Reservoir-derived subsidies provide a potential management opportunity for novel river ecosystems

Aquatic ecosystems world-wide are being irreversibly altered, suggesting that new and innovative management strategies are necessary to improve ecosystem function and sustainability. In river ecosystems degraded by dams environmental flows and selective withdrawal (SWD) infrastructure have been used to improve habitat for native species. Yet, few studies have quantified nutrient and food web export subsidies from upstream reservoirs, despite their potential to subsidize downstream riverine food webs. We sampled nutrient, phytoplankton, and zooplankton concentrations in outflows from the Shasta-Keswick reservoir complex in Northern California over a 12-month period to understand how SWD operation and internal reservoir conditions interact to influence subsidies to the Sacramento River. We found that nutrients, phytoplankton, and zooplankton were continuously exported from Shasta Reservoir to the Sacramento River and that gate operations at Shasta Dam were important in controlling exports. Further, our results indicate that gate operations and water-export depth strongly correlated with zooplankton community exports, whereas internal reservoir conditions (mixing and residence time) controlled concentrations of exported zooplankton biomass and chlorophyll a. These results demonstrate that reservoirs can be an important source of nutrient and food web subsidies and that selective withdrawal infrastructure may provide a valuable management tool to control ecosystem-level productivity downstream of dams

Floodplain farm fields provide novel rearing habitat for Chinook salmon.

When inundated by floodwaters, river floodplains provide critical habitat for many species of fish and wildlife, but many river valleys have been extensively leveed and floodplain wetlands drained for flood control and agriculture. In the Central Valley of California, USA, where less than 5% of floodplain wetland habitats remain, a critical conservation question is how can farmland occupying the historical floodplains be better managed to improve benefits for native fish and wildlife. In this study fields on the Sacramento River floodplain were intentionally flooded after the autumn rice harvest to determine if they could provide shallow-water rearing habitat for Sacramento River fall-run Chinook salmon (Oncorhynchus tshawytscha). Approximately 10,000 juvenile fish (ca. 48 mm, 1.1 g) were reared on two hectares for six weeks (Feb-March) between the fall harvest and spring planting. A subsample of the fish were uniquely tagged to allow tracking of individual growth rates (average 0.76 mm/day) which were among the highest recorded in fresh water in California. Zooplankton sampled from the water column of the fields were compared to fish stomach contents. The primary prey was zooplankton in the order Cladocera, commonly called water fleas. The compatibility, on the same farm fields, of summer crop production and native fish habitat during winter demonstrates that land management combining agriculture with conservation ecology may benefit recovery of native fish species, such as endangered Chinook salmon

Floodplain farm fields provide novel rearing habitat for Chinook salmon

<div><p>When inundated by floodwaters, river floodplains provide critical habitat for many species of fish and wildlife, but many river valleys have been extensively leveed and floodplain wetlands drained for flood control and agriculture. In the Central Valley of California, USA, where less than 5% of floodplain wetland habitats remain, a critical conservation question is how can farmland occupying the historical floodplains be better managed to improve benefits for native fish and wildlife. In this study fields on the Sacramento River floodplain were intentionally flooded after the autumn rice harvest to determine if they could provide shallow-water rearing habitat for Sacramento River fall-run Chinook salmon (<i>Oncorhynchus tshawytscha</i>). Approximately 10,000 juvenile fish (ca. 48 mm, 1.1 g) were reared on two hectares for six weeks (Feb-March) between the fall harvest and spring planting. A subsample of the fish were uniquely tagged to allow tracking of individual growth rates (average 0.76 mm/day) which were among the highest recorded in fresh water in California. Zooplankton sampled from the water column of the fields were compared to fish stomach contents. The primary prey was zooplankton in the order Cladocera, commonly called water fleas. The compatibility, on the same farm fields, of summer crop production and native fish habitat during winter demonstrates that land management combining agriculture with conservation ecology may benefit recovery of native fish species, such as endangered Chinook salmon.</p></div

Map of agricultural substrates in the approximately two-hectare experimental field.

<p>Shading represents different agricultural substrates. Background matrix (white) was short stubble. Arrows represent direction of water flow.</p

Location of Yolo Bypass and important landmarks including Knaggs Ranch.

<p>Location of Yolo Bypass and important landmarks including Knaggs Ranch.</p

Representative juvenile Chinook salmon before (top) and after (middle) rearing for six weeks on the Knaggs Ranch experimental agricultural floodplain on Yolo Bypass.

<p>Bottom picture is of a tagged Knaggs fish incidentally recaptured in a rotary screw trap in the Yolo Bypass Toe Drain 13 miles downstream of the release site four weeks after the termination of the experiment.</p

Proportion of zooplankton taxa found in (from left to right) the water column of the flooded rice field on February 14^th (n = 4), Feb 27^th (n = 4), the stomach contents of free-swimming juvenile Chinook salmon collected on Feb. 24^th (n = 6) and March 13^th (n = 19) and the stomach contents of enclosure-reared (e) juvenile Chinook salmon collected on March 13^th (n = 48).

<p>Proportion of zooplankton taxa found in (from left to right) the water column of the flooded rice field on February 14^th (n = 4), Feb 27^th (n = 4), the stomach contents of free-swimming juvenile Chinook salmon collected on Feb. 24^th (n = 6) and March 13^th (n = 19) and the stomach contents of enclosure-reared (e) juvenile Chinook salmon collected on March 13^th (n = 48).</p

Growth rates (mm/d, g/d) of juvenile Chinook salmon across the experimental enclosures.

<p>Dotted line represents mean value for all enclosures.</p