Metabolism of an arid region canal ecosystem

Journal of Arid Environments (1987) 12, 255-267

Metabolism of an arid region canal ecosystem Paul C. Marsh* & Stuart G. Fisher] Accepted 1 March 1985 Canals are important aquatic habitats in many arid regions, yet their biological structure and function are relatively unknown. We estimated ecosystem and component metabolism and examined biotic and physical longitudinal changes in the Arizona Canal, Southwestern U.S.A. Gross primary production (P G ) averaged 6'70 g Oz/mz/day and ecosystemrespiration (R) was 5'82 g Oz/mz/day. Ecosystem PdR was 1'16, indicating that the canal was autotrophic and thus exported or accumulated organic matter. Macrophytes contributed 20% to whole-system P G and 15% to total R, epiconcreticalgaeaccounted for 80% of P G and 55% of R, and sediment respiration was 30% of ecosystem R. Consistent downstream decline of N03-N was attributed to biotic uptake independent of physical changes in the canal. The spatial distribution of aquatic macrophytes wasdetermined by physicalfeatures of the system. Longitudinal changesin algal community structure were a result of both biotic and physicalfactors, the relative effects of which could not be separated. No consistent patterns of metabolism were observed despite longitudinal changes in autotrophs and nutrients.

Introduction Canals are conspicuous features ofthose arid landscapes occupied by man. In the deserts of Southwestern U.S.A. there are more than 11,000 km of canals, 2000 km of which are in the Phoenix metropolitan area, central Arizona. In comparison, the entire state of Arizona contains about 6000 km ofnatural rivers and streams, half ofwhich are regulated by dams. Most natural watercourses are restricted to mountainous regions, while low desert has little permanent water other than canals and large rivers such as the Colorado. Canals thus constitute a major portion of flowing waters in the Southwest. Canals are major lifelines which move water long distances to agricultural, industrial, municipal and domestic users. Although canals support a varied biota (Marsh & Minckley, 1982; Marsh, 1983; Marsh & Stinemetz, 1983), their explicit purpose is water supply. To enhance water conservation and delivery efficiency, most canals are morphometrically uniform. Because of this physical uniformity, canals may provide a unique opportunity to examine selected concepts of current interest to stream ecology. Southwestern canals are large, uniform channels which flow for many tens of kilometres with little physical change. Water width, depth, velocity and volume are constant for long periods, and no tributary or seepage inputs exist. Riparian vegetation is absent. Substrate is uniform, being largely impervious concrete, and metabolically active deep sediments

* Center forEnvironmental Studies, Arizona State University, Tempe, Arizona 85287, U.S.A. t Department of Zoology, Arizona State University, Tempe, Arizona 85287, U.S,A. 0140-1963/87/030255+ 13$03.00/0

© 1987Academic Press Inc.(London) Limited

256

P. C. MARSH & S. G. FISHER

(Hynes, 1983; Grimm & Fisher, 1984) are absent. In these nearly uniform physical systems, longitudinal patterns of structure and function might be attributed solely to biotic factors. We examined biological communities, oxygen metabolism and chemical and physical features along a 12-km reach of a major canal in Arizona. Canal metabolism has never been reported, and there are few examples of concurrent estimation of whole-system and component metabolism for any aquatic system. Our first objective was to improve our general understanding of canal ecology by investigating these aspects of canal functioning. Our second objective was to assess the utility of these canals, as a model of lotic systems, for investigating causes of longitudinal pattern. Description of the Arizona Canal system The Salt and Verde rivers provide water for use in the Phoenix, Arizona metropolitan area located in the hot Sonoran Desert of Southwestern U.S.A. River water is diverted at Granite Reef Diversion Dam (c. 400 m MSL) just below the confluence of these rivers into two large concrete structures, the Arizona and South Canals (Fig. 1). The Salt and Verde rivers together drain 35,000 km 2 of land area and, except during unusual flood events, their entire flows are discharged through the canals. We selected a relatively uniform, 12-km reach of the Arizona Canal (T2N, R6E, Maricopa Co.) immediately downstream from Granite Reef Dam. The east-west oriented channel is trapezoidal in cross-section (Fig. 2), averages 16'5 m (range 13,2-19'2 m) bottom width, and has a mean sidewall angle of 45° (ran~e 35°-54°). Design capacity is 53'8 m 3/s and approximate modal summer flow is 34 m Is. At modal discharge, water depth, surface width and current velocityrespectively average 1·7 m, 19·2 m and 1·15 m/s, and are relatively uniform throughout the reach (Fig. 3). The total submerged surface area of the canal is 25'35 ha, of which 22'1% is sidewall and 77'9% is bottom.

"f\IZONA C"1N"1L 7

5

c"'--"-B

o=

5 10 KILOMETERS

Figure 1. (a) Stateof Arizona showing majorriversand location of the Phoenix metropolitan area' (b) majorcanals of the Phoenix metropolitan area; (c) aerialview of the study reach of the Arizon~ Canal. Letters designate stations; numbersindicate km downflow.

METABOLISM OF AN ARID REGION CANALECOSYSTEM

, \

2 I

0

Station

7

r' ~ '" r,

/

o (A)

7

2

oS

2

1

\

7

.. f\ ~ \

.c

Q.

0

0

257

/

:3 (B)

/

5 (C)

2

I

~ r' \

o

7

/

9 (D)

7

2t' 1\ 0

0

5

10

/7

15

!

12 (E)

20

Width (m)

Figure 2. Scale cross-sections of the Arizona Canal, 14'2 m 3/second (14 November 1983).-Letters designate stations; numbers indicate km downflow.

,..

.-... ,'" ~ E

~-

1·5

( a)

1·5

1·0

',0

0·5

0·5

0

~-

25

"~-E

20

.c

2·0

~-

,,0

0

( b)

25 20

15

Q.E

15

(c )

2·0 1·0

0

0

6

8

9

10

II

12

Distonce downflow (km)

Figure 3. (a) Velocity, (b) width, (c) depth of the Arizona Canal, 14'2 m 3/second (14 November 1983), and (d) aerial view of the study reach.

258

P. C. MARSH & S. G. FISHER

The reach receives water inputs only at Granite Reef Dam and as runoff from small desert watersheds during infrequent, intense local storms. Regional precipitation averages 23·4 em/year (NOAA, 1982). Canals are dredged between one and three times annually to remove nuisance aquatic macrophytes and sediments. Total material removal, judged from visual estimates, seems small and the system is little affected by these maintenance efforts. The canal is drained each November-December for c. 4 weeks for cleaning and repair. The physicochemical characteristics of the influent water are determined largely by relative flows from the Salt and Verde river reservoirs. Salt River water is high in conductance (1140 uS/em), Na+ (161 mg/litre) and Cl" (234 mg/litre) compared to Verde water (510 uS/em conductivity, 30 mg/litre Na" and 19 mg/litre Cl-) (USEPA STORET, 1981; December 1950-September 1979, N = 303-522). During the study period from December 1982 to November 1983, canal water was slightly basic (pH c. 7'5); conductance varied from 250 to 835 uS/em and cr from 28 to 133 mg/litre. Nitrate-N varied from 0 to 125 J.Lg/litre and soluble reactive phosphorus (SRP) from 4 to 68 J.Lg/litre. Water temperatures vary annually from c. 10°C to 30°C; daily range in summer is less than 5°C. Local mean daily air temperatures range seasonally from c. 10°C(January) to 32°C (July); daily summer-time temperatures vary from about 21°C to 38°C, with maximum readings in excess of 43°C occurring regularly from late June into August (NOAA, 1982; Sellers & Hill, 1974). Organic sediments are locally abundant (depth to c. 15 em) throughout the canal, but comprised only 14% of total bottom area. These provide substrate where aquatic macrophytes Potamogeton pectinatus L., P. crispus L. and Zannichellia palustris L. become established, inducing further accumulations of sediment. Concrete sidewalls and bottom are colonized by Cladophora glometata (L.) Kutzing and an assemblage of epiconcretic diatoms. Blue-green algae, Nostoc sp. and Oscillatoria sp., occur in middle portions of the reach. Invertebrate taxa are few and their abundance low, as is typical of Southwestern canals (Marsh, 1983; Marsh & Stinemetz, 1983). Predominant are Turbellaria, Oligochaeta, Hydracarina, Ephemeroptera, Trichoptera, Chironomidae and introduced Asiatic clam, Corbicula fluminea (Philippi). The last is conspicuous in some areas, where dense aggregations of several thousand per square metre occur. Most macroinvertebrates are collector-gatherers or filterers; except for the scraper Helicopsyche mexicana Banks (Trichoptera), other functional groups are poorly represented. The canal supports a diverse and abundant ichthyofauna (Marsh & Minckley, 1982; unpublished data). Individuals of several fish species are of angling quality, yet no sport fishery exists. We did not assess the roles of benthos or fishes. Methods

Metabolism measurements Whole-system metabolism was estimated from diel changes in dissolved oxygen (Odum, 1956; Owens, 1969; McDiffett, Carr et al., 1972; Fisher & Carpenter, 1976) measured by modified Winkler titration on four dates in June, July, September and November 1983. Measurements at intervals of 2-4 hours were made at stations A, C and E, located respectively 0, 5 and 12 km downflow of Granite Reef Dam (Figs 1-3). Collection was scheduled based on time of flow in order to monitor sequentially the same water mass three to five times as it passed through the study reach. Diffusion was estimated from analysis of night-time oxygen curves; community respiration was assumed constant throughout the diel period. Metabolism estimates were for the entire reach (A-E) and two linearly connected segments (A-C, C-E).

METABOLISM OF AN ARID REGION CANAL ECOSYSTEM

259

Replicate water samples for chemical analyses were collected in congruence with diel O 2 sampling schedules and on 18 other dates from December 1982 to November 1983. Nitrate-N was determined by cadmium-copper reduction to nitrite (Wood, Armstrong et al., 1967), ammonia-N by the phenolhypochlorite method (Solorzano, 1969) and SRP was measured colorimetrically (Murphy & Riley, 1962). Net photosynthesis and community respiration of epiconcretic (attached to concrete substrate) algae were estimated from 0,5-1,5 hour in situ incubations between 11:00 and 13:00 hours of colonized artificial substrates in light and dark chambers. Chambers were cylindrical (5'0 x 9·5 em, 150 ml), clear glass containers mounted horizontally on concrete slabs at 0'2 m depth. Substrates were 3·8 x 7'6 cm unglazed ceramic tiles coated with concrete which simulated the canal lining. Tiles were colonized over 2-6 weeks at depths of 0,2-0,5 m on the sidewalls or on the canal bottom ate. 1·5 m depth. Metabolism was unaffected by incubation depth (0'2 versus 1'5 m), presumably because of light saturation. Chlorophyll-a on tiles was measured spectrophotometrically after freezing and extraction in 90% basic acetone (Wetzel & Likens, 1979). Metabolism within canal water was measured as dissolved O2 change in standard light and dark bottles incubated at surface and 1·5 m depth. Metabolism of aquatic macrophytes was determined from light-dark bottle incubations (chambers as above), at 0'2 mdepth, of plant leaves (Westlake, 1967; Fisher & Carpenter, 1976). Leaves from both meristem and mid-sections of Potamogeton pectinatus plants were used to estimate plant metabolism (exclusive of roots). Sediment respiration was determined as O 2 change in 0,5-1'1 hours in 100-175 ml water overlying sediments enclosed in situ by darkened cores. Measurements were made at dusk during canal draw-down when water depth over the sediments was less than (j'1 m. Because photosynthetic rates vary with insolation, we used whole-system diel curves to estimate daily net primary productivity (P N ) from hourly measurements for algae and macrophytes. Daily respiration (R) was estimated by multiplying hourly respiration by 24, and daily component gross productivity (P G) was computed as P N +R. Since we measured major subsystem components contributing to whole-system metabolism, the sum of subsystem estimates should equal whole-system values; that is, P GE = P Ga + P Gm + P Gw +PGs> where subscripts E, a, m, wand s refer respectively to ecosystem, algae, macrophytes, water and sediments. We attempted to verify this over a 12-day period in November 1983 when ecosystem and subsystem metabolic rates were estimated concurrently just prior to the canal being drained. Fresh macrophyte samples for dry mass estimation were collected with a 24'5-cm core from the drying canal, and a detailed map of substrate, algae and rnacrophyte areal coverages was compiled from nearly 3000 observations along 75 transects at 0'16-km intervals. Macrophyte coverage was also estimated from water surface measurements in June, August and September. Areal estimates were used to determine the relative contributions of subsystem components to whole-system metabolism. Algal community structure, distribution of autotrophic components, metabolism and nutrients were examined for longitudinal change within the reach.

Results Daily P GE in the 12-km study reach (A-E) averaged 6·70 g 02/m2 over four dates (Table 1). P G was highest in June and fell consistently to a minimum in November. Seasonal patterns in metabolism were probably due to differences among dates in solar radiation, or other factors (Table 2). Ecosystem respiration averaged 5'82 g 02/m2/day over the period, was highest in July and, except for the low November rate, showed no seasonal trend. PdR ranged from 0'93 to 1'69 (Table 1) and averaged 1'16, indicating that the canal ecosystem was autotrophic; organic matter was produced in excess of that

P. C. MARSH & S. G. FISHER

260

Table 1. Estimates of daily ecosystem oxygen metabolism determined from diel oxygen curves onfour dates in theArizonaCanal. Stations A, C and E are 0, 5 and 12 km downflow g Ozlm2/day Reach

Date

PN

PG

R

PaiR

2-3 June 1983

A-C C-E A-E

3·44 4'35 3·66

9·68 8'91 8·94

6·24 4'56 5'28

1·55 1'95 1'69

26-27 July 1983

A-C C-E A-E

5'46 7-87 7-42

A-C C-E A-E

8'16 6·48 7'92 5·52 6·72 6·48

0'67 1·21 0'94

7-8 September 1983

-2'71 1-39 -0'50 1·04 0·50 0'60

8-9 November 1983

A-C C-E A-E

0'08 -0,38 -0,26

3-84 3'12 3'60

1-02 0'89 0·93

6·56 7-22 7'08 3-92 2-74 3-34

1'19 1·07 1-09

Table 2. Estimates of selected parameters on dates when whole-ecosystem metabolism studies were conducted in theArizonaCanal Date 2-3,June 1983 26-27 July 1983 7-8 September 1983 8-9 November 1983

Discharge (m3/second)

Temperature COC)

Dissolved O2 (mg/litre)

NHrN (ug/litre)

Radiation* (langleys/day)

38'2 34'0 33-8 31-8

17,5-22'4 21'8-27'0 23-8-28'6 16'4-18·5

7-82-9'93 6'71-8'68 5-88-8'65 7-83-10'07

13-30 12-32 6-36

ot

721'7 611'8 525-8 332'4

* Global solar radiation at Tempe, Arizona. t Notdetected. consumed. Relative changes in estimates of P G , Rand PdR for the upstream (A-C) and downstream (C-E) segments showed no pattern among dates. Subsystem component metabolism

Epiconcretic algae at all stations were predominated by Cladophora glomerata and an attached diatom assemblage. Algal P G was 2'00-8'85 g 02/m2/day and respiration was 0,91-2,93 g/m 2/day (Table 3). PdR for algae was always high (> 1'7) and relatively constant (mean 2'84, SD 0'59; N = 18), indicating that the algal subsystem was highly autotrophic. Metabolism of this component did not vary consistently as a function of incubation depth (0'2 versus 1·5 m), location (upstream versus downstream) or period of substrate colonization (14-42 days) (Table 3). A general downstream decline in both Pc and R on 31 August and 14 October was reversed on 28 October and 11 November. Standing crop of chlorophyll-a also decreased in a downstream direction on the first two dates and increased downflow on the last two. Water-column metabolic activity was undetectable in light and dark bottle studies. Incubation periods of 0,5-1,5 hours at midday resulted in no measurable change (mean

261

METABOLISM OF AN ARID REGION CANAL ECOSYSTEM

Table 3. Estimates (mean ± SE; N = 12) of daily algaloxygen metabolism determined in light and dark chambers in theArizonaCanal. StationsA, E, C, D and E are 0, 3, 5, 9 and 12 km doumflou: g 02/m2/day Date

Colonization (days)

31 August 1983 (0'2 m depth)

23

31 August 1983 (1'5 m depth)

23

14 October 1983

14

Station

PN

PG

R

PdR

A

3-90 ± 0'94 1'94 ± 0'42 1'09 ± 0'27

5·60 2-95 2·00

1'70 ± 0'19 1'01 ± 0·08 0'91 ± 0'26

3'29 2·92

A

2'47 ± 0'17 2·26 ± 0'27 1-65 ± 0'56

4'17 3'27 2'56

1'70 ± 0'19 1·01 ± 0·08 0'91 ± 0'26

2'45 3·23 HI

A B

2'99 2'64 2'10 2'89

± ± ± ± 1'39 ± 1-69 ± 1·58 ± 4'34 ± 3-66 ± 3'29 ± 5·92 ± 6·59 ±

0'32 0·33 0·52 0'35

4'93 4·40· 3'17 4'06

1·94 1·76 1'07 1'17

± ± ± ±

0'40 0·43 0'29 0-17

2·54 2-50 2'96 3'47

0'21 0'19 0'38 0·18 0'96

3'36 4'00 2'60 6'85 5·56

1'97 ± 2-31 ± 1-02 ± 2·51 ± 1'90 ±

0'39 0'58 0'34 0'23 0·52

0'60 0'97 2·20

6'22 8'29 8·85

2'93 ± 1'43 2-37 ± 0'56 2·26 ± 0·91

1'71 1·73 2·55 2'73 2'93 . 2'12 3,50 3'92

C E

C E

C

D

28 October 1983

28

A B

C D

E

11 November 1983

42

A

C

D

NO

change 0'01 mg 02/litre/hour, SD 0'04; N= 14) from initial concentrations (mean 9'36 mg Oj/litre, SD 0'06;N = 6, T = 18·2°C). We thus considered both Pa and R to be zero for the water-column subsystem. Macrophytes included Potamogeton peainatus (84% total plant dry mass), P. crispis (trace) and Zannichellia palustris (16% dry mass). Specific metabolism of P. pectinatus leaves from terminal portions of plants was greater than ofleaves from mid-sections (P N = 7'35 and R = 1·52 mg 02/m2/hour/g dry mass (N = IS) for tips versus P N = 3'46 and R = 0,66 mg 02/m2/hour/g dry mass (N = 10)for mid-sections). We estimated a 1:9 ratio for the proportion of plant-tip leaves to mid-section leaves, and calculated a weightedmean metabolism on that basis. Standing crop in cores from macrophyte beds averaged 154'2 g dry mass/m? (SD 105·3; N = 20) (all species). Thus, daily macrophyte P N based onP.pectinatus = 7'12,PG = 9'88andR= 2·76 g02/m2/day. PdR was 2'58, indicating that the macrophyte component was highly autotrophic. These estimates do not include metabolic contributions of stems, which should be small. Inclusion of root respiration would result in a reduced value for PdR, although roots made up only 15%of macrophyte biomass. Sediment respiration was highly variable, presumably due to differences among cores in organic matter content and microbial activity. On 12 November we estimated average daily R; = 1'70 g Oim2/day (SD 1'08; N = 12). This component was entirely heterotrophic (P G = 0). Metabolic rates for each subsystem were extrapolated to whole-system rates on an areaweighted basis (Table 4). The percentage contributions to total P and R by algae, macrophtyes and sediments were similar for the upstream and downstream reaches. Algal metabolism was more than 80% of total P G and more than 50% of total R, while macrophytes accounted for about 20% of PG and 15% of R. More than 30% of system respiration was attributed to sediments.

P. C. MARSH & S. G. FISHER

262

Table 4. Estimates of dailycomponent and whole-system oxygen metabolism in the Arizona Canal, 2-14 November 1983. Component estimates are expressed as areaweighted values for each reach. Stations A, C and E are0, 5 and 12 km downfiow g Oz/mz/day Reach

Component

A-e

algae macrophytes sediments summation whole-system algae macrophytes sediments summation whole-system algae macrophytes sediments summation whole-system

C-E

A-E

PN

PG

R

PdR

1'73 0'45 -1,05 1'13 0'08

3·22 0'76 0 3-98 3'92

1-49 0·31 1·05 2-85 3-84

2·16 2'45

3'22 0·57 -0'58 3-21 -0'38

4'42 0·93 0 5·35 2'74

1'20 0'36 0'58 2'14 3'12

3-68 2·58

2-67 0'55 -0,71 2'51 -0'26

3-94 0'91 0 4-85 3-34

1'27 0'36 0'71 2-34 3-60

3'10 2·53

1-40 1·02

2'50 0'88

2'07 0'93

Summations of subsystem component estimates for P G and P N were higher than wholesystem estimates, while summation estimates for R were lower than whole-system values (Table 4). PdR from subsystem summations (1'40-2'50) were thus substantially higher than indicated by diel studies for the whole system in November (0'88-1'02).

Spatial patterns Spatial distribution and areal coverage of algae and macrophytes showed distinctive patterns in November 1983 (Fig. 4). Cladophora coverage declined abruptly from more than 80% to a minimum of9'2% between 3'6 and 4'8 km downflow, and then gradually increased to a maximum of 84·5% at 9,6-10'4 km. Blue-green algae (primarily Nostoc with some Oscillatoria) first appeared in abundance where Cladophora coverage was minimal. They followed an increase in coverage parallel to that of Cladophora, to reach a maximum of 57'7% about 7'6 km downflow, then declined abruptly to essential absence beyond 8'8 km. Aquatic macrophytes were generally scarce to absent from the canal headwaters in November (Fig. 4). They covered about 11% of the canal bottom below 1·0 km; however, macrophytes were in discrete, dense stands often downstream from bends in the canal, rather than uniformly distributed throughout. Similar percentage cover estimates (1214%) were obtained by surface observations in June, August and September. Asiatic clam, Corbicula fluminea, was rare between 0 and 6'0 km. Below that segment, coverage was about 14% of the canal bottom to a maximum of nearly 30% in the furthest downstream 1'6 km. As with macrophytes, Corbicula was found in distinct, dense aggregations. Significant downstream declines in N0 3-N were observed on 23 of 24 dates throughout the year; the average reduction at Station E was 43% of the concentration at Station A. During die! studies (Table 5), the greatest net decline between stations A and E (17' 3 I1g1 litre) occurred in November, when influent concentrations were high (86 I1g/litre); net declines were 4'5-6'0 ug/litre on dates when influent concentrations were less than 20 I1g1 litre.

18·3 31'3 31-6 33'9

6-7 10'0 19-4 81'0

± ± ± ±

± ± ± ± 4·3 14·7 7'4 3·4

1'8 2'2 2'7 7'7

C

(7) (26) (7) (9)

(7) (15) (9) (9)

t NS, not significant.

* F statistic from one-way analysis of variance for difference among stations.

(8) (26) (6) (9)

(7) 1.3 2'9 (15) (9) 2-9 11-4 (8)

Soluble reactive phosphate (fLg/litre) 16'4 ± 2-2 2-3 June 39-2 ± 15'8 26-27 July 7-8 September 26'2 ± 3'7 35-7 ± 3'4 8-9 November

10'4 ± 11-5 ± 20·2 ± 86·2 ±

A

Station

17·1 24·3 26'9 34·1

4'4 7'0 14'4 68'9

± ± ± ±

± ± ± ± (8) 2·5 10'6 (27) 3-3 (8) (9) 4·6

1-6 (8) 1-9 (14) (9) 2'3 2-1 (9)

E

0·624 7-327 1'953 0·504

28·818 10-703 11·541 6'494

F*

(2,20) (2,76) (2,18) (2,24)

(2,19) (2,41) (2,24) (2,23)

D.f.

NSt 0'005 NS NS

0'005 0'005 0-005 0·025

p<

Nitrate-N and soluble reactive phosphate onfour dates in theArizonaCanal (24-hour mean ± SE, N in parentheses). Stations A, C and E are0, 5 and 12 km dawnflaw

N itrate-N (fLgllitre) 2-3 June 26-27 July 7-8 September 8-9 November

Date

TableS.

~

w '"

tv

~

V)

...,0-<::

V)

o

rm n

~

:>

n

oZ

o

tTl

::<'

t:l

~

o"I1

en

-a::

tl:l

or-

~

a::

P. C. MARSH & S. G. FISHER

264

80

I

I

IR.

I

.d

40

~

,....0"

c> '" 0

Q; > 0 u

20

\

\ \ \

\ \

Blue-green algae

\J \

b.,_

0

Macraphyfes

20 10 0

A(O)

C(5)

E(12)

Slot ian (krn)

Figure 4. Longitudinal patterns of areal coverage by algae and macrophytes in the Arizona Canal, 14 November 1983.

Although highly variable, SRP showed a statistically significant downstream decline during 26-27 July, falling by 38% from 39 to 23 ug/litre between stations A and E (Table 5). Concentrations of SRP remained constant throughout the reach on 23 other dates, with an average difference between stations A and E of less than 2%. Ammonia-N (Table 2) showed no longitudinal pattern. Discussion Metabolism

Daily P GE in the Arizona Canal ranged from 3· 34 to 8'94 g Oz/mz/day and R E from 3'60 to 7·92 g Oz/mz/day, rates falling into the intermediate-high-productivity stream category of Fisher & Carpenter (1976). Ecosystem metabolism rates in the canal are only slightly lower than those of nearby Sycamore and Pinto creeks (Lewis & Burraychak, 1979; Busch & Fisher, 1981; Grimm & Fisher, 1984), the only other Sonoran Desert lotic systems for which metabolic data are available. In spite of metabolic similarities, the creeks and canal share few biotic and abiotic characteristics. For example, Sycamore Creek is small (1-4 m wide, flow rate <0·05 m 3/second), has metabolically active sand-gravel sediments, generally lacks macrophytes, and is subject to severe flood disturbance (Fisher, Gray et al., 1982). However, shading is low and primary producers are predominated by algae in both creeks and the canal. Autochthonous primary productivity is dominated by algae in most small streams; yet in intermediate-sized rivers, macrophyte productivity may assume an important role. However, this is highly variable. Fisher & Carpenter (1976) summarized results of several studies which indicated that macrophytes contribute 12'5-1520 g OzlmZlyear (1'2-30%)

METABOLISM OF AN ARID REGION CANAL ECOSYSTEM

265

to annual P G in a variety of stream ecosystems. Macrophytes in the canal were responsible for somewhat less than 20% (0,91 g/mz/day) of total P G in November 1983. This extrapolates roughly to an annual macrophyte P G of c. 400 g Oz/mz/year, a rate exceeded in lotic systems only by macrophytes of Silver Springs, Florida (Odum, 1957). The epiconcretic algal component in the Arizona Canal contributed about 80% of total ecosystem Pi, in November 1983. Using 80% of mean whole-system P G of6'70 g Oz/m z/ day (die1 studies, Table 1), annual algal P G is 1608 g Oz/mz/year. This rate is relatively high compared to estimates for similar algal assemblages (30-1200 g/year; McConnel & Sigler, 1959; Vannote, 1963; Sumner & Fisher, 1979; Naiman, 1983). Moreover, algal metabolism in natural stream ecosystems of similar discharge to the canal are generally much lower than measured here. R; in the Arizona Canal was about 30% (0'71 g Oz/mz/day) of whole-system R. Annually, sediments consumed about 585 g Oz/m z, a rate (1'7 g Oz/mz/day) similar to measurements of other river muds (about 2-4 g Oz/mz/day; Edwards & Owens, 1962; Edwards & Rolley, 1965). R; in the canal is about half that of deep-sediment respiration in Sycamore Creek, Arizona (3'5-4'7 g Oz/mz/day; Grimm & Fisher, 1984). Although subsystem measurements in several lotic systems have been summed to estimate whole-system metabolism (King & Ball, 1967; Mann, Britton et al., 1970; Westlake, Casey et al., 1970; Hill & Webster, 1982; Naiman, 1983), there have been few attempts to measure subsystem and whole-system metabolism concurrently. Fisher & Carpenter (1976) and Sumner & Fisher (1979) reported rates for major subsystems plus whole-system estimates, but their measurements were temporally separated. It appears that we consistently overestimated P N and underestimated R in summed subsystem measurements compared to whole-system values in the Arizona Canal (Table 4). Ii seems unlikely that the individual P N values for algae and macrophytes are overestimates; the use of static incubation chambers should result in underestimation, since flow enhances rather than depresses metabolism by current-dwelling autotrophs (Whitford & Schumacher, 1964; McIntire, 1966; Odum, 1957; Westlake, 1967). We more probably underestimated sediment R and/or failed to account adequately for other components (e.g, the clam Corbicula, macrophyte roots) that contribute to whole-system respiration. Based on other research (Aldridge & McMahon, 1978; Marsh, 1983, 1985; unpublished data), Corbicuia respiration in the canal could approach 1'0 g Oz/mz/day where clams were abundant. However, on a canal-wide basis, this would contribute only about 0'1 g Oz/mz/day to whole-ecosystem R, which is insufficient to account for observed whole-system and summation differences.

Longitudinal patterns Among the parameters measured, only N0 3-N showed a consistent pattern oflongitudinal downstream decline independent of physical changes in the canal. This pattern was observed throughout the year and at both high and low influent concentrations. In November 1983, NOrN was reduced by nearly 20 fLg/litre over the study reach. Similar decline of inorganic nitrogen is common to many Sonoran Desert streams (Grimm, Fisher et al., 1981). It is notable that net decline ofNOrN was relatively small «6 ug/Iitre) in June, July and September, when whole-system P N varied from - O'50 to 3'66 while wholesystem R remained relatively stable (5'28-6'72 g Oz/mz/day). Whole-system P N in November was similar to that in July and September, but respiration was about one-half that on other dates. Although decline in NOrN in part reflects autotrophic uptake, net decline in nitrate does not necessarily correlate with system P N • Other processes (e.g. denitrification, bacterial assimilation) may thus be important in shaping nitrogen dynamics. Is the Arizona Canal sufficiently uniform physically to allow separation of biotic and abiotic causes of longitudinal change? Compared to natural streams, the canal is remarkably

P. C. MARSH & S. G. FISHER

266

invariant, yet changes in morphometry and hydrology do occur. Segments where physical transitions were greatest (2-3 and 8-9 km, Fig. 3) coincided with abrupt changes in algal communities (Fig. 4); however, the intervening reach from 3 to 8 km was the most uniform segment of the canal and significant changes in algae occurred there also. Some changes in algal community structure presumably are due to biotic factors. Others, such as the occurrence of macrophyte beds along the inside of bends, are probably due to physical factors. Our evidence thus indicates that even small changes in morphology can result in biotic changes which might otherwise be attributed to the effects of upstream organisms on those living downstream. Even in this relatively uniform system there obviously are complex interactions among biological and physical features. Although several longitudinal patterns are well developed in the Arizona Canal, biotic and abiotic mechanisms producing change cannot be separated clearly except in the case of downstream N0 3-N decline. Influences of relatively minor physical incongruities of the system (compared to natural streams) are confounded with biologically mediated factors in shaping observed patterns. Further discrimination of biological versus physical factors as determinants of downstream change can probably be achieved only in systems which are demonstrably more uniform than the Arizona Canal. New, modern canals such as those of the Central Arizona Project currently under construction are substantially more uniform and may provide a truer test of longitudinal-change hypotheses than the older Arizona Canal. We thank N. B. Grimm and W. L. Minckleyfor constructive reviewsof the manuscript. Research wassupported by a grant from the Arizona State University Research Fund. The Salt River Project provided information on canal operations, and they and the Salt River Pima-Maricopa Indian Community permitted access to the study area. References Aldridge, D. W. & McMahon, R. F. (1978). Growth, fecundity, and bioenergetics in a natural populationof the Asiaticfreshwaterclam, Corbicula manilensis Philippi, from north central Texas. Journal of Molluscan Studies, 44: 49-70. Busch, D. E. & Fisher, S. G. (1981). Metabolism of a desert stream. Freshwater Biology, 11: 301-307.

Edwards, R. W. & Owens, M. (1962). The effects of plants on river conditions. IV. The oxygen balance of a chalk stream. Journal of Ecology, 50: 207-220. Edwards, R. W. & Rolley, H. L. J. (1965). Oxygenconsumption of river muds. Journalof Ecology, 53: 1-19.

Fisher, S. G. & Carpenter, S. R. (1976). Ecosystem and macrophyte primary production of the Fort River, Massachusetts. Hydrobiologia, 47: 175-187. Fisher, S. G., Gray, L. J., Grimm, N. E: & Busch, D. E. (1982). Temporal succession in a desert stream ecosystemfollowing flash flooding. Ecological Monographs, 52: 93-110. Grimm, N. B. & Fisher, S. G. (1984). Exchangebetween interstitial and surfacewater: implications for stream metabolismand nutrient cycling. Hydrobiologia, 111: 219-228. Grimm, N. B., Fisher, S. G. & Minckley, W. L. (1981). Nitrogen and phosphorus dynamicsin hot desert streams of Southwestern U.S.A. Hydrobiologia, 83: 303-312. Hill, B. H. & Webster, J. R. (1982). Periphyton production in an Appalachianriver. Hydrobiologia 97: 275-280.

'

Hynes, H. B. N. (1983). Groundwater and stream ecology. Hydrobiologia, 100: 93-99. King, D. L. & Ball, R. C. (1967). Comparative energetics of a polluted stream. Limnology and Oceanography, 12: 26-33. Lewis, M. A. & Burraychak, R. (1979). Impact of copper mining on a desert intermittent stream in central Arizona: a summary. Journal of the Arizona-NevadaAcademyof Science, 14: 22-29. Mann, K. H., Britton, R. H., Kowalczewski, A., Lack, T. J., Mathews, C. P. & McDonald, I. (1970). Productivity and energy flow at all trophic levels in the River Thames, England. In: Kajak, 2. & Hillbricht-Ilkowska, A.(Eds), Productivity Problems of Freshwaters. pp. 579-596.

METABOLISM OF AN ARID REGIONCANALECOSYSTEM

267

International Biological ProgrammelUnited Nations Educational, Scientific, and Cultural Organization Symposium. Warsaw & Krakow: Polish Scientific Publications. 918 pp. Marsh, P. C. (1983). Benthic invertebrates of an earthen canal in the Phoenix metropolitan area, Arizona. Journal of theArizona-Nevada Academyof Science, 18: 1-4. Marsh, P. C. (1985). Secondary production of introduced Asiatic clam, Corbicula jluminea, in a central Arizona canal. Hydrobiologia, 124: 103-110. Marsh, P. C. & Minckley, W. L. (1982). Fishes of the Phoenix metropolitan area in central Arizona. North American Journal of Fisheries Management, 2: 395-402. Marsh, P.C. & Stinemetz, C. R. (1983). Benthic invertebrates of the earthen Coachella Canal, California. California Fish and Game, 69: 198-203. McConnel, W. J. & Sigler, W. F. (1959). Chlorophyll and productivity in a mountain river. Limnology and Oceanography, 4: 335-351. McDiffett, W. F., Carr, A. E. & Young, D. L. (1972). An estimate of primary productivity in a Pennsylvania trout stream using a diurnal oxygen curve technique. American M idlandNaturalist, 87: 564-570. McIntire, C. D. (1966). Some factors affecting respiration of periphyton communities in lotic environments. Ecology, 47: 918-930. Murphy, J. & Riley, J. P. (1962). Determination of phosphate in natural waters. AnalyticaChimica Acta, 27: 31-36. Naiman, R. J. (1983). The annual pattern and spatial distribution of aquatic oxygen metabolism in boreal forest watersheds. Ecological Monographs, 53: 73-94. NOAA (1982). Climatological DataAnnualSummaryArizona 1982. Asheville, NC: National Oceanic and Atmospheric Administration, National Climatic Data Center. Odum, H. T. (1956). Primary production in flowing waters. Limnology and Oceanography, 1: 102-117. Odum, H. T. (1957). Trophic structure and productivity of Silver Springs, Florida. Ecological Monographs, 1: 85-97. Owens, M. (1969).Methods for measuring production rates in running waters. In: Vollenweider, R. A. (Ed.), A Manual onMethods for Measuring PrimaryProductivity in AquaticEnvironments. pp. 9297. Oxford: Blackwell Scientific Publications. 213 pp. Sellers, W. D. & Hill, R. H. (1974). ArizonaClimate 1931-1972. Tucson, AZ: University of Arizona Press. 616 pp. Solorzano, L. (1969). Determination of ammonia in natural waters by the phenolhypochlorite method. Limnology and Oceanography, 14: 799-801. Sumner, W. T. & Fisher, S. G. (1979). Periphyton production in Fort River, Massachusetts. Freshwater Biology, 9: 205-212. Vannote, R. L. (1963). Community productivity and energy flow in an enriched warm-water stream. Doctoral Dissertation, Michigan State University, East Lansing, MI. Westlake, D. F. (1967). Some effects of low velocity currents on the metabolism of aquatic macrophytes. Journal of Experimental Botany, 18: 187-205. Westlake, D. F., Casey, H., Dawson, H., Ladle, M., Mann, R. K. H. & Marker, A. F. H. (1970). The chalk stream ecosystem. In: Kajak, ~. & Hillbricht-Ilkowska, A. (Eds), Productivity Problems of Freshwaters. pp. fiI5-635. International Biological Programme/United Nations Educational, Scientific, and Cultural Organization Symposium. Warsaw/Krakow: Polish Scientific Publications. 918 pp, Wetzel, R. G. & Likens, G. E. (1979). Limnological Analyses. Philadelphia, London, Toronto: W. B. Saunders. 357 pp. Whitford, L. A. & Schumacher, G. J. (1964). Effect of current on respiration and mineral uptake in Spirogyra and Oedogonium. Ecology, 45: 168-170. Wood, E. D., Armstrong, F. A. J. & Richards, F. A. (1967). Determination of nitrate in seawater by cadmium-copper reduction to nitrite. Journal of the Marine Biological Association of the United Kingdom, 47: 23-31.