H41F-1052
Nutrient Uptake and Cycles of Change: the Ventura River in Southern California
California
Al Leydecker,1 Julie Simpson2 and Leigh Ann Grabowsky3
(1Bren School of Environmental Science, University of California, Santa Barbara, CA 93106, al.leydecker@cox.net; 2Dept. of Ecology, Evolution and Marine Biology,
University of California, Santa Barbara, CA 93106, simpson@lifesci.ucsb.edu; 3Santa Barbara Channel Keeper, 714 Bond St., Santa Barbara, CA93103, lag@sbck.org)
Matilija Creek
North Fork
14
15
13
Robles
Diversion
Dam
3
Ojai
10
08
Lake
Casitas
11
07
Canada
Larga
06
05
04
03
02
Pacific Ocean
01
(Santa Barbara Channel)
Ventura
6 km
60
40
20
1950
1960
1970
1980
1990
VR02-NO3
The Ventura River, a 580 sq. km mountainous coastal watershed 100 km northwest of Los Angles, Ca., has a
VR01-NO3
300
flow
nitrate (µM)
1
Mediterranean climate with an average annual rainfall of 500 mm. More than 90 % of the rain falls between Nov.
and Apr., and most of the annual discharge occurs over a few days. The seasonal and inter-annual variations are
extreme – dashed lines represent the mean (12.1 cm) and median (3.8 cm) annual runoff. Since the spring of 2001
nutrient concentrations have been sampled monthly at 15 locations, 3-times a month on the lower river (VR01, 02,
03 and 06) since March 2003. A sequential decrease in concentrations below a sewage treatment plant outfall
between sites VR06 and VR03 provides estimates of nutrient uptake under changing conditions.
flux-NO3
% plants (left scale)
% algae (left scale)
flow
VR03-NO3
2000
NO3 uptake (kg-N/km-d)
1940
1000
200
6
100
flow (cfs)
0
1930
100000
VR06-NO3
1000
10
10
flow at VR06 (cfs)
400
the flux decrease between VR03-VR01
annual runoff (cm)
80
10
100
1
0.1
Oct-00
Jan-01
Jun-01
Oct-01
Jan-02
Jun-02
Oct-02
Jan-03
Jun-03
Oct-03
100
Jan-01
May-01
Sep-01
Jan-02
May-02
Sep-02
Jan-03
May-03
Sep-03
35
100000
VR06-PO4
VR03-PO4
VR02-PO4
VR01-PO4
flow
30
25
1000
flow (cfs)
phosphate (µM)
phosphate (SRP, µM), algae
Oct-00
VR01-PO4
VR01-NO3
VR03-N/P
0.1
20
15
% algae (left scale)
flow
VR01-N/P
1000
10
10
1
0.1
10
0.1
Oct-00
10
Jan-01
Jun-01
Oct-01
Jan-02
Jun-02
Oct-02
5
2001-NO3
100000
Oct-00
Highly variable annual runoff engenders cycles of sediment deposition and removal, algal growth, and the
advance and retreat of riparian and aquatic vegetation. Major El Nino winter storms (every 10 to 12 yrs. on
average) begin a transformational cycle by completely scouring the channel of vegetation and fine sediment. The photos
show the Ventura River upstream of sampling site VR02 in July 2001, August 2002, March 2003 and Sept. 2003 (left to
right). In 2001 the river, still recovering from the scouring of the river bed during the El Nino of 1998, was open and
relatively free of vegetation. Algae were abundant in the open water with little competition from plants. Aquatic plants
began to supersede algae by the end of summer, 2001. By 2002, after a winter of low flows and no storms large enough to
uproot them, plants came to dominate the entire river channel. Winter storms in 2003 scoured the center of the channel and
re-established deeper flows, allowing algae to briefly reappear, but plants, surviving along the channel edge, soon recovered.
5
0.1
Jan-01
May-01
Sep-01
Jan-02
May-02
Sep-02
Jan-03
May-03
Sep-03
Nitrate concentrations typically peak in late winter,
presumably from rainy season mineralization and mobilization,
and decrease to a minimum by late summer, i.e., they follow the
path of the hydrograph, while phosphate remains relatively
constant. Concentrations at VR06, above the sewage plant –
yellow symbols – represent this variation. Below the sewage
outfall, the pattern, controlled by dry-season effluent discharge,
is of increasing concentration as normal river flows decrease
throughout summer and into the fall. During dry years, more
than 90 % of lower river flow originates as effluent. The blue
shading marks the mid-Nov. to mid-Mar. rainy season.
monthly flux (kg-N)
0
2002-NO3
Jan-03
Jun-03
PO4-2001
2003-NO3
100000
Oct-03
PO4-2002
flow (cfs), nitrate (µM), N/P
0
2
Channel Transformation (VR02
in Oct. 2002, and March, May, June,
Sept. and Nov. 2003 (left to right, top to
bottom): The plant is water primrose (Ludwigia
hexapetala) and it dominates the lower river from
VR03 to the estuary. By the fall of 2002 the river
was essentially a slow moving current through a
sea of Ludwigia. Storms in the winter of 2003
partially opened up the channel, but also deposited
sediment among the roots of the remaining plants,
creating confining banks in what had been an open
waterway (the May photo shows this process
during a small storm). Deeper, faster flows in the
now confined stream slowed, but didn’t stop, plant
expansion during the summer of 2003. In the last
photo the process begins anew with this year’s
rains. Channel narrowing, aquatic plants and the
shading of the waterway by growing riparian
vegetation end the cycle of dry-season algal
dominance.
(downstream of VR03
in June 2001, June
2002, Mar. 2003 and Sept. 2003
(clockwise from the upper left):
Winter storms of 2003 removed
aquatic plants and sediment from
the center of the channel, but
taller and deeper rooted riparian
species (willows and arundo,
visible in early stages of
establishment in 2001) were
relatively undisturbed. The
replacement of aquatic by
riparian vegetation increases the
threshold size of a channelscouring storm. Ending the
riparian growth cycle will
require another El Nino year
such as 1995 or 1998.
09
San Antonio
Cr.
12
4
Riparian Vegetation
Nitrate uptake (the flux decrease
between sewage outfall and
estuary; upper panel) is surprisingly
consistent, except during periods of heavy
algal growth (the summer of 2001, late spring
2003; “percent cover” is a rough measure of
abundance derived from the average of 15
lower river transects). Algal dominance
increases nitrate uptake 2 to 4-fold over
periods of aquatic plant domination. The shift
from algal to plant dominance also causes a
change from P-limitation to N-limitation.
When plants dominate, N/P ratios are well
below 30 (the estimated Redfield ratio for
terrestrial aquatic systems shown as a dashed
line in the lower panel) and the ratio decreases
going downstream (from VR03 to VR01),
nitrate concentrations are often at minimum
values, and the effluent driven increase in dryseason nitrate concentrations below the
sewage outfall is preceded by the phosphate
increase – all evidence of N-limitation.
Conversely, during algal periods phosphate
concentrations are at a minimum and N/P
ratios near or above 30.
PO4-2003
7
Differences in annual
runoff in Southern
California coastal streams,
magnified by riverine processes, can
1000
1000
produce greater than 3 order-ofmagnitude differences in monthly
nutrient export fluxes to receiving
10
10
estuaries. Differences are greatest for
dry-season nitrate concentrations and
export. In contrast, annual export
differences are greatest for phosphate
0.1
0.1
Oct
Dec
Feb
Apr
June
Aug
Oct
Dec
Feb
Apr
June
Aug
– the great majority of annual export
occurs during the rainy season and phosphate conc. are closely coupled with sediment load. The relative
ratios of runoff to nitrate to phosphate for annual export in 2001, 2002 and 2003 were 31:1:5, 29:1:5 and
7:1:3, respectively.