, given their complexity and their importance, both for nature conservation and economy. The Ebro River and its delta belong to a Mediterranean area with marked anthropogenic pressures. This study presents an assessment of the relationships between mean flows (discharges) computed at different time scales and (i) ecological quality based on fish populations in the lower Ebro, (ii) bird populations, and (iii) two shellfish fishery species of socioeconomic importance (prawn, or Penaeus kerathurus, and mantis shrimp, or Squilla mantis). Daily discharge data from 2000 to 2015 were used for analyses. Mean annual discharge was able to explain the variation in fish-based ecological quality, and model performance increased when aquatic vegetation was incorporated. Our results indicate that a good ecological status cannot be reached only through changes on discharge, and that habitat characteristics, such as the coverage of macrophytes, must be taken into account. In addition, among the different bird groups identified in our study area, predators were related to river discharge. This was likely due to its influence on available resources. Finally, prawn and mantis shrimp productivity were influenced up to a certain degree by discharge and physicochemical variables, as inputs from rivers constitute major sources of nutrients in oligotrophic environments such as the Mediterranean Sea. Such outcomes allowed revisiting the environmental flow regimes designed for the study area, which provides information for water management in this or in other similar Mediterranean zones.
Highlights
Deltas and estuaries are complex ecosystems largely recognized for their productivity and importance, both for the economy and the conservation of nature (e.g., [1,2,3])
Total annual runoff and the runoff generated by sudden flow rises each year were highly correlated (Pearson correlation coefficient: 0.98; p < 0.001; Figure S1, Supplementary Material)
This study shows relationships between water discharge, riverine fishes, bird diversity, and coastal fisheries
Summary
Deltas and estuaries are complex ecosystems largely recognized for their productivity and importance, both for the economy and the conservation of nature (e.g., [1,2,3]). The present study, developed in the lower Ebro and its delta, aimed to (i) deepen into the ecohydrological relationships found in Belmar et al [18] in order to obtain additional conclusions for water managers; and (ii) complete such conclusions using other groups of organisms relevant for ecological conservation and socioeconomic activities at the delta and its littoral zone [22]. Determine the relationship between mean discharge (instead of the original set of hydrological indices in Belmar et al [18]) averaged along the same time periods and ecological status in order to use the obtained models to calculate e-flows to preserve the good ecological status in the lower Ebro (assessing the suitability of the proposals presented up-to-date). Identify potential relationships among discharge, water quality, and two (shellfish) fishery species with socioeconomic relevance (prawn, or Penaeus kerathurus, and mantis shrimp, or Squilla mantis), as well as between flows and bird populations (ecological relevance) at the delta
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