The Vertical Dimension of Lotic Systems

Transcription

1 The Vertical Dimension of Lotic Systems The structure and function of lotic systems result from physical, chemical, and biological processes occurring in 4 dimensions (Ward The 4-dimensional nature of lotic ecosystems.) Note all 3 dimensions shown in figure.

2 Stream as a PIPE -versus- an INTERACTING ELEMENT of the catchment Hydrologists have recognized the interactive nature of catchments for a long time Ecologists began to think this way around 1960 (Bencala 1993) Orghidan (1959) coined the term Hyporheic Zone

3 Hyporheic Zone Definition: Subsurface region of streams and rivers that exchanges water with the surface Impermeable boundary Hyporheic zone ( under river ) Groundwater in fractures or permeable material (Phreatic zone)

4 Does the HZ have a distinct Boundary between surface water and groundwater or does it represent an Ecotone (transition zone)? (people disagree) How to distinguish HZ from SW and GW? Temp. stability: SW << HZ < GW D.O. levels SW > HZ >> GW Light levels SW >> HZ > GW Current Vel. SW >> HZ > GW Conductivity SW << HZ < GW Biota

5 Geomorphic setting is important! HZ best developed in alluvial streams and rivers with coarse, permeable substrate (b and c on the right) Groundwater may or may not be present Alluvium = sediments deposited and formed by fluvial forces

6 Hyporheic Zone interacts with other hydrologic compartments in a stream valley. (Figure 1, left) (Boulton et al. 1998) Alluvial Aquifer often synonymous with groundwater. Extends beyond boundary of riparian zone. Parafluvial Zone under active channel but lacks surface water. (Figure 1, below)

7 Hyporheic flow patterns: Lateral Vertical Longitudinal Why does surface water enter the hyporheic??

8 GW-SW exchange is measured with piezometers

9 Vertical Hydraulic Gradient (VHG) VHG = dh/l + - dh = difference (meters) in vertical head inside piezometer (yellow arrows) L = distance (meters) inserted into substrate (orange arrows) + VHG is upwelling - VHG is downwelling Surface Water Substrate Hyporheic Water

10 Ecological Importance of HZ Habitat Ø the Hyporheos (versus Benthos) Refuge from extremes Ø stream drying ü some invert species move into HZ Ø during spates/floods (poor evidence) Physico-chemical conditions Ø Temperature and nutrients Ø Upwelling vs. downwelling zones

11 Hyporheic fauna Defined in terms of life cycle dependency on HZ (1) occasional hyporheos (can be found there) often >50% benthos (2) obligate hyporheos (must occur during part of life cycle) Balkans: 10/34 chironomids Montana: 8/42 stoneflies (3) permanent hyporheos (complete life cycle in HZ) many crustaceans, nematodes, copepods Any insects? True GW fauna blind crustacea restricted to GW (Gibert et al. 1994)

12 Fauna and HZ Geomorphology Faunal composition varies with characteristic size of the saturated sediment. Larger bodied species less representative in sand. (Hakenkamp and Palmer 2000)

13 There are maggots in my drinking water! (Stanford and Gaufin 1974, Flathead Basin, MT) Hyporheic zone can be extensive High hydrologic connectivity with main river channel Copyright Flathead Lake Biological Station

14 Alluvial Aquifer adjacent to Flathead River, MT (Stanford and Gaufin 1974) PhD work on distribution of Plecoptera in the Flathead Basin Could not find Paraperla frontalis nymphs in river, but adults were commonly collected. Water engineer called with a strange problem: maggots in his pumped groundwater 2 km from river!!

15 How is this possible? Hydrographs 2.4 km from river!! 0.5 km from river Main channel

16 Active microbially-mediated nutrient transformations Microbes in HZ transform key nutrients that upwell to fuel algal production downwelling upwelling Nitrogen Organic N (dissolved and particulate) is transformed to NO 3 -, which is available to algae. Why aren t these nutrients taken up by algae in the HZ? Phosphorous Where interstitial environment becomes more anoxic, immobilized P is released from dissolved solids or dissolved/particulate organic matter and converted to biologically available PO 4 3-.

17 Algal response to hyporheic flow (Valett et al. 1994, Sycamore Creek, AZ) Scouring flood removed algal biomass Recovery in Upwelling zone >> Downwelling Zone Nutrient enrichment via hyporheic metabolism

18 2 scales of LONGITUDINAL Hyporheic Exchange Bedrock, channel resistant to erosion Reach scale (riffles and pools) Segment-scale (through long alluvial valley)

19 Riparian response to hyporheic exchange (Harner and Stanford 2003, Middle Fork Flathead River, MT) 1 km Canyon mouth Wide alluvial valley Canyon mouth Alluvial segment bounded by bedrock knick points Distinct DW and UW zones (losing and gaining reaches) Measured Cottonwood growth, density, and leaf nutrient concentrations from plots adjacent to both reaches

20 Riparian response to hyporheic exchange (Harner and Stanford 2003, Middle Fork Flathead River, MT) 1 Gaining reach had: 1. larger trees (faster growth) 2. More Nitrogen in leaves 2

21 Bull trout (Salvelinus confluentus) Spawning (Baxter & Hauer 2000, Flathead Basin, MT) Observation: Trout spawn in alluvial valleys of 3rd-4th order streams, but not all stream valleys with "suitable" gradient, cover, substrate are used. Sampled 9 streams. Two types of valley segments: - bounded (have knickpoints or geomorphic control on stream gradient at either/both ends of valley) - unbounded (no knickpoint, valley just gets narrow) Bounded Alluvial Valley Segment (BAVS)

22 * * Measured VHG at 2 scales: 1) Segment scale (valley scale) 2) Reach scale (riffle-pool) Valley (segment) scale hyporheic flow for 4 streams using >500 piezometers to measure +VHG at top, middle, bottom of alluvial valley [1 piezometer per 15 m] Reach scale hyporheic flow: 2 spawning reaches sampled [1 piezometer per 2 m]

23 : Trout spawn in streams with BAVS Fig. 3 shows number of spawning redds as a function of BAVS from GIS for all 9 streams Fig. 4 shows density of redds in alluvial valleys as a function of measured VHG in 4 streams. (Groundwater here is hyporheic upwelling.)

24 What s different about bounded valley segments? Bedrock, channel resistant to erosion Bedform-scale (riffles and pools) Reach-scale (through long alluvial valley) How would winter temperature vary between upwelling and downwelling zones? (Fig. 5) upwelling downwelling ice Most spawning occurs at the downstream end of BAVS, with warmer winter temps and less ice. Why spawn there?

25 : Trout select spawning sites at 2 scales Fig. 6 shows at beform (riffle-pool) scale, more redds where VHG is higher. Redd locations (yellow dots) occur where? High interstitial flows e.g., downwelling zones at heads of riffles Why? High interstitial flow = high oxygen

26 2 Scales of spawning selection within BAVS: Segment scale Valley upwelling area with warmer winter temperature Reach scale - high interstitial flow (riffle downwelling) and high dissolved oxygen Bedrock, channel resistant to erosion Bedform-scale (riffles and pools) Reach-scale (through long alluvial valley)

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