Ecological Implications of Erratic Floods in Large River Floodplains of the Andean Amazon Region

Transcription

1 Ecological Implications of Erratic Floods in Large River Floodplains of the Andean Amazon Region Jorge E. Celi & Stephen K. Hamilton Department of Zoology & Kellogg Biological Station Michigan State University

2 The Flood Pulse Concept (Junk et al. 1989) Floodplains are transitional between aquatic and terrestrial zones. Large unmodified watersheds produce flood pulses of long duration and extensive seasonal floodplain inundation. Small or modified systems produce frequent flood pulses of shorter duration. Predictable pulses allow organisms to adapt to and benefit from inundation. Unpredictable pulses act as a disturbance and impede adaptation.

3 The Natural Flow Regime concept can be extended to Flood Regimes on floodplains Poff et al The Natural Flow Regime. Naiman et al. 28. Flow variability and the biophysical vitality of river systems.

4 The Napo River Drains 1 5 km 2 of a highly diverse and largely undisturbed region of the western Amazon in Ecuador and Peru. Fringed with extensive floodplains along 8 km of lowland reaches. Exceptional levels of biodiversity may be related to flood regimes. Intermediate disturbance hypothesis

5 Andes upstream reaches middle reaches downstream reaches Amazon 1 Km Laraque et al. 29. Hydrol. Proc.

6 Jan-1 Aug-1 Mar-2 Oct-2 May-3 Dec-3 Jul-4 Feb-5 Sep-5 Apr-6 Nov-6 Jun-7 Jan-8 Aug-8 Mar-9 Oct-9 May-1 Dec-1 Jul-11 water level (cm) Mar-1 Oct-1 May-2 Dec-2 Jul-3 Feb-4 Sep-4 Apr-5 Nov-5 Jun-6 Jan-7 Aug-7 Mar-8 Oct-8 May-9 Dec-9 Jul-1 Feb-11 water level (m) Water levels of the Napo River Highly variable and less predictable flow and flood regimes (multiple short pulses). Contrasts with more predictable, mono-modal regimes of the Amazon, Orinoco, Parana rivers. Stage fluctuations range from 4 m in upper reaches to 9 m in lower reaches, vs. 16 m in the central Amazon upstream Napo - Ecuador Source: INAMHI-SENAMHI, Hybam Project Source: ANA, Brazil. Amazon- Brazil

7 jan feb mar apr may jun jul aug sep oct nov dec Precipitation (mm) Water level (m) jan feb mar apr may jun jul aug sep oct nov dec precipitation (mm) Water level (m) jan feb mar apr may jun jul aug sep oct nov dec Precipitation (mm) Water level (m) Water levels and precipitation Less pronounced seasonality in the Napo. Hydrograph coupled to precipitation events towards the upper reaches and decoupled (like the Amazon) towards the lower reaches. upstream Napo - Ecuador AVG PP(mm) AVG WL (m) Source: INAMHI-SENAMHI, Hybam Project. Central Amazon - Brazil downstream Napo- Peru AVG PP (mm) AVG WL (m) AVG PP (mm) AVG WL (m) Source: ANA, Brazil.

8 max depth (cm) jan feb mar apr may jun jul aug sep oct nov dec water level (m) Magnitude: Depth of inundation River levels upstream Napo downstream Napo central Amazon Inundation hydrology of the Napo floodplains # days flooded distal forested swamp proximal forested swamp Napo River has smaller amplitude of water levels than the Central Amazon. Lower depth of flooding. Deepest flooding occurs at floodplain sites proximal to river. Floodplain biota less adapted to cope with deeper floods?

9 median depth (cm) # days Duration: Floodplain Hydroperiod Napo floodplains depth (cm) Napo proximal floodplain - high connectivity Napo proximal floodplain - low connectivity Napo distal floodplain days flooded per year distal floodplain proximal floodplain Large variability in flood duration and depth. Proximity and hydrological connectivity to river explain depth and duration of flooding. Elevation of sampling sites relative to river unknown. Some sites may be on perched terraces?

10 water level (cm) water level (cm) 1 Frequency: Napo River hydrographs # of flow peaks/year downstream upstream upstream downstream year 29 day Source: INAMHI-SENAMHI, Hybam Project. Higher frequency of floods towards the Andes. Inundation may act more as an ecological disturbance towards the Andes % time

11 water level (cm) water level (cm) water level (cm) water level (cm) 4 3 Frequency: Floodplain hydrographs River Proximal floodplain 1 Distal floodplain upstream River River 4 Distal floodplain 4 Proximal floodplain % time Distal floodplain 1 Proximal floodplain 1 downstream River % time Distal floodplain 4 Proximal floodplain 4 Most floodplain sites showed shallow, continuous inundation

12 water level (m) water level (m) Timing and flashiness: Predictability upstream downstream day day Lower flood predictability in upstream reaches. Much flashier than Central Amazon!

13 distal floodplain 4 water level (cm) distal floodplain 1 water level (cm) proximal floodplain 4 water level (cm) proximal floodplain 1 water level (cm) proximal floodplain 2 water level (cm) Proximal floodplain 3 water level (cm) River control of floodplain inundation? In some places it may to some extent: Napo R water level (cm) Napo R. water level (cm) Napo R. water levels (cm) 1 But often it doesn t: Napo R. water level (cm) Napo R. water level (cm) Napo R water level (cm) Caveat: only modest river floods occurred during the study

14 Na + (mg/l) Sources of flood water Major ions as hydrological tracers Na + and Mg 2+ highest in Andean rivers Lowland waters are more dilute in ions although similar in proportions Napo River (Andean) Yasuní River (lowland) Mg 2+ (mg/l)

15 Na + (mg/l) Sources of flood water Floodplain waters span the range from river water to local water Most are dominated by local water Diversity of water sources may increase floodplain biodiversity Floodplain waters along Napo River Mg 2+ (mg/l)

16 Floodplain Hydrological Regimes Compared Moist Andean Amazon Central Amazon Dry Season Pantanal Arid Tropical Australia Erratic, short Riverine inundation Predictable, long

17 Ecological implications: Do erratic floods matter? Ecological phenomenon Aquatic macrophyte growth and coverage Fish life cycles tied to flood pulse Flooding resets vegetation succession Flooding as a filter for terrestrial biota Dry season stress on aquatic biota River-floodplain exchange of organic matter, nutrients Central Amazon Abundant Andean Amazon Sparse Closely Unknown perhaps less? Sometimes Severe Modest High Often (channel migration) Lower Lower Modest

18 Acknowledgements Field assistants and local guides. Local communities and organizations Napo Wildlife Center & Sani Lodge HYBAM Project - INAMHI (Ecuador) & SENAMHI (Peru) World Wildlife Fund - Russell E. Train Fellowship National Geographic Society - Committee for Research & Exploration Grant National Science Foundation - Doctoral Dissertation Improvement Grant Michigan State University: Center for Latin American Studies, Center for Water Sciences, College of Natural Sciences, Graduate School, Kellogg Biological Station