Geol 230
GW/SW interaction
Week 13 Lecture
Subsurface influence on surface biology
Reading: Ch. 16, Jones and Mulholland, pp.381-402, by Lisa Dent, John Shade, Nancy Grimm,
and Stuart Fisher
I) Introduction
- Water exchange: the primary mechanism where the hyporheic zone affects surface
biology
(As opposed to surface water affecting subsurface biology)
- Definitions:
Surface stream: Parts covered by surface water
Parafluvial zone: Region of the active channel without water (during low flow)
Riparian zone: region bordering the channel that supports longer-lived, highstanding vegetation
Hyporheic zone: The region of saturated sediments that underlies surface,
parafluvial and riparian zones, and has active exchange
Upwelling zone: Water discharges vertically upward
Downwelling zone: Water reenters the hyporheic zone
Outwelling zone: Water enters the surface from parafluvial subsurface sediments
along stream margins
Inwelling zone: Surface water infiltrates parafluvial gravel bars
- How (surface) biological patterns and processes are affected:
Physical processes: temperature, current
Chemical interactions: nutrient supply, organic matter, oxygen
Biological relationships: use of refuges, organism movement
II) Effects on primary producers
- The main effect of hyporheic flow: Nutrient delivery (supply)
At upwelling and outwelling zones
- Ex: Sycamore Creek, upwelling zones have higher algal abundance and faster recovery
after floods
Due to nutrient availability (mostly inorganic nitrate)
- Several other studies show similar results with algae AND macrophytes!
See Fig. 1 from J&M, p. 383
See Figure 2 from J&M, p. 384
- Shows post-flood recovery: more rapid in upwelling zones
- Lateral input may also be important: parafluvial gravel bars are high in nutrients
See Figure 3 from J&M, p. 385
- Discharge point is called a spring, source or “parafluvial drainage channel”
- Ex: Chara growth at downstream base of hummocks is due to hyporheic upwelling
(Hendricks and White, 1988)
- Closing note: subsurface zones may also be nutrient SINKS!
III) Effect on microorganisms and microbial processes
- A new (and largely undocumented) area of study
- 2 likely effects:
1) Subsurface flow may change availability of resources:
- organic matter or inorganic nutrients
2) Direct transport of microorganisms
- Organic supply (DOC) is probably a key issue
Subsurface can be a supply or sink for DOC
Ex #1: Rhone river: bacterial carbon demand exceeds DOC in stream,
DOC is (postulated to be) supplied by upwelling
Ex #2: Sycamore Creek: DOC is higher in surface water, rates of
microbial activity are higher in downwelling zones
- A conclusion: microbial activity may be carbon-limited
- Other studies: microbial activity may be nitrogen or phosphorus limited
- Direct physical flushing of bacteria from the subsurface may also be significant
Several studies have shown increased bacterial outflux at upwelling or outwelling
zones
- Anaerobic bacteria, fungi and protests may also be important:
Ex: nitrifying bacteria
- Sycamore Creek: nitrifying bacteria were most active at downwelling
and inwelling zones, where ammonic and oxygen were more abundant
Ex: fungi and protists
- May be more active where nutrients are more common
IV) Effects on invertebrates
- Invertebrate density is directlyl related to Periphyton (a complex matrix of algae and
heterotrophic microbes) abundance
- Nutrients and light are the controlling factors here (see above)
- Studies that directly relate invertebrate assemblages to upwelling and downwelling are
lacking!!!!!!!!!!!!!
- Hyporheic zone is a refuge for invertebrates during a flood
Serves as a source for colonists
Note: Desert systems (Sycamore Creek) may be the exception; may require
colonization from upstream
- Upwelling may dislodge inhabitants
- Downwelling may carry them deeper into sediment
V) Effects on fish
- Earliest hyporheic studies: were related to fish habitat
- Refuge from drought
Ex: lungfish burrow into sediment
- D.O. in spawning gravel
Eggs of salmonids are emplaced in gravel (but this is unusual!)
High permeability, high vertical exchange is preferable
Fine sediment infiltration is bad:
Occludes pores, lowers permeability
Upwelling may prevent eggs from freezing, or maintain cool temp in
summer
- Food availability due to enhanced algal growth in upwelling zones
See J&M Figure 4, p. 390
See Table 1 from J&M, p. 391
Shows a preference for downwelling conditions (found at the downstream ends of sandy
runs)
- Next week’s paper (Geist et al, 2000) will expand on this idea
VI) Heterogeneity and scale
Hyporheic interaction varies with time, space, scale:
A) Temporal heterogeneity:
- Surface discharge is INVERSELY correlated with storage zone volume and
residence time (especially when storage includes surface water pools and eddies)
- Hyporheic exchange is reduced during high flows
Note: My question: proportionately??
- So: streams alternate between “pipelike” (high flows) and more connected with
vertical and lateral subsystems (low flows)
- Seasonal nutrient change is also a factor:
- Nitrate concentrations change when water table rises into the rooting
zones of alders (Fall and Winter)
- DOC is highest in late Fall and Winter, when decomposition is
maximum
- Hyporheic O2 may vary seasonally
B) Spatial heterogeneity
- Over a stream reach many factors vary w/ upwelling + downwelling
- Heterogeneity in the substrate generally increases biological diversity
Ex.: May include the whole nitrogen cycle
C) Scale
- Scales of exchange vary:
- From
- Catchment to
- Reach to
- Bar form to
- mm scale
VII) Human effects
- Variables:
Land use practices
Flows
Extraction of GW or SW
See J&M Table II, p. 395
- Excess fine sediment: is a common problem