High flows in winter and summer ideal
Power et al. (2015) argue that different sequences of wet-winter and low-summer flows also greatly affect river food webs on the Eel River. During wet years, the authors argue that high magnitude winter flows remove large over-wintering cased caddisflies (Dicosmoecus spp., AKA October caddisfly) that graze on algae; this disappearance, in turn, releases algae from grazing and encourages algal growth during summer. If summer flows are of sufficient magnitude and temperature (i.e., non-drought flows), they promote the growth of nutritious filamentous and diatom algal species. These algae species are important food resources for numerous soft-bodied caddis and mayflies, which are important food sources for over-summering salmonids. According to Power et al. (2015) these are ideal conditions in the Eel River—high winter flows followed by relatively high and cool summer base flows. Such a flow sequence promotes ideal physical habitat conditions (hydrologic connectivity, cool water temperatures, etc.) and a robust aquatic food web capable of supporting over-summering juvenile salmonids.
Other flow sequences, however, are not as favorable. When high winter flows do not occur, such as during dry years, cased caddisflies are retained within the Eel River enabling them to suppress algal growth and outcompete other invertebrate grazers such as soft-bodied caddis and mayflies. This suggests that important prey items for juvenile salmonids are not as plentiful and, if summer flows are low, means that over-summering salmonids likely experience high water temperatures with fewer food resources.
The worst case flow scenario occurs when high winter flows occur (i.e., promoting bed scour and the removal large overwintering algae grazing cased caddisflies) but are followed by a precipitous decline in summer base flow. Here, pools on the Eel River can become disconnected from one another, ungrazed mats of rotting filamentous algae form on the top of the water column, water quality significantly declines, especially dissolved oxygen, and water temperatures rise dramatically. According to Power et al. (2015), these conditions encourage the proliferation of cyanobacteria within the Eel River. Thus, while filamentous algae and diatoms support healthy river food webs under certain winter-summer flow sequences, winter floods followed by drought like summers may encourage proliferation of cyanobacteria which may contain neurotoxins lethal to fish.
Photo: Daniel Kowalski