Arch. Hydrobiol. 125 I 4 385-410 I 1 Smrrgm, Okrober 1992 Hydrologic and trophic controls of seasonal algal blooms in northern California rivers By MARYE. Po-, University of California at Berkeley’ With 15 figures and 1 table in the text Abstract Ckzdophoru glomeruta L., a dominant macroalga in lakes and rivers worldwide, undergoes a marked bloom-detachment-senescencecycle in unregulated rivers of north- ern California with natural winter flood, summer drought flow regimes. In two re- gulated channels which probably did not experience scouring floods, however, low standing crops of attached Cladopboru turf persisted throughout the year. The contrast between Ckzdophora phenology in regulated and unregulated rivers suggests that Cla- dophoru cycles may be extrinsically driven by factors related to the hydrograph. Pre- liminary data on seasonal patterns of animal abundance in regulated and unregulated channels suggest that winter flooding promotes Cladophoru blooms in rivers by reducing consumer densities. A working hypothesis relating hydrologic regime, food chain length, and algal phenology in rivers is advanced. This hypothesis predicts that pro- nounced algal bloomdetachment-senescence cycles will occur in unregulated rivers in Mediterranean climates following winter flooding, and that blooms will not occur in the absence of flooding in regulated channels, or in natural rivers during prolonged drought. Introduction In rivers which are sunlit, rock-bedded, and clear at low flow, attached algae are often dominant components of ecological communities. Cludophoru gfomerutaL., a filamentous green, may be the most common and cosmopolitan macroalga in temperate rivers throughout the world (BLIJM,1956; WHITTON, 1970; WHAFSEet al., 1984). In sunlit rivers of the western United States, Cla- dophoru provides much of the physical structure in the habitat during the low flow season, and plays a driving role in food web dynamics (LAMBERTI& RF.SH, 1983; Gw,1987; FEMINELUet al., 1989; POWER,1990 a, b). In these rivers and elsewhere (notably in the Laurentian Great Lakes), Cludophoru blooms create severe management problems (BLUM,1956, 1982; BELLIS, 1967; AUERet al., 1982; MILLNER& SWEENEY, 1982; GOLDMAN& HORNE, 1983). Despite their ecological and economic importance, factors regulating growth, detachment, and senescence cycles of Ckzdophoru are still not well understood (WHI-ITON, 1970; NEIL&JACKSON, 1982). ’ Author’s address: Dept. of Integrative Biology, University of California at Ber- keley, Berkeley, CA. 94707, USA. 25 jrchiv f. Hydrobiologie, Bd. 125 0003-9136/92/0125-0385$6.50 0 1992 E. Schwcizerbm’scbe Vcrlagsbuchhmdlung.DJDXI Stuttgm 1 386 Mary E. Power Hydrologic and trophic controls of seasonal algal blooms 387 This study addresses three questions about Ckdopbora in northern Cali- fornia rivers: 1. What is the seasonal cycle of Ckzdqhora in rivers with natural summer drought, winter flood hydrographs? 2. How does this cycle compare with Cladophoru phenology in regulated rivers with artificially stabilized flow? m Eureka 3. How do abundance patterns of invertebrates and smallvertebrates asso- ciated with Cladophoru change seasonally in regulated and unregulated chan- 2 nels? To develop hypotheses about factors governing timing, magnitude, and duration of algal blooms and mat detachment, I monitored Cludophora, physical factors, nutrients, and associated biota in four unregulated and two re- gulated rivers in northern California. Results from this survey complement ex- perimental studies on smaller spatial scales of controls on this dominant river dga (LAMBEXTI& ~SH, 1983; LIGON,1986; FEMINELLAet d., 1989; POWER,1990 a, b). \ Study Sites I monitored six rivers near sites gaged by the United States Geological Survey (USGS) (Fig. 1). These rivers differed in discharge, exposure to sun, and land use in their watersheds. Two rivers were regulated upstream of the monitored sites, either by a dam (Dry Creek) or by a water diversion (from the Eel River to the East Fork of the Russian River) and had artificially sustained summer baseflow (Fig. 2 e, f, Fig. 3). Dry Creek ex- perienced stable low flow throughout the year. Dividing discharge by channel drainage area indicates that releases from the Warm Springs Dam above Dry Creek were consid- erably less than natura winter flows in channels of similar drainage area in this region (Fig. 4). Consequently, the bed of Dry Creek was stable throughout the period of study. To compare the relative intensity of bed movement for the remaining five rivers, I used two empirical generalizations about gravel bedded riven. First, bankfull discharge typically has a recurrence interval of about 1.5 years (e.g. DUNNE& LEOPOLD,1978). Sec- Fig. 1. Location of the six study reaches (numbers 1,2,4-7) and of a diversion from the ond, significant gravel bed mobility often does not occur until the flow is close to bank- Eel River to the Russian River (3) that stabilizes flow in the East Fork Russian River discharge (Pmmn, 1978). In the East Fork Russian River, elevated winter flows oc- full (site 2). Drainage areas above the six monitored sites are, for the two regulated channels: curred, but peak discharge in 1989 remained below bankfull discharge as estimated from (1) Dry Creek (USGS 11465000): 562km', and (2) East Fork Russian River (USGS flood frequency analyses (Fig. 5). Hence, it is likely that little bed movement occurred. 11461500): 239 km2. For the four unregulated channels, drainage areas are (4) Outlet This inference is consistent with visual observations that the bed of the East Fork Creek (USGS 11472200): 417kmz; (5) Middle Fork Eel (USGS 11473900): 1929 km2; (6) Russian River showed no evidence of scour over the period of study. In contrast, the South Fork Eel (USGS 11475500): 114km' and (7) Elder Creek (USGS 11475560): other four monitored rivers, which were unregulated (the South Fork Eel River, Elder 17km'. Creek, Outlet Creek, and the Middle Fork Eel River), experienced the natural summer drought, winter flood hydrograph typical of streams in regions with Mediterranean climates (Fig. 2 a-d). Each unregulated river experienced flows equal or greater than bankfull discharge during the 1988- 1989 study period (Fig. 5). Visual observations over the summer (Fig. 3). During October, the month of minimum discharge in natural chan- the winter confirmed that beds in these four rivers moved, and were subject to consid- nels, most monitored sites had current velocities slower than 5cms-'. In the two re- erable scour. gulated rivers, current velocities during October ranged from 0 to > 50cms-', and During the summer low flow season, from June through September, the four unre- were fairly evenly represented among the monitored sites (Fig. 6). Despite the large gulated rivers experienced low or no discharge (Fig. 3). In contrast, flows in Dry Creek variation in drainage areas and discharges of the six rivers (Fig. 1, legend), in summer and the East Fork Russian River were maintained at levels ranging from 2-4 m's-' over months they were all easily waded. Hydrologic and trophic controls of seasonal algal blooms 389 388 Mary E. Power Low Flow Discharge S. Fk. Eel, 1988-1989 m -mwl P -0- SouthFkEel - ElderCreek - OutletCreek - Middle Fk. Eel East Fk. Russian May Jun Jul Aug Sep outlet Creek. 1988-1989 Fig.3. Mean monthly discharge during the summer low flow season in the four unre- gulated (solid lines) and two regulated (dashed lines) channels. Note that in August and September, when there was little or not flow even in the large unregulated rivers, flow continued in the two regulated channels. salmon hatchery just below Warm Springs Dam, upstream from monitored sites. Both sources contribute nutrients to the stream, and probably account for its relatively XpBi91Pp3~so elevated levels of nitrate (Fig. 8). The East Fork Russian River near Ukiah receives water diverted from the Eel River through the Potter Valley Diversion (Fig. 1). The East Fork Dry Creak, 1988-1989 E. Fk. Russlan RIver, 1988-1989 Russian does not receive agricultural runoff, but human habitations occur very close to the river just upstream from the monitored cross sections. This river had the second I\ -m :a highest measured level of nitrate of the six studied here (Fig. 8). Insolation of the E. Fk. Russian, however, was much less than at Dry Creek due to shading during mornings and afternoons by tall alder trees and narrow valley walls. Outlet Creek is not regulated, but is otherwise heavily impacted by humans. Many human dwellings occur along the creek, which also receives sewage effluent from the town of Willits (Fig. 1). Of the unregulated streams, Outlet Creek has the highest meas- ured nitrate levels (Fig. 8). Outlet Creek is extremely open and sunlit, as the active, boulder-strewn channel kept open by winter floods is much wider than the wetted chan- nel during the summer low flow period. This is also the case along the monitored Fig.2. Monthly discharge records (from the USGS) for the four natural (a-d) and two reaches of the Middle Fork Eel. The land around the Middle Fork Eel is sparsely settled regulated (e, f) channels. Records for the South Fork Eel are from a currently monitored by humans, and subject to light cattle grazing. Crystal-clear water and low nutrient station near Leggett, CA, where the drainage area is 642 km', 5.6 times larger than at the levels (Fig. 8) suggest little human impact on this river at the monitored site. The South study site near Branscomb, where USGS monitoring was discontinued. During Fork Eel River flows for 5 km through a 3200 hectare forest preserve (the Northern 1968 - 1970 when Branscomb and Leggett stations were both monitored by the USGS, California Coast Range Preserve) and is also relatively undisturbed by humans, al- discharges were highly correlated (0.93 < r < 1.00 for 18 of 24 consecutive months, though sparse settlements and a sawmill occur upstream from the monitored site. As in 0.27 < r < 0.78 during four transitional spring or fall months (May, June, August, and Outlet Creek and the Middle Fk., winter floods open a wider channel in the S.